Weathering, Erosion, and Deposition for Middle School Science

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Weathering, erosion, and deposition is where the atmosphere and the geosphere interact. Wind and water weather, erode, and depose rocks and particles of rocks. These changes shape the world every day.

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Weathering, Erosion, and Deposition

The atmosphere and hydrosphere change the lithosphere through weathering, erosion, and deposition. Weathering breaks rocks into smaller pieces called sediments. Erosion is the process of moving the sediments, and deposition is the process of the sediments settling in a new location.

Weathering can be physical, chemical, or biotic. Wind, water, and gravity cause physical changes to rocks. Physical changes break the rock into smaller pieces, but they do not change the chemical composition of the rock. For example, sand particles in the wind scrape across rocks, slowly removing parts of the rock through abrasion.

During frost-wedging, water seeps into cracks in the rock during the day. When the temperature falls at night, the water freezes and expands, making the cracks bigger. The next day, the water melts and falls farther into the cracks. Eventually, the crack will break the rock. Salt-wedging is like frost-wedging. Water full of dissolved salts gets into the crack of a rock. When the water evaporates, the salts form crystals that push against the crack, making it larger. Frost-wedging happens in places with dramatic temperature differences between the day and night, and salt-wedging happens along seacoasts or dry climates where water evaporates quickly.

In hot deserts, rocks expand in the heat of the day and contract during the colder nights. The constant expansion and contraction of the rocks form cracks and breaks in them.

When igneous rocks that had been buried deep underground are exposed, the change in pressure causes them to form cracks parallel to the ground and break apart. This process is called exfoliation.

Chemical weathering happens when the minerals that make up the rock change their chemical structures. For example, carbonic acid forms when carbon dioxide and water in the atmosphere combine. Raindrops carry the carbonic acid to Earth, where it dissolves minerals in rocks, releasing ions. When the rocks release calcium ions, the calcium and carbon from carbon dioxide combine to form calcium carbonate. Calcium carbonate is one part of the calcite that forms stalactites and stalagmites in caves.

When an acid changes rock, it is called solution chemical weathering. Carbonic acid is a weak acid involved in solution chemical weathering. When humans burn fossil fuels, nitric acid and sulfuric acid also form in the atmosphere. These stronger acids create acid rain that dissolves more rock than carbonic acid.

Water can also dissolve minerals in rocks. This process is called hydrolysis. For example, water dissolves feldspar in granite to form clay. The clay makes the granite weaker, and the rock can break more easily.

Oxygen chemically weathers the minerals in rocks through oxidation. For example, when oxygen gas and iron combine, they form iron oxide, which is better known as rust. The rust makes the rocks look red, and they are more fragile.

When organisms change rocks, it is called biotic weathering. Biotic weathering can be both physical or chemical. A rabbit digging a den can scrape away pieces of rock with its claws. This example shows physical weathering because the structure of the rock doesn't change. Chemical weathering happens when an organism breaks down the minerals in the rock. Lichen, a symbiosis between fungi and algae, releases a weak acid that breaks down the rock under it. As a result, lichen is an essential part of forming soil in new places. Plant roots also release a weak acid as they grow through soil and rocks.

All of the types of weathering form sediments, small particles of minerals and rocks. Erosion is the process of moving these sediments. The most common agent of erosion is water. Flowing water in streams, rivers, and oceans carries sediments away from where they formed. Wind and glaciers also move sediments. Gravity takes sediments down hills and mountains.

Deposition happens when sediments stop moving. Deposition forms a variety of landforms, including dunes, sand bars, and deltas. Beaches are formed by deposition as waves deposit sand when they crash into the shore.

Without wind from the atmosphere or water from the hydrosphere, the lithosphere wouldn't look the way it does today.