Name _____________________ Class _________ Date _________ Period _________
How are igneous rocks made?
Igneous rocks are made from either lava or magma. Lava is molten rock that is found near or
above Earth’s surface. Rocks that form when this lava cools are called extrusive (exit/outside) or
volcanic rocks. Extrusive igneous rocks form from lava that is near or on the surface where the
temperature is much cooler than deep underground. This causes the rock to cool quickly, NOT
allowing a lot of time for the mineral crystals to grow. Extrusive igneous rocks have small or not
mineral crystals.
Magma is molten rock that is found deep below Earth’s surface. Rocks that form when this
magma cools are called intrusive (inside) or plutonic rocks. Intrusive igneous rocks form magma
deep underground where the temperature and pressure are much greater than on the surface.
This causes the rock to cool slowly, allowing a lot of time for the mineral crystals to grow.
Intrusive igneous rocks have large mineral crystals.
Igneous rocks that have high concentrations in aluminum or silicon are called felsic rocks. Felsic
rocks typically have a lower density and lighter color than mafic rocks. Igneous rocks that have
high concentrations in iron or magnesium are called mafic rocks. Mafic rocks typically have a
higher density and darker color than felsic rocks.
Rhyolite and granite are both made from the minerals plagioclase feldspar, quartz, amphibole,
biotite, and potassium feldspar. Even though they are made from the same minerals, they are
classified as different rocks because of the differences in their crystal size. Granite is an
intrusive igneous rock which cooled slowly deep underground. This allowed granite the time
needed for large crystals to form. Rhyolite is an extrusive igneous rock which cooled quickly
near or on Earth’s surface. This did NOT allow rhyolite the time needed, so it formed little to no
visible mineral crystals.