Forms Of Igneous Rocks.
Igneous rocks are the product of consolidation of magma i.e. molten silicates which underlie the solid crust of the earth. Igneous rock bodies are of two types:
1.Extrusive– These are those rocks which are formed by the solidification of magma poured out at the earth’s surface. Ex- lava Flows.
2.Intrusive– These are those rocks which are formed by the consolidation of magma at some depth below the Earth’s Surface. Such rocks show the variation of size and shape. Ex- Sills, Dikes, batholiths etc.
The Intrusive rocks are nevertheless characterized by the development of variety of forms, depending upon their exact mode of formation and their relation with the surrounding country rock. If the Intrusive mass cut across the structures of the pre-existing country rock those igneous rock bodies are called discordant but when the rock bodies run necessarily parallel to the structure of the country rock, such igneous rock bodies are referred to as Concordant.
Various Forms of Igneous rock bodies are:
- Batholith: It is the most remarkable discordant body which have extremely large dimensions so much that their exposures upon the surface are expressed in terms of tens or even hundreds of square miles and these have transgressive relationship with the adjacent country rock. Their outcrop at the surface is roughly circular or oval. In cross section batholiths possess steep outwardly dipping contacts and they are thought to be bottomless. The composition of batholiths is generally granitic to granodiorite.
- Lopolith: These are concordant bodies with their shape more or less similar to that of a saucer (i.e. the central portion of the mass is thicker while it thins out towards the periphery). These are produced due to the collapse of rock beds in any region and consequent upraise of the magma through the collapsing layers. Its diameter is usually 10 to 20 km and its composition is commonly basic.
- Laccolith: These are also concordant igneous bodies with their lower surface flat and upper surface arched in the form of a dome. These are naturally formed due to the accumulation of viscous magma underneath the rocks occurring upon the surface, the latter being push upwards to make room for the mass.
- Phacolith: These are crescent shaped bodies of igneous rocks which occupy the crest and trough part of folded strata as these parts of a fold are the zones of minimum stress.
- Sill: These are relatively thin tabular sheets of magma which have penetrated approximately along a horizontal bedding plane. These show nearly parallel upper and lower margins for considerable distances but as they thin out in distance the shape is flatly lenticular. The thickness may vary from a few inches to many hundreds of feet.
- Dike: It is wall like igneous body that cut across the strata of the pre-existing rock. They are often vertical to steeply incline and their thickness vary from a few cm to a hundred meters or more. Dikes tend to occur as system or swarms which are parallel to one direction or are radial to a center. Dikes are frequently more resistant to erosion than the enclosing rocks.
- Volcanic Plug: It is a vertical cylindrical shaped igneous body which has a roughly oval or circular cross section. It represents the vent of an extinct volcanic plugs range in diameter from a few hundred meters to a few kilometers.