Weathering & Erosion

Weathering is the process that breaks down rock and other substances at Earth’s surface. Erosion is the movement of rock particles by wind, water, ice or gravity.

Types of Weathering Mechanical Weathering – breaks rocks into pieces by; freezing and thawing, release of pressure, growth of plants, actions of animals, and abrasion. Chemical Weathering – process that breaks down rock through chemical changes, as rock interacts with water, oxygen, carbon dioxide, living organisms, and acid rain.

Mechanical Weathering: Freezing & Thawing Water seeps into cracks in rocks and then freezes (and expands). The ice acts as a wedge that forces rock apart. Slowly, over long periods of time can create very peculiar rock formations.

Mechanical Weathering: Release of pressure As erosion moves materials away from the surface of rock, the pressure on the rock below is reduced. Causes the outside of the rock to crack and flake.

Mechanical Weathering: Plant Growth Tree and plant roots grow into cracks and force rocks apart as the grow.

Mechanical Weathering: Abrasion Mineral and rock particles carried by wind, water, or glaciers can wear away exposed rock surfaces like sandpaper on wood.

Glaciers cause abrasion as they advance or retreat, producing and transporting large amounts of sediment in the process. Mt. Rainier – Nisqually Glacier

Mechanical Weathering: Animal Actions Animals burrow in the ground, dig for food, and tread on rocks, loosening and breaking apart rocks.

Chemical Weathering: Water Water weathers rock by dissolving it, meaning the particles mix uniformly throughout the water to make a solution.

Chemical Weathering: Oxygen Iron combines with oxygen in the presence of water in a process called oxidation. The product is rust, which appears reddish brown and makes rock soft and crumbly

Chemical Weathering: Carbon dioxide CO2 dissolves in water and seeps into soil and rock. Forms carbonic acid, which easily weathers limestone (sedimentary) and marble (metamorphic) which contain calcite minerals.

Chemical Weathering: Living organisms Plants and plant-like organisms such as lichen grow on rock and produce weak acids that weather rock.

Chemical Weathering: Acid Rain Humans burn fossil fuels that pollute the air with sulfur, carbon and nitrogen compounds, which react chemically with water vapor in clouds to form acid rain.

Chemical and Mechanical weathering often work together. The rate of weathering depends on the type of rock, generally more rapid for minerals that dissolve in water and for more permeable rock. The rate of weathering also depends on climate, generally more rapid in wet, warm climates. Soil forms as mineral and rock particles break down and mix with other materials on the surface, such as decaying organic material.

Types of Erosion Erosion by Water – movement of rock particles by falling rain, runoff, streams and rivers, waves. Sediments are deposited to create various landforms.

Erosion by Wind – the weakest form of erosion, more powerful where there are few plants to stabilize soil (i.e. deserts)

Wind erosion is a powerful force in the desert.

Erosion by Ice (glaciers)- erode land by picking up rocks as they flow and dragging them across land. Upon melting, deposit sediments carried from land to create various landforms.

Think of a glacier as a massive river of ice, flowing very slowly but carrying with it massive amounts of sediment, while carving out a U-shaped valley.

Weathering, Erosion and Deposition act in a cycle that continually reshapes the earth’s surface. END