Weathering and erosion are both terms that can be used to describe the loosening of particles at a rock's surface. Although the terms are often used interchangeably, weathering differs from erosion. Weathering refers to rock breaking apart, by chemical or mechanical processes. Erosion is the term used when these particles have been broken apart and then carried away by air, water or ice. The two processes are very closely related, with only the movement of particles used to differentiate the two terms. Erosion is also related to weathering in that it can wash away surrounding soil and other sediment, leaving greater areas of rock surface exposed to the elements and subject to weathering. The breakdown of rocks can create sand and gives soil much of its texture. Several naturally occurring circumstances and events can cause rock to break down, regardless of a rock's hardness. It is important to keep in mind that unlike metamorphism, weathering is a process occurring at or near the Earth's surface.
Mechanical Weathering: Water and Ice Forces
The most common natural agent that weathers rock is water, in either liquid or frozen form. Ocean waves crashing against a rocky cliff smooth and carve the rock through weathering, while also contributing as a force of erosion, carrying the smaller pieces out to sea. Meandering rivers carve their pathways through solid rock, forming canyons or leaving behind mighty bluffs. Even rain pounding away at a rock face causes it to slowly disintegrate. Ice is even more powerful, as water seeping into porous rocks freezes and breaks off large chunks, allowing the weathering and erosion processes to hasten.
Mechanical Weathering: Wind and Plant Forces
In addition to water and ice, forces of nature that break down rock -- forcing it to cleave or breaking off bits -- include wind and organic growth. Wind driving small particles grinds even the hardest rock faces into smooth surfaces. Trees that take hold in a crevice send roots burrowing into tiny gaps in the rock, causing larger pieces to break off. Even moss, tiny as it is, works away at the surface of the rock it attaches to.
Chemical Weathering: Acid Rain
While mechanical weathering wears rock down, it doesn't change its composition. A piece of sandstone broken apart by the freeze and thaw cycle is still sandstone, albeit in smaller pieces. But chemical weathering can actually change the chemical properties of the rock it weathers. Although acid rain can be both natural and man-made, human activities are the primary culprit. Sulfur and nitrogen are primarily the cause of acid rain effects. The past several decades have seen many different chemicals released into the air, whether from manufacturing, power plants, automobile exhausts or other pollutant sources. These chemicals rise high in the atmosphere, changing the mixture of gases and reacting with water, oxygen and other chemicals to bring about acid rain. Natural processes can also lead to acid rain. Sea spray and volcanoes are common natural sources of sulfur, which can cause acid rain. But these contributions from natural sources typically play a much smaller role in producing acid rain than those from man-made pollutants. Even when acid is weak, over time it wears away at rocks by pulling out minerals like calcite. The result is a weaker rock that is more susceptible to weathering.
Chemical Weathering: Oxidation
The chemical process whereby oxygen reacts with Earth's substances is known as oxidation. While the most commonly seen reaction involves oxidation of metallic iron to produce rust, oxygen can react with a number of elements to cause chemical weathering of rock. Attacking iron in the Earth's soil or rock, oxidation can leave behind a brownish red to red color and the discoloration and weakening of rocks due to the oxidation are commonly referred to as rust, though it differs from that of oxidation of metallic iron. However, oxidation is not limited to reaction with iron. Many other metallic ores and minerals oxidize and hydrate to produce a variety of colored deposits which can weaken rock and make rock surfaces more prone to weathering.
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