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Weathering

Weathering is the process of breaking down rocks into smaller pieces. Sometimes you will hear (or read) that rocks are broken down by erosion. This is actually not true. Rocks are broken down by weathering.

Erosion is the process of moving sediments (small pieces of rock and other debris) from one place to another. Weathering is different. There are three different types of weathering. The three types of weathering are mechanical, chemical, and biotic. Each of these types of weathering happen in a different way, but the end result is the same in each case – rocks are broken down into smaller pieces. These “smaller pieces” can range from large rocks to small pebbles to even smaller pieces of sand, silt, or clay.

Mechanical Weathering

The Wave in Arizona was partially formed by wind and water abrasion.

The Wave in Arizona was partially formed by wind and water abrasion.

Mechanical weathering is the process of breaking rocks down into smaller pieces, but not changing the chemical composition of the rock. Think of it like cutting a rock into two pieces. Both pieces are still the same type of rock, but now there are two rocks instead of one. However, most mechanical weathering does not break rocks into two pieces.

Abrasion can also happen when ocean waves continually wash across rocks along the shore.

Abrasion can also happen when ocean waves continually wash across rocks along the shore.

Abrasion is a type of mechanical weathering involving wind or water. In dry areas, like deserts, particles in the wind scrape across rocks. Over long periods of time, this will wear away the rock. Abrasion can also happen when ocean waves continually wash across rocks along the shore.

Potholes are caused by frost-wedging.

Potholes are caused by frost-wedging.

Frost-wedging happens when water falls into the cracks of rocks. The water then freezes and expands forcing the cracks to become even bigger. Over time, the rock will be broken into pieces by the large cracks that have formed. Not only does water change size based on temperature, but rocks do too. During hot weather, the rocks expand slightly. When the temperature drops, the rocks contract. The repeated expansion and contraction of the rocks weakens their structures allowing them to be more easily broken apart by other factors. This is called thermal expansion and contraction.

Rock slides can block the road.

Rock slides can block the road.

Gravity is another agent of weathering. Rock slides and landslides cause rocks to tumble down the sides of hills and mountains. As the rocks move, they scrape and bounce against other rocks. This can cause pieces of both rocks to break away.

Chemical Weathering

Feldspar is transformed into clay and silt by hydrolysis.

Feldspar is transformed into clay and silt by hydrolysis.

Chemical weathering, unlike mechanical weathering, breaks rocks down by changing their chemical structures. Hydrolysis is the process of minerals in rocks interacting with the hydrogen in rain water. This takes minerals out of the rocks and breaks them down. For example, one type of rock, feldspar, will be changed into clay and silt by hydrolysis.

The rusting of iron is a type of oxidation.

The rusting of iron is a type of oxidation.

Carbonation involves dissolved carbon dioxide forming an acid and breaking down the rocks. Oxidation is the process of oxygen combining with water and the minerals in the rocks. A common type of oxidation is the rusting of iron. This wears down the rock and eventually breaks it down.

Biotic Weathering

The roots of plants can weather rocks.

The roots of plants can weather rocks.

Biotic means “living”. Biotic weathering is weathering done by living organisms. Plants weather rocks when their roots push into the cracks in the rocks and the soil around them. Over time, these roots will break apart the rocks. Another type of biotic weathering comes in the form of animals digging. As animals dig in the dirt, they can scratch or break up rocks in the ground. Another example of biotic weathering is the process of lichens releasing acid onto rocks. The acid breaks down the rocks creating soil. Once this soil is created, grasses and mosses can take root.

Water Weathering

A glacier scrapes across the land as it moves.

A glacier scrapes across the land as it moves.

Water can weather rocks mechanically or chemically. Water is a powerful force in the weathering of rocks. Rivers and streams weather the rocks around them. Oceans weather rocks and sand on the shoreline. Rain weathers rocks as it falls on the land. Ice weathers rocks by expanding in tiny cracks. Glaciers weather rocks by scraping across them as they move downward. Even the smallest drops of water will weather rocks over time. Water is constantly changing the landscape of the Earth through weathering.