Igneous Rocks (Chapter 4)

A comprehensive set of practice flashcards covering igneous rocks, magma formation, textures, rock types, and plate tectonics concepts from the lecture notes.

What are igneous rocks?

Rocks formed from the solidification of molten rock (magma) either below the surface (intrusive/plutonic) or at the surface (extrusive/volcanic).

What is magma and what is lava?

Magma is melt beneath the surface; when it erupts onto the surface it becomes lava.

What are the main components of magma?

Liquid melt; solids (minerals) in the melt; volatiles (dissolved gases like H2O and CO2); and other elements such as Al, Ca, Na, K.

How is magma classified by silica content?

By SiO2 percentage: felsic (>65%), intermediate (53–65%), mafic (45–52%), ultramafic (<45%).

What minerals define felsic magma?

Feldspar and quartz; non-ferromagnesian silicates; continental crust is felsic.

What minerals define intermediate magma?

SiO2 53–65% with >25% ferromagnesian minerals.

What minerals define mafic magma?

SiO2 45–52%; ferromagnesian silicates and Ca-rich feldspar; common in oceanic crust and volcanic islands.

What minerals define ultramafic magma?

SiO2 <45%; predominantly ferromagnesian silicates; very rare.

How does silica content affect magma viscosity?

Higher silica increases viscosity; low-silica magmas are less viscous and flow more readily.

Which magma type has high mobility due to low silica content?

Mafic magma; low silica content yields low viscosity and high mobility.

What characterizes mafic lava compared to felsic lava?

Mafic lava has low silica and flows far; felsic lava has high silica and tends to form domes due to high viscosity.

What is the rate of cooling's effect on crystal size?

Slow cooling forms fewer but larger crystals; fast cooling forms many small crystals (or glass).

What is a porphyritic texture?

Two-stage cooling with large crystals (phenocrysts) in a finer groundmass.

What is an aphanitic texture?

Rapid cooling producing microscopic crystals; fine-grained.

What is a phaneritic texture?

Slow cooling producing visible, coarse grains.

What is a glassy texture and an example?

Rock made of natural glass formed by very rapid cooling; example: obsidian.

What is rhyolite and granite in terms of intrusion/extrusion?

Rhyolite is extrusive felsic; granite is intrusive felsic; both high in silica.

What is andesite and its intrusive/extrusive equivalents?

Andesite is extrusive intermediate; diorite is its intrusive equivalent.

What is basalt and gabbro in terms of intrusion/extrusion?

Basalt is extrusive mafic; gabbro is intrusive mafic.

What is peridotite/komatiite?

Ultramafic rocks; peridotite is mantle rock; komatiite is ultramafic volcanic rock.

What are obsidian and pumice?

Obsidian is volcanic glass, typically dark; pumice is frothy volcanic glass with many bubbles.

What are pyroclastic rocks and examples?

Rocks formed from ejected volcanic material; examples include tuff (ash-sized) and volcanic breccia.

What is an intrusive igneous structure?

Magma intrudes into surrounding rocks forming dikes, sills, laccoliths, plutons, and batholiths.

What are dikes, sills, laccoliths, plutons, and batholiths?

Dike: discordant tabular intrusion; Sill: concordant tabular intrusion; Laccolith: lens-shaped pluton arching strata; Pluton: large deep intrusion; Batholith: very large pluton (>100 km2).

What is the difference between intrusive and extrusive rocks?

Intrusive rocks crystallize at depth; extrusive rocks crystallize at or near the surface.

What does Bowen's reaction series describe?

Minerals crystallize in a predictable sequence as magma cools; early-formed minerals remove elements, changing melt toward felsic.

What processes change magma composition after formation?

Fractional crystallization, assimilation, magma mixing, and partial melting.

What is fractional crystallization?

Removal of early-formed crystals from the melt, changing its composition toward felsic.

Where does magma originate and where are magma chambers found?

Originates in the upper mantle or lower crust; magma chambers can be a few km to tens of km deep depending on tectonic setting.

What are the three main magma formation mechanisms?

Heat-transfer melting, decompression melting, and flux melting (volatiles).

What is decompression melting and where does it occur?

Melting due to a drop in pressure; common at mantle plumes, continental rifts, and mid-ocean ridges.

What is flux melting and where does it occur?

Melting caused by volatiles (e.g., water) lowering melting temperatures; occurs at subduction zones.

How is igneous activity related to plate tectonics?

Igneous activity concentrates at plate boundaries: convergent volcanic arcs, divergent mid-ocean ridges, and continental rifts.