The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place on their reaching the surface or also while the lava is still in the crustal portion. The lava that cools within the crustal portion assumes different forms called intrusive forms.
The various intrusive forms are as follows :
(i)Batholiths - A large body of volcanic material that cools at the deeper depth of crust develops in the form of large domes. They appear on the surface only after denudation process removes the overlying material. These are the cooled portions of the magma chambers.
(ii)Localiths - These are the large dome-shaped intrusive bodies with a level base and connected by a pipe like conduit from below. It resembles the surface volcanic domes of composite volcano only located at deeper depths. Karnataka plateau is spotted with dome-shaped hills of granite rocks.
(iii) Lopoliths - When lava moves upwards, portions of the same may tend to move in horizontal direction whenever it finds a weak plane. It may get rested different forms. If it develops into a saucer-shaped, concave to the sky body, it is called lopoliths.
(iv)Phacoliths - The wavy mass of intrusive rocks found at the base of synclines or at the top of anticline in folded igneous regions are called the phacoliths.
(v)Sill or sheet - Near horizontal bodies of the intrusive igneous rocks are called sill or sheet.
(vi)Dykes - When the lava cools in fissures developed in the land, such structure are called dykes.
