Weathering is a natural process that breaks down rocks, soils, and minerals on or near the Earth’s surface. It involves material disintegration or alteration without significant removal from its original location. This geological process shapes Earth’s landscapes over time. Unlike erosion, which involves the movement of weathered material, weathering occurs in place.
Freeze-Thaw Weathering
Freeze-thaw weathering is a physical process that breaks rocks apart without changing their chemical makeup. Water seeps into existing cracks or pores within a rock. When temperatures drop below freezing, water turns into ice, which expands by about 9%. This expansion exerts pressure, gradually widening and deepening the cracks.
As temperatures rise, ice melts, allowing more water into the now-enlarged cracks. Subsequent freezing cycles repeat this, causing cracks to grow further and eventually leading to rock fragmentation. This weathering is common in regions with fluctuating temperatures around freezing, such as mountainous areas or cold climates. Visible effects include jagged rock fragments and the formation of potholes.
Pressure Release and Exfoliation
Pressure release, also known as unloading, is a physical weathering process occurring when deeply buried rocks are exposed at the Earth’s surface. Subsurface rocks are under immense pressure from overlying materials. When erosion removes this overburden, pressure on the underlying rock decreases. This reduction in pressure allows the rock to expand slightly.
As the rock expands, it fractures into concentric, sheet-like layers that peel away, a process called exfoliation. This phenomenon is often compared to an onion shedding its layers. Large, rounded rock formations, such as exfoliation domes in granitic terrains, are common results. The expansion creates joints parallel to the exposed surface, leading to the characteristic peeling effect.
Chemical Weathering
Chemical weathering involves reactions that alter mineral structure and composition within rocks. Unlike physical weathering, it creates new minerals or dissolves original ones, fundamentally changing the rock. Water, atmospheric gases, and acids are key agents in these transformations. It occurs more rapidly in warm, damp climates due to water and higher temperatures.
One mechanism is dissolution, where minerals directly dissolve in water. This is noticeable with soluble rocks like limestone, especially when exposed to acidic rainwater. Oxidation involves the reaction of rock minerals with oxygen. Iron-rich minerals, for instance, react with oxygen to form rust-like compounds, often giving rocks a reddish-brown color and weakening their structure.
Hydrolysis occurs when water reacts with minerals, especially silicates. This changes the mineral’s composition, often forming new clay minerals from feldspar. For example, feldspar in granite can transform into kaolinite, softening the rock. Carbonation is a process where carbon dioxide dissolves in water to form carbonic acid. This acid then reacts with carbonate minerals, like those in limestone, dissolving them and contributing to cave formation.
Biological Weathering
Biological weathering involves rocks breaking down due to living organisms. This can occur through both physical and chemical means, often simultaneously. One physical aspect is root wedging, where plant roots grow into cracks and expand, prying the rock apart. Animals also contribute by burrowing into soft rocks or moving fragments to the surface, exposing them to other weathering agents.
Beyond physical disruption, organisms can also cause chemical weathering. Lichens and mosses grow on rock surfaces and produce organic acids. These acids react with and dissolve rock minerals, contributing to its decomposition. Microbial activity can alter rock mineral composition, making them more susceptible to breakdown. Biological activity accelerates weathering by both mechanical and chemical means.