Igneous Rocks

Magma

The molten, parent material of igneous rocks.

Magma Composition

Magma is composed of three parts: liquid (melt), solids (crystallized silicate minerals), and volatiles (gaseous portion like H2O, CO2, and SO2).

Geothermal Gradient

The rate of change of temperature, per unit depth in the Earth.

Decompression Melting

Occurs when there is a decrease in confining pressure, leading to a decrease in melting temperature.

Volatiles

Gaseous components in magma that lower the melting temperature of mantle rock.

Bowen's Reaction Series

A model sequence showing the order that minerals crystallize from magma.

Partial Melting

A process where a rock does not completely melt; the liquid portion consists largely of melted minerals at the lower end of Bowen's Reaction Series.

Extrusive Rocks

Rocks formed from magma that erupts through the Earth’s surface.

Intrusive Rocks

Rocks formed from magma that solidifies before reaching the Earth’s surface.

Types of Magma

Four basic types: Ultramafic, Mafic, Intermediate (Andesitic), and Felsic.

Ferromagnesian Minerals

Typically dark colored, rich in iron and/or magnesium, examples include Olivine and Pyroxene.

Non-ferromagnesian Minerals

Typically light colored, contain more potassium, sodium, and calcium than iron or magnesium, examples include Quartz and Feldspar.

Aphanitic

A fine-grained texture often seen in extrusive igneous rocks.

Phaneritic

A coarse-grained texture common to intrusive igneous rocks.

Tuff

A general term for all consolidated pyroclastic rocks, typically composed of ash-size fragments.

Pyroclastic Volcanism

Explosive volcanic situations caused by gas buildup inside a volcano, characteristic of high SiO2 content magma.

Gabbro

A dark colored intrusive rock typically composed of plagioclase feldspar and augite.

Basalt

An extrusive compositional equivalent of gabbro, typically dark colored and rich in magnesium and iron.

Rhyolitic Magma

Felsic (Si-rich) magma typically erupted along continental margins.

Andesitic Magma

Intermediate magma that forms as rising basaltic magma ponds beneath Si-rich crustal rocks.