Igneous Rocks
Chapter 10.2
Igneous Rocks
1. Igneous Rocks
a. Igneous rocks form when magma cools and
hardens.
b.Igneous rocks are categorized into two
groups: Intrusive & Extrusive
i. Intrusive igneous rocks form when magma from
deep below the earth’s crust cool and harden.
1. We call them intrusive because the magma that
forms them intrudes, or enters, into other rock
masses beneath the earth’s surface and slowly cools.
ii.Extrusive igneous rocks are formed from the
rapid cooling of lava, or melted rock on the
earth’s surface.
Intrusive (left) & Extrusive (right)
Igneous Rocks
c. Intrusive and extrusive igneous rocks
differ mainly in the size of their
crystalline mineral masses or grains
which is called texture.
d. The texture of igneous rocks is
determined by the cooling rate of the
magma or lava that formed the rock.
Texture
2. Texture of Igneous Rocks
a.Since intrusive igneous rocks are formed
by slow cooling and hardening of
magma underground, there is plenty of
time to form large, well developed
crystalline grains.
i. These rocks are formed of large mineral
grains and have a coarse grained texture.
ii.An example is granite.
Granite
Texture
b.When extrusive igneous rocks cool
and harden rapidly on the surface of
the earth there is not much time for
large crystalline grains to form, and
therefore produce fine grained rock.
i. An example of this type of rock is basalt.
Basalt
Texture
c. Sometimes igneous rocks form from
magma that cools slowly at first and
then rapidly as it nears the earth’s
surface.
i. This type of cooling produces large
crystals embedded within a mass of
smaller crystals.
ii.Porphyritic texture is described by large
and small crystals.
Porphyritic texture
Texture
d. When highly viscous, silica rich
magma cools rapidly, the resulting
rock has no crystals.
i. An example of this type of rock is obsidian
or volcanic glass.
e. When magma that contains a large
proportion of dissolved gases cools
rapidly, the gases may become trapped
in the rock and produce a rock full of
holes.
i. An example of this type of rock is pumice.
Obsidian & Pumice
Composition
3.Composition of Igneous Rocks
a.The mineral composition of an igneous
rock is determined by the chemical
composition of the magma from which
the rock develops.
Composition
b. Igneous rocks have been divided
into three families based on
mineral composition: felsic,
intermediate, and mafic.
i. Felsic rocks form from magmas that
are high in silica.
1.Examples of rocks in the felsic family
include granite, rhyolite, and obsidian.
Rhyolite
Composition
ii. Mafic rocks form from magmas that are
low in silica but rich in iron and
magnesium.
1. Examples of rocks in the mafic family include
basalt and gabbro.
iii.Intermediate igneous rocks contain less
quartz than rocks in the felsic family and
have a color that falls between felsic and
mafic rocks.
1. Examples of rocks in the intermediate family
include diorite and andesite.
Gabbro, Diorite, & Andesite
Rock Structures
4.Igneous Rock Structures
a.Igneous rock structures that form
underground are called intrusions.
i. Batholiths are the largest of all
intrusions and have at least 100
square kilometers of surface
exposure.
1.The word batholith means “deep rock.”
2.Batholiths form the cores of many major
mountain ranges.
Rock Structures
ii. A stock is an intrusion similar to
a batholith but covers less than
100 square kilometers.
iii.A laccolith forms when magma
flows between rock layers and
spreads upward pushing the
overlying rock into an arc.
1.Laccoliths can sometimes be
identified by small dome shaped
mountains.
Rock Structures
iv. A sill is formed when a sheet of
magma flows between the layers of
rock and hardens.
1. A sill will lie parallel to the rock layers
surrounding it, even if the layers are
tilted.
v. A dike is formed when magma
forces its way through rock layers
by following existing vertical
fractures or by creating new ones
and solidifies.
Rock Structures
Rock Structures
b. Igneous rock structures that form on
the surface are called extrusions.
i. A volcanic neck forms when a dormant
volcano weathers down to only the hard,
solidified rock in the vent.
ii.A lava plateau forms from lava that flows
out of long cracks in the earth’s surface,
spreads out over a vast area, and fills in
valleys covering hills.
1. Once the lava is hardened it forms a plateau.
Lava Plateau & Volcanic Neck