- The word igneous is derived from the Latin word ‘ignis’ meaning fire.
- Igneous rocks are formed by the cooling of highly heated molten fluid material, known as magma.
- The word magma is derived from a Greek word which means ‘dough’.
It requires a greater quantity of heat to melt the rocks under overlying pressure than at the surface. We do not know the exact depths at which magma forms but probably it is formed at different depths not exceeding 40 km. Molten rocks produce an increase in volume which is responsible for causing fractures or cracks in the crust. The overlying pressure gets weakened along these openings, thus forcing out the magma through them. Otherwise it can’t escape due to great overlying pressure.
When magma is ejected to the surface, it is called lava. Igneous rocks are formed from solidified molten magma below or on the earth’s surface.
As they comprise the earth’s first crust and all other rocks are derived from them, these are called the parent of all rocks or the ‘primary rocks’. In simple words, all rocks can be described as of igneous origin because at one time or another, they were erupted to the surface: A younger series of igneous rocks is still being formed. About 95% of the volume of outermost 16 km of the earth is composed of them. These are largely hard and massive because of their magmatic origin and are crystalline in appearances.
On the basis of their mode of occurrence, igneous rocks can be classified as –
- Extrusive or Volcanic rocks,
- Intrusive rocks.
(i) Extrusive igneous rocks are formed by cooling of lava on the earth’s surface. As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine. These rocks are also called volcanic rocks.
- Gabbro and basalt are very common examples of such rocks.
- These rocks are found in volcanic areas.
- Deccan plateau’s regur soil in India is derived from lava.
(ii) Intrusive igneous rocks are formed when magma solidifies below the earth’s surface. The rate of cooling below the earth’ s surface is very slow which gives rise to formation of large crystals in the rocks.
- Deep seated intrusive rocks are termed as plutonic rocks.
- Shallow depth intrusive rocks are termed as hypabyssal.
- Granite and dolerite are common examples of intrusive rocks.
From this point of view, therefore, igneous rocks can, in accordance with their mode of formation, be classified as –
- (a) Plutonic
- (b) Hypabyssal
- (c) Volcanic
The huge blocks of coarse granitic rocks are found both in the Himalaya and the Decean Plateau.
This figure illustrates that –
- Magma, on cooling, produces rocks of different shapes and sizes, depending on the space available after it forces itself into the crust.
- Common forms of instrusive igneous rocks are batholiths, sills and dykes etc.
- Batholiths are huge masses of solidified magma. They vary in size; some are as much as several hundred kilometers across and thousands of kilometers thick. They generally form the core of the major mountains. Their irregular dome shaped roofs sometimes appear on the surface after erosion of millions of years.
- Sill is the horizontal intrusion of solidified magma between the layers of pre-existing rocks.
- Dyke is similarly a more or less vertical formation from few metres to several kilometers in length and from few centimeter to hundreds of metres in thickness.
On the basis of chemical properties, igneous rocks are classified into acidic and basic rocks. These are formed as a result of solidification of acidic or basic lava.
- Acidic igneous rocks are composed of 65% or more of silica. These rocks are light coloured, hard and very strong. Granite is an example of an acidic rock.
- Basic igneous rocks contain less than 55% of silica and have more of iron and magnesium. These rocks are dark coloured and weak enough for weathering. Gabbro, basalt and dolerite are examples of basic rocks.
Bibliography : NIOS – Geography
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