Insolation or Thermal Expansion Weathering

Insolation or thermal expansion weathering refers to rock breakdown that happens when rocks contract and expand. Some authors call it thermal weathering or thermoclasty.

This breakdown process is a physical or mechanical form of weathering. It is driven by cyclic temperature variation during the day when the sun shines and at night when it gets colder.

Also, heat from nuclear explosions or bushfires can cause it. This explains the use of fire in quarrying in India and Egypt.

Why does it occur? Insolation weathering occurs for two reasons, i.e., 1) temperature differences on the surface and inner rock and 2) varying expansion and contraction of individual mineral crystals in rocks.

These differences will cause mechanical or internal stress. Repeated internal stress will, over time, progressively make rocks crack, flake, or spall and undergo granular disintegration, including from fatigue.

Spalling occurs when rock layers, some only a few millimeters thick, break off or flake from the surface of a rock mass. These shells, sheets, or layers often have curvilinear to linear fractures parallel to the surface of the outcrops or exposed rocks.

Why does it happen? Spalling happens due to the shear stress beneath the surface. These stresses arise from a variation of rock surface and inner temperature differences.

However, spalling isn’t unique to insolation weathering. It occurs in other physical weathering forms, like frost weathering and salt wedging. Also, unloading or sheeting can cause it.

On the other hand, granular disintegration occurs along individual grain boundaries. It happens when individual grains expand or contract at different rates.

Granular disintegration will form grus. Grus is a German word translated into grit, fine-grained debris, or gravel of unconsolidated angular, coarse-grained individual mineral grains. It happens in coarse-grained rock-like granites.

Lastly, oriented cracks can happen from differential heating as the sun moves across the sky. These cracks are not due to rock variations, such as bedding being oriented north-south. 

Why and how does thermal expansion weathering occur?

Scientists have debated for a long time whether insolation or variation in temperature causes weathering. However, recent studies suggest it happens.

For instance, some studies have shown that a steep temperature gradient or difference between shaded and sunny areas can cause cracking.

From basic science, you know that heating causes expansion and cooling contraction. However, if uniform, it wouldn’t necessarily cause cracking or disintegration.

Why and how does it then happen? Thermal expansion or insolation weathering happens because rocks are 1) low heat conductors and 2) differently colored minerals.

Here is how each of these two reasons makes rocks weather.

1. Rocks are poor or low heat conduction

Rocks are low thermal conductors. When heated, only a few outer millimeters or centimeters will get hotter than the inner portion. Also, during cooling, this outer part will cool faster than the inner.

This causes a thermal gradient, i.e., a temperature difference between the surface and interior.

Therefore, when heated, the outer hotter portion will expand more than the inner cooler and shrink faster during a cooling cycle. The differences in expansion will cause shear stress that will crack, spall, and disintegrate rocks.

2. Varying rock mineral behavior to heating and cooling

Rocks, especially plutonic ones like granites and diorites, have different minerals. These minerals have different colors and expand, contract, and transfer heat at different rates.

Put differently, rock minerals have different thermal coefficients or thermally expand differently. Thermal coefficients predict how materials behave to temperature change.

Therefore, they show differential thermal response to heating and cooling cycles. For instance, dark minerals absorb and transmit more heat than lighter-colored ones and cool faster.

Also, the various rock-forming minerals, like calcite, quartz, amphiboles, and biotite, may expand or contract at different rates. Some are anisotropic, i.e., their properties vary in different directions.

Consequently, these minerals expand or contract differently when heated, even at a small temperature change. The difference in expansion and contraction rates will cause internal stress.

Usually, expansion causes compression force and contraction tensile. These two forces will cause microfractures along grain boundaries, causing granular disintegration. They do so by loosening their structure before they crumble.

Factors affecting insolation weathering

Factors that influence insolation weathering include temperature and rock properties.

1. Temperature

Higher temperatures will cause more expansion. This will speed rock weathering. For instance, rocks in deserts such as Sudan can reach 82°C and Atacama 50°C.

Even in humid tropical, temperatures can reach 50°C. Such places will have faster rock breakdown.

Don’t be surprised by the high values. The rock surface will get hotter than the ambient temperature.

Secondly, daytime and night (hot and cold) temperature differences will affect the rate of insolation weathering. A higher fluctuation will favor a faster breakdown.

This explains why it is high in semi-arid areas like the Mojave and Sahara deserts. In these areas, temperature fluctuates daily at about 20-30°C.

Lastly, a higher rate of temperature fluctuation will speed weathering. Engineering and ceramic research show that even minor variations in temperature over short periods can most effectively disintegrate rocks.

2. Rock properties

Rock properties like color, texture, grain/crystal size, minerals or composition, or their crystal structure can affect thermal weathering speeds. Also, the kind of cementing or bond that exists has an influence.

For instance, darker minerals will absorb more heat faster and get more. The difference in expansion between darker and lighter minerals will cause stress that favors quicker disintegration.

Similarly, different minerals expand to a different extent. Also, their size can affect the resulting stress from expansion. These two will cause differential stress.

Lastly, some rock-forming minerals expand more in a specific direction due to their crystalline structure.

3. Moisture

Experiments show that cracking happens more in the presence of moisture compared to dry hair. This is true even in deserts with fog, night dew, and storms. Such will quickly cool rocks on contact.

However, the higher rates may be from wetting or drying weathering, also known as slaking.

Where does it occur?

Thermal expansion weathering can occur anywhere with cyclic heating and cooling, including humid areas.

However, rock outcrops in arid and semiarid places like the Mojave and Sahara deserts are most susceptible. Such places have high temperatures and fluctuations.

Insolation in such places will produce desert pavements or angular stones and shard-like fragments.

Also, it is common in Arctic environments with clear skies. Although they have low ambient temperatures, cooling happens quickly.

Chemical weathering may play a role.

Some studies suggest chemical weathering causes splitting, flaking, and disintegration, not physical.

Evidence to support this hypothesis includes a study on fallen granite columns in the Egyptian desert near Cairo that receives low rainfall. These 3600-year-old columns are weathered more in portions of shady areas than in those exposed to the sun.

Frequently Asked Questions

References

• Elorza, M. G. (2013). Geomorphology. Taylor & Francis.
• Turkington, A. (2004). Mechanical weathering. In Goudie, A. (ed.) Encyclopedia of geomorphology (vol. 1, pp-657-659). Routledge.
• Dixon, J. C. (2004). Weathering. In Goudie, A(ed.) Encyclopedia of geomorphology (vol. 1, pp 1108-1109). Routledge
• Huggett, R. J. (2011). Fundamentals of geomorphology (3rd ed.). Routledge.
• Bierman, P. R., & Montgomery, D. R. (2014). Key concepts in geomorphology. W.H. Freeman and Company Publishers