Ch. 4 - Igneous Rocks and Volcanoes

Igneous rocks

Formed by the cooling and solidification of magma or lava.

Intrusive igneous rocks

Form below the surface, cool slowly, coarse-grained.

Extrusive igneous rocks

Form on/above the surface, cool quickly, fine-grained.

Characteristics used to classify igneous rocks

Texture (crystal size), Composition (mineral content).

Cooling rate and crystal size relationship

Slower cooling = larger crystals; Faster cooling = smaller crystals.

Coarse-grained rocks

Indicates slow cooling and intrusive origin.

Porphyritic texture

Indicates two-stage cooling: slow cooling (large crystals) followed by rapid cooling (fine matrix).

Vesicles in basalt

Gas bubbles preserved in volcanic rock; vesicles in basalt indicate volatile-rich lava that cooled quickly.

Felsic rocks

High silica, low Fe/Mg; light-colored (quartz, feldspar).

Intermediate rocks

Mixed composition (andesite, diorite).

Mafic rocks

Low silica, high Fe/Mg; dark (basalt, gabbro).

Ultramafic rocks

Very low silica, very high Fe/Mg (peridotite).

Trends from ultramafic to felsic

Silica, Na, K increase; Fe, Mg, Ca decrease.

Bowen's Reaction Series

Model describing the order of silicate mineral crystallization from a melt as it cools.

Silicates that crystallize first

Dark ferromagnesian minerals (mafic) crystallize first; felsic minerals like quartz form last.

Ways magma can form

Decompression melting: Divergent boundaries; Flux melting: Subduction zones; Heat-induced melting: Hotspots, continental crust.

Factors influencing initial magma composition

Composition of the source rock.

Processes changing magma composition

Fractional crystallization, Assimilation, Magma mixing.

Controls on magma viscosity

Silica content, Temperature.

Viscosity comparison of felsic and mafic magma

Felsic magma is more viscous due to higher silica content.

Igneous rock compositions in environments

Mid-ocean ridge: Mafic; Continental rift: Mafic to intermediate; Ocean-ocean subduction: Mafic to intermediate; Ocean-continent subduction: Intermediate to felsic; Continental collision: Felsic; Hotspots: Ocean - mafic; Continent - felsic to intermediate.

Types of volcanoes

Scoria (cinder) cones: Small, steep, short-lived, gas-rich mafic lava; Shield volcanoes: Broad, low-sloped, mafic lava flows; Stratovolcanoes: Steep, tall, alternating layers, intermediate-felsic; Lava domes: Small, steep-sided, viscous felsic lava; Calderas: Large depressions formed by collapse after massive eruptions; Flood basalts: Extensive flat layers from fissure eruptions of low viscosity mafic lava.

Magma compositions by volcano type

Scoria cones: Mafic; Shield volcanoes: Mafic; Stratovolcanoes: Intermediate to felsic; Lava domes: Felsic; Calderas: Intermediate to felsic; Flood basalts: Mafic.

Gentle slopes of shield volcanoes

Low viscosity mafic lava flows easily and spreads far.

Steep slopes in scoria cones and stratovolcanoes

Higher viscosity traps gases, resulting in explosive eruptions and steep buildup.

Viscous magma and pyroclastic eruptions

Traps gases, causing pressure build-up and explosive release.

Flood basalts and extinctions

Long-lasting eruptions release massive CO2 and SO2, causing climate change.

Danger of calderas, domes, stratovolcanoes

More explosive, larger eruptions, more ash and pyroclastics, higher population impact.

Volcanic hazards

Lava flows: Destroys infrastructure; Ash fall: Respiratory hazard, collapses buildings; Pyroclastic flows: Fast-moving, deadly; Lahars: Mudflows of ash and debris; Landslides: From slope collapse; Volcanic gases: CO2, SO2 - health hazards; Tsunamis: From landslides or caldera collapse.

Volcano monitoring methods

Gas emissions; Ground deformation (tilt meters, GPS); Seismic activity; Thermal imaging.

Precursors to eruption

Increased gas emissions; Earthquakes; Ground swelling; Temperature changes.