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8/3/2019 The Geological History of Deep-sea Volcanoes
1/14
Oceanography V.23, N.158
M o u N ta i N s i N t h e s e a
the GeoloGical historyo Deep-sea VolcaNoesBiosphere,
hyDrosphere,aND lithosphere
iNteractioNs
Oceanography V.23, N.158
B y h u B e r t s t a u D i G e l a N D D a V i D a . c l a G u e
emgn bmn vn n N, tng, M 18, 2009. T n
mn d v bmn n, b g
(g d) d zn b nd j mg
m n mn. Photo credit: Dana Stephenson/Getty Images
8/3/2019 The Geological History of Deep-sea Volcanoes
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Oceanography M 2010 59
aBstract. Te geological evolution o seamounts has distinct inuence on their
interactions with the ocean, their hydrology, geochemical uxes, biology, resources,and geohazards. Tere are six geological evolutionary stages o seamounts: (1) small
seamounts (1001000-m height), (2) mid-sized seamounts (>1000-m height, > 700-m
eruption depth), (3) explosive seamounts (< 700-m eruption depth), (4) ocean islands,
(5) extinct seamounts, and (6) subducting seamounts. Troughout their lietimes,
seamounts oer major passageways or uid circulation that promotes geochemical
exchange between seawater and the volcanic oceanic crust, and seamounts likely
host signicant microbial communities. Water circulation may be promoted by
hydrothermal siphons in conjunction with the underlying oceanic crust, or it may
be driven by intrusions inside seamounts rom Stage 2 onward. Geochemical uxes
are likely to be very large, primarily because o the very large number o Stage 1seamounts. Intrusive growth o seamounts also initiates internal deormation that
ultimately may trigger volcano sector collapse that likely peaks at the end o the main
volcanic activity at large seamounts or islands. Explosive activity at seamounts may
begin at abyssal depth, but it is most pronounced at eruption depths shallower than
700 m. Wave erosion inhibits the emergence o islands and shortens their liespans
beore they subside due to lithosphere cooling. Once volcanism ends and a seamount
is submerged, seamounts are largely unaected by collapse or erosion. Troughout
their histories, seamounts oer habitats or diverse micro- and macrobiological
communities, culminating with the ormation o coral rees in tropical latitudes.
Geological hazards associated with seamounts are responsible or some o the largest
natural disasters recorded in history and include major explosive eruptions and large-
scale landslides that may trigger tsunamis.
iNtroDuctioN
Seamounts are the least explored major
morphological eatures on Earth.
Hundreds o thousands o them are
dispersed through the ocean basins, with
less than 0.1% known rom any direct
observation such as an echosounding,
or any kind o sampling. Most o what
we know about seamounts comes rom
satellite observations that sense them
indirectly rom the centimeter-size
deections they impose on sea surace
topography (e.g., Wessel et al., 2010).
Tis indirect method reliably identi-
es only eatures larger than 1500 m
in height and does not oer any topo-
graphic detail below this scale. Tis state
o exploration reveals nothing about
a seamounts geology. In act, this lack
o detail would be considered entirely
unacceptable or topographic eatures
on land, eectively all o which are
now sampled and mapped by direct
measurements with a resolution better
than 30 m in vertical and 100 m in
horizontal dimensions. Seamounts are
the last major rontier in our exploration
o Earths surace.
Te minimal level o seamount explo-
ration contrasts with their signicance
to science and society. Tey have rich
sheries (Pitcher et al., 2010), are marine-biological and microbiological hotspots
(Danovaro et al., 2009; Emerson and
Moyer, 2010), and have a signicant
geochemical impact, with chemical uxes
that allow mantle-derived materials to
interact with the hydrosphere (Fisher
and Wheat, 2010). Hence, seamounts
are key points o intersection among the
biosphere, hydrosphere, and lithosphere,
and they are globally relevant.In this paper, we explore how
seamounts grow, collapse, and age,
and how these processes interact with
the hydrosphere and impact biological
processes. Te reader is reerred to earlier
reviews by Batiza and White (2000),
Schmidt and Schmincke (2000), and a
recent monograph (Pitcher et al., 2007).
DeiNiNG a seaMouNt
Tere is no uniorm denition or the
term seamount that equally satises all
scientic disciplines. Denitions reect
diverse interpretative perspectives and
disciplinary approaches that can be quite
specic to the needs o a particular scien-
tic community (see Box 1 on page 20
o this issue [Staudigel et al., 2010a]).
We use here a denition that centers
on seamounts as isolated geological
eatures on the seaoor. Seamounts are
typically volcanoes that orm on top o
the oceanic crust with their own central
magma supply and hydrothermal system
Tis simple distinction can become more
complex when isolated volcanoes orm
right on the mid-ocean ridge axis, such
as Axial Seamount on the Juan de Fuca
8/3/2019 The Geological History of Deep-sea Volcanoes
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Oceanography V.23, N.160
Ridge (see Spotlight 1 on page 38 o this
issue [Chadwick et al., 2010a]).
Isolated volcanoes on the seaoor
come in a range o sizes whereby the
smallest ones are typically reerred to
as abyssal hills and the larger ones asseamounts. We use 100 m as the tallest
abyssal hill and smallest seamount,
ollowing a denition o Schmidt and
Schmincke (2000). By this denition,
seamounts may orm in any tectonic
setting; they may have conical, at-
topped, or complex shapes; and they
may or may not have carbonate or
sedimented caps. We also include
oceanic islands in our denition o aseamount, considering them as very
large seamounts that breached the sea
surace. Viewing islands as a subgroup
o seamounts is unusual because most
views ocus on the distinct nature
o islands in terms o their volcanic
eatures, subaerial weathering, and
land-based biology (see Gillepsie and
Clague, 2009). However, our grouping
makes sense rom a geological point o
view in which volcanic islands are the
summit regions o very large seamounts
that are temporarily exposed above sea
level. Most oceanic islands start out as
seamounts and turn back into seamounts
toward the ends o their lie cycles.
GeoloGical settiNG
Seamounts orm in an oceanic geological
setting that is much more dynamic than
the continents, resulting in much larger
vertical and horizontal displacements.
Oceanic crust exes and bends relatively
easily rom the processes involved in
seamount ormation (see Koppers and
Watts, 2010). It may bulge upward
rom the buoyancy o rising mantle or
due to heating o the lithosphere rom
below. However, as soon as a seamounthas reached a critical size, subsidence
becomes the dominant vertical orce,
caused by the bending o the oceanic
crust into the mantle as it yields to
the seamount load (Watts, 2001). For
example, data rom drowning coral
terraces on Hawai`i suggest a subsid-
ence rate o 2.6 mm yr -1 or the Island
o Hawai`i (Moore and Fornari, 1984;
Moore et al., 1996). However, on a muchlonger time scale extending millions o
years, seamounts also subside owing to
the cooling o the lithosphere they are
built upon. Most seamounts are ormed
within a ew hundred kilometers o
mid-ocean ridges at roughly 2.5-km
water depth. Tese seamounts will
subside with the underlying cooling
lithosphere to about 5-km water depth
within 60 million years and possibly
down to 10-km depth when they arrive
at subduction zones. Tis journey rom
mid-ocean ridge to subduction zone may
involve thousands o kilometers o hori-
zontal displacement. For example, the
majority o the seamounts in the central-
western Pacic originally ormed in the
South Pacic 80140 million years ago.
GeocheMistry
Seamount volcanoes are ormed by two
principal mantle melting processes.
Mid-ocean ridge (MOR) basalt and
oceanic intraplate volcanoes (OIVs)
are ormed by decompression melting
o mantle materials that buoyantly rise
in upward convective ow and/or in
response to plate extension and thin-
ning. Subduction zone volcano magmasare commonly ormed by the lowering
o the melting point o the sub-arc
mantle due to addition o volatiles rom
the descending slab. Tese dierent
melting processes have a proound
impact on the chemistry o seamount
rocks and the chemical evolution o
the volcanoes ormed.
Seamounts close to MORs and in OIV
settings erupt mostly basaltic (tholeiiticand alkalic) magmas, whereas subduc-
tion zone volcanics are predominantly
composed o calc-alkaline (andesitic or
arc-theoleiitic) magmas. All seamount
rocks are ormed by mantle melting
processes that involve smaller degrees
o melting than are characteristic or
the generation o the oceanic crust
itsel. As a result, their incompatible
element abundances may be an order o
magnitude higher, and seamount uxes
o these elements may dominate global
mantle uxes even though seamounts are
substantially less voluminous than the
elongated volcanoes orming the MORs
themselves. Subduction zone magmas
tend to have overall higher Si, alkali,
and volatile abundances, and lower Mg
contents, than OIVs and MORs. As a
result, they are more explosive and tend
to have higher viscosities.
OIVs display a characteristic
geochemical evolution over their
520-million-year volcanic histories
(Clague and Dalrymple, 1987). Four
geochemical stages are commonly
distinguished, originally dened or
the Hawaiian Islands but also ound to
Hubert Staudigel ([email protected]) is Research Geologist and Lecturer, Institute
of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of
California, San Diego, La Jolla, CA, USA. David A. Clague is Senior Scientist, Monterey Bay
Aquarium Research Institute, Moss Landing, CA, USA.
8/3/2019 The Geological History of Deep-sea Volcanoes
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Oceanography M 2010 61
characterize many other OIVs (Konter
et al., 2009). OIV ormation at Hawai`is
most recent active volcano, L`ihi, starts
with small erupted volumes o basaltic
rocks with diverse compositions gener-
ated by small degrees o mantle melting.Te subsequent main, shield-building
stage then produces up to 98% o the
entire volcano typically rom rocks
ormed by relatively large melt rac-
tions in the mantle, yielding tholeiitic
or mildly alkalic basalts. Tis is closely
ollowed by a cap o alkalic rocks that
are ormed by smaller degrees o partial
melting than the shield magmas. Some
OIVs have a last stage o post-erosionalor rejuvenated volcanism, aer a
volcanic hiatus o 1.510 million years.
Rejuvenated volcanism typically consists
o very small volumes o highly alkalic
lavas that oen include mantle xenoliths.
Tese stages o geochemical evolution
provide insights into the temporal evolu-
tion o mantle melting with substan-
tial implications or the geodynamic
processes involved. An interesting act
is that the majority o seamounts have
completed their geochemical evolutions,
and hence their suraces most likely
display the latest stages o volcanism.
As a consequence, the majority o their
earlier shield-building evolution may be
largely hidden rom direct sampling.
litholoGical rock types
Seamounts, like all volcanoes, are ormed
by intrusive and eruptive processes, prob-
ably in roughly equal proportions, at least
in larger volcanoes where these processes
have been studied in more detail.
Intrusive rocks at seamounts include
dikes, sills, and gabbroic to ultramac
intrusions. Te intrusion o dikes is likely
to be one o the key means o growth o
volcanoes with pronounced ri zones
such as Hawai`is Kilauea Volcano
(Dieterich, 1988; Leslie et al., 2004).
Kilaueas East Ri displays volcanic
spreading rates o about 10 cm yr -1,
pushing the whole volcano outwardrom Mauna Loa along a near-horizontal
slip surace at 89-km depth (Hill and
Zucca, 1987; Denlinger and Okubo, 1995;
Delaney et al., 1998). Such spreading
may be entirely intrusive, as indicated by
the common earthquake swarms on the
ri zones o Kilauea that are not associ-
ated with eruptions (Klein et al., 1987).
Erosion has also exposed the ri zones
on several extinct Hawaiian volcanoes,particularly Koolau on Oahu, that also
show large volumes o dikes (Walker,
1987; Figure 1). At the uplied La Palma
seamount series (Canary Islands), sills
are the dominant orm o intrusion.
Teir cumulative thickness suggests
upli o the overlying volcano by about
2 km (Staudigel and Schmincke, 1984;
Staudigel et al., 1986). Sills may also
play a key role in the growth o some
Galpagos volcanoes (Amelung et al.,
2000; Chadwick et al., 2006).
Te deeper portions o seamounts arelikely to contain magma reservoirs that
crystallize to gabbroic and ultramac
intrusions (Upton and Wadsworth,
1972). Such intrusions may occur at a
range o depths below the summit and
are commonly sampled as xenoliths
in later lavas (Clague and Dalrymple,
1987). At La Palma, such intrusive rocks
have been ound at stratigraphic depths
o almost 5 km below the likely originalsummit o the seamount (Staudigel et al.,
1986), whereas active magma chambers
at Kilauea may be only 800 m below
the summit (Almendros et al., 2002).
Intrusive activity at seamounts results in
ination, over-steepening o anks, and
ultimately ank collapse.
g 1. D nn (v nd nd ) n k vn d, o, h`.
p n dng d nn d dnd b , nzn
. a n n nd (d) .
8/3/2019 The Geological History of Deep-sea Volcanoes
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Oceanography V.23, N.162
Eruptive rocks dominate the surace
and the upper portions o a seamount,
hence they are the major apparent
component o volcano growth, with two
major submarine types: lava ows and
volcaniclastics (Staudigel and Schmincke1984; Batiza and White, 2000; Schmidt
and Schmincke, 2000).
Pillow lavas are the most common
submarine lava ows and may be ound
in almost any subaqueous setting
(Figure 2A,B). Less abundant are sheet
ows and massive ows that tend to
indicate higher mass eruption rates.
Sheet ows typically are ound on
shallow slopes and settings where they
may be ponded (Figure 2C; Staudigeland Schmincke 1984, Clague et al., 2000,
2009). All submarine lava ows have
pronounced, quenched, glassy margins.
Pillow lavas are named aer the curved,
occasionally round, cross section o the
lava tubes that orm these ows. Pillow
lava cross sections commonly display
characteristic V-shaped bottoms as
the lava tubes mold themselves to the
underlying pillow lavas. Individual
lava tubes may occur in a range o tubediameters, rom a ew tens o centimeters
to 200 cm in diameter (Clague et al.,
2009), decreasing in size with distance
rom the eruption center and upward
in a section (Figure 7 in Staudigel
and Schmincke, 1984).
Seamount volcaniclastic rocks
are sedimentary rocks ormed by
mechanical breakup o lava ows or by
explosive-eruptive ragmentation mech-anisms (Fisher and Schmincke, 1984);
they are the most common volcanic
rocks in shallow submarine volcanoes
(e.g., Staudigel and Schmincke, 1984;
Garcia et al., 2007). Mechanical rag-
mentation prevails during the collapse
o unstable volcanic rock ormations
and spallation o pillow margins.
Explosive ragmentation may be caused
by the expansion o gas bubbles inside
the magma, magma interaction with
external water, or grain-to-grain inter-
action during an eruption. Although
mechanical ragmentation processes
have no (conning) pressure depen-
dence, explosive ragmentation does. Te
water-depth dependence o explosive
ragmentation processes is due to pres-
sure dependence o magma volatile solu-
bility and the pressure dependence o
the extraordinarily large volume change
in water when it transorms rom liquid
to gas. At one atmosphere pressure, this
volume change is by a actor o 1600. It is
important that the pressure dependence
o gas solubility varies substantially with
the dissolved species. CO2 may outgas at
relatively high pressures, whereas H2O
3 m
2 m
50 m
50 cm
1 m
50 m
A B
C D
E F
g 2. d nd mg bmn vn : (a) p v m bmn
x Vn smn (1600m d). (B) Dd mn m an
v n png, r. (c) s bmn f mbng b
f m Vn smn (1712m d). (D) p gmn b m l` smn,
1000m d. (e) tb m nn n g mgn m D s Dng
pj s 396B, 20r3, 13, 108112 m. () sgmnd b n nd b d
b m td . tb n (e) nd () nd md b
mb m dng (sdg ., 2008).
8/3/2019 The Geological History of Deep-sea Volcanoes
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Oceanography M 2010 63
and SO2 tend to outgas at much lower
pressures. Sulur may separate rom
magma as an immiscible liquid at a wide
range o pressures, but it contributes to
the explosivity o lava only at low pres-
sure when SO2 bubbles can orm in themagma. Geological observations and
theoretical considerations point to a
relatively consistent critical depth or the
onset o major explosive activity. A more
than 1800-m-thick section o seamount
volcanic rocks at the La Palma seamount
series shows a dramatic increase in
volcaniclastic rocks at about 1000-m
water depth (Staudigel and Schmincke,
1984). Teoretical considerations placethe explosive transition closer to 700 m
or explosions caused by the interaction
o external water with magma (Peckover
et al., 1973) as well as the explosive
outgassing o magma itsel (McBirney,
1963). However, some variance may be
expected as a result o magma composi-
tion and eruption rates, and minor pyro-
clastic activity can be widespread even at
abyssal depths (Clague et al., 2009).
Pillow ragment breccias are prob-
ably the most common mechanically
ragmented volcaniclastic rocks at
seamounts, oen ormed during the
collapse o steep pillow lava ow ronts
(Figure 2D). Pillow breccias may orm
when pillow lavas intrude into their own
ne-grained volcaniclastics, with highly
irregular, amoeba-shaped pillow cross
sections that are completely enclosed in
a hyaloclastite matrix. Pillows may also
intrude into ne-grained epiclastic sedi-
ments (peperites). Explosive seamount
volcaniclastics rom magmatic outgas-
sing may orm highly vesicular scoria
that may occur as nearly pumiceous
deposits or be broken up into ner
particles. Te ragmentation o dense
spatter may also be responsible or the
ormation o well-sorted, ne-grained
hyaloclastite (glass sand; Clague et al.,
2009). Most submarine volcaniclastics
are missing most o the ne grain-size
raction, which remains suspended inturbulent waters and is deposited slowly
in more distant areas surrounding the
seamount eruption site. Tis lack o ne
grains gives seamount volcaniclastics a
relatively high permeability.
Lithology, particularly the abundance
o coarser-grained volcaniclastic deposits,
has a substantial impact on chemical
uxes and on the development o micro-
bial habitats. Key controls include thehigh permeability o these deposits,
which allows or seawater circulation,
and a high abundance o volcanic glass,
which is highly reactive with seawater.
Te alteration o volcanic glass is the
primary process that controls low-
temperature alteration uxes between
seawater and basalt (e.g., Staudigel and
Hart, 1983), and glass is a particularly
suitable material or colonization by
microbial communities (Figure 2E,F),
up to the moment they are sealed o by
authigenic mineral deposition.
hyDrotherMal aND
BioloGical actiVity
Seaoor volcanism generally produces
hydrothermal systems that are likely to
be associated with substantial microbial
and possibly macrobiological activity.
Seamount hydrothermal water ow may
be driven by intrusive activity, or the
seamount may act as a hydrothermal
siphon (Fisher and Wheat, 2010),
allowing the recharge o seawater to the
oceanic crust and the escape o oceanic
crustal uids that may be otherwise
trapped by a blanket o impermeable
sediments. Hydrothermal circulation at
seamounts is signicant, because they
are made o rather permeable volcani-
clastic rocks, and sediment cover does
not signicantly inhibit circulation.
Seamounts also have a topographicadvantage in that water can enter on
the anks and then rise within the
seamount due to its raised geothermal
gradients resulting rom hydrothermal
activity (Schiman and Staudigel,
1994) or simply by their topography.
Furthermore, seamount volcanic rocks
are rich in volcanic glass, and their
alteration contributes preerentially to
hydrothermal chemical uxes. Hence,seamount hydrothermal uxes may
be substantially more important than
suggested by their volume relative to
MORs. Some indications or large uxes
come rom the large uxes o3He or CO2
(L`ihi; see Spotlight 3 on page 72 o this
issue [Staudigel et al., 2010c]; Lupton,
1996; Hilton et al., 1998) or Mn and 3He
anomalies (Vailuluu; see Spotlight 8 on
page 164 o this issue [Koppers et al.,
2010]; Hart et al., 2000; Staudigel et al.,
2004). ogether, seamount and oceanic
crust hydrothermal systems play key roles
in buering the chemical composition
o seawater, but the contribution rom
seamounts remains less well known and
is likely to vary among elements.
Microbial communities have been
documented rom a range o seamounts,
and there is abundant evidence that they
might play a role in a potentially very
large deep oceanic crustal biosphere.
Emerson and Moyer (2010) review
much o the evidence rom microbio-
logical studies at seamounts, showing
that there are very diverse microbial
communities, oen with novel organisms
with metabolic capabilities (including
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Oceanography V.23, N.164
heterotrophy) and microbes that oxidize
(and reduce) iron, manganese, and sulur.
Microbial communities may thrive, in
particular, on the reducing potential o
these elements in basalt, which is strongly
enhanced by the high abundance o keynutrients in these rocks that are depleted
in seawater (Fe, Mn, Ni, P). Fluids
extracted rom a sealed drillhole into
Baby Bare Seamount (Cascadia Basin,
Northeast Pacic Ocean) yielded a highly
diverse microbial community (Cowen
et al., 2003), suggesting that microbes
are active inside the crust as well. Te
latter is also suggested by the study o
microbial trace ossils in volcanic glassalteration that imply that bioalteration
dominates glass alteration in the upper
oceanic crust, at least to a depth o 500 m
(Figure 2E,F; Furnes and Staudigel, 1999;
Staudigel et al., 2008).
Te interplay o ocean circulation
and hydrothermal activity at seamounts
also has a substantial impact on the
macroauna ound at their suraces.
For example, the crater o Vailuluu
Seamount displays substantial metazoan
mortality in the deepest part o the
crater, whereas the summit o a new
volcano growing in the crater displays
a thriving population o eels (Staudigel
et al., 2006). Similar complex benthic
zonations likely occur at many other
seamounts, such as the liquid sulur
lakes at Daikoku and Nikko volcanoes
(Embley et al., 2007).
the structural eVolutioN
o seaMouNts
Seamounts evolve in six stages that
are structurally distinct (Figure 3).
(1) Small seamounts (< 1-km tall)
and abyssal hills (< 100-m tall) are
seaoor volcanoes built on the oceanic
crust largely by eruptive processes.
(2) Mid-size seamounts are taller than
about 1000 m and remain at eruptive
depths deeper than 700 m, representing
largely nonexplosive volcanoes with
magmatic plumbing systems thatbecome an increasingly important part
o the seamount itsel. (3) Explosive
seamounts orm at ocean depths shal-
lower than 700 m and are mostly covered
by volcaniclastics. (4) Islands orm when
a seamount breaches the sea surace,
shiing to subaerial processes, including
subaerial weathering, soil ormation,
and erosion. (5) Extinct seamounts
have completed all volcanic activity andsubsided below sea level, sometimes
orming atolls. (6) Seamounts meet
the end o their geological lie during
closure o an ocean basin or by subduc-
tion. Although these six stages describe
the complete development o very large
seamounts, most seamounts never reach
the island stage. Smaller seamounts may
be entirely buried by sediments by the
time they reach a subduction zone.
sg 1
Small seamountswith total heights o1001000 m are by ar the most common
type o seamount, even though they
are oen difcult to distinguish rom
the roughness o the abyssal oceanic
crust topography that may occur on a
similar scale. Te key distinctive eature
is their oen circular morphology that
is superimposed on a seaoor abric
that largely is made up o elongated
MOR volcanoes or tectonic blocks.
Tere may be as many as 25 million o
them worldwide (Wessel et al., 2010).
Te majority o these seamounts orm
on relatively young crust, at water
depths between 23 km, on bare rock,
or on thinly sedimented seaoor. Teir
dominant eruptive lithologies include
mostly pillow lavas and approximately
20% volcaniclastics (as ound at Ocean
Drilling Program/Deep Sea Drilling
Project Site 417A and the deep-watersection at La Palma; Staudigel and
Schmincke, 1984). It is likely that
seamounts on sedimented seaoor may
initially orm intrusions into the sedi-
ments and/or peperites, beore they
erupt pillow lavas that quickly cover
these rocks. Small seamounts are likely
to include almost no intrusive rocks,
except or their eeder dikes, which has
important consequences. Te underlyingoceanic crust controls these seamounts
plumbing systems and tectonics. Tere
also is no or very little intrusive ination
o the reestanding part o the seamount,
which makes it very unlikely that these
volcanoes develop any major collapse
eatures. And, nally, the reaction zone
or their hydrothermal uids is located
inside the oceanic crust, and their hydro-
thermal venting is strongly linked to the
hydrothermal cooling o the underlying
oceanic crust. Intense low-temperature
hydrothermal circulation in small
seamounts results in extreme enrich-
ments in seawater-derived chemical
components, particularly in volcaniclas-
tics but also in lava ows (e.g., Site 417A;
Staudigel et al., 1996). It is quite likely
that the very large number o small
seamounts and their extreme alteration
makes them major players in global
geochemical cycles and as globally rele-
vant habitats or microbial communities.
sg 2
Mid-size seamountsare dened hereas volcanic eatures that are tall enough
(at least 1000 m in height) to have a
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Oceanography M 2010 65
magma plumbing system above the level
o the oceanic crust, but that have not
yet shoaled enough to be dominated by
shallow-water explosive activity (which
starts at ~ 700-m water depth). Tere
may be hundreds o thousands o suchseamounts (Wessel et al., 2010), and
some o them may be quite largeall
Hawaiian seamounts had to grow to
4000-m heights above the seaoor
beore they became explosive in Stage 3.
Mid-size seamounts are likely to be
mostly made o eruptive rocks, with
an increasing raction o intrusives as
they increase in size. Based on observa-
tions at La Palma, the eruptive rocks on
typical mid-size seamounts may consist
o a majority o pillow lavas with about
20% volcaniclastics.
As seamounts increase in size, theybegin to develop internal magma
reservoirs above the oceanic crust. Tis
attribute has two important conse-
quences. First, intrusions begin to
inate the volcano, and this ination
soon dominates its internal stresses and
determines the development o ssures
and ri zones as well as, ultimately, the
extent and geometry o volcano collapse.
Second, intrusions introduce heat into
shallower portions o the volcano, estab-
lishing hydrothermal convective cooling
systems that are increasingly decoupled
rom the underlying oceanic crust. AtVailuluu Seamount, earthquake loca-
tions cluster below hydrothermal vents
along main structural trends dened
by ri zones (Konter et al., 2004), indi-
cating a close relationship among the
development o permeability, intrusive
geometry, and hydrothermal cooling.
Te depth o the earthquakes suggests
1
2
3
4
5
6
g 3. sx g
mn vn
dnd b mn
z, b nnng
m
n, nd b x
. sg 1
n m
mn (1001000 m),
mgm mbng
m d n
n , b vn. sg 2 nd
mdz vn
xdng 1000 m n
g nd
v d g n
700 m,
d bgnnng
xv v. sg 3
db xv
g mn bv
700m n d,
nd sg 4
mn mgd
mm mng ndnd bn .
sg 5 nd
mn v
d nd
mgm v. sg 6
b
mn b
nmd b bdn.
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Oceanography M 2010 67
photosynthesis becomes a viable energy
source or carbon xation. In tropical
regions, this can result in the ormation
o massive carbonate rees. Such rees are
among the most diverse and productive
habitats on Earth.
sg 4
Islands are born when shallow subma-
rine volcanism allows a seamount to
emerge above sea level and to transi-
tion into subaerial volcanic activity.
Tis transition is a prolonged process.
Historically, most newly born islands
have disappeared shortly aer their
emergence (e.g., Metis Shoal o onga,Myojinsho o the Izu Ogosoawara
group south o Japan, Graham Island
in the Mediterranean Sea). Tey disap-
pear because the continuous power
o sea-surace wave erosion tends to
out-compete constructive volcanic
growth by minimally alteration-resistant
volcaniclastic rocks. In act, persistent
emergence may only be possible during
periods o very high eruption rates,
and the ormation o a more resistant
lava veneer that protects the underlying
clastic rocks rom wave erosion. It is also
conceivable that emergence is critically
aided by intrusive activity that may li
the top o a seamount.
Emergence o a volcanic island has
important geological consequences in
terms o eruptive and intrusive processes
and volcano collapse, erosion, and
weathering. Eruptive rocks at ocean
islands are dominated by lava ows that
cool at substantially slower rates than
their submarine equivalents. However,
many o these ows enter the sea, where
they produce large quantities o glass-
rich volcanic rocks that are deposited on
lava benches, submarine island anks, or
the surrounding seaoor. In particular,
ocean arcs, but also ocean islands, may
produce substantial explosive (Plinian)
eruptions that can distribute volcanic ash
as discrete layers up to 2000 km down-
wind rom the source (oba/Sumatra).Tese explosive eruptions constitute
major volcanic hazards, particularly
when hydrovolcanic processes resulting
rom contact with seawater ampliy the
eruptions. Major catastrophic erup-
tions include the 1630 BCE Santorini
cataclysm that is likely to have termi-
nated the Minoan culture, or the 1883
Krakatoa eruption that was associated
with more than 30,000 casualties in theIndian Ocean area.
Intrusive activity at islands is demon-
strably a major actor in their evolu-
tion and collapse. Collapse eatures
are most likely and most prominent in
the largest seamounts and islands with
well-established, long-lived magmatic
plumbing systems. Tey are most
commonly ound near the point o
emergence o major ri zones romthe main body o a volcano (Figure 4).
Sector collapse may mobilize volumes
up to 3000 km3 (Satake et al., 2002), and
the debris rom such ows may cover an
ocean area o 10,00015,000 km2 with
> 1-km-size blocks up to 180 km rom
their site o origin (Moore and Clague,
2002). Such events leave a major scar on
the side o the volcano, have a substantia
eect on benthic lie around the volcano,and result in catastrophic tsunamis that
may have runups reaching inland to
localities up to 400 m above sea level,
constituting a major ocean-basin-wide
-6000 -5000 -4000 -3000 -2000 -1000
Depth (m)
15400'E 15420'E
1700'N
1640'N
1620'N
1720'N
DEBRIS
HEADWALL
SCARP
g 4. Dgm
fn Vnd
smn n nn
dg mm m
(k ., 1998). M
n 10 m3 mn
fn mbzd nd
dd > 6 m m
b.
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Oceanography V.23, N.168
natural hazard (e.g., at Kohala Volcano
in Hawai`i; McMurtry et al., 2004).
As soon as islands are ormed, they
are subjected to weathering, soil orma-
tion, and erosion, as well as the develop-
ment o groundwater, particularly inenvironments with signicant rainall.
Weathering and erosion products intro-
duce a range o new materials into the
marine environment, including organic-
rich soils and sands with highly reactive
components (olivine, glass) that can
support lie in the ocean. In addition,
the ormation o groundwater on islands
commonly creates a submarine resh-
water lens that alters the hydrothermalreactions rom a seawater-dominated
environment to a reshwater system
(Clague and Dixon, 2000). Other conse-
quences o the reshwater lens include
reshwater seeps in shallow marine
settings, and the potential lubrication
o aults at depth due to increased hydro-
static pressure rom a groundwater table
that commonly is located at elevations
well above sea level.
Ocean islands and their coastal
shallow submarine systems are biologi-
cally complex environments that benet
in particular rom the availability o
photosynthesis as an energy source, as
well as rom the ertility o volcanic soils.
Ocean islands also oen provide nesting
grounds or seabirds whose presence can
result in deposition o guano that may be
introduced to the ocean by weathering
or runo, resulting in increased ertility
on land and in the surrounding shallow
ocean. Islands and volcanic rees in
tropical and subtropical climates provide
substrates or carbonate rees, that may
range rom minor shoreline rees or
massive coral rees that eventually may
cover the entire subsiding volcano. I
conditions are met, rees can get estab-
lished airly quickly, as is evident at the
relatively recent Hawaiian volcanoes
o Hualalai, Kohala, and Mauna Kea.
Overall, coral rees and volcanic ocean
islands are among the most productivesheries on Earth. Tis productivity
may be a result o many processes and
circumstances, including the constant
volcanic input, photosynthesis, and
complex local ocean circulation, due to
the islands roles as ocean stirring rods.
staGe 5
Once volcanism ceases, a volcanic island
will eventually drown due to erosionand subsidence. Coral rees will meet a
similar ate i coral growth does not keep
pace with subsidence. In some cases,
volcanically dormant islands may show
a short period o volcanic rejuvenation
that may be reected in the ormation
o volcanic cones that rise above the
erosion surace o its otherwise at
top. Following cessation o volcanism,
drowning islands and rees eventually
will become part o a diverse group o
extinct seamounts that includes many
that never reached Stages 24. Former
islands and coral rees can be recog-
nized as seamounts with at summits
(guyots). Aer their submergence,
all the extinct seamounts remain intact
and virtually unchanged until they
are consumed by subduction or ocean
basin closure. Some may be older than
140 million years, giving us the oldest,
most stable landorms on the planet.
Despite the absence o volcanism or
erosion, extinct seamounts neverthe-
less serve some signicant geological,
hydrological, and oceanographic unc-
tions. With time, seamount suraces
are encrusted with erromanganese
oxides and phosphorites, depending
on their chemical environment. Tese
mineral groups potentially include very
high concentrations o elements that
are very scarce in seawater (Mn, P, Co,
Ni, i, Pt, T, Pb, rare earth elements;Hein et al., 1997), and thus seamount
suraces are likely to play a role in the
geochemical budgets o these elements
in seawater. Such suraces provide some
unique substrates or microbial growth
and the basis or potential seaoor
mining, in particular or Co and some
high-tech metals such as tellurium
(Hein et al., 2010).
Hydrologically, extinct seamountsmay continue to act as long-term hydro-
thermal siphons (Fisher and Wheat,
2010), possibly providing the dominant
mechanism or the exchange o uids
between seawater and sediment-covered
old oceanic crust. Tis chemical exchange
remains to be quantied but may be
proound in terms o global geochemical
budgets and in providing a habitat or
microbial activity. Large submerged
seamounts can substantively impact
ocean circulation, including vertical
mixing and the generation o turbulence
that determines the sedimentation
patterns at and down-current rom the
seamount (Lavelle and Mohn, 2010).
Extinct seamounts are considered
biological hotspots (Danovaro et al.,
2009), with thriving microbial commu-
nities and sessile megaauna colonizing
their suraces (see Box 7 on page 128 o
this issue [Etnoyer, 2010]). Microbial
communities are known to be associated
with the venting or seepage o uids
rom the seamount, but it is also likely
that they orm communities inside
the seamount, taking advantage o a
range o chemical nutrient or energy
8/3/2019 The Geological History of Deep-sea Volcanoes
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Oceanography M 2010 69
sources provided by hydrothermal
circulation. Surace-colonizing auna
include, in particular, lter eeders such
as deepwater corals, especially along
ridges and ri zones where they are
exposed to maximum ow o seawater.Deepwater corals are valuable recorders
o paleoclimate or at least the past
200,000 years (Robinson et al., 2007),
with individual corals recording climate
inormation or their lie spans o up to
several thousand years.
sg 6
Seamount lie spans end when they
reach a subduction zone or when theirhost ocean basin closes due to the colli-
sion o two continental plates. ectonic
processes that aect geohazards, as
well as chemical uxes and biological
processes, mainly control this phase.
Seamount subduction may ollow
dierent paths, depending on subduc-
tion geometry. In arc systems with
pronounced accretionary wedges,
seamount subduction leaves promi-
nent scars in the rontal portion o
the wedge (e.g., Costa Rica). In the
western Pacic, seamount subduction
commonly begins with aulting and
breakup (e.g., Capricorn and Osborne
seamounts) beore the seamount
descends into the trench. As seamounts
become entirely subducted, they can
temporarily li up segments o the over-
lying arc system, potentially creating a
blockage to continued subduction that
may be released in rupture and associ-
ated seismic activity (e.g., Scholz and
Small, 1997; Watts et al., 2010). Descent
o a major topographic eature, like a
seamount chain, may have a proound
impact on a subduction zone, potentially
liing up the entire arc and/or creating
a slab window as in the Costa Rica arc
at its intersection with the Cocos Ridge.
Te tectonism involved in seamount
subduction also results in the opening
o new uid pathways and brings cold
sediments and seamount materials
into contact with warmer materials in
the subduction system. Tese eects
combine, cause top-down prograde
metamorphism, and allow uids to
escape, potentially with consequences or
substantial geochemical uxes in an arc
system (Staudigel et al., 2010b).
Although there is almost nothing
known about the biology o this nal
stage o seamount evolution, it is very
likely that the enhanced uid ow also
results in substantial microbial activity,
perhaps providing a link between micro-
bial activity and prograde metamor-
phism in the subducting slab.
coNclusioNs
Seamounts are geological and biological
hotspots that connect lithosphere,
hydrosphere, and/or biosphere in ways
that evolve systematically as a seamount
grows. Tese processes are likely to have
a proound impact on geochemical uxes
and the development o seaoor commu-
nities and deep biosphere communi-
ties, and, nally, they may result in
hazards to humankind.
Geochemical uxes between seawater
and seamounts are acilitated by their
role as hydrothermal siphons and by
their own hydrothermal convection
systems. Te latter is driven initially
by intrusive activity in the underlying
oceanic crust and then by intrusions
into seamounts as they develop their
magma plumbing systems. Fluxes are
high because o the high permeability
o the volcaniclastic rocks and their
large volume raction, and the lack o
sedimentary cover that would impede
uid ow. Fluxes are relevant to ocean
chemistry, arc mass balances, and the
chemical heterogeneity o the mantle
through seamount subduction.
Reactive suraces and the chemical
uxes rom hydrothermal ow support
the development o biological communi-
ties that take advantage o the chemical
energy and nutrients provided by
seamounts. Small seamounts mostly
act as portals to the underlying oceanic
crust, whereas larger seamounts
are likely to include their own deep
biosphere. Both are likely to be impor-
tant. Counting the larger seamounts
alone, they represent a biome the size o
Europe or larger (see Box 12 on page 206
o this issue [Etnoyer et al., 2010]).
Signicant geohazards may be
associated with very large seamounts,
which pose navigational hazards and
produce explosive activity and tsunamis
roM oVer hal a ceNtury o MoDestresearch eorts, it is clear that seaMouNts areMost rewarDiNG aND iNterestiNG tarGets oruture oceaNoGraphic research.
8/3/2019 The Geological History of Deep-sea Volcanoes
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Oceanography V.23, N.170
associated with sector collapse. Tese
hazards are responsible or some o
the largest geologic disasters recorded
in human history.
All three processes are likely to be
globally relevant, but none are known tothe extent needed to make meaningul
estimate o uxes, total biomass, extent
o primary carbon xation, or a realistic
probability o geohazard threats. Tis
lack o detail results rom less than
one percent o all seamounts having
been studied even in a cursory way,
and much o what we know o these
processes is drawn rom a ew, oen
singular, well-studied examples. Fromover hal a century o modest research
eorts, it is clear that seamounts are
most rewarding and interesting targets
or uture oceanographic research. Tis
body o work points the way to a series
o targeted investigations that have
much potential or better understanding
how Earth works.
ackNowleDGeMeNts
We acknowledge Anthony Koppers,
John Mahoney, Bob Duncan, and Rodey
Batiza or helpul and insightul critical
reviews and assistance with illustrations
by Patty Keizer.
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