Chapter 4: Igneous Rocks and Intrusive Activity - Vocabulary Flashcards

A set of vocabulary-focused flashcards covering textures, rock types, processes, and key terms from Chapter 4 on Igneous Rocks and Intrusive Activity.

Glassy texture

A texture formed by very rapid cooling of lava, producing an amorphous, glassy solid with no visible crystal structure (obsidian is a natural glass example).

Vesicular texture

An extrusive rock texture with numerous voids (vesicles) left by escaping gas bubbles as lava solidifies (pumice is a frothy example).

Porphyritic texture

Texture with two distinct crystal sizes: large crystals (phenocrysts) embedded in a finer groundmass, indicating two-stage cooling.

Phaneritic texture

Coarse-grained texture with large mineral grains visible to the naked eye, formed by slow cooling underground.

Aphanitic texture

Fine-grained texture with crystals too small to identify without a microscope, formed by rapid cooling at/near the surface.

Pyroclastic texture

Texture produced by the consolidation of volcanic fragments (ash, pumice, blocks) ejected during explosive eruptions.

Obsidian

Dark, glassy volcanic rock formed when high-silica lava cools rapidly at the surface.

Pumice

Glassy, vesicular rock formed from gas-rich lava; highly porous and often floats in water.

Magma

Molten rock stored beneath Earth’s surface; originates from partial melting and crystallizes to form igneous rocks.

Lava

Magma that erupts onto the surface of the Earth.

Extrusive igneous rock

Rocks formed from the rapid cooling of lava at or near the surface; often fine-grained, glassy, or pyroclastic.

Intrusive igneous rock

Rocks formed when magma crystallizes below the surface, typically coarse-grained.

Pluton

A general intrusive igneous body formed when magma solidifies underground.

Dike

A tabular, discordant intrusion that cuts across rock layers, injected into cracks.

Sill

A tabular, concordant intrusion that parallelly intrudes along bedding planes.

Batholith

A very large intrusive body, often hundreds of kilometers long, with surface exposure >100 km2; typically felsic to intermediate.

Stock

A smaller intrusive body similar to a batholith but smaller in size.

Laccolith

A mushroom-shaped or lens-like intrusion that arches overlying strata and inflates the rock above.

Pegmatitic texture

Exceptionally coarse-grained texture formed during late-stage crystallization; rocks are pegmatites.

Granite

Granitic, coarse-grained felsic igneous rock with ~10–20% quartz, ~50% K-feldspar, and minor dark silicates.

Rhyolite

Extrusive, fine-grained (aphanitic) equivalent of granite; light-colored silicates; buff to pink/gray.

Andesite

Andesitic (intermediate) rock; medium-gray, fine-grained, often porphyritic; volcanic.

Diorite

Intrusive intermediate rock; coarse-grained; resembles gray granite but lacks visible quartz.

Basalt

Basaltic, fine-grained mafic rock; dark color; mainly pyroxene and calcium-rich plagioclase; common extrusive.

Gabbro

Intrusive mafic rock; coarse-grained; similar composition to basalt; common in oceanic crust.

Peridotite

Ultramafic mantle rock dominated by olivine and pyroxene; major constituent of the upper mantle.

Ultramafic

Rocks dominated by ferromagnesian minerals (high Mg and Fe; low silica).

Felsic

Rocks rich in light-colored silicates (quartz, potassium feldspar) with high silica content; granitic in composition.

Mafic

Rocks rich in dark ferromagnesian silicates (Fe, Mg); higher density; basaltic/gabbroic in composition.

Intermediate

Igneous rocks with composition between felsic and mafic, often 25% or more dark silicates.

Ultramafic

Very low silica rocks dominated by olivine and pyroxene, e.g., peridotite.

Silica content

Percentage of SiO2 in a rock; controls magma viscosity and affects the classification (felsic > intermediate > mafic > ultramafic).

Magmatic differentiation

Process by which a single parent magma evolves into different compositions via crystallization, settling, assimilation, and mixing.

Crystal settling

Early-formed, denser crystals sink, changing the composition of the remaining melt.

Assimilation

Magma incorporates surrounding rock, melts it, and changes the magma’s composition.

Magma mixing

Two chemically different magmas blend, producing intermediate compositions.

Partial melting

Incomplete melting of rocks; the melt is enriched in minerals with the lowest melting temperatures.

Decompression melting

Melting caused by a drop in pressure as rocks rise, common at divergent boundaries and hotspots.

Addition of water

Volatiles (especially H2O) lower melting temperatures, promoting melting, especially at subduction zones.

Primary (primitive) magma

Magma formed by partial melting that hasn’t undergone significant evolution.

Tuff

Most common pyroclastic rock; cemented ash-sized fragments.

Welded tuff

Ash particles fused together while still hot during deposition.

Volcanic breccia

Pyroclastic rock composed of larger angular fragments cemented together.

Stoping

Process during pluton emplacement where blocks of country rock break off and sink into magma.

Xenolith

Foreign rock fragment enveloped within a pluton, providing evidence of stoping and assimilation.