Igneous Rocks

Igneous rocks form from the solidification or crystallization of minerals in molten material.

If the rock form below the Earth’s surface it is considered magma

Molten rock that hardens on the Earth’s surface is called lava

Igneous rocks form from the cooling of molten materials◦They are composed of interlocking and randomly

oriented crystals◦These crystals may be clearly visible to the

naked eye (e.g. granite)

◦Or require a microscope for viewing (e.g. rhyolite)

The Various Form of Igneous Rock

Classification of Igneous Rock

Two properties used to classify igneous rock

◦Texture (crystal size)

◦Mineral composition

Generally Igneous rocks fall into two main groups◦Intrusive – form from the crystallization of

magma, as it occurs below the surface. Also known as plutonic (Pluto – Greek god of the

underworld)

Being below the surface the minerals it cools slowly, which gives the crystals time to grow

Intrusive rocks are coarse-grained or phaneritic texture. Crystals are visible to the naked eye.

Two Main Groups

◦Extrusive - igneous rocks formed from lava at the earth’s surface These rocks are referred to a volcanic.

Lava tends to cool quickly since it is exposed to atmospheric temperatures. Crystals have a short or no time to grow.

Extrusive rocks are fine-grained or aphanitic. The mineral crystals are not visible to the naked eye.

Porphyritic material

This is a rock which shows both visible and no-visible crystals.◦This indicates that the rock cooled at two

different rates.

These rocks are known as porphyritic

Glassy texture

Some extrusive igneous rock cool so quickly that they develop a glassy texture with no visible crystals.◦A good example of this is obsidian.

Frothy texture

Where there is a release of gases in the rock during the cooling of the lava a frothy texture appears.◦Pumice is a fine example of this feature.

Porphyritic Textures

This picture to the rightDepict the various textures present in igneous rocks as a function of the cooling rate.

Mineral Composition

In terms of mineral composition, magmas are characterized by the amount of silica (SiO2) present in the rock

There are four categories into which igneous rocks fall into based on the percentage of silica in the rock.◦Mafic 45-52%◦Ultramafic less than 45%◦Felsic greater than 65%◦Intermediate 53-65%

Naming of Igneous Rocks

The naming of an igneous rock takes into consideration the rock’s texture and mineralogy as shown in the table below.