Download Section 13.2

The Changing Earth
Chapter Thirteen: Formation of
Rocks
• 13.1 The Composition of Rocks
• 13.2 Igneous Rocks
• 13.3 How Rocks Change
Investigation 13B
Igneous rocks
• How are igneous rocks classified?
13.2 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.
13.2 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.
13.2 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.
13.2 Comparing igneous rocks
• Granite makes up
continental plates.
• It is less dense than
basalt, is made of highsilica 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.
13.2 Comparing igneous rocks
• An igneous rock that
forms above Earth’s
surface is called an
extrusive rock.
• An igneous rock that
forms within Earth’s
crust is called an
intrusive rock.
Which rocks are more likely
to have large crystals?