Chapter 1-8: Igneous Rocks and Melting Concepts (Vocabulary)

Vocabulary flashcards covering igneous rock textures, compositions, melting processes, and plate-tectonics concepts from the notes.

Igneous rocks

Rocks formed by the cooling and solidification of molten rock (magma); can crystallize underground (intrusive) or at/near the surface (extrusive).

Intrusive

Igneous rocks that crystallize below the surface, cooling slowly to form large mineral grains (coarse-grained).

Extrusive

Igneous rocks that erupt or cool at/near the surface, cooling quickly to form small mineral grains (fine-grained) or glass.

Texture

The size and arrangement of mineral grains in a rock, reflecting its cooling history and environment.

Coarse-grained

Texture with large, visible mineral grains; indicates slow cooling underground (phaneritic).

Fine-grained

Texture with tiny mineral grains not visible to the naked eye; indicates rapid cooling at/near the surface (aphanitic).

Glassy texture

Texture with no minerals visible; formed when lava cools extremely rapidly (obsidian is a common example).

Vesicular texture

Texture featuring bubble-like holes from trapped gases in rapidly erupted lava.

Porphyritic

Texture with large crystals (phenocrysts) in a finer-grained groundmass, indicating two-stage cooling.

Welded texture

Texture in pyroclastic rocks where hot ash/fragments fuse together during deposition; very light-weight rocks.

Mafic

Silica-poor, iron/magnesium-rich rocks that are typically dark-colored (examples: basalt, gabbro).

Felsic

Silica-rich, light-colored rocks containing minerals like quartz and feldspar (examples: rhyolite, granite).

Ultramafic

Very silica-poor rocks rich in magnesium and iron; mantle-derived (example: peridotite).

Silica content (SiO2)

proportion of silicon dioxide in minerals; higher SiO2 → felsic; lower SiO2 → mafic; ultramafic is very low in SiO2.

Bowen's reaction series

The sequence in which minerals crystallize from cooling magma, from high-temperature (early) to low-temperature (late) minerals.

Partial melting

Melting where minerals melt at different temperatures, producing a melt with a different composition than the original rock.

Decompression melting

Melting caused by a decrease in pressure, typically at divergent boundaries or hot spots, producing mafic magma from mantle sources.

Flux (fluid-induced) melting

Melting aided by water/volatiles that lower the solidus, common at subduction zones; yields intermediate to felsic magmas.

Subduction

Plate boundary where one plate sinks below another, introducing water and promoting melting of the overlying mantle/crust.

Divergent plate boundary

Plate boundary where plates move apart; mantle upwelling and decompression melting create new oceanic crust.

Mantle peridotite

Ultramafic mantle rock composed mainly of olivine and pyroxene; green-black in appearance.

Basalt

Mafic, fine-grained extrusive rock; common in oceanic crust; fast-cooling lava at the surface.

Gabbro

Mafic, coarse-grained intrusive rock; forms when basaltic magma crystallizes underground.

Andesite

Intermediate composition rock; can be fine-grained extrusive or coarse-grained intrusive (between basalt and rhyolite/diorite).

Diorite

Intermediate, coarse-grained intrusive rock (salt-and-pepper appearance).

Granite

Felsic, coarse-grained intrusive rock; light-colored with pink feldspar; forms underground.

Rhyolite

Felsic, fine-grained extrusive rock; composition equivalent to granite but with a glass-like or fine texture.

Peridotite

Ultramafic mantle rock, green-black in color; dominant minerals include olivine and pyroxene.