Weathering, Erosion, & Deposition

While Internal Processes, like Plate Tectonics, push the Earth's surface upward, the External Processes wear the surface back down. In this way, Earth's surface is constantly changing.

Weathering is a process that happens on Earth's surface, mostly due to weather conditions but not only weather. The rate of weathering depends on climatic factors like temperature as well as the characteristics of the exposed rock. Elements of the atmosphere, hydrosphere, and biosphere act on rocks on the surface in different ways that cause them to gradually break down into smaller pieces. There are 2 types of weathering:

• Mechanical Weathering (also called Physical Weathering) involves making physical changes to the rocks, such as thermal expansion & contraction, collisions with other rocks, pressure from plant roots, and water freezing within cracks. These physical changes are seen as breaks, cracks, fractures, holes, or changes to the surface of a rock such as smoothing and decreasing in size. Rocks are eventually broken down so much that they become tiny particles of sediment: sand, silt, or clay (depending on the type of rock they came from originally).
• Chemical Weathering involves chemical reactions of minerals with substances such as water, oxygen, carbonic acid, and other acids produced through pollution. These chemical reactions are seen as color changes, like rust, pock-mark indentations, and the "eating" away of the surface of the rock. One of the best examples of chemical weathering is a cave. Caves are created with acidic groundwater eats away weak calcite/limestone from its surrounding rock to create tunnels and holes in the ground. Within caves, the evidence of the acidic water eating away the limestone can be seen in the reformation of structures like stalagmites and stalactites - created when the water deposits some of the calcium is has worn away from the rocks above.

Agents of Weathering are responsible for changing the rocks on the surface. These agents come from the atmosphere, hydrosphere, and biosphere. Solar energy and gravity get these agents moving!

• Atmospheric agents include wind, rain, snow, and sleet. (Wind that picks up sand or salt is especially effective.)
• Hydrospheric agents include rivers, seas/oceans, glaciers, runoff, and groundwater.
• Biological agents include lichens, plants, and animals. (One could also argue that human activity be included.)

These agents often work on rocks of the surface at the same time. Elements of weather and changes in temperature can cause large rocks to become smaller, piece by piece, over time. Thermal expansion and contraction occurs in places with extreme temperature differences, such as deserts. Rocks heated during the day expand just a little, then they contract in the colder conditions at night. This back-and-forth transition weakens the rocks until they break. Biological agents like plants roots can creep into cracks in rock, widening the cracks to the breaking point as they grow. (This is how paper beats rock!) Lichens perform chemical weathering by excreting acids onto the rocks on which they grow, causing them to break off in flakes over time. Ice Wedging happens when liquid water seeps into cracks in the rock then freezes, expanding the crack each time. The types of weathering acting on rocks is determined by the climate conditions of the region in which the rock is found.

Erosion is a process that moves weathered rock fragments on the surface. Whether fragments are moved to the bottom of the rock, to the ground nearby, or are transported to new locations further away, erosion is dynamic. Weathering breaks rocks down to create sediments. These sediments are moved by mobile agents of erosion. The processes of weathering and erosion often happen simultaneously. The greater the density and/or speed of the agent, the greater the amount of erosion it will cause. The characteristics of the rock also play a role in how vulnerable they are to erosion. Sedimentary rocks tend to be less resistant to external processes than Igneous or Metamorphic rocks.

The agents of erosion are mainly wind and moving water, including moving ice like glaciers. These agents pick up and carry rock debris and sediments, sometimes causing further weathering during transport. How far the rock debris and sediment can be moved depends on the mobile agent's energy. The more power behind the agent, the larger the rock pieces they can move and/or the further they can move smaller pieces of sediment. Transport can be in bed-loads, moved by running water. As the debris moves along the bottom of the river or stream, it often bounces and breaks further in a frictional process called saltation. Transport by wind is a suspended-load. Sediments carried in suspension can remain airborne for longer periods of time, travelling longer distances.

As the process of erosion transports sediments, it can cause more weathering to the surface rocks it passes over. Wind causes erosion is 2 ways: deflation and abrasion. Deflation is the lifting and removal of fine sediments. Abrasion is really a combination of weathering and erosion as the windblown sediments cut and polish rock. This action often creates interesting desert rocks called Ventifacts, where the windblown side is clearly more degraded and the bottom of the rock is more worn away that the top.

Water can reshape the land as it erodes sediments. Formations in the land created by runoff include Rills, Gullies, Ravines, Canyons, and Hoodoos. Lack of vegetation allows the water to cut into the surface as it runs, carving out rills, gullies, and canyons. Hoodoos are formed when rainwater runs off rock that is less resistant at the bottom than at the top, creating interesting features of a big rock sitting atop a smaller column that varies in size. Rivers can form canyons, cutting into rock over thousands of years like the Grand Canyon. Glaciers can cause erosion as they slowly grow and retreat into valleys. Underneath the moving ice, rock debris is dragged along as it moves.

To bring these surface processes full circle, the final process builds new landforms from the decomposed rocks. Deposition is the process by which sediments that have been transported to a new location are dropped and begin building up. The sediments are deposited when the mobile agents moving them lose energy, often due to encountering an obstacle. Most of the locations in which deposits are made are referred to as sedimentary environments. These include places like beaches, river mouths, glacial valleys, and the bottom of bodies of water.

Deposition landforms created by windblown sediment include Sand Dunes and Loess. Sand dunes are mounds that form when the windblown sand hits an obstacle. These are mobile formations that vary in size and shape depending on the energy of the wind and terrain. They are often found in deserts and on beaches, and can be used to determine the average direction of the wind. The side of the dune that is smooth up to the crest in facing the direction where the wind is coming from, while the other side of the dune is concave and sheltered from wind. Loess are created by windblown silt, particles finer than sand. They take thousands of years to build up and are often covered with grasses. These can produce fertile soil in plains but are very susceptible to erosion.

Deposition landforms created by water include Alluvial Plains/Fans, Deltas, Estuaries, and Marshes. Alluvial Plains, also called Alluvial Fans, are mostly created by floods. When a river overruns its banks, sediments are deposited in the flat area of the river's valley. Alluvial fans can also be created at locations where a river suddenly loses most of its energy usually by entering an area that is less narrow, such as at the end of a steep valley or canyon that opens to a plains area. Alluvial Fans are so named because of the shape they make when depositing sediment. Deltas and Estuaries are also created by rivers. Deltas are created by rivers that are carrying more sediment than the sea currents at the mouth of the river can carry away. Small island-like landforms are often built up by the excess sediments. Estuaries are formed when the river mouth empties into ocean ares that are subject to strong tides, currents, or waves. Estuaries are usually wider and deeper than deltas, and no island-like formations are created where the river and ocean waters mix. Marshes are more like flood plains. They are water-logged wetland areas close to oceans, lakes, and rivers that blend aquatic and terrestrial lands and are mostly deposits of peat - an organic-based sediment of decayed material and fine particles.