Classification of Rocks

Rocks

The rocks are the solid mineral materials forming a part of the surface of the earth and other similar planets. The earth’s crust (Lithosphere) is composed of rocks. A rock is an aggregate of one or more minerals. Rock is an important natural resource and is found in solid state. It may be hard or soft in nature. An estimation reveals that there are 2,000 different types of minerals found on the earth surface out of which only 8 basic minerals commonly found all over the earth. Minerals are chemical substances which exist in nature. They may occur either in the form of elements or compounds.

Classification of Rocks

According to the mode of formation the rocks are classified into three types as follows.

1. Igneous Rocks

2. Sedimentary Rocks and

3. Metamorphic Rocks

1. Igneous Rocks

The igneous rocks are formed by the solidification of molten magma. These rocks are also called as the ‘Primary Rocks’ or ‘Parent Rocks’ as all other rocks are formed from these rocks.

The word Igneous is derived from the Latin word ‘Ignis’ means ‘fire’

Characteristics of Igneous Rocks

1. These rocks are hard in nature

2. These are impermeable

3. They do not contain fossils

4. They are associated with the volcanic activities

5. These rocks are useful for construction works

Types of Igneous Rocks

Igneous Rocks are of two types. They are:

1. Extrusive Igneous Rocks

2. Intrusive Igneous Rocks

1. Extrusive Igneous Rocks

Can you visualize the lava comes out from a volcano? Lava is actually a fiery red molten magma comes out from the interior of the earth on its surface. After reaching the earth surface the molten materials get solidified and form rocks. Rocks formed in such a way on the crust are called Extrusive igneous rocks. These rocks are fine grained and glassy in nature due to rapid solidification. Basalt found in the north western part of peninsular India is the example for this type of rock.

2. Intrusive Igneous Rocks

The molten magma sometimes cools down deep inside the earth’s crust and becomes solid. The rocks formed this way is called ‘Intrusive Igneous Rocks’. Since the cool down slowly and form crystals. Hence they are called 'crystalline rocks'. Intrusive Igneous rocks are two types. They are, 1. Plutonic rocks 2. Hypabysal rocks. The deep seated rocks are called 'Plutonic rocks' and the ones formed at shallow depths are called 'Hypabysal rocks'. Granite, Diorite and Gabbro are the example of plutonic rocks and dolerite is an example of hypabysal rocks.

Some major Active Volcanoes: Mount Vesuvius, Mt. Stromboli and Mt. Etna in Italy and Mauna Loa and Mauna Kea in Hawaii Islands.

2. Sedimentary Rocks

The word ‘S e dimentar y ’ has been derived from Latin word ‘Sedimentum’ means settling down . The sedimentary rocks are formed by the sediments derived and deposited by various agents. Due to high temperature and pressure, the undisturbed sediments of long period cemented to form sedimentary rocks. Sedimentary rocks consist of many layers which were formed by the sediments deposited at different periods. As it consists of many strata, it is also known as ‘Stratified rocks’.

Sedimentary rocks are the important source of natural resources like coal, oil and natural gas.

Characteristics of Sedimentary rocks

1. They have many layers.

2. They are non-crystalline rocks.

3. They contain fossils.

4. They are soft and get eroded easily

Oldest sedimentary rocks of the world has been identified in Greenland and estimated as 3.9 billion years old.

Types of Sedimentary Rocks

1. Organic Sedimentary Rocks

These rocks are formed as a result of the decomposition of dead plants and animals. It contains fossils. Chalk, Talc, Dolomite and Limestone rocks are of this category.

2. Mechanical Sedimentary Rocks

These rocks are formed due to the disintegration of igneous and metamorphic rocks. The natural agents erode and transport these rocks and deposit them at some places. After a long period of time, they cemented to form rocks. Sandstone, Shale and Clay are the examples of rocks of this type.

3. Chemical Sedimentary rocks

These are formed by precipitating of minerals from water. It is formed usually through evaporation of chemical rich solutions. These rocks are also called as evaporates. Gypsum is an example of this kind.

3. Metamorphic Rocks

The word Metamorphic is derived from two Greek words “Meta” and “Morpha”, Meta means change and Morpha means shape. When Igneous and sedimentary rocks subject to high temperature and pressure, the original rocks get altered to form a new kind of rock called metamorphic rocks. Metamorphism is of two types. They are

1. Thermal Metamorphism

2. Dynamic Metamorphism

If the change in the rocks is mainly caused by high temperature, the process is called as 'Thermal Metamorphism'.

If the change in the rock is mainly caused by high pressure, the process is called as 'Dynamic Metamorphism'.

One of the world wonders Taj Mahal in India was built with White Marbles a metamorphic rock.

Formation of Metamorphic Rocks from Igneous rocks

1. Granite into gneiss caused by dynamic metamorphism.

2. Basalt into slate caused by thermal metamorphism.

Formation of Metamorphic Rocks from Sedimentary rocks

1. Sandstone into quartz caused by thermal metamorphism.

2. Shale into slate caused by thermal metamorphism.

Characteristics of Metamorphic Rocks

1. Metamorphic rocks are mostly crystalline in nature.

2. They consist of alternate bands of light and dark minerals.