Igneous Rock Notes

• Igneous rock forms when magma cools and solidifies.

Formation

• Igneous rocks are classified by texture and composition.

• Texture• The sizes, shapes, and positions of the grains that a rock

is made of

• Composition– The minerals or elements that make up a rock

Classification

• Composition• Felsic or Mafic

• Texture– Fine-grained (extrusive)

or coarse-grained (intrusive)

Classification

General Composition

–The types of minerals found in an igneous rock (the composition) impact the color and density of the rock.

Felsic Composition

• Felsic – light colored, less dense– Rich in aluminum,

potassium, silicon, and sodium.

Granite

Rhyolite

Gabbro

Basalt

Mafic Composition

• Mafic – dark colored, more dense– Rich in calcium, iron, and

magnesium

Composition

Intrusive Texture• Intrusive Igneous Rock– Form inside Earth’s crust

• Cool slowly– Coarse-grained (large particles)

• Intrusive Igneous Rock Formations– Batholiths: The largest of all igneous

intrusions– Stocks: Intrusive bodies that are

exposed over smaller areas than batholiths

– Dikes: Sheet-like intrusions that cut across previous rock units

– Laccolith: Sheet-like intrusions that run parallel to previous rock units

– Volcanic neck: A column of igneous rock formed when magma hardens with in a vent

Intrusive Texture

Extrusive Texture• Extrusive Igneous Rock– Form on Earth’s surface

• Cool quickly– Fine-grained (small particles)

• Extrusive Igneous Rock Formations– Lava Flow: Formed when lava

erupts from a volcano– Fissure: Long cracks in Earth’s

crust where lava can flow

Extrusive Texture

Igneous Rock Flow Chart

rock melts

rock cools

igneous rock

pressure

temperature

compositionSlow cooling (coarse-grained)

Fast cooling (fine-grained)