Chapter 8
Weathering and Soil Formation
Weathering
1. Weathering occurs when rocks are exposed to the atmosphere.
2. Weathering is an interaction between the lithosphere and the atmosphere and hydrosphere.
3. The weathering process results in physical and/or chemical breakdown of the rock material.
4. Uniformitarianism- The geological principle that states that the same processes that operate
today operated in the past.
Physical Weathering
1. Any process that causes a rock to crack or break into pieces is called physical weathering.
2. Physical weathering of rock may occur as the result of alternate freezing and melting of water,
a process called frost action. (ice wedging)
3. Physical weathering may occur when cracks in rocks are widened by the forces exerted by
growing plants.
4. Physical weathering may occur when rock split along planes of weakness, usually into slabs, when
the pressure of overlying material is removed, a process called exfoliation.
5. Physical weathering may occur when rock grind with other rocks by friction in a process called
abrasion.
Chemical Weathering
1. Chemical weathering changes the chemical composition of the minerals in rocks and usually
weakens the rocks as a result of those changes.
2. The agents of chemical weathering include oxygen, water, carbon dioxide, and acids.
3. Water is the most important factor in chemical weathering because it speeds up chemical
reactions, dissolves many materials, and reacts
directly on mineral substances.
Rates of Weathering
1. The rate of weathering is affected by climate, the size of the particles being weathered, and
the type of material being weathered.
2. At high latitudes (and high altitudes) frost action is the major form of weathering. In warm
climates, chemical weathering is most important. However, low humidity will slow the rate of
weathering in both cases.
3. Local climate conditions- winds, nearness to cities, etc.- can affect the expected weathering
rate for a region.
4. A sample consisting of small rock particles will weather faster than a single large sample of
the same mass because surface area is exposed by the small particles.
5. Rocks and minerals weather at different rates because of
their different physical and chemical properties.
How Soil Forms: The Products of Weathering & Soil
Physical and chemical weathering combine to break down into
loose particles called sediment. The order of particle size
(from larger to smaller) is boulders, cobbles, pebbles, sand,
silt, clay, and colloids.
Soil is the loose, weathered material on Earth’s surface in
which plants can grow.
Bedrock is the solid layer of rock beneath the soil.
Soil gradually develops layers called horizons. A soil profile shows the distinct layers, called soil
horizons, which differ in color, texture, and mineral composition, etc. A soil horizon is a layer of
soil that differs in color and texture than those above it or below it
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The A horizon is made up of topsoil. (rich dark soil made of humus, clay and other
materials).
The B horizon is called subsoil. (clay and other particles washed down from the A
horizon).
The C Horizon is made up of partially weathered rock.
Directly below the C layer is the bedrock.
Soil profiles develop as a result of weathering processes and biological activity, and therefore
contain weather rock and organic material.
Regional soils form in place and remain there while transported soils are formed at one location
and move to another.
Transported soils are more common in PA because much has been deposited by continental
glaciers moving south during glacial periods.
Soil Conservation
1. Natural resource-anything in the environment humans use.
2. Soil is Earth’s most valuable resource because every living thing
depends upon it.
3. Sod-the thick mass of tough roots at the surface of the soil.
The value of soil is reduced when soil loses its fertility.
The value of soil is reduced when topsoil is lost due to erosion.
Topsoil can be lost whenever it is exposed to water, wind.
Dust Bowl-an area in the Midwest that lost its soil to over farming, over
ranching. In the 1930’s most of the soil had washed away and turned to dust.
Soil Conservation-the management of soil to prevent its destruction.
1. Contour plowing-farmers plow fields along the curves of a slope
to prevent soil from washing away.
2. Conservation plowing- dead leaves, weeds are not pulled so the
soil can retain moisture and nutrients.
3. Crop rotation- different crops planted in different fields each
year.