• weathering

• mechanical weathering

• chemical weathering

• oxidation

Weathering

The mechanical and chemical processes that change objects on Earth’s surface over time are called weathering.

Over thousands of years, weathering can break rock into smaller and smaller pieces, such as sand, silt, and clay.

Weathering and Its Effects

When physical processes naturally break rocks into smaller pieces, mechanical weathering occurs.

The chemical makeup of a rock stays the same during mechanical weathering.

Mechanical weathering can be caused by ice wedging, abrasion, plants, and animals.

Mechanical Weathering

An example of mechanical weathering is when the intense heat of a forest fire causes nearby rocks to expand and crack.

When something is broken into smaller pieces, it has a greater surface area.

Surface area is the amount of space on the outside of an object.

Mechanical Weathering (cont.)

Chemical weathering changes the materials that are part of a rock into new materials.

These granite obelisks show how chemical weathering can affect some rock.

Chemical Weathering

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Water is important in chemical weathering because most substances dissolve in water.

The process of dissolving breaks up the minerals in the rock into small pieces. The small pieces mix with water to form a solution and are washed away from the rock.

Acids are also agents of chemical weathering and cause more chemical weathering than pure water does.

Chemical Weathering (cont.)

Oxidation combines the element oxygen with other elements or molecules.

The product of oxidation is called an oxide.

When rocks that contain iron oxidize, a layer of iron oxide forms on the outside surface.

Chemical Weathering (cont.)

The environment helps determine the rate of weathering.

Mechanical weathering occurs fastest in locations that have a lot of temperature changes.

Chemical weathering is fastest in warm, wet places.

The type of rock being weathered also affects the rate of weathering and determines what kinds of products result.

What affects weathering rates?

• erosion

• deposition

The Erosion-Deposition Process

A combination of constructive processes and destructive processes produce landforms.

Constructive processes build up features on Earth’s surface.

Destructive processes tear down features on Earth’s surface.

Reshaping Earth’s Surface

The breakdown of rock—weathering—is one type of destructive process that changes Earth’s surface.

Chemical weathering alters the chemical composition of rock.

Physical weathering is the breaking of rock into pieces, called sediment, without changing the chemical composition of the rock.

Water, wind, and ice are agents, or causes, of weathering.

A Continual Process of Change

ErosionErosion is the removal of weathered

material from one location to another.

Agents of erosion include water, wind, glaciers, and gravity.

Factors that affect the rate of erosion include weather, climate, shape of the land, and type of rock.

Erosion (cont.)

The presence of plants and the way humans use the land affect the rate of erosion.

The rate of erosion sometimes depends on the type of rock.

Weathering breaks some types of rock into large pieces. Other rock types easily break into smaller pieces that are more easily transported.

Deposition (cont.)

Deposition is the laying down or settling of eroded material.

As water or wind slows down, it has less energy and can hold less sediment, which can result in some of the sediment being deposited.

Sediment is deposited in locations called depositional environments, such as swamps, deltas, beaches, and the ocean floor.

Landforms created by deposition are often flat and low-lying.

An apron of sediment, called an alluvial fan, often forms where a stream flows from a steep, narrow canyon onto a flat plain at the foot of a mountain.

Interpreting Landforms (cont.)