Download Weathering and Soil Formation

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
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
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–
–
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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
Draw this diagram on the facing page (label it “Soil Formation”)
Source: http://www.eoearth.org/article/Soil_forming_factors
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 weathering
5. Describe five factors in soil formation
Copy the questions into your notebook (right-side) and answer them