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Chapter 8-I: Weathering
Weathering is the break-up of rock due to exposure to the atmosphere
When rocks that formed deep underground are moved to the surface by
Earth's forces, they are subjected to water, oxygen and lower
Types of Weathering:
Mechanical Weathering (disintegration) takes places when rock is split or
broken into smaller pieces of the same material without changing its
Chemical Weathering (decomposition) takes place when the rock's
minerals are changed into different substances (usually by water)
Types of Mechanical Weathering
Common mechanical weathering processes are frost action, wetting and
drying, action of plants and animals, and the loss of overlying rock and soil.
Water takes up about 10% more space when it freezes. Water held in the
cracks or rocks wedges the rocks apart when it freezes --> ice wedging
Ice wedging occurs mostly in porous rocks, and in rocks with cracks
Repeated wetting and drying is effective at breaking up rocks that contain
Clays swell up when wet and shrink when dry, causing rocks that contain
clay to fall apart.
Small plants such as lichens grow on rocks and wedge tiny roots into
pores and crevices, when the root grows, the rock splits.
Granite is formed deep underground. It is exposed when large masses of
rock are lifted to form mountains and the rocks above the granite are worn
Upward expansion of the granite leads to long curved breaks, or joints.
This is known as sheet jointing.
From time to time large sheets of loosened rock break away from the
outcrop, in a process called exfoliation.
Chemical Weathering
Results mainly from the action of rainwater, oxygen, carbon dioxide, and
acids of plant decay.
The chemical reaction of water with other substances is called
- ex. Feldspar, hornblende, augite form clay
The chemical reaction of oxygen with other substances is called
- iron-bearing minerals are the ones most easily attacked by oxygen
Carbon dioxide dissolves easily in water, forming carbonic acid.
Carbonic acid attacks many common minerals.
- Carbonic acid has the greatest effect on calcite, dissolving it completely.
Resisting Weathering
Quartz does not react very much to water, oxygen, or acids. It is almost
unchanged by chemical weathering.
Quartz is hard and does not have cleavage, so it also resists mechanical
weathering. In time, quartz is broken into pebbles and sand grains.
Feldspar, hornblende, biotite mica, augite, calcite, and gypsum are all
affected by mechanical and chemical weathering.
Most igneous rocks and many metamorphic rocks weather more rapidly
in wet climates than in dry ones.
Sedimentary rocks are only as resistant as the cements that hold them
The Rate of Weathering
Rocks weather at different rates:
Less resistant rocks weather more quickly, more resistant rocks (granite
and gneiss) take much longer to weather away.
When rocks of different resistance are in the same place, spectacular
landscapes are sometimes formed.
An important factor that affects the rate at which rock weathers is the
amount of rock surface that is exposed.
Smaller pieces have greater surface area exposed, and more surface is
exposed to weathering, increasing the rate.
Climate is an important factor in rock weathering: warm, wet climates
favour chemical weathering processes. Cold or dry climates favour
mechanical weathering processes.
Weathering is a very slow process. Limestone dissolves as little as 1/20 of
a cm in 100 years.