Igneous Rocks
“Liquid Hot Magma!”
Igneous Rocks
• Rocks formed from cooling of lava or magma
• Lava-Melted rock erupted from volcanoes and
deposited on the earth’s surface. Cools quickly.
• Magma-Melted rock that sits inside the earth in a
magma chamber. Cools slowly.
Magma Chamber
Types of Igneous Rocks
• 2 major types: Intrusive and Extrusive
• Intrusive igneous rocks cool and harden inside
the earth.
• Extrusive igneous rocks cool and harden on
the earth’s surface or outside the earth.
Intrusive Igneous Rocks
• Form when magma cools
before reaching the
surface.
– Cool very slowly
– Intrusive igneous rocks
have large, interlocking
crystals because they
cooled slowly inside the
earth.
– Examples are Granite,
Gabbro, Diorite, and
Unakite (Virginia’s State
Rock!)
Granite
Unakite
Extrusive Igneous Rocks
• Form when lava erupts and
hardens on the surface.
– Cool very quickly
– Extrusive igneous rocks have
fine (small) to no crystal
grains at all.
– A rock with no visible grains is
called a volcanic glass
(examples are pumice and
obsidian).
– Examples are Obsidian,
Pumice, Scoria, Basalt, Tuff,
Porphyry (cools inside then
erupts), Rhyolite, Andesite.
Rhyolite
Obsidian
Texture of Igneous Rocks
• Grain Size (Glassy, Fine, Medium, Coarse)
• Grain Shape (Irregular, Angular)
• Sorting:
– Glassy: Looks like glass (Obsidian, Pumice)
– Aphanitic: Can’t see crystals (Scoria, Tuff)
– Porphyritic: Large crystals in fine matrix
(Porphyry)
– Phaneritic: Medium to Large interlocking crystals
(Granite, Diorite, Gabbro)
Glassy
Aphanitic
Textures
Phaneritic
Porphyritic
Vesicles
• Vesicles are holes found ONLY in Igneous
rocks.
• Formed from trapped gasses when lava erupts
• Examples: Scoria, Pumice, Basalt (sometimes)
Tectonic Plate Boundaries
• Places where tectonic plates converge,
diverge, or slip past each other.
Igneous Rocks form at Convergent
Boundaries
• Ocean-Continent
Convergent Boundary.
• Forms volcanoes like
those in Washington
State.
Igneous Rocks form at Divergent
Boundaries
• Divergent Boundary or
Spreading Ridge
• Forms volcanoes like
those that make up
Iceland and the midAtlantic Ridge.
Mid-Atlantic Ridge
Igneous Rocks Form at Hot Spots
• A hot spot is where
magma rises to the
surface in the middle of
a plate and not a plate
boundary.
• Forms volcanoes like
those in Hawaii or
Yellowstone.
Hot Spots
Hawaii
Yellowstone