Most soil contains minerals and
these minerals come from rock. The rock found on the earth surface varies
depending on their mode and time of formation. Each rock type consists of
different combination of minerals according to the way they were formed. There
are three types of rock:-
1.
Igneous
rock
2.
Sedimentary
rock
3.
Metamorphic
rock
1.
Igneous Rock: When a solid rock
melts to form a thick liquid called molten magma as a result of the hotness of
the earth interior. This thick liquid comes out of the earth interior in a
volcanic eruption, it cools down, solidify to form igneous rock, e.g. granite,
basalt, diorite e.t.c.
2.
Sedimentary Rock: Sedimentary rock
is formed from accumulation and sedimentation of weathered rock materials
carried and deposited by wind, water e.t.c sedimentary rocks are formed in
three different ways:
a.
Mechanically
formed sedimentary rock is formed as described above e.g. mud stone, sand stone
shale.
b.
Chemically
formed sedimentary rock is formed as described above e.g. mud stone, sand
stone, shale.
c.
Originally
or biologically formed sedimentary rock: These are rocks formed as a result of
decay and remains of plants and animals e.g. limestone. It could also be as a
result of great heat and pressure action on buried plant over a long time e.g.
coal sedimentary rocks are always in layers and they are not as hard as
metamorphic or igneous rock.
3.
Metamorphic Rock: Metamorphic rock are formed from already
existing rocks that changed their composition due to great heat and pressure
getting on them e.g. gness formed
from granite which is of igneous origin. Schist and slate are formed from
shale, mud stone which is of sedimentary origin.
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