Classifying Igneous Rocks

Authors Avatar

CLASSIFYING IGNEOUS ROCKS

An igneous rock is a rock formed by the cooling and crystallization of magma intrusively in the crust or extrusively on the surface. The location and speed of cooling and the type of magma determine which igneous rock is formed. Igneous rocks can be classified according to their colour, texture and density, the minerals present, and the chemical composition of the rock.

COLOUR

Colour is useful as a diagnostic tool for course-grained igneous rocks but not fine grained. The colour index of a rock indicates the amount of dark ferromagnesian minerals that are present. These include olivine, pyroxene and biotite mica. They are rich in iron and magnesium and usually associated with constructive plate margins. The pale minerals in igneous rocks are called felsic minerals. These include feldspars, quartz and muscovite mica. They are rich in silica and aluminium and associated with continental areas.

Join now!

The colour index measures the proportion of ferromagnesian or mafic minerals present in the rock. Rocks with less than 30% of dark mineral are pale and described as leucocratic. Rocks with 30-60% are mesocratic and tend to be grey in colour. Those with over 60% mafic minerals are known as melanocratic.

TEXTURE

The texture of a rock describes the relationships between the grains. Grain size, shape and orientation, degree of crystallinity, and the ways in which grains come into contact with each other are all textural features. The texture of a rock depends upon the physical conditions in which the magma ...

This is a preview of the whole essay