Igneous Rocks
An igneous rock is one that is
formed from the cooling and
solidification of molten (liquid)
rock.
Extrusive vs. Intrusive Igneous
Rocks
Extrusive (volcanic) rock is one that forms from the cooling of
lava (liquid rock) found at Earth’s surface.
Intrusions
Extrusions
Extrusive rocks are igneous rocks that have cooled quickly over time.
Lava flows and volcanoes are examples of igneous extrusions.
Intrusive (plutonic) rock is one that forms from the cooling of magma (liquid rock)
within the lithosphere of the Earth.
Intrusive rocks are igneous rocks that have cooled slowly over time.
Batholiths, laccoliths, dikes and sills are examples of igneous intrusions.
Formation of
Igneous Rocks
Inter-grown crystals of several
minerals formed by
crystallization.
Igneous rocks form as a result of the solidification of liquid rock into solid rock.
Most igneous rocks are made through a process of solidification known as
crystallization.
As an igneous rock cools into a solid form, different minerals form crystals that
are inter-grown with other mineral crystals. This is known as crystallization.
Formation of Igneous Rocks – Crystal Size and Texture
The texture of an igneous rocks and the type of crystals it
exhibits depends on the environment in which it had formed.
GRANITE
Granite and Gabbro
are both examples of
coarse-textured
igneous rocks.
GABBRO
Both rocks have very
large crystals that are
easy to see.
The development of LARGE CRYSTALS in an igneous rock indicate a
LONG and SLOW PERIOD OF COOLING.
Coarse-grained igneous rocks cooled deep within the lithospshere
(intrusions) where the temperature and pressure are higher than that
of Earth’s surface.
Formation of Igneous Rocks – Crystal Size and Texture
RHYOLITE
BASALT
OBSIDIAN
vesicles
Rhyolite and Basalt are both examples of fine-textured igneous rocks.
Both rocks have very small crystals that can only be seen through magnification.
glassy
texture
The development of SMALL CRYSTALS in an igneous rock indicate a QUICK PERIOD OF
COOLING.
Fine-grained, igneous rocks cooled on top of the Earth’s surface (extrusions) where the
temperature and pressure are not as extreme as deeper within the Earth.
Some fine-grained, igneous rocks have vesicles – large air pockets that formed within the
rock when it is cooled quickly. Sometimes igneous rocks cool so quickly that mineral
crystals do not have time to form. Instead igneous rocks form a non-crystalline solid known
as glass.
Identification of Igneous Rocks
RT = Pg. 6
Identification of Igneous Rocks
Which igneous rock is intrusive
and has crystal sizes greater than
10 mm?
PEGMATITE
Describe
RHYOLITE.
-- Extrusive (volcanic)
-- Crystal Size is less than 1 mm
-- Fine texture
-- Non-vesicular
-- Light in color
-- Low Density
-- Felsic composition (rich in Si and Al –
silicon and aluminum)
-- Mineral Composition consists of
potassium feldspar, quartz, plagioclase
feldspar, biotite and amphibole.