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g These rocks do not have remains of animals g These rocks are mostly formed under water
or plants. and have marks of waves and mud-cracks.
g These rocks do not allow water to percolate g These rocks also contain remains of animals
through them. and plants, called fossils, between the
g Other types of rocks are formed from layers.
igneous rocks. g The important examples are sandstone,
g Valuable minerals such as iron, gold, silver limestone and coal.
and aluminum are found in these rocks. Metamorphic Rocks
Sedimentary Rocks The metamorphic rocks make up a large part of
The sedimentary rocks are formed by the deposi- the earth’s crust and are classified by texture and
tion and subsequent cementation of that material by chemical and mineral assemblage. They may
within bodies of water and at the surface of the be formed simply by being deep beneath the
earth. The process that causes various organic earth’s surface, subjected to high temperatures
materials and minerals to settle in a place is and the great pressure of the rock layers above it.
termed as sedimentation.
Metamorphic Rocks
Sedimentary Rocks
Metamorphic rocks arise from the transformation
The particles that form a sedimentary rock by
of existing rock types, in a process called meta-
accumulating are called sediment. Before being
morphism, which means “change in form”. The
deposited, the sediment was formed by weather-
original rock is subjected to heat with tempera-
ing and erosion from the source area and then
ture greater than 150°C to 200°C and pressure
transported to the place of deposition by water,
around 1500 megapascals, causing profound
wind, ice, mass movement or glaciers, which are
physical and/or chemical change.
called agents of denudation. Sedimentation may
Metamorphic rocks have the following character-
also occur as minerals precipitate from water
istics :
solution or shells of aquatic creatures settle out of
suspension. g These rocks after change remain in their
original position.
Sedimentary rocks have the following character-
istics : g They are very hard and have a high specific
gravity.
g These rocks are softer than igneous rocks.
g These rocks do not have empty spaces in
g These rocks have horizontal layers,
them.
arranged one above the other.
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