IGNEOUS ROCKS
IGNEOUS ROCKS are formed as a result of the cooling and solidification of molten magma either on the surface or in the earth crust.
When the temperature of the earth is very hot, it melts the rock into thick liquid called molten magma, and when it cools, it becomes solid which forms igneous rock. Examples of Igneous rocks are basalt, granite, quartz, diorite, pumice etc. Igneous rocks contain the following materials biotite, horn-blend, quartz and feldspar.
SEDIMENTARY ROCKS
SEDIMENTARY ROCKS are formed from the deposits of igneous rock or metamorphic rock pieced together for many years and hardened to form sedimentary rocks.
This is to say that sedimentary rocks are formed from small pieces of rock that have settled on the earth’s surface or in lakes or in shallow seas. Sedimentary rocks are also formed as a result of the deposition and cementation of weathered materials from pro-exiting rocks.
They are often called water formed rocks because the weathered particles are transported an deposited by moving water.
METAMORPHIC ROCKS
METAMORPHIC ROCKS are rocks formed as a result of subjecting an already existing rock (igneous or sedimentary) to heat, pressure or chemical change.
Examples are
i. Shale changes to slate.
ii. Limestone to marble.
iii. Sandston to quartzite
iv. Granite to gneiss.
v. Schists formed from sandstone, shale and limestone.
vi. Clay changes to slate.
vii. Coal changes to graphite.