Continental Margins What is weathering and erosion? Weathering is basically the breakdown and alteration of rocks and minerals at or close to the Earth's surface into products that are more in equilibrium with the conditions found in its environment. Erosion is "the removal of weathered rocks by moving water, wind, ice, or gravity" (Thomas and Turk). These two processes together help make the Earth's surface how it is today. Weathering is a process that is going on most all of the time and involves little or no movement of the decomposed minerals and rocks. There are two main types of weathering that occur on Earth's surface: physical, and chemical.

Physical weathering, also called mechanical weathering, is the break down of rocks into smaller sized pieces without altering the composition of the rocks and minerals. Some types of physical weathering include, exfoliation, frost wedging, and thermal expansion. Exfoliation is also often referred to as unloading. This is a process by which pieces of rocks split off into sheets or leaves beside joints that are parallel to the ground surface. This type of weathering is due to expansion of rock caused by uplift and wearing away of the rock. Frost wedging is the process that happens when water fills a crack in the rock, then freezes, and then expands.

The expanding of the ice presses against the rock and wedges the crack open. For example, a rock called talus, are often subjected to this process. Thermal expansion is another type of physical weathering. Thermal expansion is the result of rocks heating up and cooling daily. When rocks heat they expand, but when they cool, its causes them to contract. Since different minerals expand and contract at different rates, this causes stress and eventually breaking along mineral borders.

The next type of weathering is called chemical weathering. Chemical weathering is "the decomposition of rock by the chemical breakdown of minerals" (Weathering and Soils) which results in a change in the rock composition, therefore speeding up the break down of the rock. There are three main types of chemical weathering: dissolution, hydrolysis, and oxidation. Dissolution is essentially when rocks and minerals become dissolved by water, then the dissolved matter is carried away leaving a gap in the rock. This process is often seen in the formation of caves in limestone regions.

The rock halite, is also often seen to be affected by this process. Next, hydrolysis is a chemical process in which water reacts with a particular mineral to form a new mineral with the water as part of its crystal structure. One of the most common minerals affected by this process is feldspar. Feldspar weathers by this process to form kaolinite (clay) and Na and K ions. Oxidation is another common type of chemical weathering. This is when "the loss of electrons from a compound or element during a chemical reaction.

In the weathering of common minerals oxidation is usually occurs when a mineral reacts with molecular oxygen" (Thompson and Turk G. 13). Several elements react with atmospheric oxygen. For example, iron rusts when it reacts with oxygen and water. Rusting is a good example of this process. As you can clearly see, these processes greatly affect the Earth's surface.

Both physical weathering (also known as mechanical weathering) and chemical weathering significantly shape and change mountains, caves, etc., into what they have become today. Without these processes, Earth's most magnificent rock formations would not be here. For example, Rainbow Bridge at Lake Powell, in Utah, would not be here today if it weren't for the water and minerals in the water that ate away at the rock to form one of the most stunning, natural occurring land formations. These processes are natural and therefore are a beautiful part of the processes that make Earth, Earth.


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