Definition of “Rock”: Rock may be defined as aggregate of minerals that makes up part of the earth's crust
Classification of Rocks
Based on the mode of formation, all the rocks are classified into 3 groups. They are:
1) Igneous rocks or primary rocks
2) Sedimentary rocks or secondary rocks
3) Metamorphic rocks.
General characters of igneous rocks:
i) Generally hard, massive, compact with interlocking grains
ii) No fossils present
iii) No bedding planes present
iv) Surrounding rocks are baked
v) Contains much feldspar group of minerals
2. Sedimentary rocks: These rocks will be formed through the process of weathering, transportation and deposition of preexisting rocks. The deposited loose sediments will undergo a process called lithification and diagnensis to form sedimentary rocks.
Based on their place of formation, sedimentary rocks are classified into 2 types.
Residual deposits: Formed at the site of preexisting rocks
Transported deposits: Formed at far off place from the preexisting rocks.
General characters of sedimentary rocks:
i) Generally soft and layered
ii) Fossils are common
iii) No effect on the surrounding rocks
3. Metamorphic rocks: These rocks are formed by the alteration of preexisting rocks by the action of Pressure and temperature.
General characters of metamorphic rocks:
i) Generally hard, interlocking grains and bedded (if derived from sedimentary rocks)
ii) Foliated
iii) Rarely preserves fossils in case of sedimentary origin
