Source of Magma:
Magma is originated from the upper mantle, which lies just under the crust, and is nearly molten. A slight drop in pressure caused by dee faulting etc, completes the melting process. The basaltic magma originates in this manner, while the “granitic magma” can be produced in two ways: (a) from melting of continental crustal rocks, and (b) from differentiation of primary basaltic magma. The continental crust is much thicker than the oceanic crust. At considerable depths, the temperature is high enough to melt the continental rocks. It is also believed that pocets of radioactive elements generate enough heat to melt the rocks in the earth’s crust. In this way the granitic manga may be produced.
Process of intrusion of Magma:
As the magma is lighter than the surrounding rock material, therefore, it rises slowly to the surface and makes room for itself in the following manner.
- The magma may melt or react and dissolve the overlying rocks and create room for itself. This process is known as “magmatic assimilation”. The magmatic assimilation can not explain the emplacement of large igneous bodies which have quite a sharp contact with the country rocks.
- The overlying country rocks may also be pushed by magma to create a space for magma. The process of pushing up the surrounding rocks is known as the “intrusion”. Laccoliths, sills, and dykes are believed to have been emplaced by this method.
- In some cases the magma penetrates into the cracks of the overlying rocks and breaks off the blocks of the various sizes. In this way a room is created by quarrying the overlying rocks. This process is known as the “magmatic stoping”. Major intrusive bodies, such as batholiths, have been emplaced by this method.