Experimental simulation of rapid rock block disintegration by sodium chloride in a foggy coastal desert

( )Journal of Arid Environments 1998 40: 347]355Article No. ae980465

Experimental simulation of rapid rock blockdisintegration by sodium chloride in a foggy

coastal desert

A. S. Goudie & A. G. Parker

School of Geography, University of Oxford, Mansfield Road,Oxford OX1 3TB, U.K.

( )Received 30 March 1998, accepted 16 October 1998

An environmental chamber was used to simulate a 24-h cycle of rock surfacetemperature and relative humidity, a stone pavement with differing levels ofsodium chloride content, and differing levels of fog precipitation to assesssome of the controls of rock weathering in the vicinity of Swakopmund in theNamib Desert. The rock employed was Cretaceous Chalk. Some treatmentswere effective at causing disintegration after 76 temperaturerhumidity cycles

.( )and 10 fog cycles. Small amounts of fog moisture 0 5 mm per event wereassociated with the greatest amounts of breakdown, whereas the amount ofsalt in the simulated pavements appeared to be of less significance. Theexperiment showed that sodium chloride can be an effective agent ofweathering in a coastal foggy environment.

q 1998 Academic Press

Keywords: salt weathering; fog moisture; sodium chloride; Namib

Introduction

( )Field observations in the Namib Desert Goudie et al., 1997 demonstrated that blocksof limestone placed on the desert surface underwent severe disintegration after only 2years of exposure. This phenomenon was attributed to the fact that this coastal deserthas two main characteristics that create highly aggressive ground conditions: sodiumchloride-rich surface materials combined with frequent wetting and drying cyclescaused by a high frequency of fog precipitation events.

The purpose of the experiment described in this paper was to determine if this greatrapidity of weathering could be simulated in the laboratory, and to ascertain whetherrates of breakdown were controlled by the amount of sodium chloride present insurface materials andror by the quantity of precipitation during fog events.

To this end, rock blocks were placed on a simulated desert surface with a range ofsalt and sand contents, subjected to simulated temperature and relative humiditydiurnal cycles in an environment cabinet, and sprayed with a range of simulated fogprecipitation amounts.

0140]1963r98r040347 q 09 $30.00r0 q 1998 Academic Press

A. S. GOUDIE & A. G. PARKER348

The Central Namib

( )The Central Namib Desert in the vicinity of Swakopmund 228419 S, 148319 E ishyperarid. Rainfall averages about 10]20 mm per annum over the long-term, butrather larger amounts of precipitation are deposited by fogs which on average precipitateon around 65 days of the year. The area around Swakopmund is the foggiest part of the

( )desert and has over 100 fog days per year Olivier, 1995 . Temperatures are stronglyoceanic in character and are controlled by the cool Benguela Current offshore. There

( )are thus low seasonal and daily ranges 58C and 88C, respectively .The environment and geomorphology of the area has been described previously by a

( )number of workers e.g. Wilkinson, 1990 , and many of these workers have noted that(bedrocks in the area have been affected by salt weathering e.g. Goudie, 1972; Selby,

)1977; Lageat, 1994 . In addition, some engineering structures in the region have( )undergone catastrophic weathering-induced failure Bulley, 1986 .

Fog in the Namib

Fog may contribute to salt weathering by adding significant quantities of moisture torock surfaces. The amounts involved are sufficient to cause cycles of solution andcrystallization but are not generally sufficient to cause wholesale salt removal byleaching. It is also possible that some salt may be deposited by fogs, though the Namibfogs, contrary to some earlier reports based on contaminated collectors, appear to bechemically very pure and are probably not themselves a major source of evaporite ions( )Eckardt, 1996 .

In the Central Namib there are nine weather stations providing useful fog data( )Table 1 . As a result of moist oceanic air flowing over the upwelled cold BenguelaCurrent, the effects of fog are felt more than 100 km inland. At a number of stationsprecipitating fog occurs on between about 50 and 90 days in the year, though thenumber of days when fog is observed may be greater than this and figures as great as300 days in the year have been given. Fog water precipitation exceeds mean annualrainfall and increases from the coast inland to a distance of about 35]60 km from thesea, after which it decreases further inland. Annual fog precipitation values at two

y1 ( )inselberg stations exceed 180 mm year compared to around 20 mm annual rainfall .Thus, the mean amount of fog precipitated per foggy day or night can be several

( )Table 1. Fog data for the Central Namib Desert Source: Lancaster et al., 1984

Mean annual Mean annual Mean quantity offog number of days precipitation

precipitation with precipitating per foggy day( ) ( )Station mm fog mm

. . .Flodden Moor 65 13 55 50 1 17

. . .Ganab 2 67 2 76 0 97

. . .Gobabeb 30 79 37 23 0 83

. . .Narabeb 35 91 38 45 0 93

. . .Rooibank 80 19 75 64 1 06

. . .Swakopmund 33 94 64 68 0 52

. . .Swartbank 183 62 86 71 2 12

. . .Voglefederberg 183 48 77 37 2 37

. . .Zebra Pan 15 11 16 00 0 94

RAPID ROCK BLOCK DISINTEGRATION 349

millimetres. At Swakopmund itself, fog occurs on around 65 days of the year with each.foggy day producing an average of 0 52 mm of precipitation.

Salt weathering

In recent years many studies have demonstrated the role of salt weathering as aneffective agent of rock disintegration in coastal, urban, polar and arid environments.

( )These studies have been reviewed by Goudie & Viles 1997 , and include both fieldobservations and laboratory simulations. Although salts can contribute to chemicalchanges in a rock mass, much of the observed disintegration has been attributed toeither salt crystallization or, in the case of certain salts, to salt hydration.

Field observations of rock block disintegration in the central Namib

( )In a field experiment reported by Goudie et al. 1997 , pre-weighed cuboid blocks of aJurassic limestone were exposed on the ground surface just north of Swakopmund for aperiod of 2 years. Some of the blocks suffered from extensive disintegration, and

(laboratory analyses indicated that the weathered samples had a high halite sodium)chloride content. It was concluded that frequent wetting and drying associated with

the many fog events of the area cause numerous cycles of halite crystallization, and thatrocks exposed at the surface absorb salts from the desert surface. This surface was astone pavement over bedrock, was capped with a fine layer of aeolian sand andcontained appreciable quantities of halite and gypsum.

The environmental cabinet

The simulation was undertaken in a Fisons FE 300 environmental cabinet, which canbe programmed through its microprocessor unit to give independent cycles oftemperature and humidity. Heating is by convection, and the ambient conditionswithin the chamber are continuously monitored by built in sensors and chart recorders.

The synthetic Swakopmund cycle

( )The cycle used in the experiment Fig. 1 was designed to represent typical rocksurface temperature and relative humidity diurnal cycles. It was constructed on thebasis of available air temperature and relative humidity data obtained from the weather

( )station at Swakopmund Lancaster et al., 1984 adjusted to give ground surfaceconditions based on some data-logger determinations undertaken in the field withthermistors and a humidity probe. It is designed to replicate in general terms theconditions experienced by a surface rock outcrop on a typical day. Given that thecoastal situation leads to little seasonal variability, it is believed to have general validityfor most of the year but it is a synthetic cycle in the sense that it is not an actually

(observed cycle for one day of the type used in some other experiments e.g. Goudie &) ( )Viles, 1995 . The cycle replicates a low night-time temperature 158C which is

( )accompanied by high relative humidity levels 99% RH , and a high daytime( ) ( )temperature 358C accompanied by a low relative humidity 20% RH .

The other aspect of the treatment was to subject the samples and the simulateddesert surface to simulated fog precipitation. Three different quantities of fog

.precipitation were applied manually by a fine spray gun using deionized water: 0 5 mm,


Figure 1. The synthetic 24-h temperaturerhumidity cycle for the ground surface of theSwakapmund area.

1 mm and 2 mm. Control samples received no spray. As fog precipitation does notoccur on every day of the year, the spray was applied once every 7 days at that point inthe 24-h cycle when temperatures were lowest and relative humidity levels werehighest. This is because fog precipitation tends to take place in the early morning.Deionized water was used because, as already mentioned, Namib fogs appear to bechemically rather pure.

The simulated desert surface

The desert surface upon which rapid block disintegration had been seen to take place inthe field was predominantly sandy and contained variable amounts of sodium chloride.A simulated desert surface was therefore created by thoroughly mixing a range ofdifferent combinations of salt and sand and placing them in trays above a layer of flint

( )gravel c. 2 cm thick . In order that heating was from above, the trays were insulated.( )on the sides and bases with polystyrene 2 5 cm thick and the thickness of the

sandrsalt mixture was 4 cm. The ratios of salt to sand used were as follows: 1:1, 1:3,1:5, 1:10 and 1:20.

In addition, a control of pure sand was used. Three blocks of chalk were placed oneach tray and each tray was subjected to one of three levels of fog moisture application:. . .0 5 mm, 1 0 mm and 2 0 mm.

The rock blocks

The rock blocks used in the simulation were blocks of Upper Chalk from theCretaceous of southern England that were freshly quarried and were cut by a diamond


. .bladed saw. Their dimensions were 4 5 cm = 4 cm = 3 5 cm. It is a soft white( )limestone purity c. 98]99% CaCO with a fine-grained texture and a water adsorption3

capacity of c. 15%. Prior to the simulation the blocks were dried to constant weight at408C and 50% relative humidity and any loose particles were brushed off beforeweighing. Chalk was used, not because it occurs in the Namib, but because it has beenobserved in other experiments to respond relatively quickly to salt attack and so waslikely to give a range of responses to different treatments in a manageable length oftime.

Assessment of change

Signs of weathering appeared very rapidly and very early on in the simulation, withsome blocks showing crumbling, blistering or cracking after only one fog application.

The experiment ran for 76 days, and thus included 10 simulated fog applications. At( )the end of 76 days before an eleventh fog application would have taken place the

samples were removed from the environmental cabinet and were immediately weighed.They were then dried at 408C and 50% RH to constant weight, and reweighed toascertain how much moisture was driven off. They were then leached in hot water for 3days to remove as much incorporated salt as possible, and then dried for 3 days at 408Cand 50% RH and reweighed. This was done to obtain a measure of weight lossresulting from weathering and also to release fragments that may have been fracturedby salt growth but also cemented by salt action. A record was also made of the numberof splits that had occurred to produce fragments more than 3 g in mass. A mean figureof weight change was obtained for each trio of samples that had been subjected to thesame treatment of spray application and salt level in the simulated pavement, giving 15trios in all. The trios were then ranked according to the degree of change that had takenplace.

Three samples were used for each treatment to reduce the effects of any inherentvariability that there might be in the rock. Two samples of the 48 did split to producetwo large fragments, but the potential effects of this were overcome by only regardingweight loss as involving particles that had a mass of less than 3 g. With thoseexceptions there was relatively little inter-trio variation in the performance of theblocks. This is evident in Fig. 2, for example, where all three chalk blocks subjected to

.0 5 mm of fog precipitation show a similar and marked degree of breakdown by. .granular disintegration in comparison to the blocks that received 1 0 and 2 0 mm of

precipitation.

Results

The main results of the experiment are summarized in Table 2..( )The five treatments with the lowest amount of fog application i.e. 0 5 mm had on

.( ( ))average the greatest amount of weight loss mean rank 4 4 Fig. 2 . Those samples.( )with the intermediate ranking in terms of weight loss mean rank 9 4 also had an

.( )intermediate amount of fog application 1 0 mm , whereas those samples with the.( ) (largest amount of fog application 2 0 mm had the least amount of weight loss mean

. )rank 10 2 . The control samples showed no loss in weight.The picture of weight loss with respect to the saltrsand ratios in the simulated

pavement is complex. There is far less range in this parameter, with a range between. .6 7 and 9 3. On average the most aggressive salt environment was that with a 1:20 ratio

of salt to sand and the least aggressive that with a 1:10 ratio of salt to sand. The amountof salt present does not seem to have a simple linear effect on the degree of breakdown,even though the salt contents have such a large range.


Figure 2. The arrangement of chalk blocks on the simulated pavement, showing the state of the. . .( ) ( ) ( )blocks subjected to a fog precipitation of a 0 5 mm, b 1 0 mm and c 2 0 mm with a saltrsand

mixture of 1:3 after six fog cycles.


Table 2. Ranking of weight change following leaching*

( )Amount of fog application mmSalt to sand ratio of. . .simulated pavement 0 5 1 0 2 0 Mean rank

.1:1 4 13 5 7 3

.1:3 1 14 10 7 3

.1:5 2 11 12 8 3

.1:10 7 6 15 9 3

.1:20 8 3 9 6 7. . .Mean rank 4 4 9 4 10 2

* Rank 1 is the greatest weight loss and rank 15 is the least weight loss.

In terms of actual weight loss the four treatments that had a weight loss greater than.10% of their initial weights were the following: 0 5 mm spray, 1:3 saltrsand ratio s

. . . .17 12%; 0 5 mm spray, 1:5 saltrsand ratio s 15 50%; 1 0 mm spray, 1:20 saltrsand. . .ratio s 12 50%; and 0 5 mm spray, 1:1 saltrsand ratio s 11 07%.

The results of the study of moisture content of the weathered samples is shown in.( )Table 3. On average, the samples sprayed with the least simulated fog 0 5 mm had the

.lowest moisture contents at the end of the experiment and those sprayed with 2 0 mmthe highest. Salt content also appears to play a role here in that the lowest moisturecontents for any particular spray application were for those samples with a saltrsandratio of 1:20. Once again, however, there is some complexity in the pattern.

Table 4 shows the ranks of weight loss against moisture content for each trio. The.Spearman rank correlation coefficient is 0 734, which is highly significant. A possible

(explanation for this relationship is that samples that remain moist either because of theapplication of a large quantity of fog, or because they contain a large amount of

)hygroscopic sodium chloride, or because of a combination of the two have relativelylow rates of breakdown because they never dry sufficiently for salt crystallization cyclesto be fully effective.

Finally, four samples which suffered little weight loss due to liberation of granulardebris, suffered cracking, while some others showed signs of incipient cracking. Thebulk of the granular disintegration took place on the upper portions of the blocks.

Conclusions

The laboratory simulation reported in this paper appears to have replicated the

Table 3. Ranking of moisture change*

( )Amount of fog application mmSalt to sand ratio of. . .simulated pavement 0 5 1 0 2 0 Mean rank

.1:1 2 9 15 8 7

.1:3 3 11 14 9 3

.1:5 5 13 12 10 0

.1:10 4 7 10 7 0

.1:20 1 5 8 4 7. . .Mean rank 3 0 9 0 11 8

* As determined by comparison of weight at end of treatment with that after drying at 408C.


Table 4. Rankings of weight loss and moisture content

Saltrsand Fog application Rank of Rank of( )ratio mm weight loss moisture content

.1:1 0 5 4 5

.1:1 1 0 13 9

.1:1 2 0 5 14

.1:3 0 5 1 1

.1:3 1 0 14 11

.1:3 2 0 10 13

.1:5 0 5 2 2s

.1:5 1 0 11 10

.1:5 2 0 12 12

.1:10 0 5 7 2s

.1:10 1 0 6 7

.1:10 2 0 15 15

.1:20 0 5 8 6

.1:20 1 0 3 4

.1:20 2 0 9 8

extremely rapid rate of rock breakdown observed in the field. Only a very limitednumber of cycles of salt crystallization are required to cause disintegration to occur.

Second, the experiment has confirmed that sodium chloride can be effective as a( )cause of rock breakdown. This has been postulated before e.g. Chapman, 1980 and

(has also been directly monitored on playa surfaces dominated by halite Goudie &)Watson, 1984 , but most experimental simulations have for the most part failed to

demonstrate its efficacy in comparison to other common salts such as sodium sulphate( )or sodium carbonate Goudie, 1993 .

Third, the experiment has shown that salt weathering can operate quickly without( )samples without being fully immersed in saline solutions Goudie, 1993 .

Fourth, the experiment has confirmed the important role fog plays in stimulating salt( )weathering in coastal deserts, thereby confirming the suppositions of Lageat 1994 and

( )Abele 1983 .Finally, and perhaps most significantly, the experiment has revealed that the

quantities of fog moisture applied and the salt content of the desert surface have acomplex and non-linear relationship with the amount of breakdown achieved. Inparticular, the application of large amounts of fog moisture reduces the rate ofweathering by reducing the opportunities for drying and salt crystallization. The

.optimal fog application was 0 5 mm, which is almost exactly the amount precipitatedby the average fog at Swakopmund.

We are grateful to Chris Jackson for laboratory assistance, to Martin Barfoot for the plates and toJan Magee for typing the manuscript.

References

( )Abele, G. 1983 . Flachen hafte Hanggestaltung und Hangzerschneidung im Chilenisch]Peruanischen Trockengebiet. Zeitschrift zur Geomorphologie Supplement Band, 48: 197]201.¨

( )Bulley, B.G. 1986 . The engineering geology of Swakopmund. Communications Geological SurveySW AfricarNamibia, 2: 7]12.


( )Chapman, R.W. 1980 . Salt weathering by sodium chloride in the Saudi Arabian Desert.American Journal of Science, 280: 116]129.

( )Eckardt, F. 1996 . The distribution and origin of gypsum in the Central Namib Desert,Namibia. Unpublished D.Phil. thesis, University of Oxford.

( )Goudie, A.S. 1972 . Climate, weathering, crust formation, dunes and fluvial features of theCentral Namib Desert, near Gobabeb, South West Africa. Madoqua, Series II, 1: 15]31.

( )Goudie, A.S. 1993 . Salt weathering simulation using a single-immersion technique. EarthSurface Processes and Landforms, 19: 369]376.

( )Goudie, A.S. & Viles, H.A. 1995 . The nature and pattern of debris liberation by saltweathering: a laboratory study. Earth Surface Processes and Landforms, 20: 437]449.

( )Goudie, A.S. & Viles, H.A. 1997 . Salt Weathering Hazards. Chichester: Wiley.( )Goudie, A.S. & Watson, A. 1984 . Rock block monitoring of rapid salt weathering in southern

Tunisia. Earth Surface Processes and Landforms, 9: 95]98.( )Goudie, A.S., Viles, H.A. & Parker, A.G. 1997 . Monitoring of rapid salt weathering in the

central Namib Desert using limestone blocks. Journal of Arid Environments, 37: 581]598.( )Lageat, Y. 1994 . Le desert du Namib central. Annales de Geographie, 103: 339]360.´ ´

( )Lancaster, J., Lancaster, N. & Seeley, M.K. 1984 . Climate of the Central Namib Desert.Madoqua, 14: 5]61.

( )Olivier, J. 1995 . Spatial distribution of fog in the Namib. Journal of Arid Environments, 29:129]138.

( )Selby, M.J. 1977 . On the origin of sheeting and laminae in granitic rocks: evidence fromAntarctica, the Namib Desert and the Central Sahara. Madoqua, 10: 171]179.

( )Wilkinson, M.J. 1990 . Palaeoenvironments in the Namib Desert. The lower Tumas Basin inthe Late Cainozoic. University of Chicago Research Paper, 231: 196.

Documents

Experimental simulation of rapid rock block disintegration by sodium chloride in a foggy coastal desert