Geology 1010: Igneous Processes & Volcanoes

Igneous Rock

Formed when liquid rock freezes into a solid rock.

Extrusive Rock

Igneous rock cooling and forming outside of the Earth.

Intrusive Rock

Igneous rock cooling and forming inside of the Earth.

Vesicular Rock

An extrusive rock filled with small bubble structures, frozen in place as gases escaped from cooling lava.

Volcanism

The process in which lava is erupted.

Phaneritic Texture

Large, easy-to-see crystals, common in intrusive rocks.

Aphanitic Texture

Small, microscopic, hard-to-see crystals, common in extrusive rocks.

Extrusive lava that cools so rapidly it doesn't develop crystals.

Porphyritic Texture

Texture with a mix of coarse-grained minerals surrounded by a matrix of fine-grained material.

Phenocrysts

A large crystal within an igneous rock.

Groundmass

The fine-grained, not-discernible part of an igneous rock.

Pegmatite

A rock with unusually large crystals, typically forming in veins as the last dredges of magma crystallize.

Volatiles

Components of magma which are dissolved until it reaches the surface.

Pumice

Low-density, highly vesiculated volcanic rock typically arising from felsic volcanism.

Obsidian

A rock consisting of volcanic glass.

Conchoidal Fracture

Fractures that have a circular appearance.

Tephra

Solid, fragmented material erupted from a volcano.

Pyroclastic Rocks

Rocks formed from explosive volcanism.

Tuff

Rocks made from pyroclastic tephra.

Felsic Composition

Light-colored silicate minerals, enriched in silicon, oxygen, sodium, potassium.

Intermediate Composition

Gray in color, containing equal amounts of light and dark minerals.

Mafic Composition

Darker in color, containing more dark minerals and some plagioclase feldspar.

Ultramafic Composition

Extremely low silica composition composed almost entirely of olivine and pyroxene.

Dike

A narrow igneous intrusion that cuts through existing rock.

Sill

A type of dike that is parallel to bedding planes within the bedrock.

Magma Chamber

A large underground reservoir of molten rock.

Diapir

A ductile material that moves toward the surface.

Stoping

The process of surrounding bedrock being broken off and passed through magma.

Batholith

A large mass or chain of many plutons or intrusive rocks.

Stock

Type of pluton with less surface exposure than a batholith.

Bowen's Reaction Series

Describes the temperature at which minerals crystallize when cooled.

Geothermal Gradient

The rate at which the temperature increases with depth.

Partial Melting

Where some minerals are melted and some remain solid.

Decompression Melting

Melting that occurs as material moves upward and pressure is released.

Flux Melting

Process in which volatiles enter the mantle wedge, lowering the melting temperature.

Heat-Induced Melting

Transforming solid mantle into liquid magma by applying heat.

Magmatic Differentiation

The process of changing a magma's composition.

Xenoliths

Unmelted country rocks within an igneous rock mass.

Pillow Basalts

Pillow-shaped structures formed by basaltic lava erupting underwater.

Cinder Cone

Small volcanoes made of pyroclastic fragments ejected from a central vent.

Stratovolcano

Steep flanked, symmetrical cone shape volcano typically with felsic to intermediate magma.

Caldera

Steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice.

Pyroclastic Flow

A mix of lava blocks, pumice, ash, and hot gases racing down steep flanks.

Tephra

Ejected rock materials that pose danger during volcanic eruptions.

Lahar

Volcanic mudflow that forms from rapidly melting snow or glaciers.