Types of Rock Exogenic Processes and Erosion: Weathering, Transport, and Deposition
The processes that shape the Earth's surface, known as exogenic processes, are primarily driven by external forces such as wind, water, and ice. These forces are responsible for breaking down and transporting rocks, eventually depositing them in new locations. This article will delve into the types of rock exogenic processes, the mechanisms of weathering, and the modes of erosion and deposition.
Weathering: The Delicate Breakdown of Rocks
Weathering is the process that causes rocks to break down at the Earth's surface. There are several types of weathering, including mechanical (or physical) weathering and chemical weathering.
Physical Weathering
Physical weathering involves the breakdown of rocks through physical processes without changing their chemical composition. Wind erosion can be a significant contributor to this. If the sediments are light enough, they can be moved by the wind, leaving scratch marks or polish on the rocks. Glacial abrasion is another form of physical weathering. Glaciers can leave various geometric forms on rocks and even strip them down to powders, which may or may not become lithified later. The free-thaw process of glaciers is an ebb and flow cycle of ice expansion within cracks, which then widen, or meltwater hydrating rock grains.
Chemical Weathering
Chemical weathering involves the change in the chemical composition of rocks through the action of chemical reactions. Water is the primary agent in chemical weathering, capable of transporting sediments if they are light, bulldozing them further down the path, or working with the atoms in sediments or rocks through hydrolysis or hydration. Other agents, such as heat dilatation and solution, also contribute to chemical weathering.
Erosion: The Power of Natural Forces
Erosion is the process by which natural forces move weathered rock and soil from one place to another. The main forces of erosion are gravity, running water, glaciers, waves, and wind. The material moved by erosion is referred to as sediment. Deposition occurs when the agents of erosion, such as wind or water, lay down sediment.
Processes of Erosion
Heat dilatation involves the expansion of materials due to heat, which can contribute to the breakdown of minerals. Solution is another form of chemical weathering, where rocks dissolve in water. Abrasion by water and wind is another method of erosion, where the force of water or wind wears down rocks and sediments. Freezing and thawing cycles can also cause rocks to break apart, especially in areas where the temperature fluctuates significantly.
Transport and Deposition of Sediment
Once rocks have been weathered and eroded, they are transported by various means. Gravity can cause rocks to roll or slide downslope, while running water, wind, and glaciers can transport sediments over varying distances. Some sediments, such as clastic material, are primarily transported by gravity, water, or wind. However, dissolved materials are not carried by the wind but are transported by water.
Deposition occurs when the agents of erosion, such as wind or water, lay down sediment. This can be identified through various characteristics of the sedimentary rock, such as bedding, grain size, and mineral content. For example, if sediment is deposited from water, it may have bedding and sharp grains, indicating flowing water. If the sediment has cross-bedding and gradation, it may have been deposited in the air or by wind. However, if the grains are rounded and biotite and muscovite minerals are found, the sediment is more likely to have been deposited by wind or near a swamp or shoreline in an anaerobic environment. Differentiating these processes can be crucial in understanding the geological history of an area.
Understanding rock exogenic processes, weathering, erosion, transport, and deposition is essential for sedimentary geologists. These processes shape the Earth's surface and play a crucial role in creating the diverse landscapes we see today. By studying these processes, geologists can gain insights into the history and evolution of the Earth.