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!)

Unakite

Granite

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)

TexturesGlassy

Aphanitic

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 mid-Atlantic 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