CLASSIFICATION OF ROCKS
Rocks are the main constituent of the earth’s crust. A rock may be defined as any mass of natural deposit present in the solid mass of the earth’s crust.
Most rocks are made of aggregates of minerals. These minerals are specifically referred to as the rock-forming minerals. A mineral is a naturally occurring inorganic (i.e. has a nonliving base) substance possessing certain physical properties, a definite chemical composition and a definite atomic structure.
Many minerals have tendency to form crystals which are bounded by plane surfaces arranged in a regular and symmetrical manner. Some physical properties like cleavage, hardness, specific gravity and colour are useful in the identification of minerals. Usually, minerals are composed of two or more than two elements, but some minerals have only one element. For instance, sulphur, graphite, gold etc. are called one-element minerals.
Most of the minerals are oxides, silicates and carbonates. On the basis of their origin, the rocks can be classified as igneous, sedimentary and metamorphic rocks.
- Igneous rocks are formed from the solidification of molten rock material. Some form below Earth’s surface. Some form on or above Earth’s surface. We describe these two basic types:
Intrusive igneous rocks crystallize below Earth’s surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are: diabase, diorite, gabbro, granite, pegmatite, and peridotite.
Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include: andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff.
- Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks.
Clastic sedimentary rocks form from the accumulation and lithification of mechanical weathering debris. Examples include: breccia, conglomerate, sandstone, siltstone, and shale.
Chemical sedimentary rocks form when dissolved materials preciptate from solution. Examples include: chert, some dolomites, flint, iron ore, limestones, and rock salt.
Organic sedimentary rocks form from the accumulation of plant or animal debris. Examples include: chalk, coal, diatomite, some dolomites, and some limestones.
- Metamorphic rocks have been modified by heat, pressure, and chemical processes, usually while buried deep below Earth’s surface. Exposure to these extreme conditions has altered the mineralogy, texture, and chemical composition of the rocks.
There are two basic types of metamorphic rocks.
Foliated metamorphic rocks have a layered or banded appearance that is produced by exposure to heat and directed pressure. Examples of foliated rocks include: gneiss, phyllite, schist, and slate
Non-foliated metamorphic rocks do not have a layered or banded appearance. Examples of nonfoliated rocks include: hornfels, marble, novaculite, quartzite, and skarn.