Sandstone is a sedimentary rock composed of sand-sized grains, typically quartz, bound together by a cementing agent or matrix. Weathering involves the natural breakdown of this rock near the Earth’s surface through physical disintegration and chemical decomposition. Sandstone is susceptible to all three major forms of weathering: mechanical, chemical, and biological.
Mechanical Forces that Fragment Sandstone
Mechanical weathering involves physical forces that break the sandstone into smaller pieces without altering the chemical composition of the minerals. These forces primarily exploit the existing pores, cracks, and microfractures within the rock fabric.
Frost wedging is one of the most powerful physical processes, occurring in environments where temperatures frequently fluctuate around the freezing point of water. Water seeps into the rock’s joints and pores, and when it freezes, it expands by approximately nine percent, exerting immense pressure against the rock walls. This repetitive freeze-thaw cycle gradually widens the cracks, eventually forcing fragments of the rock apart.
Abrasion is another significant mechanical force, involving the grinding action of particles carried by wind, water, or ice. Sandstone on coastlines or in deserts is constantly bombarded by sand grains, which acts like natural sandpaper, slowly wearing away the rock surface. This continuous friction and impact smooths the rock and removes loose grains, exposing fresh material to further weathering.
Pressure release, also known as unloading, contributes to the mechanical breakdown of deeply buried sandstone formations that are later exposed at the surface. As the weight of overlying rock is removed by erosion, the sandstone expands, causing fractures to form parallel to the surface. This process can cause sedimentary rocks to fracture along pre-determined planes, aiding in their disintegration.
The Chemical Vulnerability of Sandstone Cement
While quartz is highly resistant to chemical weathering, the rock’s overall vulnerability is determined by the cementing agent that holds the grains together. Common cementing materials—silica, calcite, iron oxides, and clay—each react differently to chemical attack.
Dissolution
Dissolution is a major threat to sandstones cemented with calcite (calcium carbonate). Acidic rainwater, which forms when atmospheric carbon dioxide dissolves in water to create a weak carbonic acid, readily reacts with the calcite matrix.
This reaction dissolves the cement, transforming the solid mineral into soluble calcium and bicarbonate ions that are then carried away by water. The removal of this matrix causes the highly resistant quartz grains to become disaggregated and fall away as loose sand.
Oxidation
Sandstones with iron-oxide cements, which often give the rock a distinctive red, yellow, or brown coloration, are susceptible to oxidation. Oxidation occurs when oxygen, typically dissolved in water, reacts with ferrous iron (Fe²⁺) within the cement to form ferric iron (Fe³⁺) compounds like hematite or limonite.
This process creates a characteristic rusty appearance and slightly changes the volume of the cement, weakening the rock’s structural integrity and making it easier to crumble.
Hydrolysis
Hydrolysis, the reaction of minerals with water, is another chemical process that targets less stable components within the sandstone. While quartz is largely unaffected, any trace feldspar or clay minerals present in the matrix are susceptible to hydrolysis.
This reaction converts the original silicate minerals into new, softer clay minerals, which further weakens the binding force of the rock. The resulting clay is easily eroded, accelerating the overall disintegration of the sandstone structure.
Biological Agents of Degradation
Living organisms contribute to the breakdown of sandstone through a combination of physical force and the release of corrosive chemical compounds. Biological activity is especially pronounced in environments that support plant and microbial growth.
Root Wedging
One of the most noticeable biological mechanical actions is root wedging, where plant roots grow into existing fractures and joints in the rock. As the roots thicken and expand, they exert immense pressure, forcefully widening the cracks and physically prying the rock apart.
This allows more water to infiltrate and accelerate other forms of weathering.
Microorganisms
Microorganisms, including bacteria, fungi, and lichens, contribute significantly to the chemical degradation of sandstone. These organisms colonize the rock surface and release chelating agents and weak organic acids, such as oxalic or citric acid, as metabolic byproducts.
These acids react with and dissolve the sandstone’s cementing agents, particularly carbonate or iron-oxide matrices, effectively etching the rock surface and dissolving the glue that holds the grains together.
The metabolic processes of these organisms can also lead to changes in the chemical environment immediately surrounding the rock surface. For instance, the production of carbon dioxide by root respiration combines with water to form carbonic acid, which accelerates the dissolution of calcite cement.