Igneous Rocks

Rock vs. Mineral

• Rock- a naturally formed consolidated material composed of grains of one or more minerals.

• Mineral- a naturally occurring element or compound with a relatively constant chemical and structural composition.

Three Rock Types1. Igneous: Rock formed from the cooling of

magma

2. Sedimentary: Rock formed from the accumulation and lithification (cementing) of weathered material.

3. Metamorphic: Rocks formed by the altering existing rock by heat and/or pressure.

The Rock Cycle

How do igneous rocks form?

IntrusiveExtrusive

Identification of Igneous Rocks

Texture: the size of the grains that make up the rock

Mineralogy: the minerals present in the rock

Textures:

Phaneritic (Fan-er-itic): all large grains Aphanitic (A-fan-itic): all small grains

Porphyritic (Poor-for-itic): large crystals surrounded by fine-grained matrix

Terms to explain to chemistry of igneous rocks:

• Mafic: (May-fik) Silica deficient igneous rocks with a high content of Magnesium, Iron, and Calcium. More dark minerals than light minerals.

• Felsic: (fell-sick) Silica rich igneous rock with a high percentage of potassium and sodium. Usually lots of quartz and feldspar.

Mineral content of Igneous Rocks

Intrusive Structures: Rock formations caused by the cooling of magma beneath the Earth’s Surface.

Shiprock, New Mexico

Formed when a volcano goes extinct (loses its heat source). The magma that was inside the volcano cools and hardens into a rock much harder than that on the surface.

Eventually the volcano is weathered away, exposing the rock.

Devil’s Tower, WY

Dikes: Magma from below is pushed up into cracks in the existing rock.

Dikes from Intrusion

Magma Formation• molten rock created near the subduction zone.

• granitic magma ~ 650o C

• geothermal gradient – 3o C/100 m

• melting point increases with pressure; pressure increases with depth.

• mineral melting point is lowered by water under pressure.

Bowen’s Reaction Series• sequence in which minerals crystallize in

cooling magma.

Volcanism and Extrusive Rocks

• pyroclastic (pyro = “fire”; clast = “broken”)• pyroclastic flow- mixture of gas and pyroclastic

debris.

• caldera-• pumice-• vesicles-• cinder cone-

Extrusive Rockspyroclastic flow(click the picture below for a short video pumice with vesicles

Pyroclastic Flows

volcanos

cinder cone caldera