Prepared by Iggy Isiorho for
Weathering and Soil
On pg. 103
Weathering – The group of processes that change rock
at or near the earth’s surface.
Erosion – The physical removal of rock by an agent
such as running water, glacial ice, or wind.
Transportation – The movement of eroded particles by
agent such as rivers, waves, glaciers,or wind.
Mechanical – The physical disintegration of
rock into smaller pieces.
Chemical – The decomposition of rock resulting
from exposure to water and atmospheric gases.
Effects of Weathering
We tend to think of weathering as destructive because it mars
statues and building fronts. As rock is destroyed, however,
valuable products can be created. Soil is produced by rock
weathering, so most plants depend on weathering for the soil they
need in order to grow. Weathering products dissolved in the sea
make seawater salty and serve as nutrients for many marine
organisms. Some metallic ores, such as those of copper and
aluminum, are concentrated into economic deposits by chemical
Differential weathering – Varying rates of weathering resulting
from some rocks in an area being more resistant
to weathering than others. (Fig. 5.5)
Frost Action – Mechanical weathering of rock by freezing water.
Frost Wedging – A type of frost action in which the expansion of
freezing water pries a rock apart.
Fig. 5.6 Fig. 5.7
Frost Heaving – The lifting of rock or soil by the expansion of
Another process that can mechanically weather
rock is abrasion, the grinding away of rock by
friction and impact during transportation. As
loose fragments of rock are picked up and moved
by a stream, they tumble against one another and
against the rocky stream bottom. (Fig. 5.8)
Glaciers, waves, and even wind are other agents
that carry and abrade rock fragments.
Pressure Release – A significant type of mechanical weathering that
causes rocks to crack when overburden is removed.
Sheet Joints – Cracks that develop parallel to the outer surface of a
large mass of expanding rock, as pressure is released during
Exfoliation – The stripping of concentric rock slabs from the outer
surface of a rock mass.
Exfoliation domes – A large, rounded landform developed in
massive rock, such as granite, by the process of
Pressure Release II
Several other processes mechanically weather rock but in most
environments are less effective than frost action, abrasion, and
pressure release. Plant growth, particularly roots growing in cracks
(Fig. 5.12A), can break up rocks, as can burrowing animals. Such
activities help to speed up chemical weathering by enlarging
passageways for water and air. The pressure of salt crystals formed
as water evaporates inside small spaces in rock also helps to
disintegrate desert rocks (Fig. 5.12B). Extreme changes in
temperature, as in a forest first, can cause a rock to expand or
contract until it cracks. Whatever processes of mechanical
weathering are at work, as rocks disintegrate in to smaller fragments
the total surface area increases (Fig. 5.13), allowing more extensive
chemical weathering by water and air.
The processes of chemical weathering, or rock
decomposition, transform rocks and minerals exposed to
water and air into new chemical products.
Minerals change gradually at the surface until they come
into equilibrium, or balance, with the surrounding
Soil – A layer of weathered, unconsolidated material on
top of bedrock; often also defined as containing
organic matter and being capable of supporting
Loam – Soil containing approximately equal amounts
of sand, silt, and clay.
Soil horizon – Any of the layers of soil that are distinguishable by
characteristic physical or chemical properties.
O horizon – Dark-colored soil layer that is rich in organic material
and forms just below surface vegetation.
A horizon – The top layer of soil, characterized by the downward
movement of water; also called zone of leaching.
B horizon – A soil layer characterized by the accumulation of
material leached downward from the A horizon above;
also called zone of accumulation.
C horizon – A soil layer composed of incompletely weathered parent
Residual and Transported
Residual soil – Soil that develops directly from
weathering of the rock below.
Transported soils – Soil not formed from the
local rock but from parent material
brought in from some other region and
deposited, usually by running water,
wind, or glacial ice.
Organisms contribute to soil development. Plant roots
break up rocks and burrowing organisms such as ants,
worms, and rodents bring soil particles to the surface and
created passageways for water and air to get underground,
thus speeding up chemical weathering.
Once soil begins to develop on a newly exposed rock, it
attracts plants and soil organisms that increase chemical
weathering, accelerating the rate of soil development.
Partially decayed organic matter provides plant nutrients,
increasing soil fertility.
Soils and Climate
Pedalfer – A soil characterized by the downward movement of
water through it, downward leaching, and abundant
humus. Found in humid climates. (Fig. 5.23)
Pedocals – A soil characterized by little leaching, scant humus,
the upward movement of water through it, and the
precipitation of salts. Found in dry climates. (Fig.
Laterite – Highly leached soil that forms in regions of tropical
climate with high temperatures and very abundant
rainfall. (Fig. 5.25)