Download 2.5 Igneous Rocks

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

Two properties used to classify igneous
rock
◦ Texture (crystal size)
◦ Mineral composition
Classification of Igneous Rock

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.

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
Porphyritic material

Some extrusive igneous rock cool so
quickly that they develop a glassy texture
with no visible crystals.
◦ A good example of this is obsidian.
Glassy 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.
Frothy texture
This picture to the
right
Depict the various
textures
present in igneous
rocks as
a function of the
cooling rate.
Porphyritic Textures

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%
Mineral Composition

The naming of an igneous rock takes into
consideration the rock’s texture and
mineralogy as shown in the table below.
Naming of Igneous Rocks