Igneous Rocks

Igneous Rock

Rock formed by the cooling and solidification of molten rock material.

Magma

Molten rock beneath Earth's surface; forms intrusive (plutonic) rocks.

Lava

Molten rock on Earth's surface; forms extrusive (volcanic) rocks.

Intrusive (Plutonic) Rock

Igneous rock formed below Earth's surface from slowly cooling magma.

Extrusive (Volcanic) Rock

Igneous rock formed at Earth's surface from quickly cooling lava.

Bowen's Reaction Series

The sequence in which minerals crystallize from cooling magma.

Discontinuous Series

Branch of Bowen's series where minerals change in distinct steps (olivine → pyroxene → amphibole → biotite).

Continuous Series

Branch of Bowen's series where plagioclase feldspar changes gradually from calcium-rich to sodium-rich.

First Mineral to Crystallize

Olivine (forms at highest temperatures).

High-Temperature Minerals

Olivine, pyroxene, calcium-rich plagioclase (rich in Fe and Mg).

Low-Temperature Minerals

Quartz, muscovite, potassium feldspar (orthoclase).

Mineral Composition Trend During Cooling

Early minerals are rich in Fe and Mg; later minerals contain more Na, K, and silica.

Mafic

Igneous rocks rich in iron (Fe) and magnesium (Mg); dark colored (example: basalt).

Felsic

Igneous rocks low in Fe and Mg, high in silica; light colored (example: granite).

Phaneritic Texture

Coarse-grained texture with large visible crystals; forms from slow cooling (intrusive).

Aphanitic Texture

Fine-grained texture with microscopic crystals; forms from rapid cooling (extrusive).

Porphyritic Texture

Two-stage cooling; large crystals (phenocrysts) embedded in fine-grained groundmass.

Phenocryst

Large crystal formed during early slow cooling stage of magma.

Groundmass

Fine-grained material surrounding phenocrysts in porphyritic rocks.

Glassy Texture

No crystals; forms when lava cools extremely quickly (example: obsidian).

Vesicular Texture

Texture containing gas bubbles (vesicles) formed by escaping gases (example: pumice or scoria).

Pyroclastic Texture

Texture formed from volcanic ash and rock fragments (example: tuff).

Why Intrusive Rocks Have Large Crystals

Slow cooling allows crystals time to grow.

Why Extrusive Rocks Have Small Crystals

Rapid cooling prevents large crystal growth.

How Igneous Rocks Are Classified

By texture (cooling history) and composition (mineral content).