Igneous Rocks and Magmatic Processes – Vocabulary Flashcards

Key igneous concepts, rocks, textures, and tectonic settings based on the lecture notes.

Magma

Molten rock beneath Earth's surface formed by partial melting of rocks in the crust or upper mantle.

Partial Melting

Melting of only part of a rock, producing magma that is more silica-rich than the solid residue.

Decompression Melting (Pressure Release),

Melting caused by a drop in pressure as rock ascends, with little or no change in temperature.

Heat Transfer (Magmatic Context)

Rising magma transfers mantle heat to surrounding rock, raising its temperature and causing melting.

Volatile Addition

Introduction of volatiles (e.g., H2O, CO2) that lower the melting temperature of rocks.

Volatiles (H2O, CO2)

Gases released or added to rock that promote melting at lower temperatures.

Assimilation

Magma melts and incorporates surrounding country rock, altering its composition.

Magma Mixing

Two or more magmas blend in a magma chamber to form a mixed magma.

Plutonic

Rocks that crystallize below the surface (intrusive) in large bodies.

Intrusive

Rocks formed by crystallization of magma below the surface.

Extrusive

Rocks formed by crystallization of lava at or above the surface.

Dike

A discordant, vertical intrusion that cuts across rock layers.

Sill

A concordant, horizontal intrusion that pushes between rock layers.

Plutons

Large, deep igneous bodies; can coalesce into a batholith.

Batholith

A very large plutonic body that forms when several plutons merge.

Texture

The size, shape, and arrangement of minerals in a rock.

Crystalline

Texture composed of interlocking mineral crystals.

Phaneritic

Coarse-grained, intrusive texture where crystals are large enough to be seen.

Aphanitic

Fine-grained, extrusive texture with small crystals.

Porphyritic

Texture with large crystals (phenocrysts) in a finer-grained groundmass.

Glassy

Texture formed by very rapid cooling, producing volcanic glass (e.g., obsidian).

Fragmental (Pyroclastic)

Rocks composed of erupted fragments (ash, pumice, tuff, breccia).

Rhyolite

Felsic volcanic rock; fine-grained to glassy; high silica content.

Andesite

Intermediate volcanic rock; moderate silica content.

Basalt

Mafic volcanic rock; relatively low silica and high iron/magnesium.

Granite

Coarse-grained felsic intrusive rock; high silica and light-colored minerals.

Diorite

Coarse-grained intermediate intrusive rock.

Gabbro

Coarse-grained mafic intrusive rock; basaltic composition.

Peridotite

Coarse-grained ultramafic mantle rock rich in olivine and pyroxene.

Komatiite

Ultramafic, high-temperature extrusive rock (rare in modern geology).

Olivine, Pyroxene, Amphibole, Mica, Quartz

Common silicate minerals in igneous rocks; olivine/pyroxene in mafic; quartz/mica in felsic rocks.

Felsic

Silica-rich igneous rocks with light-colored minerals; high viscosity.

Intermediate

Igneous rocks with silica content between felsic and mafic.

Mafic

Silica-moderate rocks with higher Fe and Mg; lower viscosity than felsic.

Ultramafic

Very high Mg+Fe content; lowest silica; mantle-derived compositions.

SiO2 Percent Ranges

Felsic 66–76%, Intermediate 52–66%, Mafic 45–52%, Ultramafic 38–45%.

Bowen’s Reaction Series

Sequence of mineral crystallization during magma cooling from ultramafic to felsic.

Olivine, Pyroxene, Amphibole, Mica, Quartz (Crystallization Order)

Minerals that crystallize at different temperatures; olivine first, quartz last in Bowen’s series.

Mantle Source vs Crustal Source

Mantle melts tend to be ultramafic/mafic; crustal melts yield mafic→felsic magmas depending on crustal composition.

Tuff

Volcanic ash that has fallen and solidified.

Volcanic Breccia

Coarse pyroclastic rock composed of larger volcanic fragments.

Obsidian

Glassy volcanic rock formed by rapid cooling of lava.

Pumice

Frothy, vesicular volcanic glass; floats in water.

Scoria

Vesicular mafic rock with abundant vesicles.

Hot Spots

Decompression melting under oceanic or continental crust, producing often mafic to intermediate rocks.

Subduction Zone

Region where one plate sinks beneath another; volatile addition triggers melting.

Mid-Ocean Ridge

Divergent plate boundary where decompression melting forms new oceanic crust (mafic).

Continental Rift

Region where continents are pulling apart; decompression melting yields mafic to intermediate rocks.