Chapter 5

Prepared by Iggy Isiorho for

Dr. Isiorho

Weathering and Soil

Index

On pg. 103

Weathering 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.

Different Weathering Mechanical – The physical disintegration of

rock into smaller pieces. Fig. 5.1 Chemical – The decomposition of rock resulting

from exposure to water and atmospheric gases. Fig. 5.2

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 weathering.

Differential weathering – Varying rates of weathering resulting from some rocks in an area being more

resistant to weathering than others. (Fig. 5.5)

Mechanical WeatheringFrost Action

Abrasion

Pressure Release

Frost Action

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 freezing water.

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Fig. 5.6

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Fig. 5.7

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Abrasion

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.

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Fig. 5.8

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Pressure Release Pressure Release – A significant type of mechanical weathering that

causes rocks to crack when overburden is removed. Fig. 5.9 Sheet Joints – Cracks that develop parallel to the outer surface of a

large mass of expanding rock, as pressure is released during unloading.

Fig. 5.10 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 exfoliation.

Fig. 5.11

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Fig. 5.9

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Fig. 5.10

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Fig. 5.11

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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.

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Fig. 5.12

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Fig. 5.13

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Chemical Weathering

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 conditions.

Soil Soil – A layer of weathered, unconsolidated material on

top of bedrock; often also defined as containing organic matter and being capable of supporting plant growth.

Loam – Soil containing approximately equal amounts of sand, silt, and clay.

Soil Horizons

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 material. Fig. 5.21

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.

Organic Activity

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. 5.24)

Laterite – Highly leached soil that forms in regions of tropical climate with high temperatures and very abundant rainfall. (Fig. 5.25)

Index

Fig. 5.1

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Fig. 5.2

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Fig. 5.5

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Fig. 5.21

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Fig. 5.23

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Fig. 5.24

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Fig. 5.25

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