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- Igneous Rocks form from magma
found deep under Earth’s surface.
- Igneous means “fire formed”
Igneous
 means
fire formed!
Video Intro to
Igneous Rocks
Magma reaching the surface flows
from a volcano as lava.
Magma trapped below the surface forms
large-grained intrusive igneous rock when it
cools. (It takes longer for magma to cool
beneath Earth’s surface, thus the mineral
grains are large).
Magma cooling at or near Earth’s surface
forms small-grained extrusive igneous
rock. (This magma cools quicker and
thus the mineral grains are smaller)
Pumice, obsidian, and scoria are
examples of volcanic glass.
These rocks cooled so quickly that few or no
mineral grains formed.
Sometimes gases are trapped in the molten
material as it cools. The gases eventually
escape and leave holes in the rock. (Pumis)
-Igneous rocks are classified in two ways.

1. Where they formed
– intrusive (under the Earth’s surface)
– Extrusive ( on the surface)
– Crystal size, large or small, can help
identify an igneous rock as intrusive or
extrusive.
Magma type: basaltic, granitic, or
andesitic
-#2 Three main types of igneous
rock
Basaltic igneous rocks are dark-colored
and dense.
Contain iron and magnesium but very
little silica
Basaltic lava flows freely from a
volcano.
How does this explain the black sand
beaches in Hawaii?

Granitic igneous rocks are lower density
and lighter color
– Contain more silica and less iron and
magnesium
– Granitic magma is thick and stiff and
contains lots of silica but lesser
amounts of iron and magnesium.
Andesitic rocks have a mineral
composition between those of
basaltic and granitic rocks
Basaltic, Intrusive: Gabbro
Andesitic, Intrusive: Diorite
Granitic, Intrusive: Granite
Basaltic, Extrusive: Basalt,
Andesitic, Extrusive: Andesite
Granitic, Extrusive: Rhyolite,
Pumice, Obsidian
Igneous rocks can be identified by:
1. texture= mineral crystal size
(coarse, fine, glassy)
2. mineral content = color
dark = lots of Mg and Fe
light or pale = lots of quartz and feldspar
(silicates)
Igneous rock uses: all hard: can
take lots of abuse
obsidian: arrowheads, knife blades
and scalpels, jewelry
pumice: in toothpaste, ‘lava’ soap
granite: buildings (Washington
Monumument), tombstones