Igneous Rocks
• Igneous rock begins as magma (mixture of
many minerals)
• Magma can form:
• When rock is heated
• When pressure is released
• When rock changes composition
• Magma freezes between
700 °C and 1,250 °C
http://www.fi.edu/fellows/payton/rocks/create/igneous.htm
Igneous Rocks
• Igneous rocks begin to form when rock melts
in Earth’s mantle.
• A good way to describe hot, solid mantle rock
is that it is like stiff putty that takes millions of
years to move.
• Decreased pressure and the addition of water
lower the melting temperature of mantle rock
so that it melts.
Crystals in igneous rocks
•
•
Minerals in magma or lava form crystals as the
melted rock cools.
Crystallization is the process by which crystals
form and grow in size.
Comparing igneous rocks
• Crystal size can tell us a lot
about how a rock formed.
• Basalt and gabbro are made
from the same low silica
magma.
• Basalt has crystals too small
to see, and gabbro has large
crystals.
Comparing igneous rocks
• Extrusive Rocks: forms
when magma erupts onto
the Earth’s surface (lava),
cools quickly with very
small or no crystals
formed
• An igneous rock that
forms within Earth’s crust
is called an intrusive rock,
magma pushes into
surrounding rock below
the Earth’s surface
Which rocks are more likely to
have large crystals?
Igneous Rocks
• Felsic: light colored rocks that are rich in
elements such as aluminum, potassium,
silicon, and sodium
• Mafic: dark colored rocks that are rich in
calcium, iron, and magnesium, poor in silicon
• Coarse-grained: takes longer to cool, giving
mineral crystals more time to grow
• Fine-grained: cools quickly with little to no
crystals
Igneous Rocks
Coarse-Grained
Fine-Grained
Felsic
Granite
Rhyolite
Mafic
Gabbro
Basalt
Comparing igneous rocks
• Granite makes up continental
plates.
• It is less dense than basalt, is
made of high-silica magma,
and has large crystals.
• Obsidian is so smooth that it
is called volcanic glass.
• Obsidian contains almost no
crystals because it cools very
quickly.
Igneous Rocks
Obsidian is a dark-colored volcanic glass that forms from the very rapid
cooling of molten rock material. It cools so rapidly that crystals do not form.
Is this rock Felsic or
Mafic?
Is it fine-grained or
coarse-grained?
Is this rock Intrusive or
Extrusive?
Mafic, fine grained, extrusive