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Rocks and Soils
Rock types
• The Earth’s crust has many different types of rocks.
• These rocks are usually divided into 3 different types.
1.
Igneous Rocks
2.
Sedimentary Rocks
3.
Metamorphic Rocks
Igneous Rocks
• These result from volcanic activity. They consists of crystals
which formed as the volcanic rock cooled down.
• E.g Granite, Basalt.
Sedimentary Rocks
• These have been layered down in layers.
• They usually consist of small particles that have been eroded and
transported.
• E.g. Sandstone and shale.
• They can also be formed from the remains of plants and animals.
• E.g. Limestone, Chalk and Coal
Metamorphic Rock
• These rocks are those that have been altered either by
extremes of pressure e.g. Shale is compressed into slate, or by
extremes of heat.
• E.g. limestone is changed into marble.
Uses of rocks
Igneous
Rock
Formation
Uses
Granite
Magma cools within the Earths
Crust
Buildings, Pottery, sites
for reservoirs, some
tourism
Basalt
Sedimentary
Metamorphic
Fertile soil, tourism,
foundation for roads
Coal
Fossilised remains of trees
Fuel for power stations
Sandstone
Grains compressed together
Building material
Chalk
Remains of shells and skeletons
of animals
Cement and lime
Marble
Limestone changed by heat and
pressure
Monuments
Slate
Clay changed by pressure
Building material for
roofs
Rock Structure
• The structure of a rock can among other things, affect its
resistance to erosion and its permeability to water.
Resistance – how hard or soft a rock is.
Permeability – how much rock allows water to pass through it.
Resistance
• The harder a rock is, the more resistant it is likely to be to
erosion.
• Harder rocks thus usually form hills, mountains and steep
cliffs.
• Soft rocks are more liable to be worn away or broken up.
• Valleys are formed in soft rocks.
• On coasts, resistant rocks form steep cliffs and stand out as
headlands, whereas the softer rocks form bays.
Permeability
• Rocks can be permeable or impermeable
• Permeable rocks allow water to pass through
• Impermeable rocks do not allow water to pass through.
• Permeable rocks may be porous – meaning it has a lot of small
pores.
• Or contain areas of weakness, such as bedding planes, along
which water flows. Horizontal bedding planes, on the other
hand, separate individual layers of rock.
Weathering and Mass
movement
• Weathering is the disintegration (breaking up) and
decomposition (decay) of rocks in situ – that is in their place of
origin.
• Weathering, unlike erosion, does not involve the movement of
material.
• There are two main types of weathering
1. Physical weathering – rocks are broken down into smaller
pieces. There is no change in the chemical composition of
the rock. E.g. Freeze-thaw, exfoliation, biological weathering
2. Chemical weathering – rocks are decomposed due to
chemical changes in the rock. Mostly found in warm, moist
climates. An example is limestone solution.