The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals. On the basis of mode of formation the rocks are divided into three categories
- A) Igneous rocks
- B) Sedimentary rocks
- C) Metamorphic rocks.
A) Igneous rocks
As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks. The igneous rocks are formed when magma cools and solidifies. When magma in its upward movement cools and turns into solid form it is called igneous rock. The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth. Granite, gabbro, pegmatite, basalt,volcanic breccia and tuff are some of the examples of igneous rocks. Igneous rocks are roughly hard rocks and water percolates with great difficulty. They do not have strata and are less affected by chemical weathering. They don’t contain fossils. The number of joints increases upwards. They are mostly associated with volcanic activity.
B) Sedimentary rocks
Rocks (igneous, sedimentary and metamorphic)of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction into rocks. This process is called lithification. In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc. Depending upon the mode of formation, sedimentary rocks are classified into three major groups: (i) mechanically formed – e.g. sandstone, conglomerate, shale, loess etc. (ii) organically formed – e.g. chalk, limestone, coal etc. (iii) chemically formed – e.g. chert, halite, potash etc. These rocks are formed due to aggregation and compaction of sediments. These rocks contain fossils of plants and animals. They cover 75 percent of surface area of the globe. However they form only 5 percent of the volume of earth’s crust. They contain several layers or strata but these are seldom crystalline rocks. They are seldom found in original and horizontal manner.They may be well consolidated, poorly consolidated and even unconsolidated. They are characterized by different sizes of joints. Most sedimentary rocks are porous and permeable.
C. Metamorphic rocks
- Sometimes igneous or sedimentary rocks metamorphize or change due to great ‘pressure, intense temperature or the action of water and chemical activity.
- Examples of metamorphic rocks formed from different rocks are:
Metamorphic – Rock Made From
- Slate       Shale and mudstone
- Quartzite   Sandstone
- Gneiss     Granite
- Marble        Limestone, dolomite or chalk
- Schist      Shale
- Anthracite   Coal
