Salt Crystal Growth, Wedging, or Salt Weathering

Salt weathering or salt crystal growth is physical or mechanical weathering. It is caused by the growth, expansion, or hydration of salt crystals inside cracks, pores, or interstices on rocks.

The growth, expansion, and hydration of salt crystals exert internal stress or pressure on the confining walls of cracks, pores, or interstices. This stress will, over time, cause rocks to spall, flake, break, disaggregate, or undergo granular disintegration.

Granular disintegration occurs along grain boundaries or interstices of crystalline rocks like granite, diorite, or gabbro. This forms grus, i.e., angular, coarse-grained fragments of mineral crystals.

Remember, rocks will only break or disaggregate when the internal force or stress exerted exceeds the tensile strength of the rock. Additionally, the internal stress will widen the cracks and pores.

Salt weathering is also known as haloclasty. Other authors refer to it as salt wedging. It may occur with other physical weathering processes like ice wedging, insolation, wetting, and drying, or chemical and biological weathering.

Lastly, acid and base reactions are what forms salts.

How does salt crystal growth weathering occur?

Salt weathering begins when saline or salt solution seeps or percolates into cracks, pores, fractures, cleavage planes, or interstices on rocks and crystallizes.

Crystallization can occur from cooling or evaporation (desiccation). Also, mixing different salt solutions can cause precipitation.  

Salt crystalizing causes weathering in three ways, i.e., it causes internal pressure or stress from 1) crystal growth, 2) saturation or hydration, and 3) thermal expansion.

Here are details on how each happens:

1. Salt crystal growth

Weathering from salt crystal growth is the most potent form of salt weathering. It happens when gradually growing salt crystals from a saline solution in pores or interstices apply pressure stress against the confining joint, pore, crack, or interstice walls.

This pressure will enlarge the cracks, pores, or interstices or cause disaggregation if the force exceeds the rock’s tensile strength over time.

The presence of a thin film of saline solution between the crystal and rock interface can help transmit generated pressure throughout the pore.

Factors that influence salt crystal weathering include supersaturation ratio and where crystallization occurs.

For instance, some can occur on the surface. Such isn’t damaging. Also, some may block pores, preventing fluid migration. Such will cause nondamaging sub-florescence.

2. Salt hydration

Hydration or absorption of water by some anhydrous salt crystals will make them expand or increase in volume. This occurs as they incorporate water into their crystal lattice.

Crystal expansion will exert tremendous hydration pressure or disruptive forces on confining pores, cracks, or interstice walls. This will make the voids enlarge and cause rocks to break or disintegrate.

Different salts will exert different hydration pressures on hydration because they expand to various degrees. For instance, sodium sulfate can expand as much as 313% on hydration. Also, sodium carbonate increases volume considerably.

Usually, hydration can happen from the wetting of rocks, dew, mist, or when relative humidity is high. For instance, humidity

Lastly, hydration is a chemical process. However, its effect is physical.

3. Salt thermal expansions

Heating of salts inside cracks or pores can cause them to expand. Most expand more than the rock. This expansion will exert thermal stress on the walls, confining them.

This thermal pressure will enlarge cracks, pores, or interstices. Also, it can cause rock to break or disintegrate.

How much each salt expands depends on its thermal properties, expansivity, and temperature.

However, this differential expansion isn’t as effective as other salt weathering mechanisms. This happens because heat penetration in rocks is poor or shallow.

Therefore, daily temperature changes will not affect subsurface rocks. Only deposits in pores or cracks close to the surface can create enough force to cause rocks to crack.

Evidence of salt weathering from thermal expansion includes more weathered rocks on sunny slopes than shaded in the Northern Hemisphere. This can be due to the thermal expansion of sodium chloride. However, it can also be due to typical insolation weathering.

Where is salt weathering likely to occur?

Salt weathering is likely to happen in arid, semi-arid, coastal, and urban environments where salt is readily available. However, such places should have wetting and drying cycles, mist or dew.

For example, salts are high in arid and semi-arid areas. It accumulates in lacustrine depressions and saline lakes. Also, it occurs in interior fluvial channels and on rock or sediment surfaces as saline efflorescences, i.e., whitish deposits.  

Also, it is common in shorelines of coastal regions because seawater has various salts dominated by sodium chloride. Wave action will spray this saline water on rocks or sea cliffs. The water will then penetrate cracks or pores.

Lastly, deicing salt in urban areas makes it abundant in these areas that experience winter.

Impact or effect of salt weathering

Salt weathering causes the cracking of roads in urban areas and some of the engineering concrete structures.

Usually, people use it to melt snow on roads. It will dissolve in water formed as ice melts and percolate into cracks or voids on the roads.

Desiccation or evaporation will make it crystallize, applying pressure to crack or void walls and causing fractures or disintegrations. Also, its hydration and consequent expansion will contribute to weathering.

Lastly, where conditions favor it, salt weathering can damage monuments and building stones, including in urban areas.

Factors that influence haloclasty

Some of the factors that influence salt weathering include:

1. Rock properties

The various rock properties, such as porosity, structure, texture, and strength or cementation, can influence the rate of salt weathering.

Also, transmissivity, or the rate at which a rock allows the saline solution to pass through, has an impact. It depends on numerous factors, including porosity, clay mineral present, adsorption capacity, etc.

For instance, sedimentary rocks like sandstones and limestones are more susceptible than igneous or metamorphic.

2. Salt type or properties

Salt characteristics like solubility, viscosity, thermal expansivity, hydration capacity, crystallization strength, and other physical properties can affect weathering.

Experiments confirm that sodium and magnesium sulfate cause most weathering as they significantly increase the hydration volume.

For instance, the hydration of thenardite (sodium sulfate – Na₂SO₄) to mirabilite (hydrous sodium sulfate – Na₂SO_4.10H₂O) is more disruptive than frost weathering or water freezing. It increases in volume significantly.

Similarly, anhydrite (calcium sulfate CaSO₄) to gypsum (calcium sulfate dihydrate – CaSO₄.2H₂O) will increase volume significantly, i.e., by about 39%.

Another factor is their thermal expansion coefficient. For instance, halite expands more than calcite granites or quartz.

Therefore, halite can cause tensile stress in rocks, especially in deserts and coastal areas with a significant ambient temperature variation.

3. Salt availability

Lithology and other factors can affect salt availability. Chlorides are common in coastal deserts, sulfates are arid, and carbonates are found in semi-arid zones. Other salts involved are sodium, calcium, magnesium, and potassium nitrates.

For instance, halite occurs in Australia, while the Atacama Desert has halite and calcium sulfate.

Ebro Depression of Spain has sodium chlorides and sulfate, and salty lakes in mid-eastern Africa have sodium carbonate.

4. Climate effect

Temperature, humidity, winds, and moisture or precipitation can affect wetting and drying, level of thermal expansion, hydration, and crystallization. This will, in turn, affect the rate of salt weathering.

For instance, high temperature, low humidity, or windy conditions will favor evaporation, causing the crystallization of saline solutions in rocks. Cooling, on the other hand, can also cause crystallization in some.

On the other hand, salts like sodium sulfate, magnesium sulfate, and sodium nitrate have lower solubility at lower temperatures. Therefore, cooling such will cause crystalize them.

Also, evaporation or desiccation will saturate the saline solutions in voids. This will also cause crystallization.  

For instance, high temperatures will dehydrate salts during the day, making them shrink. When nighttime temperatures drop and humidity increases, they rehydrate and expand at night. Such conditions occur in deserts.

Lastly, climate can affect water capillaries, generating saline efflorescence if they reach the surface.

Frequently asked questions

References

• Huggett, R. J. (2011). Fundamentals of geomorphology (3rd ed.). Routledge.
• Turkington, A. (2004). Mechanical weathering. In Goudie, A. (ed.) Encyclopedia of geomorphology (vol. 1, pp-657-659). Routledge.
• Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
• Elorza, M. G. (2013). Geomorphology. Taylor & Francis.
• Migoń, P. (2013). Weathering Mantles and Long-Term Landform Evolution. In Shroder, J. (Ed). Treatise on geomorphology (Vol. 4 pp. 137-139 ). (2013). Elsevier.
• Dixon, J. C. (2004). Weathering. In Goudie, A(ed.) Encyclopedia of geomorphology (vol. 1, pp 1108-1109). Routledge.