Intrusive rock

Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, such as batholiths, dikes, sills, laccoliths, and volcanic necks.

The relative amounts of quartz, alkali feldspar, plagioclase, and feldspathoid are particularly important in classifying intrusive igneous rocks, and most plutonic rocks are classified by where they fall in the QAPF diagram.

However, dikes of basaltic composition often show grain sizes intermediate between plutonic and volcanic rock, and are classified as diabases or dolerites.

Rare ultramafic hypabyssal rocks called lamprophyres have their own classification scheme.

[10] Because their crystals are of roughly equal size, intrusive rocks are said to be equigranular.

These differences show the influence of the physical conditions under which crystallization takes place.

[13] Intrusions vary widely, from mountain-range-sized batholiths to thin veinlike fracture fillings of aplite or pegmatite.