ROCK

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.