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Geology - Weathering and Erosion

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    Study Guide Geology – Weathering and Erosion 1. Processes of Mechanical Weathering Mechanical weathering is the process that breaks rocks into smaller pieces without changing their chemical makeup . It happens slowly, but over long periods of time, it can completely change the landscape. 1.1 Ice: Freezing and Thawing One of the most powerful agents of mechanical weathering is ice . Water can seep into tiny cracks and joints on the surface of rocks. When temperatures drop, this water freezes and expands. As it expands, it pushes against the sides of the crack. Over thousands of years, this repeated freezing and thawing slowly forces t he crack to widen and deepen. This process is called frost wedging . Eventually, pieces or even large slabs of rock break off. Frost wedging works best in climates where temperatures regularly move above and below freezing. Another ice - related process is frost heaving . This happens in soil. Water beneath rock fragments freezes first and forms ice. As freezing and thawing happen again and again, the growing ice pushes rocks upward. Over time, the rocks are slowly lifted to the surface of the soil. 1.2 Exfolia Ɵ on: Peeling Away Rock Layers Exfoliation occurs 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. If tectonic uplift raises this rock and erosion removes the overlying layers, the pressure on top is suddenly released. However, pressure from below still remains . This imbalance causes the rock to expand outward. This process is known as unloading . As the outer layers expand the most, cracks called sheet joints form. These cracks run parallel to the curved surface of the rock. Over time, curved layers of rock break off, much like the peeling of an onion. This is exfoliation.

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    Study Guide The rounded landforms created by this process are called exfoliation domes . Well - known examples include Stone Mountain in Georgia and Half Dome in Yosemite National Park. 1.3 Fric Ɵ on and Impact: Rocks Colliding Rocks can also break apart through friction and repeated impacts . When rock fragments are carried by rivers, they constantly bump into other rocks and scrape along the 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 by wind or carried by glaciers . In all cases, constant motion and repeated contact cause rocks to wear down. 1.4 Other Causes of Mechanical Weathering Several 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 cracks to 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 and contract. In places where temperatures change by as much as 30°C in a day, this repeated movement weakens rocks. • Extreme heat: Sudden temperature changes, like those caused by forest fires, can cause rocks to crack or shatter. 2. Processes of Chemical Weathering Chemical weathering happens when rocks change their mineral composition as they react with substances in the environment. This process usually begins when a rock that formed deep inside the Earth is finally exposed at the surface — sometimes millions or even billions of years later.

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    Study Guide The minerals that formed under high temperature and pressure underground are not stable at the Earth’s surface . Over time, they react with air, water, and other chemicals and slowly break down into new minerals that are stable in surface conditions. The main agents of chemical weathering are water, oxygen, and acids . 2.1 How Chemical Weathering Works When water, oxygen, or acids react with surface rocks, the original minerals are changed into new minerals. Any leftover ions from these reactions are washed away by water, especially if the water is slightly acidic. For example, feldspar , one of the most common minerals in rocks, weathers into clay minerals . During this process, elements such as silica, potassium, sodium, calcium, and hydrogen are released into 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 be cemented together when minerals like calcite or quartz precipitate from calcium - rich or silica - rich water. This process helps form sedimentary rocks. 2.2 Surface Area and Weathering Speed The rate of chemical weathering depends on how much rock surface is exposed . The more surface area there is, the faster chemical reactions can occur. This is why chemical weathering is closely connected to mechanical weathering . Mechanical weathering breaks large rocks into smaller pieces, which increases the total surface area. More exposed surface means more opportunities for chemical reactions to take place. 2.3 Water: The Most Important Agent Chemical weathering is strongest in areas with abundant 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 • Quartz is very resistant and weathers very slowly. In tropical climates , intense weathering often breaks rocks down so completely that quartz grains are the only minerals left unchanged in the soil. In contrast, dry desert climates slow chemical weathering. Minerals like calcite, which normally weather easily in wet conditions, can survive much longer in dry environments. 2.4 Acids: Breaking Minerals Apart Acids are substances that release hydrogen atoms when dissolved in water. These hydrogen atoms can replace other elements in mineral structures, causing the minerals to break down and form new ones. The most common natural acid is carbonic acid , which forms when carbon dioxide dissolves in water. Rainwater usually contains dissolved carbon dioxide, making it slightly acidic. Human activities increase acidity in the environment. Burning coal, oil, and gasoline releases carbon dioxide, nitrogen, and sulfur into the atmosphere. These substances react with rainwater to form carbonic, nitric, and sulfuric acids , leading to acid rain , which can damage rocks, soils, plants, and buildings. Other acids also play a role: • Organic acids come from plants and decaying organic material in soil. • Strong natural acids are rare but can form during volcanic activity or around hot springs, such as sulfuric and hydrofluoric acids. 2.5 Solu Ɵ on Weathering: Dissolving Rock Solution weathering occurs when minerals are dissolved by acidic water. A common example is calcite in limestone. Carbonic acid easily dissolves calcite. As rainwater moves through cracks and fractures in limestone, it slowly dissolves the rock. Over long periods of time, these cracks can widen and eventually form caves and underground passageways .

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