3.2 – Igneous Rocks
I.
Formation of Igneous Rocks (igneous  comes from the Latin word ignis, which means “fire”)
A.
B.
Intrusive igneous rocks – rocks that form when magma hardens beneath the Earth’s
surface (they intrude into the existing rocks)
i.
would never see these rocks if not for erosion stripping away the overlying rock
ii.
Magma…the building block of igneous rocks
magma contains mainly the elements silicon and oxygen, plus
aluminum, iron, calcium, sodium, potassium, and magnesium.
b.
Also contains some gasses including water vapor.
c.
These gases are kept within the magma by the pressure of the
surrounding rocks.
d.
These gases make magma less dense than the surrounding rocks,
causing it to slowly work its way to the surface.
e.
As magma rises it cools, allowing the elements to combine and form
minerals.
f.
Gradually, the minerals grow in size, forming a solid mass of interlocking
crystals.
Extrusive igneous rocks – rocks that form from hardened lava (they are extruded onto
the surface)
i.
II.
a.
When magma reaches Earth’s surface it becomes lava. Extrusive igneous rocks
are formed from this lava.
Classification of Igneous Rocks – texture and composition are two characteristics used to
classify igneous rocks
A.
B.
Coarse-Grained Texture
i.
Slow cooling results in the formation of large crystals.
ii.
Large crystals exhibit a coarse-grained texture
Fine-Grained Texture
i.
Rapid coolingresults in the formation of small crystals.
ii.
C.
D.
E.
Glassy Texture
i.
result of lava spewing onto Earth’s surface
ii.
Ions in the lava do not have the time to arrange themselves into a network of
crystals, so the solids produced this way are made of randomly distributed ions.
Porphyritic Texture
i.
It is possible for some crystals to become quite large before others even start to
form.
ii.
The resulting rock can have large crystals surrounded by fine-grained minerals
(small crystals)
Granitic Compostion
i.
F.
Those that are made almost entirely of the light-colored silicate minerals quartz
and feldspar.
Basaltic Composition
i.
G.
Small crystals exhibit a fine-grained texture
Rocks that contain many dark silicate minerals and plagioclase feldspare.
Other Compositional Groups
i.
Andesitic composition - rocks with a composition between granitic and basaltic
ii.
ultramafic – composed almost entirely of dark silicate minerals