Weathering. Weathering: the physical and chemical breakdown of rocks into smaller particles called sediments.

Weathering

Weathering: the physical and chemical breakdown of rocks into smaller particles called sediments.

Physical Weathering• Breaking a rock by force• The smaller pieces of rock have the

same composition as the rock they came from.

Causes of Physical Weathering1. Freezing & Thawing (ice wedging):

a) Water seeps into cracks in rocks and freezes when the temperature drops.

b) Water expands when it freezes.c) With repeated freezing and thawing, the cracks

expand until pieces of the rock break off.

Ice Wedging• Happens the most in areas where

the temperature swings above and below freezing

2. Release of Pressure (exfoliation): a release of pressure can cause outside layers of a rock to crack and flake off like an onion

Causes of Physical Weathering

3. Plant Growth: roots of plants can enter tiny cracks in rock and force the cracks further apart.

Causes of Physical Weathering

4. Abrasion: occurs when sediments carried by streams, wind, glaciers, or gravity cause the particles to collide into each other and the surrounding rock.

Causes of Physical Weathering

Chemical Weathering• Rock material is changed into another

substance by reacting with a chemical.

Causes of Chemical Weathering1. Carbon Dioxide: dissolves in rainwater to

form carbonic acid.- Carbonic acid in surface water and ground water easily dissolves some rocks

Caves form as a result of carbonic

acid weathering

rock

Sinkholes formed as a result of chemical weathering

Limestone bedrock

Carbonic acid rain water seeps into the limestone bedrock through the cracks. This water dissolves the limestone rock.

1. Sinkholes2. Columns3. Stalactites4. Stalagmites

2. Water: weathers rock by DISSOLVING it. - most important cause of chemical weathering

Weathering Granite

Causes of Chemical Weathering

Causes of Chemical Weathering

3. Oxygen: oxygen in the air can combine with minerals in a rock.– For example, when oxygen combines with iron

minerals, iron oxide (RUST) forms. – The chemical change of the mineral weakens the

rock and the rock crumbles.

Causes of Chemical Weathering4. Living Organisms: plants can produce acids that chemically breakdown rock.

Lichens chemically weathering a fallen tree

5. Man-made Acids – Gases produced by humans can dissolve in the water droplets of a cloud to produce acid rain.

Causes of Chemical Weathering

Acid Rain formation

What affects the RATE of weathering?

1. Type of Rock: some minerals break down more easily than others.

What affects the RATE of weathering?

2. Climate: weathering occurs FASTER in locations with wet climates.

Due to climate and different weathering processes, landscapes

develop differently.

Arid climate Humid climate

What affects the RATE of weathering?

3. Particle Size: Weathering takes place on the outside surface of rocks. –The more surface area that is exposed to

weathering, the faster the rock will be broken down. –As a rock breaks into smaller pieces, the surface

area increases, therefore the rate of weathering increases.

A full solid block has the least surface area. The interior is safe from exposure.

A smashed piece has the greatest surface area exposed. The interior can now be attacked.

What affects the RATE of weathering?

4. Exposure: the closer a rock is to the surface, the faster it will weather. – Weathering is dependent on exposure to air, water,

and actions of living things

• Soil is a combination of weathered rock and organic matter.

• Humus = decayed plant and animal material found in soil. Soil that contains 20-30% humus is considered rich soil for plant growth.

Soil Formation

Soil Development

1. The surface of the rock gets attacked by weather and broken into smaller pieces

2. Over many years, you wind up with heavily weathered sediments at the surface.

3. As you dig deeper, you find less and less weathering and the materials become coarser.

TIME

Soil Layers1. Topsoil or A Horizon = the top

layer of soil that contains more humus than the layers below.

2. Sub soil or B Horizon = consists of clays and dissolved minerals that have been washed down from above. Contains less humus.

3. C Horizon = consists of weathered rock fragments, usually from the parent rock below.

4. Bedrock = the layer of rock beneath the soil. Frequently the parent rock of the soil above.

Residual vs. Transported Soil

• If the bedrock matches the rock fragments in the soil, the soil is most likely residual.

• If it does not, the soil is most likely transported from another location.–Most soil on Earth’s surface is

transported.