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 a
great number of years to move (plastic.)
• Decreased pressure and the addition of
water lower the melting temperature of
mantle rock so that it melts.
13.2 Crystals in igneous rocks
•
Crystallization is the process by which
crystals form and grow in size. Minerals in
magma or lava form crystals as the melted
rock cools.
13.2 Comparing igneous rocks
•
•
Crystal size can tell us a lot
about how a rock formed.
Larger crystals take longer to
form, so rock with larger
crystals cooled slowly.
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 has large
crystals because it
cools slowly beneath
the surface of the
earth.
• Obsidian is so smooth
that it is called volcanic
glass. It contains
almost no crystals
because it cools very
quickly on the surface
of the earth.
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?