Geology - Weathering and Erosion - Quick Guides | Geology

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Study GuideGeology–Weathering and Erosion1. Processes of Mechanical WeatheringMechanical weathering is the process that breaks rocks into smaller pieceswithout changing theirchemical makeup. It happens slowly, but over long periods oftime, it can completely change thelandscape.1.1 Ice: Freezing and ThawingOne of the most powerful agents of mechanical weathering isice.Water can seep into tiny cracks and joints on the surface of rocks. When temperatures drop, thiswater freezes and expands. As it expands, it pushes against the sides of the crack. Over thousands ofyears, this repeated freezing and thawing slowly forces the crack to widen and deepen.This process is calledfrost wedging. Eventually, pieces or even large slabs of rock break off. Frostwedging works best in climates where temperatures regularly move above and below freezing.Another ice-related process isfrost heaving. This happens in soil. Water beneath rock fragmentsfreezes first and forms ice. As freezing and thawing happen again and again, the growing ice pushesrocks upward. Over time, the rocks are slowly lifted to the surface of the soil.1.2 ExfoliaƟon: Peeling Away Rock LayersExfoliationoccurs when large bodies of rock deep underground are exposed at the Earth’s surface.When molten rock cools below the surface, it is under great pressure from the rocks above it. Iftectonic uplift raises this rock and erosion removes the overlying layers, the pressure on top issuddenly released. However, pressure from below still remains. This imbalance causes the rock toexpand outward. This process is known asunloading.As the outer layers expand the most, cracks calledsheet jointsform. These cracks run parallel to thecurved surface of the rock. Over time, curved layers of rock break off, much like the peeling of anonion. This is exfoliation.

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Study GuideThe rounded landforms created by this process are calledexfoliation domes. Well-known examplesinclude Stone Mountain in Georgia and Half Dome in Yosemite National Park.1.3 FricƟon and Impact: Rocks CollidingRocks can also break apart throughfriction and repeated impacts.When rock fragments are carried by rivers, they constantly bump into other rocks and scrape alongthe riverbed. Each collision chips away small pieces, slowly breaking the rock into smaller fragments.This process also smooths the rock’s edges.Similar breaking happens when rocks are moved bywindor carried byglaciers. In all cases,constant motion and repeated contact cause rocks to wear down.1.4 Other Causes of Mechanical WeatheringSeveral smaller processes also contribute to mechanical weathering:•Animal activity:Burrowing animals disturb rocks and soil, exposing rocks to air and water.•Plant roots:Roots grow into small cracks and expand as the plant grows, forcing the cracksto widen.•Bacteria:Some bacteria can break down certain minerals, such as metal sulfides.•Temperature changes:Daily heating and cooling cause minerals in rocks to expand andcontract. In places where temperatures change by as much as 30°C in a day, this repeatedmovement weakens rocks.•Extreme heat:Sudden temperature changes, like those caused by forest fires, can causerocks to crack or shatter.2. Processes of Chemical WeatheringChemical weathering happens when rockschange their mineral compositionas they react withsubstances in the environment. This process usually begins when a rock that formed deep inside theEarth is finally exposed at the surface—sometimes millions or even billions of years later.

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Study GuideThe minerals that formed under high temperature and pressure underground arenot stable at theEarth’s surface. Over time, they react with air, water, and other chemicals and slowly break downinto new minerals that are stable in surface conditions.The main agents of chemical weathering arewater, oxygen, and acids.2.1 How Chemical Weathering WorksWhen water, oxygen, or acids react with surface rocks, the original minerals are changed into newminerals. Anyleftover ions from these reactions are washed away by water, especially if the water isslightly acidic.For example,feldspar, one of the most common minerals in rocks, weathers intoclay minerals.During this process, elements such as silica, potassium, sodium, calcium, and hydrogen are releasedinto solution. These dissolved elements are commonly found in surface water and groundwater.Some of these dissolved materials later become useful again. Newly deposited sediments can becemented togetherwhen minerals like calcite or quartz precipitate from calcium-rich or silica-richwater. This process helps form sedimentary rocks.2.2 Surface Area and Weathering SpeedTherate of chemical weathering depends on how much rock surface is exposed. The moresurface area there is, the faster chemical reactions can occur.This is why chemical weathering is closely connected tomechanical weathering. Mechanicalweathering breaks large rocks into smaller pieces, which increases the total surface area. Moreexposed surface means more opportunities for chemical reactions to take place.2.3 Water: The Most Important AgentChemical weathering is strongest in areas withabundant water.Different minerals react to water at different rates, and climate plays a major role:•Ferromagnesian minerals(rich in iron and magnesium) weather quickly.

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Study Guide•Quartzis very resistant and weathers very slowly.Intropical climates, intense weathering often breaks rocks down so completely that quartz grainsare the only minerals left unchanged in the soil.In contrast,dry desert climatesslow chemical weathering. Minerals like calcite, which normallyweather easily in wet conditions, can survive much longer in dry environments.2.4 Acids: Breaking Minerals ApartAcidsare substances that release hydrogen atoms when dissolved in water. These hydrogen atomscan replace other elements in mineral structures, causing the minerals to break down and form newones.The most common natural acid iscarbonic acid, which forms when carbon dioxide dissolves inwater. Rainwater usually contains dissolved carbon dioxide, making it slightly acidic.Human activities increase acidity in the environment. Burning coal, oil, and gasoline releases carbondioxide, nitrogen, and sulfur into the atmosphere. These substances react with rainwater to formcarbonic, nitric, and sulfuric acids, leading toacid rain, which can damage rocks, soils, plants, andbuildings.Other acids also play a role:•Organic acidscome from plants and decaying organic material in soil.•Strong natural acidsare rare but can form during volcanic activity or around hot springs,such as sulfuric and hydrofluoric acids.2.5 SoluƟon Weathering: Dissolving RockSolution weatheringoccurs when minerals are dissolved by acidic water.A common example iscalcitein limestone. Carbonic acid easily dissolves calcite. As rainwatermoves through cracks and fractures in limestone, it slowly dissolves the rock. Over long periods oftime, these cracks can widen and eventually formcaves and underground passageways.

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