Chapter 11
Igneous Rocks, Intrusive Activity and the
Origin of Igneous Rocks
What is a rock?
 Rocks are naturally formed consolidated material composed of
one or more minerals.
The Rock Cycle
 KNOW THE ROCK CYCLE!
 If you can explain the rock cycle you will have the basic knowledge
that you need to understand rocks.
Igneous Rocks
 Igneous rocks are those that form from the crystallization of
magma. Magma is molten rock within the earth. Note that lava is
magma on the surface.
 Igneous rocks are either
 Intrusive – crystallizing into a solid below the surface – or
 Extrusive – crystallizing on the surface
 Intrusive and extrusive igneous rocks are mineralogically and
chemically the same
 Texturally they are different
 Extrusive rocks are generally fine grained. The magma cools
quickly on the surface so the mineral crystals don’t have time to
grow large. The common extrusive rocks are basalt, andesite
and rhyolite.
 Intrusive igneous rocks are coarse grained because the magma
has time to cool slowly and the mineral crystals grow larger than
on the surface. These rocks are also referred to as Plutonic.
Granite, diorite and gabbro are the common intrusive igneous
rocks.
 Other textures used in igneous rocks are:
 Pegmatite – These are intrusive igneous rocks with very large
mineral crystals (over 5 centimeters)
 Porphyritic – Texture of larger crystals in a fine grained matrix
 Xenoliths – fragments of country rock inclosed in intrusive
igneous rock
Identification of igneous Rocks
 Igneous rocks are identified on the basis of texture (grain size) and
mineral content
Intrusive Bodies
 Intrusions are classified on their shape, size and their relationship
to the country rock
 Shallow intrusions include
 Volcanic necks
 Dikes and Sills
 Intrusions that crystallize at depth are called plutons. When
plutons are exposed on the surface they are classified by size.
 Batoliths are greater then 100 square Km
 Stocks are less than 100 sq. Km
 Granites are the most common intrusive rock. They form most of
the continental “basement” rock.
Bowen’s Reaction Series
Igneous Rocks and Plate Boundaries
 Divergent boundaries (See figure 11.24 page 307)
 Convergent boundaries