Igneous Rocks

Igneous rocks originate from the magma chamber. When the magma or molten rocks reach the surface, from below the ground, it is called lava. It is the magma or molten rocks that form igneous rocks above and below the earth’s surface. The cooling rate of the lave helps determine how large individual mineral grains are able to grow in the rock, the grain size makes a difference to the appearance and the texture of the rock, the composition of the magma determines the final mineral assemblage in the cooled rock (2008).

The two main categories regarding the formation and origination of igneous rocks are volcanic and plutonic rocks. Volcanic rocks form when the lava comes into contact with much cooler air or water above the earth’s surface. Volcanic rocks cool rapidly, causing large mineral grains to not have time to form within the rock. Not all the lava reaches the surface, instead it stays trapped in the magma chamber until it crystallizes and hardens to form igneous rocks (2008).Plutonic rocks forms when magma crystallizes underground, this is a much slower process which gives time for large mineral grains to form in the rock.

The distinguishing characteristics between the two categories of rocks are that volcanic rocks have a glassy look because it is unable to form into crystalline due to its rapidly cooling rate, while plutonic rocks have time to form large mineral grains that can be seen by any unaided eye (2008). Volcanic rocks that have fine grained texture are called aphanitic, and some volcanic rocks that are called porphyritic, they are the large mineral grains in the aphanitic matrix. This happens when magma is already crystallized into large grains before it erupts, after the eruption when the lave cools to form an aphanitic, the large mineral grains that were formed earlier, become embedded together with the liquid lava to form an aphanitic matrix. Plutonic rocks are large mineral grains that sometimes are