Igneous Rocks and Volcanic Features: Key Concepts and Classifications

Igneous rock

Rock formed by the cooling and solidification of molten material (magma or lava).

Rock cycle

The continuous process in which rocks are formed, broken down, and transformed between igneous, sedimentary, and metamorphic forms.

Magma

Molten rock beneath the Earth's surface that contains gases and crystals.

Lava

Magma that has reached the Earth's surface through volcanic eruptions.

Xenolith

A fragment of foreign rock trapped within an igneous body during its formation.

Intrusive rock

Igneous rock formed when magma cools slowly beneath the Earth's surface (e.g. granite).

Extrusive rock

Igneous rock formed when lava cools quickly on the surface (e.g. basalt).

Crystal size categories

Classifications based on grain size; large crystals form from slow cooling, small from fast cooling.

Euhedral

Crystals with well-formed, perfect faces.

Subhedral

Crystals that are partially well-formed with some imperfect faces.

Anhedral

Crystals with no well-formed faces; irregular shape.

Equigranular

Rock with crystals of roughly equal size.

Porphyritic

Texture with large crystals (phenocrysts) embedded in a fine-grained matrix.

Aphanitic

Fine-grained texture; crystals too small to see with the naked eye.

Phaneritic

Coarse-grained texture; crystals visible to the naked eye.

Vesicular

Texture with many gas bubbles (vesicles), typical of volcanic rocks.

Felsic

Silica-rich igneous rocks, light in color, high in quartz and feldspar (e.g. rhyolite, granite).

Intermediate

Rocks with moderate silica content, between felsic and mafic (e.g. andesite, diorite).

Mafic

Silica-poor, iron- and magnesium-rich rocks, dark in color (e.g. basalt, gabbro).

Ultramafic

Very low silica, very high in magnesium and iron (e.g. peridotite).

Leucocratic

Light-colored, high in felsic minerals.

Mesocratic

Intermediate color and mineral composition.

Melanocratic

Dark-colored, rich in mafic minerals.

Streckeisen diagram

A ternary diagram used to classify plutonic (intrusive) igneous rocks based on mineral composition.

Ternary diagram

A triangular graph showing the proportions of three components.

Bowen's Reaction Series

A model describing the sequence in which minerals crystallize from cooling magma.

Making magmas

Processes that generate magma, including melting due to heat, pressure decrease, or addition of volatiles.

Solidus

The temperature below which a rock is completely solid.

Liquidus

The temperature above which a rock is completely molten.

Partial melting

When only part of a rock melts, producing magma different in composition from the original rock.

Fractional crystallization

Sequential crystallization and removal of minerals from magma, changing its composition.

Magma mixing

The blending of two or more magmas with different compositions.

Assimilation

When magma melts and incorporates surrounding rock material.

Viscosity

The resistance of magma to flow; higher silica = higher viscosity.

Pluton

A large intrusive igneous body that crystallized below the surface.

Sills

Horizontal sheet-like intrusions that form parallel to rock layers.

*****

Vertical or steeply inclined intrusions that cut across rock layers.

Shield volcano

Broad, gently sloping volcano built from low-viscosity lava flows (e.g. Mauna Loa).

Columnar jointing

Polygonal columns formed when thick lava flows or sills cool and contract.

Stratovolcano (composite volcano)

Steep-sided volcano built from alternating layers of lava and ash.

Lava dome

A mound formed by viscous lava piling up near the vent.

Flow banding

Layered textures in volcanic rocks caused by flow of viscous lava.

Caldera

A large, basin-like depression formed when a volcano collapses after an eruption.

Pyroclastic flow

A fast-moving, deadly flow of hot gas, ash, and volcanic debris.

Tephra

Fragmented volcanic material ejected during an eruption, including ash, lapilli, and bombs.

Ignimbrite

Rock formed from the deposition and welding of pyroclastic flow material.

Tuff

Rock formed from compacted volcanic ash.