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Methods for measuring rock surface weathering and erosion: a critical
review.
Cherith Moses, David Robinson and John Barlow
Department of Geography
University of Sussex
Brighton
UK
Corresponding author [email protected]
Abstract
Studies of rates, processes and modes of rock surface, and near-surface,
deterioration, and also hardening, are central to rock weathering and building
stone research, conservation and management. There is a need to measure and
monitor weathering at the rock-atmosphere interface to facilitate understanding of
climatic, environmental and lithological controls on the evolution and
development of surface weathering features. This paper reviews long-established
and recently developed field and laboratory methods used by geomorphologists
to monitor and measure the impact of weathering and erosion on physical and
mechanical properties of exposed rock surfaces and their immediate sub-
surface. Key advances are highlighted, their application to multi-scalar
understanding and modelling of rock surface weathering in different contexts is
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discussed and potential future advances to provide new insights into rock
weathering, durability and materials conservation are identified. In highlighting
key advantages and disadvantages of a wide range of methods to the broader
earth science community, the paper aims to contribute to further innovative
thinking across disciplines to develop new methods for measuring and monitoring
rock weathering.
Keywords: rock weathering, geochemical, geotechnical, laser scanning,
microscopy, tomography
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1. Introduction
The alteration of rock surfaces exposed to atmospheric conditions is one of the
most fundamental of geomorphic processes and is generally referred to by the
term ‘weathering’. Alteration occurs by physical, chemical and biological
processes which result in changes that most commonly weaken the rock surface
resulting in what is variously termed rock deterioration, decay, crumbling,
decomposition, rotting, disintegration, disaggregation or breakdown (Hall et al.,
2012) which lead to erosion of the surface. However, in some cases weathering
may harden the surface layers of the rock, at least temporarily (Robinson and
Moses, 2011). Whilst such hardening may in the short term protect a rock
surface from erosion, it may in the longer term contribute to rock deterioration. A
hardened crust, for example, may respond very differently to environmental
stresses than the underlying rock thus leading to decay of the surface layer. This
is quite a common occurrence in both natural and urban environments (e.g.
Smith, 2003).
This paper focuses on methods used by geomorphologists to monitor and
measure the impact of weathering on exposed rock surfaces and their immediate
sub-surface by measuring changes to the form and physical properties of rock
surfaces that result from weathering. Its purpose is to demonstrate how long
established and recently developed methods are complementary in providing
new insights into rock weathering, rock durability and materials conservation that
facilitate modelling of weathering processes and contribute to further innovative
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thinking across disciplines. It does not consider either weathering at depth, to
produce a regolith (e.g. Thomas, 1994; Taylor, 2011; Migón and Thomas, 2002;
Dosseto et al., 2008; Hilley et al., 2010) or soil (e.g. Ferrier et al., 2010; 2012), or
weathering fluxes associated with rivers and oceans (e.g. Vance et al., 2009).
The focus is on bare rock surfaces, because it is these soil free surfaces that are
directly impacted by atmospheric processes. A range of methods have been
developed to monitor weathering-induced surface and sub-surface (a few
millimetres to ~ 10 centimetres, depending on the method used) changes on
rocks and on natural building stones and to measure their rates of surface
erosion (Viles, 2000). This paper reviews developments in these methods and
assesses their contribution to one of the key challenges in geomorphology –
quantifying rock surface weathering, via the measurement of surface and near
surface weathering impacts on rock physical and mechanical properties and on
rates of material loss.
Rock weathering processes operate synergistically with erosion processes, with
the latter removing weathered materials to reveal the fresh rock surface to
further, continuing interaction with atmospheric conditions. The operation of
weathering processes and their synergies with erosion are discussed in detail by
Robinson and Moses (2011) and Viles (2013b). A key element to understanding
how weathering and erosion processes combine to influence rock surfaces is the
measurement and monitoring of their impact. The range of methods developed to
do this has evolved quite rapidly over recent years. This is partly as a
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consequence of weathering scientists applying their knowledge of natural rock
behaviour to buildings conservation (e.g. Pope et al., 2002; Prikyl and Viles,
2002; Smith, 2003; Siegesmund et al., 2004; Turkington and Paradise, 2005;
Přikyl, 2007; Smith et al. 2008; Viles, 2013a) and to understanding the potential
impacts of climate change on rock surfaces (e.g. Viles, 2002; McCabe et al.,
2011; Smith et al., 2011; Viles and Cutler, 2012). The methods are therefore of
relevance to those in the broader field of materials degradation and stone
conservation, including architects, designers, manufacturers, test laboratory
personnel, materials engineers, failure and forensic specialists, and others who
require an understanding of the effects of weather on materials and products
(e.g. Viles and Wild, 2003; Wypych, 2008; Doehne and Price, 2010; Warke et al.
2010; Viles 2013a). The development of methods has been aided by
technological advances that allow rock weathering to be examined remotely and
at much higher spatial resolutions than before. This paper focuses on
measurement methods for two reasons: first, to link long established and widely
used measurement methods to innovative new technologies, explaining how they
help to facilitate understanding of rock surface weathering; second, to provide a
resource to the wider scientific community of the advantages and disadvantages
of methods that can be used to measure and monitor rock surface and near-
surface weathering and the removal of weathered material.
1. Measuring rates of surface change.
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A range of direct contact and non-contact measurement techniques are currently
used to measure rates of surface change (Table 1).
2.1 Direct and contact measurement methods
2.1.1 Direct measurement relative to a datum
Estimates of rates of rock surface recession caused by weathering were for
many decades restricted to measurement of relative rates, either between
different components of a rock, between different rock types, or of rock surfaces
relative to some other datum point. Rates of surface change can be measured
relative to a fixed, or reference, point or plane on the rock surface that is of
known age. For example, rates of surface lowering on crystalline rocks have
been measured using resistant quartz veins as the reference plane (Dahl 1967,
André 2002, Nicholson 2008). Others have measured rock surface recession
rates relative to glacial erratics and tsunami boulders of known age (e.g. Trudgill
1986; Goldie, 2005; Matsukura et al., 2007). The same principle has been used
to calculate weathering rates on different rock types and in different
environments on engineering structures of known age where the depth of
recession relative to the original surface of the structure is measured
(Mottershead 1997, 2000). Other types of fixed point include human emplaced
artefacts that remain static relative to the rock surface. Examples include the lead
plugs inserted into balustrade stones on St Paul’s cathedral that have been used
to measure historical rates of weathering (Trudgill et al. 1989), lead lettering on
gravestones (Meierding 1993, Cooke et al. 1995, Inkpen and Jackson 2000) and
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rock art of known age that protects the rock surface (Häusselman 2008). The
height of rock pedestals beneath sedentary organisms of known age have also
been used to calculate rock surface downwearing rates (Trudgill, 1983). In all of
these cases, the artefact protects the rock surface creating a step, the height of
which can be measured. The height of the step, divided by the age of the artefact
in years yields an annual rate of surface lowering.
The major limitation of this method is that the majority of natural rock surfaces do
not have fixed or reference surfaces from which recession measurements can be
taken. Figures calculated from natural fixed points are minimum values because
the reference surface is also likely to have experienced recession, albeit at a
much slower rate than the surface being measured. Also, the rates calculated by
such methods are usually decadal or longer timescales and so do not allow
annual or seasonal variations to be quantified. In some cases, however, it is
possible to measure the impact of individual catastrophic events, such as fire
(Dorn, 2003) or lightening (Wakasa et al., 2012), that cause the rock surface to
spall and where the depth of spall relative the original rock surface provides a
measure of the erosion caused.
2.1.2 Contact measurement: erosion meters
Since the 1960s there have been significant developments in the design of
equipment to directly measure weathering and erosion over much shorter annual
or seasonal timescales. The most widely used instrument is the Micro-Erosion
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Meter (MEM; High and Hannah, 1970) and its modified, more sophisticated
version, the Traversing Micro-Erosion Meter (TMEM; Trudgill et al. 1981). The
contribution of the MEM to understanding landform evolution is reviewed in
Stephenson and Finlayson (2009). The basic triangular instrument, constructed
of rust resistant marine grade steel, rests on three hemispherical studs that are
permanently fixed into the rock surface, 150 mm apart, and constitute a
measurement site. In the original design these studs were ball-bearings that were
glued either directly to the rock surface or to the top of a hexagonal bolt held in
the rock by a bolt fixing device such as the Rawltamp flat (High and Hannah,
1970). Most studies now use dome headed screws fixed into the rock, because
of problems with ball-bearing detachment. An engineers’ height gauge is used to
measure the distance from the instrument to the rock surface. The studs remain
fixed in the rock surface and the instrument can be accurately replaced, using the
Kelvin Clamp principle, so that changes in the rock surface elevation can be
measured over time. By rotating the frame round the three studs, the standard
MEM measures three separate points at each measurement site, though the
number can be increased by installing additional, adjacent sites. Each adjacent
site can use two studs from the initial site and requires the fixing of only one
further stud. This enables a network of adjacent measurement sites to be quickly
and efficiently installed. More point measures from a single site, without the
installation of extra studs, can be obtained by mounting additional gauges and
probes on the base plate, which may also be enlarged (Ellis, 1986). However,
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this makes the instrument bulkier and is a modification that has not been widely
adopted.
In comparison, the TMEM has proved more popular. This has an engineer’s
gauge mounted on a moveable structure located at the centre of three cylindrical
arms set at 120o to each other. These arms rest on a triangular metal frame with
ball bearings, of the same diameter as the arms of the block, along each edge.
The probe can be moved to many positions by resting the arms of the block
between different pairs of balls, by rotating the block and by rotating the frame on
the studs. The number of possible positions depends on the relative sizes of the
frame and balls, but one constructed by Trudgill et al. (1981) with a 300 mm
equilateral triangular frame allowed up to 1000 readings to be taken within the
area defined by the three legs. These values can be plotted to create contour
maps that indicate variations in surface lowering at individual measurement sites
(Smith et al. 1995).
A further modification has been to drill a series of holes into the metal plate of a
standard MEM to increase the number of measurement points. For example, the
MEM used to collect the most recent set of erosion measurements in a long-
running study on St Paul’s Cathedral (Trudgill et al., 2001) can measure 42
points via 14 pre-drilled holes (Inkpen et al., 2012). Until the mid 1990s, readings
were recorded manually from analogue engineers’ gauges. This still often occurs
but the process of collection of large numbers of measurements, such as are
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produced by the TMEM, and as required for example to produce micromaps, has
been made less laborious with the introduction of digital gauges that allow
automatic data collection when connected to a laptop or other digital storage
devices either via a cable or by a wireless connection (Stephenson 1997;
Stephenson and Finlayson, 2009).
The development of the MEM transformed studies of rates of rock weathering by
enabling direct measurements of annual, and in some cases seasonal, rates of
downwearing of rock surfaces. MEMs are relatively cheap and easy to construct,
and all parts can be obtained with little difficulty. All variants (MEM, TMEM and
modified MEM) are small and easily portable, simple to use, reasonably reliable
and robust, which makes them suitable for use in a wide variety weathering and
erosional environments (Swantesson et al., 2006). They can be used on flat,
sloping and even on vertical surfaces if they are carefully held in place. Dial
gauges with a resolution down to 0.001 mm are commercially available and,
taking into account environmental factors, instrument wear and operator errors,
instrument readings are thought to be correct to 0.05 mm if the instrument is
carefully calibrated (Spate et al., 1985; Swantesson et al., 2006). There are two
main drawbacks to using MEMs: first, the probe itself may cause erosion of very
soft rock surfaces and this has been noted by the authors on chalk shore
platforms, although it does not constitute a problem where erosion rates are rapid
(Foote et al., 2006; Swantesson et al., 2006); second, the screws onto which the
instrument sits are inserted into holes drilled into the rock surface and this would
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be problematic at heritage or conservation sites. Despite this, the MEM has been
used successfully to measure the recession of bare rock surfaces in many
environments and locations around the world, and on a range of lithologies,
although most commonly on limestone (Stephenson and Finlayson, 2009). It has
even been used to estimate the persistence of oil pollution on a rocky shore
(Mottershead 1981).
MEMs are particularly effective for measuring relatively rapid surface recession
on less resistant rocks, such as limestone, mudstone or schist. There are
limitations, however, where the lithology offers either very high or very low
resistance to weathering and erosion. For example, if weathering and erosion
lowers the surface very rapidly, the hemispherical studs on which the instrument
rests may become loose or detached between measurement intervals making
the measurement site redundant or requiring replacement. This has happened
over annual timescales on the chalk of the eastern English Channel coast (Ellis,
1986; Andrews, 2000; Foote et al., 2006) and over decades, e.g. on limestone
and mudstone at Kaikoura Peninsula, New Zealand (Stephenson and Kirk, 1996;
Stephenson et al., 2010) and on reef limestone on Aldabra Atoll, Indian Ocean
(Viles and Trudgill, 1984). Despite its potential shortcomings, the MEM has been
used to collect long term datasets that span up to several decades (e.g. Smith et
al., 1995; Stephenson and Kirk, 1996; Stephenson et al., 2010; Moses and
Robinson, 2011; Inkpen et al., 2012; Stephenson et al., 2012) and is still widely
used.
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Rapid surface recession rates on less resistant materials, such as glacial till and
sedimentary mudstones such as the London Clay, pose a particular
measurement problem. A modified version of the MEM has been used to
measure the recession of glacial till surfaces on the shore of Lake Ontario,
Canada. Constructed from aluminium, rather than marine grade steel, the
engineers dial gauge is replaced with a steel scale with millimetre precision. This
means that, unlike the standard MEM, it can be used underwater. In addition, the
foot of the probe is 4 cm in diameter, which is considerably larger than a
standard MEM (Askin and Davidson-Arnott, 1981; Davidson-Arnott and
Ollerhead, 1995). Clay rich, cohesive materials on marine shore platforms
experience weathering and erosion rates that are beyond the vertical
measurement capabilities of the standard MEMs, and so the Traversing Erosion
Beam (TEB), has been designed for these situations (Charman et al., 2007). A
horizontal linear beam is inserted into a fixed point on the shore platform and
height readings are recorded sequentially in four directions at right angles to
each other from a digital height gauge. Measurements can be made every
millimetre, providing a total of 1500 readings per profile. This is not often possible
in tidal environments where time is limited and so there is usually a trade-off
between measurement density and the length of time needed to complete the
measurements on each profile line. Measurements recorded at 50 mm intervals
provide a total of 28 readings per profile. The TEB, with an instrument error of 2.5
mm, facilitates longer-term measurements and can be used to show changes in
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the surface morphology with time and roughness values can be extracted from
the dataset. At the other end of the spectrum, MEMs are not sensitive enough to
detect changes on very resistant lithologies such as granite and gneiss, except in
very aggressive erosion environments or over very long timescales.
2.2 Non-contact and indirect measurement methods
2.2.1 Laser scanning
A range of Portable Laser Scanners have been designed to measure very low
rates of weathering and erosion. One of the earliest, designed specifically to map
microtopographic changes caused by weathering and erosion of igneous
lithologies, is the Swantesson Laser Scanner that can measure areas of up to 40
x 40 cm (Swantesson, 1989; 1994). A laser gauge probe is mounted close to the
rock surface, approximately 10 – 15 cm above, on an aluminium frame that has
adjustable legs approximately 30 cm in height, and is moved in the x and y
directions around the mapping area by means of two stepping motors that are
also mounted on the frame. The movement of the laser is controlled via specially
designed software on a portable computer that is linked to the instrument via a
cable. Unlike the MEM, the Swantesson Laser Scanner cannot be accurately
replaced on the measurement site, but to counter this, at least four, usually eight,
metal studs are fixed into the rock surface within the measurement area. The
positions and heights of these studs are measured to a resolution of 0.025 mm
and accuracy of ± 0.04 mm, and act as reference points for calculating changes
in the height of the rock surface. In addition to allowing the calculation of rock
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surface recession rates, the laser scanner can provide detailed surface micro-
maps, generated from up to 100 million measurement points depending on the
area of the scan and spacing of the points, that indicate areas of the rock surface
that are subject to the most rapid rates of recession. It can also be used to obtain
a measure of changes in surface roughness and produce 3D images of rock
surfaces and rock surface change using standard software.
The Swantesson Laser Scanner has since been used to investigate surface
recession rates and patterns on a range of lithologies including chalk, limestone,
dolomite, sandstone, gneiss and granite (Swantesson, 2005; Swantesson et al.
2006). The use of this Laser Scanner and its value in comparison to the MEM is
reviewed by Williams et al. (2000) and Swantesson et al. (2006). It has been
used to map small scale rates and patterns of surface change on the surfaces of
horizontal or gently sloping natural rock outcrops. It cannot be used on steeply
sloping or vertical rock surfaces because it is too cumbersome and the frame is
slightly flexible. A key advantage is that it offers a close-range, non-contact
method to monitor rock surface changes in great detail. Key disadvantages are
that the rock surface needs to be dry in order for the laser beam to register a
reading, high resolution scans can take two or more hours, and it cannot be used
on vertical surfaces or dissected surfaces e.g. within deep runnels in rock
surfaces. There are, in addition to the Swantesson Laser Scanner, a range of 3D
non-contact digitizers that are capable of collecting hundreds of thousands of
measurments in only a few seconds. For example, Meneely (2009) reports
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accuracies of 0.05 mm for weathering of building stone using a Konica Minolta
Vi9i scanner. Smith et al. (2009) used the same scanner, with brass screws as
reference points to monitor salt weathering in marble blocks. It has been used
also to quantify and analyse surface roughness and surface morphology of
weathered boulders (Ehlmann et al., 2012). In this instance the boulders were
too large for transport and the laser scanning was not possible in the field and so
plaster moulds of relatively horizontal boulder surfaces were made in the field
and subsequently scanned in the laboratory. The Konica Minolta Vi9i scanner
has a maximum footprint of 1495 × 1121 mm at a range of 1750 mm both of
which exceed those of the Swantesson Laser Scanner. To date the Konica
Minolta Vi9i scanner has been used only on relatively flat rock surfaces of
building and laboratory specimens and its use for measuring weathering and
erosion on dissected surfaces, where a greater vertical range is needed, remains
to be investigated.
Other remote sensing methods do not have the same problems associated with
vertical range but provide lower precision and accuracy. For example, Light
Detection and Ranging (LiDAR) makes use of time-of-flight to determine the
range between a reflective surface and the instrument. LiDAR instrumentation is
typically divided into Airborne Laser Scanning (ALS) and Terrestrial Laser
Scanning (TLS). LiDAR is an active sensor technology in that it emits pulses of
electro-magnetic energy and is therefore not susceptible to shadowing, a
significant source of error when using optical sensors in high relief areas (Kaab,
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2002; Liu 2008). However, as the lasers are typically in the near-infrared
wavelength, water vapour in the atmosphere can drastically reduce the range of
the instrumentation and wet surfaces tend to provide poor reflectivity due to
absorption (Huising & Gomes Pereira, 1998). ALS systems consist of a laser
instrument connected to an inertial measurement unit (IMU) and a global
positioning system (GPS) antenna to provide highly precise positional data to the
laser at the time of emission (Liu, 2008). Precision and accuracy are dependent
upon the height above ground level and is typically established via calibration
flights prior to survey. Typical values are between 15 cm vertically and 20 cm
horizontally (Liu, 2008) but in highly sloping terrain these errors can be much
greater (Hodgson & Brenahan, 2004). These data characteristics are such that
sequential ALS data is typically used to quantify geomorphic change at the basin
scale (Starek et al., 2011) and is not suitable for measuring small scale rock
surface weathering. A detailed listing of available sensors and their technical
specifications is given by Mallet & Bretar (2009).
TLS systems are typically set up at discrete stations and require multiple targets
of known coordinates to georeference the data (Armesto et al., 2009). However,
increasingly, TLS units are being mounted on vehicles in tandem with GPS
antennae and IMUs in a similar configuration to that used for airborne systems.
Modern TLS systems are capable to sampling at up to 122 000 points/second
and require specialist software for storage and processing. Long range systems
now exist that can scan out to 6 km under ideal conditions with strongly reflective
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targets. Long range systems have a precision and accuracy of 1 cm and 1.5 cm
respectively whereas shorter range systems are capable of sub-centimetre
precision and accuracy. TLS systems have been used, via the creation of Digital
Terrain Models (DTMs): to document cultural heritage sites (Rüther et al., 2009;
Guarnieri et al., 2010); to monitor 3D changes to buildings and monuments e.g.
Armesto-González et al. (2010), Cecchi et al. (2000), English Heritage (2007),
González-Jorge (2012), Palombi et al. (2008); to monitor rock art deterioration
e.g. Barnett et al. (2005), Díaz-Andreu et al. (2006); to detect and measure slope
deformation (Monserrat and Crosetto, 2008; Abellán et al., 2009) and to map
patterns of coastal cliff surface recession and quantify volumes lost in individual
events (Lim et al. 2005, 2010). Fluorescence Lidar has been used to monitor
biological colonisation on rock surfaces (Wakefield and Brechet, 2000). The
value of TLS for measuring amounts and rates of microscale rock surface
change, as opposed to monitoring and mapping change, has been investigated
via experimentally induced changes in a laboratory setting e.g. Moropoulou et al.
(2003), Birginie and Rivas (2005), Gomez-Heras et al. (2006), Bourke et al.
(2008), Gomez-Heras et al. (2008). These experiments are helping to develop
suitable methodologies for measuring rock surface weathering by identifying
specific issues in their use for monitoring and measuring rock surface change.
For example, the laser camera scanner used by Birginie and Rivas (2005) has
been shown to be more reliable on monomineralic rocks, such as limestone, than
polyminerallic ones, such as granite, where translucent minerals absorb rather
than diffuse the light.
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The application of Laser Scanning, a fast evolving form of technology, although
noted as having potential to measure rates of rock surface recession caused by
weathering (Bridges et al., 2005; Heslop et al., 2004; Laity and Bridges, 2009;
Várkonyi and Laity, 2012) is only relatively recently applied in this context and
available datasets cover very short time periods unlike the longer term MEM
database. The level of detail provided by this method, with several hundred
thousand measurement points per scan, is transforming our understanding of
rock surface recession. Its value lies in the high levels of precision, the ability to
micromap rock surfaces to indicate where weathering loss has occurred and the
relatively large area of rock surface that can be mapped very rapidly with some of
the field based lasers. Accuracy, however, depends on distance of the instrument
from the rock surface and on the size of the laser footprint. For example, Lim et
al. (2005, 2010) have used a Measurement Devices Limited LaserAce 600
terrestrial laser scanner to monitor surface changes on areas of cliff face of up to
4100 m2. The instrument was up to 70 m away from the cliff face giving an
accuracy of ± 60 mm. Bourke et al. (2008) used a Minolta 900 ‘triangulation’
laser scanner that scans from a much closer range of 60 to 100 cm to create a
CAD model of rock surfaces with a resolution of 0·23–0·40 mm. Schaefer and
Inkpen (2010), also using a Minolta 900 laser scanner found that it was possible
to detect surface changes to 0.2 mm when scanning an object or surface straight
on, with a medium range lens and at a scanning distance greater than 1800 mm.
The precision and accuracy of measurements made using laser scanners are
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influenced by factors including the scan distance, lens configuration, lighting
conditions, scanning angle and the nature of the topography of the scanned
surface. In a study of building stone using a Leica HDS 3000 laser scanner, Scott
and Young (2007) report that in addition to using lasers to measure distance to
create 3D images and monitor erosion, reflection intensity data can be used to
distinguish between clean, soiled and weathered granite surfaces. Interpretation
of intensity levels is complex but the method offers possibilities for remotely
assessing the severity of surface weathering, especially of building stone. Laser
Scanning is becoming more accessible as the hardware becomes smaller, and
therefore more portable, and less expensive. It allows the quantification of
material loss and rock surface retreat in ways that have not previously been
possible. As the technology becomes more accessible many more researchers
will be able to use it in a wide range of situations.
2.2.2 Repeat photography and digital photogrammetry
Laser scanning requires the use of specialist and expensive equipment that is
beyond the budget of many researchers. Less expensive, non-contact methods
such as repeat photography and digital photogrammetry have also been used to
quantify rock surface recession, though these still require a skilled operator. For
example, Pentecost (1991) successfully monitored the loss of surface grains
from sandrock faces using a standard 35 mm camera with close-range, high
magnification lenses to obtain data on rates of surface weathering and erosion of
the faces of sandstone cliffs in southeast England. The technique can be applied
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only to granular rocks and to relatively planar surfaces because close-range
photography has a very limited depth of focus (fractions of a millimetre).
Pentecost (1991) used a standard film SLR and the advent of digital cameras
that produce high quality digital images has facilitated a new phase of mapping
and visual representation of micro-scale rock surface weathering and erosion.
For example, polynomial texture mapping is an image processing method that
enables the representation of subtle surface features using a normal digital
camera (Malzbender et al., 2001). It is used to visualise, rather than to measure,
the rock surface. The camera and rock sample or surface both remain fixed in
position whilst a single point light source is moved around either manually or
robotically. This can be carried out in a laboratory, using samples, or in a field
setting. The technique requires a series of photographs, typically 40-80, taken
under differing illumination conditions and produces an RGB, red-green-blue
intensity, value for each image pixel based on the variance in luminance (Earl et
al., 2010a). It allows virtual relighting and modification of rock surface reflective
properties in order to bring out subtle detail. The method has been used in a
number of archaeological studies (e.g. Mudge et al., 2006; Earl et al., 2010b) as
well as for visualising fossils (Hammer et al., 2002). As with any digital
photograph the image can be draped over a 3D model of the surface to enhance
visualisation. The method enhances minute variations in surface topography and
so is applicable to mapping complex rock surface microtopographies and
monitoring surface change. Its application for heritage conservation is described
in detail in Duffy (2013).
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Advances in analytical and digital photogrammetry techniques has led to their
application to the analysis of rock surfaces undergoing weathering. Close-range
photogrammetry can be used to assess both relative changes in weathering
forms and the alteration of weathered surfaces, including measurements of rates
of surface loss across a variety of scales (e.g. Inkpen, et al. 2000). Thornbush
and Viles (2004) combine photography and digital image processing to assess
the development of soiling of smaller scale limestone wall surfaces in an urban
environment. At a larger scale Dornbusch et al. (2010) have used soft copy
photogrammetry to measure chalk coast shore platform erosion on decadal time
scales.
2.2.3 Cosmogenic dating
The calculation of rates of rock surface weathering and erosion over very much
longer timescales of millennia, something of a ‘holy grail’ in geomorphology, has
been facilitated by cosmogenic dating of rock surfaces (Nishiizumi et al., 1989;
Bishop, 2007). The physics controlling the production of cosmogenic nuclides at
the surface of the earth is discussed in detail by Lal (1991), Grosse & Phillips
(2001) and more recently by Dunai (2010). Galactic cosmic radiation, composed
primarily of high-energy protons, interacts with atoms in the atmosphere to
produce a particle flux of high energy neutrons and muons incident upon the
surface of the earth (Grosse & Phillips, 2001). High energy neutrons colliding
with atomic nuclei remove protons and neutrons resulting in the production of
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cosmogenic nuclides. These spallation reactions are highest at the rock surface
and decrease exponentially with depth (Dunne et al, 1999). Muons are less
reactive but penetrate more deeply such that at depths greater than ~3m,
muogeic production is greater than that produced through spallation reactions
(Dunai, 2010). Production rates are also strongly influenced by geomagnetic
latitude (Lal, 1991), altitude (Stone, 2000), through shielding by surrounding
topography (Dunne et al., 1999; Choi et al., 2012; Regard et al., 2012), and
through seasonal shielding of snow, sand, or peat (Grosse & Phillips, 2001). As
the production rate for any given scenario acts as the “clock”, a precise
understanding of production rates for a given area is critical to the process of
cosmogenic dating. Different nuclides may be used to investigate weathering and
erosion rates over different timescales. For example, stable, e.g. 3He, 21Ne, and
short-lived nuclides, e.g. I4C, are appropriate for the recent 10,000 years, stable
and longer-lived nuclides such as 10Be, 26Al, 36Cl, 41Ca, 53Mn and 129I are
appropriate for >Myr (Nishiizumi et al., 1993; Matsushi et al., 2010). The type of
nuclide used is also highly dependent upon the rock type. The most commonly
used isotopes, 10Be and 26Al require quartz while 36Cl can be extracted from a
variety of minerals including carbonates and feldspar. A detailed listing of the
differing nuclides and their usefulness can be found in Dunai (2010).
The removal of rock mass via weathering and erosion results in the exposure at
the surface of previously shielded rock. Therefore, a single measurement will
provide a minimum age estimate based on the assumption of zero erosion (Lal,
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1991). This limitation can be overcome in one of two ways. The first is to take
multiple samples in a vertical column in order to establish the concentration with
depth profile of a given isotope (Dunai, 2010). The profile can then be used to
optimize exposure models to the best fitting weathering and erosion rates.
Sampling in this way typically requires unconsolidated materials such as soils
and glacial deposits (e.g. Schaller et al., 2009). For bedrock surfaces, the most
common method of estimating erosion rates is via the use of isotopic ratios. This
requires two nuclides that have significantly different half-lives (Lal, 1991; Gosse
& Phillips, 2001). The most commonly used pair are 10Be and 26Al as these can
be measured together in quartz, although any combination is possible (Dunai,
2010). A ratio plot of the isotopes shows the concentration expected for various
erosion rates as well as those associated with more complex exposure histories
or with problems with sample preparation and measurement (Gosse & Phillips,
2001). A description of how to deploy the methods of cosmogenic dating
operationally is given by Balco et al. (2008). Cosmogenic isotope analysis has
also been used, in addition to measuring erosion, to calculate rates of rock
varnish build up, or accretion and development, by dating individual laminations
that make up the microstratigraphy of varnishes (Dorn, 1983; Lui and Broecker,
2007).
2.3 Methods for measuring values that represent weathering rates
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It is not always possible to measure rates of surface recession on natural rock
outcrops and other approaches have been used, including surface form, surface
roughness indicators and exposure experiments.
2.3.1 Surface form
Rock surface form is often used as an indicator of relative rates of weathering
e.g. Robinson and Williams 1994, 1996, 1999; Williams and Robinson 2000;
Robinson and Moses 2002. Surface roughness, for example, can be measured
on a range of scales and in a variety of ways. A number of simple and cheap
methods have been devised to measure surface roughness in field settings either
directly e.g. by calculating surface roughness indices using dimensions that have
been measured by manually tracing the profile collected by pressing a simple
mechanical contour gauge, of the type that can be bought in a hardware store
and accurate to within a millimetre, against the rock surface (Crowther and Pitty,
1983; Crowther 1996, 1997, 1998), using a ruler and tape measure (Ley, 1979)
or a micro-roughness meter (MRM; McCarroll 1992, 1997; Nesje et al. 1994;
Whalley and Rea 1994; McCarroll and Nesje 1996) or indirectly by comparing
Schmidt Hammer (see section 3.1) rebound values (McCarroll 1991). Micallef
and Williams (2009) have used a mechanical rock profiler to record profiles
relative to fixed points on a limestone shore platforms on the Maltese coast to
assess changes in micro-relief and also to measure erosion rates. In the
laboratory the Talysurf instrument, which draws a stylus across the surface of the
rock, can be used to record surface roughness profiles at micrometre scale from
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which indices such as the roughness average (Ra) can be calculated. This
technique has been used in combination with rock block exposure trials to
examine microenvironmental controls on rock weathering (e.g. Moses 1994,
2000; Fornós et al. 2011). Roughness parameters can also be extracted easily
from readings collected by laser scanners (Huang and Bradford, 1992;
Swantesson 1992, 1994; Pardini and Gallart, 1998; Moropoulou et al. 2003,
Birginie and Rivas 2005; Gómez-Pujol et al., 2006).
2.3.2 Microcatchment and rock exposure experiments
Microcatchment experiments are a method of studying rock weathering that
involves measuring the chemical inputs and outputs from a rock surface as it
weathers under ambient or controlled environmental conditions. They may be
conducted in the laboratory or in the field. The output occurs as runoff which can
be collected in bottles for subsequent chemical analysis. This enables the loss of
material from a stone or area of stone to be monitored and from this a weathering
rate is derived (Reddy, 1988; Halsey, 2000). Their value lies in the fact that,
unlike other techniques used to measure rates of surface recession, they provide
valuable data about the physical and chemical losses resulting from rock surface
weathering. Microcatchments typically use cut rock slabs of ~ 30 x 30 cm that sit
on a perspex base from which runoff is collected via an outlet hole where a
discharge tube is attached to a collection bottle. Detailed experimental
procedures are outlined in Reddy (1988) and Halsey (2000). It is usual for the
microcatchment to be tilted by ~ 300 from the horizontal to prevent surface
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ponding of water, and where a series of microcatchments are used they usually
all face the same aspect. O’Brien et al. (1995) modified the design to overcome
the effects of directionality by cutting the stones into hemispheres of 23.5 cm
diameter. Others have used rocks with a variety of different surface treatments to
investigate particular processes such as the potential for bioprotection of rock
surfaces, as described in Carter and Viles (2005) and the impact of vehicular
emission particulates on Portland Limestone, a commonly used building stone in
the UK (Searle and Mitchell, 2006). Whilst these individual studies are
informative about specific processes or rock types their results are not directly
comparable because of the variety of microcatchment dimensions, preparation
methods and experimental protocols used. In addition, some microcatchment
studies have been conducted in the laboratory. For example, Shelford, et al.
(1996) simulated weathering of limestone by salt and acid rain, focusing on the
spatial variability of surface change resulting from the weathering, which was
assessed through photogrammetric analysis of the slabs. Microcatchments can
also be set up directly on the surface of natural rock outcrops in the field. This
method has not been widely undertaken but has been used to investigate
solution features that develop on limestone e.g. solution flutes known as
rillenkarren (Mottershead and Lucas, 2001) and lichen weathering (Zambell et al.
2012). It has the advantage of measuring the weathering of a wholly natural, as
opposed to a pre-prepared rock surface, but a key disadvantage is that it is more
difficult to ensure comparability if more than one microcatchment is used i.e. pre-
prepared blocks can be prepared to the same dimensions and exposed at the
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same orientation and angle whilst it may be more difficult to do this on natural
rock outcrops. To address this, a few studies have combined field and laboratory
based microcatchment experiments e.g. Fiol et al., 1992; Fiol et al., 1996;
Aghamiri and Schwartzmann, 2002.
The deliberate exposure of samples of rock to investigate how they weather in a
particular environment over a period of time has become an established method
of studying rock weathering via rock block, rock tablet or rock disc exposure
trials. The rock samples used may vary from small discs only 5mm thick and 20
mm in diameter to large cubes 200 x 200 x 200 mm or more in size. They have
been employed widely for studying weathering under a variety of atmospheric
conditions, on shore platforms, in soil environments and to examine lithological
controls via the exposure of a range of rock types - although limestone is the
predominant rock type that has been used (Moses, 2000). The rate of weathering
is usually expressed as a percentage weight loss but, if the density of the
material is known, the weight loss can be converted to a surface lowering
equivalent (Trudgill, 2000). Surface lowering may also be measured directly on
exposure blocks using an MEM (e.g. Moses et al., 1995; Smith et al., 1995;
Furlani et al., 2010). The value of using exposure blocks to study rock weathering
and erosion lies in the fact that a range of rock types, of identical size and shape,
can be exposed to range of environmental or laboratory experimental conditions.
It is possible to subject blocks to a specific process or combinations of processes
in the laboratory in order to isolate particular controls on rock weathering in
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different environments. Often, however, different researchers use exposure
blocks of different sizes and shapes and different preparation methods making it
difficult to compare results across studies. Some also pre-stress their exposure
blocks in order to investigate the role of weathering histories on the current
operation of weathering and erosion processes. In addition, relatively little is
known about the influence of the surface finish of the block on the rate of
operation of processes. For example, Moses (1996) exposed rock discs, that
were cut and then smoothed with carborundum, in a range of microenvironments.
Every six months over a period of three years the surface roughness average of
the discs was, measured using a Talysurf, showing that the surface polish
appeared to retard weathering processes for the first year of exposure.
Despite these drawbacks, exposure blocks are used to provide information on
relative rates of weathering and can be examined by a range of destructive and
non-destructive methods to assess the impact of weathering processes. Rock
tablets have been used to investigate weight loss due to different weathering and
erosional processes (Trudgill 1975, 1976, 1977) and microenvironmental controls
on weathering (e.g. Gams, 1985; Goudie, 1986; Jaynes and Cooke, 1987;
Goudie et al., 1992; Goudie and Viles 1995; Moses 1996; Williams and
Robinson, 1998; Thorn et al., 2007), the influence of pollution histories and
climate on surface soiling (Viles et al., 2002), potential weathering rates in soils
under different types of vegetation cover (Dixon et al., 2006; Thorn et al. 2002,
2006a, 2006b), operation of freeze-thaw and thermal stress events (Hall 1999,
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2004) and the influence of stress histories on rock weathering and sediment
release (Warke, 2007).
3. Measuring sub-surface change.
Many weathering processes effect change not only on the exposed surface of
rocks but also within the rock just beneath the surface. For example, weathering
rinds often have a zone of weakened material directly underneath them (e.g.
Robinson and Willliams, 1987), both salt and frost weathering are most effective
in pore spaces and fractures within rock (Smith et al., 1994; Williams and
Robinson 1998; Matsuoka, 2001) and microorganisms may weaken the rock just
beneath the surface (Viles, 1987; Moses et al., 1995; Viles et al., 2000). The
latter is particularly important in extreme hot, cold or dry environments where
endolithic microorganisms are present (Friedmann and Ocampo, 1976;
Friedmann, 1982; Bell, 1993; Smith et al., 2000). The changes that take place
beneath the surface may be simply physical, such as an increase in porosity or
the creation of fractures, or chemical, either as the result of precipitation of
compounds such as salts or the selective removal of compounds by leaching.
These changes are more difficult to measure and a range of methods are used
(Table 2). The simplest way to examine subsurface changes is to fracture the
rock perpendicular to the surface or drill into the rock with a coring device.
Changes can then be identified visually or using laboratory based techniques
including microscopy, strength testing and chemical analyses, but the sample is
destroyed making it difficult to chart changes over time unless repeat sampling is
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carried out. In addition, the sampling methods can induce artefacts that might be
mistaken for real weathering features. For example, fracturing rocks to produce a
cross section might induce cracks and/or spread microorganisms across the
exposed surfaces. To avoid this, samples may first be impregnated with resin
(Ehlmann et al., 2012). In many instances, however, such destructive sampling is
not possible, for example where outcrops are protected by a conservation order.
A useful guide to sampling strategies is provided by Smith and McAlister (2000).
3.1 Changes in rock strength
Because subsurface weathering influences the porosity and changes the
compressive strength and elasticity of a rock, one of the most commonly
employed approaches for assessing the impact of weathering is to measure rock
strength. In the laboratory bulk sample strength can be measured by
compressive testing using standard methods such as the Triaxial Hoek Cell (e.g.
Allison, 1988) or the Point Load Tester (e.g. Moses et al., 2006), which can also
be used in the field. Both of these tests are destructive. More sensitive strength
tests that can be used to measure individual rock components include the
Vickers microhardness tester (e.g. Oguchi, 2004) and the Knoop hardness test
(e.g. Benavente et al., 2007a, b). Both of these tests are laboratory based
indentor tests that operate at the scale of individual minerals. A pyramidal
diamond point is pressed into the material and the size of the indent, measured
using a microscope, is used to calculate the mineral hardness. The need to
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measure subsurface changes caused by weathering has led to the development
of a range of non-destructive techniques.
The most widely used non-destructive method for measuring rock strength in the
field is the Schmidt Hammer. Originally designed in 1948 for testing concrete,
this rebound hammer has been used by geomorphologists since the 1960s,
predominantly for studying the effects of weathering (Day, 1980; Day and
Goudie, 1977; Goudie, 2006) but also as a relative dating tool (Matthews and
Shakesby, 1984; Shakesby et al., 2006; Guglielmin et al., 2012) that allows the
selection of sites for more precise dating methods to be applied (e.g. Sanjurjo
Sánchez et al., 2009). The Schmidt Hammer is portable and allows rapid non-
destructive testing of rock strength in the field and the laboratory. It measures the
rebound of a spring-loaded mass from the rock surface. The rebound value can
be converted to give the compressive strength of the rock. There are three
models of Schmidt Hammer with low, medium and high impact energies: L-type
(0.735 Nm impact energy), N-type (2.207 Nm impact energy) and M-type (which
is rarely used due to its bulk and weight; 29.43 Nm impact energy). The L and N
type Schmidt Hammers weigh approximately 1 kg whilst the M-type is
approximately ten times heavier (Stanley, 2010). They can be used across a
wide range of rock types and material hardness. The standard plunger spreads
the impact across a small area (diameter ~1.5 cm). In the L type hammer this can
be replaced with a mushroom shaped plunger that spreads the impact over a
greater area (diameter ~ 3 cm) so that softer and/or friable rocks can be
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measured. The more recently introduced Silver Schmidt Hammer, also available
in L and N types, is lighter than the classic version (~ 600 g) and stores the
readings in digital form for downloading later (Viles et al. 2011). Problems with
the use of the Schmidt Hammer include the fact that surface roughness affects
the readings, as do edge effects and rock moisture content, and even the lowest
impact model, with the mushroom plunger fitted, may not work on very soft
materials such as Chalk and London Clay or may be destructive in such cases
(Williams and Robinson, 1983; McCarroll, 1991; Sumner and Nel, 2002). The
classic Schmidt Hammer must be held so that the plunger impacts at right angles
to the test surface. Values vary if this does not occur and may vary also
according to whether it is used on horizontal, vertical, sloping or over-hanging
surfaces. The Silver Schmidt Hammer is said not to have this problem in relation
to impact direction (Viles et al. 2011). More recently, the Equitop hardness tester
that is sensitive enough to measure the hardness of very soft materials such as
fruits, has been used to measure variations in rock hardness associated with
very thin surface weathered layers (Aoki and Matsukura, 2007, 2008), changes in
rock strength linked to variations in moisture content (Viles et al. 2011) and
abrasion of weathered rock surfaces (Feal-Pérez and Blanco-Chao, 2012). It is
light and portable (780 g plus a 120 g battery pack; Viles et al. 2011). It can also
be used over much smaller areas than the Schmidt Hammer because it is a
much smaller instrument n which the rebound impact is focused on a ball-shaped
indenter with a radius that is typically 3 – 5 mm. Equotip hardness testers are
available, like the Schmidt hammer, in a range of models with different impact
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energies and indenter ball radius: G device (90 Nmm, 5 mm), D device (11 Nmm,
3mm) and C device (3 Nmm, 3mm). The 11 Nmm impact device is also available
in a range of models: the D device, already listed, is the standard model; the DL
device has a slim front section and the DC device is shorter than the other
models and both are designed for use in confined spaces e.g. recesses and
holes respectively; the E device has a diamond ball indenter and the S device
has a ceramic ball indenter, both for use on very hard materials. The D device
and its compact version, the Piccolo, have been tested by Viles et al. (2011). The
impact energy of the D type is approximately 1/200 that of the Schmidt Hammer
N-type, and 1/66 that of the Schmidt Hammer L-type and so causes less damage
to the surface being tested. However, it does not perform well on rough and/or
friable rock surfaces. Both the Schmidt Hammer and Equotip hardness tester are
easily used in the field and allow many readings to be collected over relatively
large areas in quite a short period of time.
It is well known that rock surface roughness influences the readings of the classic
Schmidt hammer. There has been some debate over whether the rock surface
should be smoothed before readings are recorded, although variations in surface
roughness and hence Schmidt hammer readings have also been used to indicate
the degree of weathering (Williams and Robinson 1983; McCarroll 1991; Goudie
2006) and as a consequence the relative ages of exposed rock surfaces
(Shakesby et al., 2006; Sánchez et al. 2009; Shakesby et al. 2011; Guglielmin et
al. 2012). Viles et al. (2011) have investigated the influence of rock surface
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roughness on the values recorded by the Equotip and the Piccolo by comparing
values measured by these instruments and the classic and Silver Schmidt
hammers on ‘natural’, rough rock surfaces compared with rock surfaces
smoothed with carborundum in order to remove any surface irregularities,
weathered areas, etc. The results indicate that differences between Schmidt
Hammer and Equotip readings may be used to investigate degrees of weathering
and case hardening and that a sequence of hardness measurements with
progressive carborundum treatments could be used as a way of extracting further
information about the rock surface weathered zone (Viles et al. 2011). Viles et al.
(2011) acknowledge that carborundum treatment of the rock surface is not
appropriate in all cases e.g. it would not be permissible on many heritage science
projects. The debate as to whether rock surfaces should be smoothed prior to
recording hardness values with rebound instruments of any type is ongoing.
It has been suggested that a more accurate means of measuring internal
changes caused by weathering is by measuring ultrasonic wave propagation or
mechanical resonance frequency. These are non-destructive tests that indirectly
measure the strength and elasticity of the rock mass and so incorporate
subsurface as well as surface changes caused by weathering. Allison (1987,
1988, 1990), Goudie et al. (1992), Allison and Goudie (1994), Prick (1997), Viles
and Goudie (2007) and Viles et al. (2010) have used the Grindosonic apparatus,
which measures the vibration pattern set up within a rock when it is excited by
being struck lightly. From the decay of this vibration pattern, the modulus of
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elasticity Ed, Young’s Modulus, and Poison’s ratio vd are computed to provide an
indirect measure of rock strength and it is possible also to determine the shear
modulus G. The results correlate well with strength measurements carried out
using the traditional Triaxial Hoek Cell but, because the test is non-destructive, it
enables repeat measurements to be made on, for example, samples subjected to
simulated weathering processes such as fire (Goudie et al., 1992; Allison and
Goudie 1994) and frost (Prick, 1997). The equipment is designed for use in the
laboratory on samples of accurately created dimensions with carefully prepared
smooth surfaces and known moisture content. The samples may comprise bars,
cylinders or circular discs, but bars are most frequently used and in this form, for
accurate results, the ratio of length to thickness needs to be greater than three
and the width of the bar should be less than one third their length (Allison, 1987,
1988; Prick 1997). Unfortunately this requires the collection or extraction of
samples in the field, which for conservation reasons may not always be possible.
However, Allison (1990) tested the equipment in the field on natural blocks of
limestone and on samples roughly cut to appropriate dimensions with a hand
rock saw. He obtained values that showed better correlations to Young’s
Modulus values obtained using the same apparatus on carefully prepared
laboratory samples and conventional triaxial Hoek Cell measurements, than were
obtained using the Schmidt Hammer.
Another instrument widely used is the Portable Ultrasonic Non-destructive Digital
Indicating Tester (PUNDIT). This instrument measures the transition time and
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velocity of an elastic pulse travelling between two points in a rock. Transducers
attached to the rock enable both compressional and shear wave (P and S waves)
velocities to be calculated and from these elastic constants such as Young’s
Modulus can be determined. Under controlled laboratory conditions, ultrasonic
equipment such as the PUNDIT have been shown to produce results that
correlate well with more traditional means of measuring compressive strength
and the modulus of elasticity and to distinguish levels of weathering (Benavente
et al., 2006; Christaras et al., 1994; Murphy, Smith and Inkpen, 1996; Svahn,
2006; Vasconcelos et al., 2007) with the advantage of being non-destructive.
The results can help also to identify changes within rocks, such as the expansion
of pores and the development of sub-surface flaws and cracks that result from
weathering but which may not be visible at the surface. Warke et al. (2006),
used a Pundit to measure the porosity of sandstone blocks used in laboratory
experiments as a means of understanding their subsequent response in
weathering experiments The equipment is light and portable but the ultra-sonic
pulse intensity measured is strongly influenced by moisture levels in the rock and
by the quality of the contact between the transducers and the rock surface. This
poses problems for its use on many natural rock outcrops, especially where the
surfaces are very uneven or covered with lichen or algal growth. Nevertheless,
Sobott (2004) successfully used ultrasonic testing to assess, in situ, the
degradation by weathering of limestone in a medieval church at Naumberg in
Germany.
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3.2 Changes in porosity and permeability
Subsurface changes associated with weathering often involve changes to the
rock porosity (percentage of void space in the rock) and permeability (ability of
fluids to move through the rock). Changes to rock surface and subsurface
porosity and permeability influence the ability of moisture to penetrate into and
move within the rock, but relatively little is known about changes in pore
dimensions caused by weathering. To date this has been studied predominantly
via laboratory experiments. For example, Nicholson (2001) and Ruedrich and
Siegesmund (2006) have used standard laboratory porosimetry measurement
methods, such as mercury intrusion porosimetry (MIP), to measure the influence
of internal rock breakdown caused by frost and salt weathering on modifications
to porosity. MIP is also used to help assess the quality of replacement building
stones (Graue et al., 2011) and to measure the impact of atmospheric pollutants
on building stones (Sanjurjo-Sánchez and Alves, 2012; Sanjurjo-Sánchez and
Vázquez, 2013). A disadvantage of MIP, however, is that it may damage narrow
pores because of the high pressures required to fill them due to the viscosity of
the mercury. Some researchers therefore recommend using nitrogen adsorption
isotherms instead to measure pore specific surface area and size distribution
(e.g. Iñigo et al. 2000; Warscheid and Braams, 2000) because the intrusion of
gas is thought not to damage the rock. Tuğrul (2004) evaluated changes in
sandstone pore geometry in response to weathering, again using destructive
methods, by examining thin sections using an optical microscope and making
Scanning Electron Microscopic (SEM) observations of unweathered compared
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with moderately and highly weathered samples. Our ability to measure rock
permeability in field, as well as laboratory, conditions is greatly enhanced by the
range of commercially available gas-driven probe permeameters that are non-
destructive and field portable, e.g. the Autoclam Permeability System (Beggan et
al., 1996; Rusell et al., 2001), used to assess the response of sandstone blocks
to experimental salt and frost weathering (Warke et al., 2006; McKinley et al.,
2006; McKinley and Warke, 2007; Buj et al., 2011) and the Ergotech and Tiny
Perm II (Alikarami et al., 2013; Filomena et al., 2014). Subsurface cracking has
been measured using geophysical techniques, Electrical Resistivity Tomography
(ERT) that utilises direct electrical current to measure subsurface resistivity in 3D
space (e.g.Schueremans et.al., 2003; Abu-Zeid et al., 2006) and acoustic
monitoring (Krautblatter and Hauck, 2007; Amitrano et al., 2012; Draebing and
Krautblatter, 2012; Menéndez and David, 2012).
3.3 Internal stress and strain
During some weathering processes rocks experience internal stresses that may
ultimately lead to fracture and possibly failure of the rock surface. These internal
stresses are not usually measured directly, but rather measurements that
indicate the possibility for internal stressing and fracturing to occur are made
instead e.g. temperature gradients between the rock surface and subsurface
(Gómez-Heras et al., 2006; Hall et al., 2008) or rate of temperature change
(thermal shock; Hall and Hall, 1991; Hall, 1999; Hall and Andre, 2001, 2003;
McKay et al., 2009). Fibre optic sensors have been used to monitor internal
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strain in engineering materials (e.g. Martin-Pérez et al., 2010), but, to the
authors’ knowledge, do not appear to have been applied in a geomorphic setting
to study strains exerted by rock weathering.
3.4 Moisture distribution
Water is essential for a variety of weathering processes and so there is an
interest in measuring water movement into and out of rocks, and its distribution
on rock surfaces, in order to link this with rates and patterns of weathering.
Measuring the movement of moisture has proved problematic and many
researchers have used porosity and permeability as a surrogate to indicate the
potential for moisture movement. Field measurements of moisture distribution
can be made using a simple Perspex infiltration tube, sealed onto the rock
surface and filled with water. The change in water level over time gives an
indication of relative rates of water ingress on different rock surfaces (e.g.
Robinson and Williams, 1987; 1989). Karsten tubes, commercially available
glass tubes with a graduation marked onto the side and which can be placed on
horizontal or vertical rock surfaces, are used in the same way (Török, 2003;
Siedel, et al., 2011).
In the laboratory, the moisture content of a rock surface can be measured
gravimetrically but this involves destructive sampling of the surface (e.g. Hall,
1986; Sass, 2005). Non-destructive measurement can be made using moisture
meters. Eklund et al. (2013) have tested relatively inexpensive, handheld
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moisture meters (one resistance and two capacitance meters) on Portland
limestone. Whilst the results from different meters were not comparable, each
moisture meter produced reliable readings that could be calibrated against the
absolute moisture content, calculated as a percentage of the oven-dry weight of
samples of known dimensions. Microwave-based methods, that work on a similar
principal to capacitance methods (Dill, 2000), have been tested on building
materials and found to produce variable results because the readings are
influenced by inhomogeneities and material defects (Camuffo and Bertolin,
2012). Sass (2005) has experimented with more sophisticated, and therefore
more expensive, methods including Electrical Resistivity, Time Domain
Reflectometry and 2D resistivity. There are practical difficulties in using these
methods in the field, such as insertion of probes, and 2D resistivity has proved
the most promising, offering insights into temporal and spatial variations in
moisture content on and within the near surface of rocks (Sass and Viles, 2010).
Fibre optic sensors are being used to monitor water ingress into building stone
(Sun et al., 2012).
The practical difficulties encountered in using high-tech methods for measuring
moisture in the field are less limiting under laboratory conditions. For example,
Murton et al. (2000) used a combination of dielectric sensors and pore pressure
transducers to measure water movement in a laboratory experiment designed to
assess the role of ice segregation in periglacial weathering (Murton et al., 2006);
LaBrecque et al. (2004) have used electrical resistivity to measure moisture
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changes as a surrogate for changes in rock strength as a consequence of
deformation during an applied stress; Jouniaux et al. (2006) have used variations
in electric conductivity to model small scale water flow through a fractured rock
matrix; Srinivasan et al. (2010) and Smith et al. (2011) have experimented with a
combination of electrical resistance and fibre optic sensors to measure moisture
movement in stone samples in laboratory conditions. Experiments with portable,
hand-held, resistance and capacitance moisture meters indicate that they provide
reliable readings that can be related to the absolute moisture content of stone
samples measured in the laboratory (Eklund et al., 2013).
4. Examining surface and subsurface change using microscope
techniques
Microscopy now provides the opportunity to examine rock surfaces at
magnifications that are high enough to view individual crystals and their surface
features. Traditionally light and petrological microscopes have been used and
improvements in technology over the last twenty years have facilitated a
combination of chemical analyses with high resolution views that have enhanced
understanding of the nature of rock weathering (Table 3). For example, sodium
chloride crystals are usually less than 5 μm in width and are too small to be
clearly discernible by light microscope. They are easily seen at the higher
magnifications provided by the Scanning Electron Microscope (SEM) and this
has facilitated an examination of their role in salt weathering (Mustoe, 2010).
Extremely high magnifications, such as are permitted for example by Atomic
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Force Microscopy (AFM), enable weathering to be observed at molecular scale
(e.g. Ruiz-Agudo et al., 2009).
Weathering studies have undoubtedly benefited from the more detailed
examination that microscopy facilitates of residual weathered surfaces (e.g.
McGreevy, 1985; Mottershead and Pye, 1994; Viles and Moses, 1998; Velbel,
2009); of the debris produced by weathering (e.g. Pye and Sperling, 1983;
Moses and Smith, 1993; Warke, 2007) and of the structure and composition of
weathering rinds and case hardening (e.g. Conca and Rossman 1982; Robinson
and Williams, 1987; Dorn 1998, 2003; Viles and Goudie 2007; Dorn 2011).
Different microscope techniques are complimentary and are often used together.
Lower magnification light microscopy may be used to select samples for higher
magnification examination using the SEM (e.g. Viles, 1988; Viles et al., 2000)
and the examination of thin sections using a petrological microscope often
complements SEM (e.g. Moses et al., 1995; Velbel, 2009) and geochemical
analysis (e.g. Warke, 2007; Morrison et al., 2009). In an experimental setting,
microscope techniques can be used to examine changes due to particular
weathering processes (e.g. Urzi et al. 1999; Smith et al., 2000) or weathering
cycles (e.g. Goudie and Viles, 1995; Viles and Goudie, 2007; Warke et al., 2006).
The development of scanning electron microscopy in back-scattered mode
(SEM-BSE) has allowed a detailed examination of the interaction of
microorganisms and the underlying rock. Ascaso et al. (2002) have used it
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together with Transmission Electron Microscopy (TEM) to identify calcium
biomobilsation on minerals situated adjacent to cyanobacteria. When SEM is
combined with X-ray energy dispersive spectroscopy (EDS) biochemical, as well
as biophysical, processes can be examined (de los Ríos and Ascaso, 2005;
Duane, 2006; Navarre-Sitchler and Thyne, 2007; Navarre-Sitchler et al., 2011).
Advances in understanding biological weathering have also been facilitated by
techniques that minimise disturbance of the sample during preparation for
viewing. For example, wet samples can be viewed using the Environmental
Scanning Electron Microscope (ESEM; e.g. Rao et al., 1996, Chiari and Cossio,
2004) and cryofixation of biological samples preserves the structure of the
organic material for viewing using the cryo-SEM or Low Temperature SEM
(LTSEM; e.g. Barker et al., 1998).
Microscopy is used either as a diagnostic tool, whereby the presence or absence
of a particular feature is used to infer process (Krinsley and Doornkamp, 1973;
Moses et al., 1995), or in a more quantitative way to measure rate or intensity of
weathering (Moses, 1996; Viles and Moses 1998). The SEM is most commonly
used and there are established methodologies for ensuring that appropriate
sampling strategies are employed (Taylor and Viles, 2000). Most of the
techniques described so far have been used to provide a snapshot in time and
one of the key challenges has been to investigate temporal variations in the
operation of weathering processes at this scale. Samples are usually examined
at different stages during an experiment (e.g. Moses, 1996; Viles and Moses,
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1998; Thornbush and Viles, 2007). When using the SEM, however, samples are
usually gold-sputter-coated for viewing and so the same sample cannot be
viewed more than once. Opportunities to view weathering processes operating in
real-time are provided by the ESEM (Rao et al., 1996; Ruiz-Agudo et al., 2007;
Luque et al., 2011). Image resolution, however, is not as good as the SEM and
so other non-destructive techniques are being developed to provide high
resolution views of real-time rock weathering. Fluid cells used with AFM allow in
situ experiments to be conducted enabling nano-scale observations of mineral
surfaces reacting with fluids (Ruiz-Agudo et al., 2009, 2010). When combined
with vertical scanning interferometry (VSI), time-lapse changes in mineral crystal
surface topography can be mapped with a vertical resolution of < 2 nm (e.g.
Arvidson et al., 2006; Vinson et al., 2007). VSI is essentially an optical
microscope equipped with interferometer objectives and motorized stage
controller. The interferograms are digitized and analyzed to produce a
topographic surface map. The combined use of a white light source, scanning
mode, and internal reference surface creates a system with a large field of view,
very high vertical and lateral resolution, and the ability to measure absolute
height differences, making it ideal for the purpose of quantitative analysis of
changes in mineral surface topography during reaction. Confocal scanning laser
microscopy and multiphoton laser scanning microscopy have been combined to
quantify biofilm coverage and the interactions between organisms and their rock
substrates (Naylor and Viles, 2002). Together they produce a 3D image that can
be quantified in transverse (x, y) and axial (z) planes. A key advantage is that,
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unlike the SEM which is operator controlled, the analyses are largely machine
controlled thus reducing the possibility of operator bias (Naylor and Viles, 2002).
X-ray computed tomography (CT) is a non-destructive technique that allows high
resolution 3D visualisation of the internal structure of rock. Unlike most other
microscopy techniques, CT does not require sample preparation and a high
resolution 3D model of the sample’s internal structure is obtained within minutes.
Micro-CT has a resolution down to 10 μm and nano-CT has a resolution down to
200 nm. The technique enables porosity to be quantified and weathering
phenomena visualised in 3D (Cnudde and Jacobs, 2004; Cnudde et al., 2006; De
Graef et al., 2005; Doehne et al., 2005). More recently CT has been combined
with X-ray Fluorescence (XRF) to provide information on the spatial distribution
of chemical elements (Dewanckele et al. 2009). Some of these techniques, as
detailed by Young (2012), are used on synchrotron beams and thus allow very
high energy levels of X rays and very good depth penetration.
Another method, linked to the use of laser and SEM technologies, of gaining
insight into the operation of weathering processes on rock surfaces has been the
application of Fourier Transform and Infra Red Raman Spectroscopy and related
forms of spectroscopy (FTIRS; Jorge-Villar et al., 2006). Organic chemicals can
be detected and quantified using this method (e.g. Hall et al., 2005). The study
of bio-weathering and bio-deterioration of rock surfaces has benefited from
FTIRS because it detects surface alteration products created by, for example,
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lichens (Ascaso and Wierzchos, 1995; Adamo and Violante, 2000; Chen et al.,
2000; Edwards et al. 2002; St Clair and Seaward, 2004). Spectroscopy
techniques have also proved valuable in the study weathering rinds and
varnishes (Gordon and Dorn, 2005a, b; Broz et al., 2007) and more generally to
identify both inorganic and organic mineral weathering transformations on, and
in, rock surface and near surface environments at a variety of scales, in a variety
of terrestrial and extra-terrestrial environments including dimension stones in
urban environments (Friolo et al., 2003) and rinds on the surface rocks on Mars
(Bishop and Murad, 2004; Lanza et al., 2012).
5. Discussion and Conclusion
There is now a wide range of established field and laboratory-based methods
available to scientists wishing to study rock weathering; from simple low tech
methods for measuring rates of surface weathering to more sophisticated high-
tech methods for mapping the surface and near-surface distribution of
weathering and for diagnosing the processes responsible for either reducing or
enhancing rock strength via weathering. Scientific advances in directly measuring
rates of rock surface weathering and erosion are being made with the application
of high-tech non-contact laser scanning and digital photogrammetric methods
that allow monitoring of surface change at a variety of spatial and temporal
scales and facilitate the production of digital terrain models of weathering
surfaces and surface change. A key advantage lies in the fact that these are non-
contact and so, unlike the long established MEMs, there is no risk that the
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measured rate is a function of probe erosion particularly on very soft rocks. They
are also non-intrusive and can be deployed without the need to drill directly into
the rock surface, so can be used at heritage and conservation sites as well as on
buildings. In addition, unlike the MEMs, they facilitate the mapping of surface
form over time providing insights into the evolution of weathering features that
has not previously been possible. However, they require expensive equipment
and skilled operators. Care needs to be taken to avoid measurement errors in
image acquisition, in ground control, establishing orientations and in data capture
(Inkpen et al., 2000). It is also necessary to compare surface maps created with
the actual rock surface to ensure accurate identification of ‘real’ forms from any
that are artefacts of the techniques employed. Most importantly, however, these
methods have the advantages of generating large datasets that facilitate more
sophisticated analyses than has previously been possible. For example,
magnitude-frequency data derived from multiple TLS and digital photogrammetry
datasets are being used to model multiscalar loss of material from cliff faces by
applying negative power law scaling to the distribution (Barlow et al., 2012). A
key disadvantage is that their relatively recent application means that available
datasets cover relatively short time spans, often only two or three years. In
contrast, MEM datasets are available over longer time periods of up to and
including decades (e.g. Inkpen et al., 2012; Moses and Robinson, 2011;
Stephenson and Finlayson 2009; Stephenson and Kirk 1996; Stephenson et al.,
2010; Stephenson et al., 2012; Viles and Trudgill, 1984). These longer term
datasets are being used to interrogate existing models of weathering rates e.g.
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Inkpen et al.(2012) and could also be used to validate models derived from the
high resolution datasets generated by laser scanning and digital
photogrammetry. Another key advantage of the MEM is that it is small and highly
portable. More than half a century after its original design, the MEM remains the
most commonly used instrument to measure rock surface recession simply
because it is cheap to make, it is portable and it is known to record meaningful
readings over the relatively short time period of most research projects. There is,
however, little doubt that the increasing miniaturisation of powerful and accurate
non-contact monitoring and recording systems will lead to further rapid advances.
Not only will this produce lighter equipment for use in the field, such as laser
scanners, but enable the use of small airborne platforms that can be controlled
remotely by the investigator (e.g. James and Robson, 2012).
Nevertheless, a particular problem remains in assessing the link between the
results of measurement and monitoring of surface weathering at the small scale
to larger scale landform and landscape evolution (Warke and McKinley, 2011).
Most studies of rock weathering rates are conducted on small surface areas and
over short time periods of two to three years. Continual records over decades are
unusual, making it very difficult to up-scale the results because of uncertainty as
to their spatial and temporal applicability. This is particularly the case with
microcatchments that are generally used over short time periods of two or three
years. As is noted above, where long term data sets of weathering and erosion
rates do exist they have been used to inform predictive modelling and to
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investigate scale issues (e.g. Smith et al., 1995; Stephenson and Finlayson,
2009; Stephenson et al., 2010; Moses and Robinson, 2011; Inkpen et al. 2012;
Stephenson et al., 2012). Data on weathering rates collected over a few years
may not represent long term trends, especially as the relative contribution of
‘average’ conditions compared to more extreme events, such as occasional
periods of unusual cold with severe frosts in temperate environments (Robinson
and Jerwood 1987a and b) or extreme heat such as during grass or forest fires
(Allison and Goudie, 1994; Dorn, 2003), remains poorly understood; not least
because the recurrence interval of such events is poorly documented. It is
possible, therefore, that rates of surface recession recorded over a period of a
few years are, in fact, erroneous. Viles and Trudgill (1984), for example, report
longer term MEM measurements recorded over a thirteen year period on Aldabra
Atoll, Indian Ocean and discuss comparisons of rates collected over two and
eleven year periods. Although they found no consistent time trend and cautioned
against using short term data for interpretation and extrapolation, a tendency for
the higher, short term erosion rates to become lower rates in the longer term was
noted. This was thought to reflect a cycle of granular disintegration, with periods
of rapid grain dislodgement followed by periods of surface stability. Viles and
Trudgill (1984) also suggested that a previous interpretation of the short term
results, that the most rapid erosion was in the upper intertidal (Trudgill, 1976),
had been erroneous and that erosion focussed instead on lower and mid
intertidal wave action and abrasion. On the other hand, Stephenson et al. (2012)
find no statistical difference between erosion rates measured on a shore platform
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on the Otway coast in south eastern Australia over a two year compared with a
thirty two year period and suggest that shore platform erosion rates measured
over a two year period are representative of decadal timescales. The combined
use of datasets from established methods, such as the MEM, and recently
developed methods, such as laser scanning and photogrammetry, will inform our
understanding of multiscalar variations in weathering and erosion rates over
decadal timescales. With the application of cosmogenic dating methods to
measure rates of rock surface weathering and erosion we are just beginning to
gain insights into the response of rock surfaces to atmospheric conditions over
millennial timescales.
The problem of assessing the contribution of small scale weathering to larger
scale landform and landscape evolution is exacerbated by the presently limited
understanding of how weathering processes interact and how weathering and
erosion interact at a range of scales. Viles (2001) identifies four key issues in
rock weathering studies: first, whether there are characteristic spatio-temporal
scales of landforms and processes; second, whether scales of process
observation are the same as the scales of process operation; third, how to up-
and downscale observations (e.g. between microscopic scale, < 1 mm, to
weathering landform scale, centimetre to metre to tens of metres); fourth, how
and if different scales of processes and events interact to produce the
geomorphology we see around us. Such questions provide the basis for ongoing
discussions in rock weathering research. For example, Smith et al. (2002a)
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caution against upscaling from process to landscape scale suggesting that it may
be more appropriate to first identify the key issues associated with the
explanation of landscape change, before drawing upon process studies i.e.
working from landscape to process. Although efforts have been made to
understand geographical variations in weathering by focusing on the micro-scale
boundary layer between the rock–atmosphere–hydrosphere–biosphere (Pope et
al., 1995), it is suggested that clear linkages still need to be established between
microscale and landscape scale enquiries (Turkington et al., 2005). Key issues
that remain to be addressed include more detailed specifications of spatial and
temporal scales and of the rates, durations, and frequencies of weathering and
related processes, forms and relationships (Phillips, 2005). Significant
contributions are likely to be made through the use of numerical modelling of
weathering processes (e.g. Walder and Hallet, 1985, 1986; Barlow et al., 2012;
Hallet, 2006; Murton et al., 2006; Trenhaile, 2008) aided by recent and ongoing
developments in monitoring and measuring rock surface weathering described in
this paper.
Rapid technological advances are also transforming our ability to record the
surface and near-surface physical and chemical impacts of rock weathering in
much greater spatial and temporal resolutions on rock surfaces and also in 3D.
Although weathering usually weakens rocks and is thereby a precursor to
erosion, it can also strengthen some rock surfaces, at least temporarily, through
the development of relatively indurated surface crusts (Robinson and Williams,
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1976, 1987; Alexandrowitz and Pawlikowski, 1982; Marszalek et al., 2012).
These crusts can help to conserve stone monuments and prehistoric inscriptions
cut into rock surfaces. Many such crusts are fragile and are vulnerable to
damage by climbers and trampling by visitors (Swantesson, 2005; Williams,
2007). In most cases crusting is a cyclical phenomenon, rock surfaces develop
crusts that then deteriorate and often fall away, before another cycle of formation
and destruction occurs (Robinson, 2007; Turkington and Phillips, 2004).
Advances in field methods for measuring rock subsurface changes, for example
using more sensitive strength testing such as the Equotip (Viles et al., 2010),
more detailed measurement of permeability and porosity (e.g. McKinley et al.,
2006; 2007) and of moisture variations (e.g. Eklund et al., 2013) will provide new
insights into the processes causing such weathering and erosion cycles. The
dating and timing of such cycles remains a challenge for the future, but measures
of the downwearing of crusted surfaces over years or even decades may be of
little relevance in terms of weathering and downwearing over centuries or
millennia.
Ongoing developments in the application of microscopy, tomography and
analytical techniques promise exciting insights into the operation of weathering
processes on rock surfaces and their internal structures. These are important
developments that contribute to the scientific understanding of weathering
(Robinson and Moses, 2011; Hall et al., 2012) that is of global significance in
landform and landscape development, carbon drawdown and climate change,
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buildings and heritage conservation and contributes to our understanding of
planetary geomorphology. In all of these cases, understanding what happens on
and in the rock surface and near-surface is critical because this represents the
direct interface with the atmosphere. Technological advances in methods for
measuring and monitoring changes in rock surfaces and near-surface in
response to weathering and erosion enable not only the diagnoses of weathering
processes but increasingly high resolution monitoring of weathering over time.
Rock weathering research has benefited from it becoming an increasingly
interdisciplinary field of study that includes, for example, geologists,
geomorphologists, engineers, materials scientists, architects, conservators,
archaeologists and botanists, using increasingly sophisticated equipment that
requires high level technical skills for both design and use. Many existing
techniques use equipment originally designed for other purposes. For example,
methods designed by engineers for materials testing e.g. concrete, metal, wood
have been applied by geomorphologists to the study of rock weathering. These
include the Schmidt Hammer, grindosonic, Pundit, Autoclam and Equotip, fibre
optics and resistivity meters. Technological advances in laboratory based
microscopy methods are facilitating increasingly high resolution imaging and
chemical analyses, including 3D imaging of the physical properties (e.g. Cnudde
and Jacobs, 2004; Cnudde et al., 2006; De Graef et al., 2005; Doehne et al.,
2005) and spatial distribution of chemical elements (Dewanckele et al. 2009) of
near surface of rock samples.
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Such developments in surface rock weathering and erosion research have been
facilitated in large part by interdisciplinary groups and organisations. For
example, in the context of buildings conservation, SWAPnet (Stone Weathering
and Air Pollution Network), ASMOSIA (Association for the Study of Marble and
Other Stones in Antiquity), research institutes such as the Getty Conservation
Institute in Los Angeles, international research sponsors such as the European
Union and UNESCO all contribute innovative thinking that develops new
methods for measuring and monitoring rock weathering (Pope et al., 2002;
Doehne and Price, 2010). In the context of natural hazards research, rock
weathering is recognised as making an important contribution to erosion
processes e.g. it is recognised as a precursor to slope and cliff failure (Borelli et
al., 2007; Bourrier et al., 2012; Lim et al., 2010; Schneider et al., 2011; Viles,
2012) and this has led to the development of high tech measuring and monitoring
methods such as Lidar, 2D and 3D resistivity and capacitance geophysical
techniques and fibre optics. A key purpose of this paper is to update the broader
scientific community on the range of methods that are used by geomorphologists
to measure and monitor rock surface and near-surface weathering and erosion.
In consequence it identifies opportunities for further innovative thinking across
disciplines to build on long-established and more recently developed methods to
continue to improve our ability to monitor and measure rock weathering and
erosion.
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A particular interdisciplinary challenge lies in understanding the contribution of
biological processes to the weathering of rock surfaces, their interaction with
other weathering processes and their resulting impact on the evolution of rock
surfaces over time (Viles, 1988; Cox, 1989; Naylor et al., 2002; Corenblit et al.,
2008). Studies of the interface between microorganisms and rock surfaces have
been greatly assisted by advances in microscopy techniques and a wide range of
studies have been conducted to examine the chemical interactions of bacteria,
fungi, archaea, algae and lichens with individual rock types and minerals.
Considerable progress has been made in understanding these interactions on
natural rock outcrops (e.g. Viles, 1995; Chen et al., 2000) on building stones and
on other forms of stone-based cultural heritage (e.g. Seaward, 1997; Warscheid
and Braams, 2000; Gaylarde and Morton, 2002; Liscia et al., 2003; Crispim and
Gaylarde, 2004; St.Clair and Seaward, 2004; Gaylarde et al., 2007; Scheerer et
al., 2009). Of the organisms that make up these lithobiontic communities, the role
of bacteria and archaea are least well understood are (Scheerer et al., 2009).
Biofilms may be involved not only in weathering rock surfaces but in some cases
actually protecting them, and there is a growing interest not only in the role of
microorganisms in the bioprotection of stonework (e.g. May, 2003) but also in
their potential for the bioremediation of stone surface deterioration caused by
other weathering processes (Webster and May, 2006).
Advances in methods for measuring and monitoring rock surface weathering
have helped also to improve understanding of the likely influence of past, present
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and future global environmental and climate change on natural rocks and
building stones, including the impacts of various forms of pollution (e.g. Goudie,
2005; Lee and Fookes, 2005; McCabe et al., 2011; Smith and Atkinson, 1976;
Smith et al., 1995; Smith et al., 2011; Viles, 2002; Viles and Cutler, 2012; Viles
and Goudie, 2003; Inkpen et al. 2012). Efforts have been made, for example, to
predict and map the impact of climate change on building stones e.g. Yates and
Butlin (1996), Brimblecombe and Grossi (2008, 2009), Grossi et al. (2008),
Bonazza et al. (2009), Sabbioni et al. (2010). Recent research indicates that
there remain considerable uncertainties, not only about the influence of climatic
changes on rock weathering (McCabe et al., 2011; Smith et al., 2011; Viles and
Cutler, 2012), but also in linking studies conducted at a range of scales (Viles
2001; Turkington et al., 2005). Inkpen et al. (2012) have used decadal limestone
erosion rates from a 30-year MEM study of St Paul’s Cathedral, London to
validate predictive indices that are used to assess future weathering of buildings.
At much larger spatial and temporal scales it is also recognised that chemical
weathering may influence atmospheric carbon dioxide levels and climate via the
global carbon cycle (e.g. Kump et al., 2000; Lui and Zhao., 2000; Gombert, 2002;
Lerman et al., 2007) but the contribution of surface and near surface weathering
to this remains unclear – ‘At the heart of many of the controversies over the
relationship between weathering and the carbon cycle is lack of data, or lack of
data collected at appropriate scales’ (Goudie and Viles, 2012, p. 69). Not only
may data on weathering rates collected over several years be unrepresentative
of long term trends, data collected on individual rock surface, hillslope or
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drainage basin scales may yield quite different results and the linkages between
ecological and geological systems remain poorly understood at all scales (Pope
et al., 2005; Viles et al., 2008; Goudie and Viles, 2012). The ability to measure
long term exposure and weathering of surfaces using cosmogenic nucleides,
particularly 10Be, offers the opportunity to obtain insights into long term
weathering rates over timescales of 103 to 106 years (Bierman and Nichols, 2004;
Brandmeier et al., 2011) and may help to improve our present limited
understanding of the contribution that weathering makes to the longer term global
carbon cycle.
Measurement and analyses of terrestrial rock surfaces are making major
contributions to the understanding of weathering and erosion of planetary
surfaces e.g. Bishop et al. (2004), Bridges et al. (2004a, b), Heslop et al. (2004),
Bourke and Viles (2007), Bourke et al. (2007, 2008), Viles et al. (2007), Chan et
al. (2008), Lanza et al. (2012). Technological improvements in successive
explorations both from remote platforms and from ground based Rovers are
generating a wealth of information on the planet’s surface geochemistry (e.g.
Hurowitz et al., 2006, 2010), morphology (e.g. Thomas et al., 2005; Bourke and
Viles, 2007) and evidence of surface and near-surface water chemistry in the
planet’s geological history (e.g. Golombek et al., 2006; Hausrath et al., 2008;
Hausrath and Brantley, 2010; Tosca et al., 2011). Equipment designed for these
explorations such as the Mars Advanced Radar for Subsurface and Ionospheric
Sounding (MARSIS) and the Thermal Emission Imaging system are further
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advancing the range of equipment for gathering information on surface and sub-
surface weathering and offer possibilities for wider application.
Recent developments in rock surface weathering research have been aided by
technological advances in techniques and methods and benefited from
interaction with other disciplines. The focus on measuring of rates of surface
change and furthering the scientific understanding of modes and mechanisms of
weathering continues and recent developments are making some key
contributions. Understanding of the ability of rock weathering processes to
strengthen, as well as weaken, rock surfaces is improving. Improvements in
measuring rates of weathering, and monitoring surface and near-surface physical
and chemical changes, over short and long timescales are contributing to a
larger dataset. Because rock surfaces interface directly with the atmosphere they
are highly sensitive to any changes in its composition and our improving
understanding of rock surface weathering and erosion will therefore help to
facilitate a better understanding of the behaviour of biogeochemical cycles via
landscape evolution, carbon drawdown and climate change. The development of
non-intrusive methods for measuring and monitoring rock surface and near-
surface weathering and erosion are particularly applicable to materials
conservation science, facilitating the study of materials and sites previously
precluded by more intrusive and destructive methods. The ability to measure and
monitor rock surface and near-surface weathering and erosion over an
increasing range of spatial and temporal scales is improving our ability to
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produce and validate multi-scalar models of the development of weathering
feature, landforms and landscapes over timescales of up to millennia. These
improvements in our understanding of terrestrial rock surfaces will contribute to a
better understanding of weathering and erosion of planetary surfaces. There are
many technological advances, described in this paper, that have only recently
been applied in the field of rock weathering. As a consequence, the future
prospects for the development of rock surface weathering research by
geomorphologists working in an interdisciplinary context promise new insights
into understanding of rock surface weathering across a wide range of spatial and
temporal scales. Critical to this is the development of challenging models to test
and interpret the increasing volumes of data being collected using both long-
established and recently developed methods.
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