Introduction to Igneous Rock Types and Formation

Igneous Rocks

Formed from cooling and crystallization of molten rock.

Plutonic Rocks

Intrusive rocks formed below Earth's surface.

Volcanic Rocks

Extrusive rocks formed on or near Earth's surface.

Magma Components

Includes melt, solids, and volatiles.

Cooling Rate

Influences crystal size in igneous rocks.

Coarse-Grained Texture

Larger crystals formed by slow cooling.

Fine-Grained Texture

Smaller crystals formed by rapid cooling.

Glassy Texture

Very rapid cooling produces no crystals.

Dark Silicates, lower % of SiO2

Mafic minerals are rich in magnesium and iron—lower K feldspar. More calcic plagioclase.

Light Silicates, greater % of SiO2

Felsic minerals are high in silica and potassium. More sodic plagioclasese.

SiO₂ Content

High SiO₂ indicates felsic minerals.

Geothermal Gradient

Temperature increases ~25°C per km depth.

Decompression Melting

Melting due to pressure reduction during ascent.

Volatiles in Melting

Water addition lowers melting temperature.

Bowen's Reaction Series

Minerals crystallize based on melting temperatures.

Magmatic Differentiation

Evolving magma composition as minerals crystallize.

Basaltic Magma

Common in oceanic settings like mid-ocean ridges.

Andesitic Magma

Typical in continental settings, intermediate composition.

Granite

Felsic rock, coarse-grained, high SiO₂ content.

Basalt

Mafic rock, fine-grained, low SiO₂ content.

Obsidian

Glassy igneous rock formed from very rapid cooling.

Phaneritic

course grained, visible minerals, intrusive.

Aphanitic

fine-grained, no naked eye crystals, extrusive.

Felsic

light silicates, non-ferromagnesian.

Mafic

Dark silicates, ferromagnesian

Upper athenosphere

rocks are close to melting

adiabatic cooling

process of reducing heat through change in air pressure caused by volume expantion.

Partial Melting

T and which rocks begin to melt.

Adiabatic rise

no conductive heat loss

Volatiles

dissolved gas in the melt