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Earth Science Notes
Weathering and Soil
Weathering
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Weathering : surface processes that work to
breakdown rock into sediment
Types of sediment – sand, silt, clay
Types of Weathering
Types of Weathering
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Mechanical Weathering - the physical break down
of rocks
The chemical make up of the rock remains
unchanged.
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Ex: burrowing animals, growing plants, expanding ice
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Ice wedging – water will enter cracks in rock and then freeze
pushing the rock apart.
This increases surface area of rocks and allows
them to be exposed more to chemical agents.
Mechanical Weathering increases
Chemical Weathering
Mechanical
Weathering
Types of Weathering

Chemical Weathering – chemical reactions
dissolve rocks
Agents of chemical weathering
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Carbonic acid (H2CO3) – acid produced from
carbon dioxide in air and soil. Dissolves
limestone.
Oxygen – reacts with iron containing minerals to
form rust. Gives rocks and soil its reddish color.
Sulfuric acid (H2SO4) – acid produced by sulfates
from factories. ACID RAIN.
Chemical Weathering
BIG IDEA: Weathering creates SOIL!!!
SOIL

Soil is a mixture of weathered rock, decayed
organic matter, mineral fragments, water, and
air.
Weathering and Soil Formation
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Recall from the last Unit: weathering
produces sediments.
Below are the classifications of sediments
Clay  silt  sand  pebble  Cobble  Boulder

Sand, silt, and clay are the main sediments
that make up soil.

These sediments give soil its TEXTURE.
Weathering and Soil Formation
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The relative proportions of sand, silt, and clay
allow us to classify soils.
The USDA has a model it uses for classifying
soil types based on the sediment composition
(the triangle chart – next slide).
These soil classifications allow tell us how a
soil will accept and retain moisture.
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Sandy – doesn’t retain water
Clay – doesn’t accept water
Weathering and Soil Formation
Soil Formation
The weathering of Rock into soil occurs in stages (seen below)
Soil Profiles and Soil Horizons

Over time (and much weathering) distinct
layers form in the soil. These layers are
called soil horizons.
Soil Profiles and Soil Horizons

The first soil horizon to form is the C
horizon. Again, weathering process
convert the exposed bedrock to smaller
sediments.
Soil Profiles and Soil Horizons
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After the C horizon, the A horizon is the
next to form. Weathering continues.
Plants begin to colonize the sediments and
die. Their remains contribute to a growing
layer rich in organic matter. This is the A
horizon.
Soil Formation
Plants and Animals play a part in soil
formation
Soil Profiles and Soil Horizons

The last layer to form is actually the B
horizon. It forms as a result of minerals
leaching downward from the A horizon to
the C horizon.
Leaching

Removal of minerals by water
Soil Profiles and Soil Horizons

A soil profile
is a crosssection of
earth that
shows the
different soil
horizons.
Soil Formation

Soils vary greatly in composition. In addition
to sediment size (i.e. sand, silt, clay), the
parent materials from which these soils
come are very diverse.
Soil Formation is influenced by
vegetation and climate.
Topography and Soil Formation
Soil Formation is influenced by Topography
 Steeper land  influenced by force of gravity
 Flatter land  water is more apt to settle
Ways to Reduce Soil Erosion

Terracing / No-till / Contour farming
Factors Affecting Soil Erosion

Exposure to elements (weathering)
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Type of Parent material
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Weight of material, Density, etc.
Farming

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Wind, rain, freezing, frost, etc.
Overgrazing / tilling
Foresting and mining
Climate
Topography
Factors Affecting Soil Erosion
Factors Affecting Soil Erosion
Weathering and Soil
Summary
 What is weathering?
 Types of Weathering
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Weathering Creates Soil
Soil Formation
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Mechanical and Chemical
Factors Affecting Soil Formation
Soil Profiles
Factors Affecting Soil Erosion