Weathering and Soil Formation

• Weathering – a physical or chemical surface process that

breaks rocks into smaller pieces. – Two types

• Physical• Chemical

Weathering and Soil Formation

• Weathering and Soil Formation• Physical Weathering

– Breaks rocks into smaller pieces without changing them chemically. The small pieces are identical in composition to the original rock

Weathering and Soil Formation

• Weathering and Soil Formation• Physical Weathering

– Ice Wedging or Frost wedging

» Water enters cracks in rocks from runoff or rain.

» In some areas of the world, air temperature drops low enough to freeze water.

» Then, when the temperature rises, the ice thaws.

» This freezing and thawing cycle breaks up rocks because water expands as it freezes (ice is less dense than water)

Weathering and Soil Formation• Weathering and Soil Formation

• Physical Weathering– Animals

» Animals that burrow in the ground weather sediment or soft sedimentary rock, breaking it apart.

– Plants

» Plant roots grow deep into cracks in rock where water collects and wedge rocks apart when roots become thicker and longer

– Abrasion

» Occurs when sediments carried by water, wind or ice come in contact with rock and cause it to wear away by friction

Weathering and Soil Formation

• Weathering and Soil Formation– Weathering

• Chemical Weathering– When the chemical composition of rock

changes– Happens faster where climate is hot and wet– When chemical weathering causes minerals to

dissolve, the remaining rock is weakened, and it can break into smaller pieces.

Weathering and Soil Formation

• Weathering and Soil Formation– Weathering

• Chemical Weathering

– Natural Acids» When water mixes with carbon dioxide in air or soil,

carbonic acid forms» Some rocks, like limestone, dolomite, and marble,

dissolve when exposed to carbonic acid.

– Plant Acids» Many plants produce a substance called tannin,

which dissolves to form tannic acid. » This acid dissolves some minerals in rocks.

Weathering and Soil Formation

• Weathering and Soil Formation– Weathering

• Chemical Weathering– Acid rain (from fossil fuel combustion)

» Sulfur in fossil fuels combines with oxygen and water vapor in the air to form sulfuric acid

» Nitrogen gas in the air combines with oxygen to form nitric acid

» Both can damage rock including stone statues and buildings

– Oxidation (effect of oxygen)» When some iron-containing minerals are exposed to

oxygen in air, they can weather to minerals that are like rust.

» This leaves the rock weakened, and it can break apart.

Weathering and Soil Formation• Weathering and Soil Formation

– Soil• Soil is a mixture of weathered rock, organic

matter, water, and air that supports the growth of plant life.

– Organic matter includes decomposed leaves, twigs, roots, and other material.

– Humus, a dark-colored fertile material, develops when plants and animals die and are decomposed by fungi and bacteria.

• Five factors are involved in soil formation– Parent rock– Slope– Climate– Time– Organisms

Weathering and Soil Formation

• Weathering and Soil Formation– Soil

• Parent rock– Parent rock is the rock from which the soil was made

– Permeable rock allows water to enter easily and breaks down more quickly that non-permeable rock

– Where limestone is chemically weathered, clayey soil is common because clay is left behind when the limestone dissolves.

– In areas where sandstone is weathered, sandy soil forms.

Weathering and Soil Formation

Weathering and Soil Formation

• Weathering and Soil Formation– Soil

• Slope of the land– The topography, or surface features, of an area also

influence the types of soils that develop.

– On steep hillsides, soil has little chance of developing, because rock fragments move downhill constantly.

– In lowlands where the land is flat, wind and water deposit fine sediments that help form thick soils.

Weathering and Soil Formation

• Weathering and Soil Formation– Soil

• Climate– If rock weathers quickly, deep soils can develop

rapidly.

– This happens in tropical regions where the climate is warm and moist and much organic material is present

– Soils in desert climates contain little organic material.

Deep soils can develop rapidly in tropical regions where the climate is warm and moist.

Source: kidsgeo.com

Weathering and Soil Formation

• Weathering and Soil Formation– Soil

• Time– It can take thousands of years for some soils to form.

– As soils develop, they become less like the rock from which they formed.

– In young soils, the parent rock determines the soil characteristics.

– As weathering continues, however, the soil resembles the parent rock less and less

Source: emeraldinsight.com

Weathering and Soil Formation

• Weathering and Soil Formation– Soil

• Organisms– Lichens are small organisms that consist of an alga and

a fungus that live together for mutual benefit. – As they grow, they take nutrients from rock that they are

starting to break down, forming a thin soil, which allows other plants to grow

– The roots of these plants further break down the parent rock.

Soil Formation

Source: http://www.eoearth.org/article/Soil_forming_factors

Draw this diagram on the facing page (label it “Soil Formation”)

Classwork 1– Weathering and Soil Formation

1. What are the four components of soil?

2. What can cause chemical weathering to happen faster?

3. Describe three sources of physical weathering

4. Describe three sources of chemical weathering5. Describe five factors in soil formation

Copy the questions into your notebook (right-side) and answer them