GEOL1303_Module 4&5

M4 — Magma

Molten rock beneath Earth's surface; the parent material of igneous rocks.

M4 — Lava

Magma that reaches Earth's surface.

M4 — Intrusive (plutonic) igneous rock

Forms at depth from slowly cooling magma; typically has large visible crystals.

M4 — Extrusive (volcanic) igneous rock

Forms at or near the surface from rapidly cooling lava; typically has small crystals or glass.

M4 — Phaneritic texture

Coarse-grained texture with crystals visible to the naked eye; forms by slow cooling underground.

M4 — Aphanitic texture

Fine-grained texture with crystals too small to see easily; forms by rapid cooling at the surface.

M4 — Porphyritic texture

Two distinct crystal sizes produced by two stages of cooling.

M4 — Phenocryst

A large crystal in a porphyritic igneous rock.

M4 — Groundmass

The finer-grained material surrounding phenocrysts in a porphyritic rock.

M4 — Glassy texture

Forms when lava cools so rapidly that crystals do not have time to grow.

M4 — Vesicular texture

Texture containing holes left by gas bubbles escaping from lava.

M4 — Pyroclastic texture

Fragmental texture made of volcanic rock, mineral, and glass fragments from explosive eruptions.

M4 — Obsidian

Dense volcanic glass; commonly felsic and glassy.

M4 — Pumice

Very vesicular, frothy volcanic glass; commonly felsic.

M4 — Felsic composition

High-silica, generally light-colored composition rich in quartz, K-feldspar, and Na-rich plagioclase.

M4 — Intermediate composition

Composition between felsic and mafic; commonly rich in plagioclase feldspar and hornblende.

M4 — Mafic composition

Lower-silica, dark composition rich in iron and magnesium minerals such as pyroxene and olivine.

M4 — Ultramafic composition

Very low-silica composition dominated by olivine and pyroxene.

M4 — Granite

Felsic + phaneritic; intrusive equivalent of rhyolite.

M4 — Rhyolite

Felsic + aphanitic; extrusive equivalent of granite.

M4 — Diorite

Intermediate + phaneritic; intrusive equivalent of andesite.

M4 — Andesite

Intermediate + aphanitic; extrusive equivalent of diorite.

M4 — Gabbro

Mafic + phaneritic; intrusive equivalent of basalt.

M4 — Basalt

Mafic + aphanitic; extrusive equivalent of gabbro.

M4 — Peridotite

Ultramafic intrusive rock composed mainly of olivine and pyroxene; common mantle rock.

M4 — Bowen's Reaction Series: first minerals to crystallize

Olivine and Ca-rich plagioclase crystallize at the highest temperatures; pyroxene also crystallizes early.

M4 — Bowen's Reaction Series: last minerals to crystallize

K-feldspar, muscovite, and quartz crystallize at low temperatures.

M4 — Which minerals melt first when rock is heated?

Low-temperature felsic minerals such as quartz, muscovite, and K-feldspar melt first.

M4 — Which minerals melt at the highest temperatures?

Olivine and Ca-rich plagioclase.

M4 — Partial melting

Only some minerals in a rock melt; the melt is generally more felsic than the source rock and the residue more mafic.

M4 — Partial melting of mantle peridotite produces what?

Basaltic (mafic) magma.

M4 — Decompression melting

Melting caused when hot mantle rises and pressure decreases without a major temperature drop; common at divergent boundaries and hot spots.

M4 — Wet melting in subduction zones

Water released from the subducting slab lowers the melting temperature of mantle rock and helps generate magma.

M4 — Magmatic differentiation

Crystallization and removal of early minerals change magma composition; remaining melt becomes more silica-, Na-, and K-rich.

M4 — Differentiation sequence

Basaltic magma → intermediate/andesitic magma → felsic/granitic magma.

M4 — Assimilation

Rising magma incorporates and melts surrounding host rock, changing the magma composition.

M4 — Dike

Tabular intrusion that cuts across existing rock layers; discordant.

M4 — Sill

Tabular intrusion parallel to existing rock layers; concordant.

M4 — Laccolith

Lens-shaped intrusion that arches overlying rock layers upward.

M4 — Batholith vs. stock

Both are plutons; batholiths cover more than 100 km², stocks less than 100 km².

M5 — Viscosity

A material's resistance to flow.

M5 — Effect of temperature on magma viscosity

Hotter magma is less viscous and more fluid.

M5 — Effect of silica on magma viscosity

Higher silica content increases viscosity; lower silica magma is more fluid.

M5 — What makes an eruption more explosive?

High viscosity combined with abundant dissolved gases.

M5 — Basaltic magma characteristics

High temperature, low silica, low viscosity, generally fluid and less explosive.

M5 — Andesitic magma characteristics

Intermediate silica and viscosity; commonly associated with convergent boundaries and composite volcanoes.

M5 — Rhyolitic magma characteristics

High silica, lower temperature, very high viscosity, often explosive.

M5 — Viscosity order

Rhyolitic > andesitic > basaltic.

M5 — Pahoehoe

Smooth, ropey basaltic lava; hotter and more fluid than aa.

M5 — Aa

Rough, jagged, blocky basaltic lava.

M5 — Ash

Pyroclastic particles smaller than 2 mm.

M5 — Lapilli

Pyroclastic fragments 2–64 mm in diameter.

M5 — Volcanic bomb

Pyroclastic fragment larger than 64 mm that was molten or fluid while airborne.

M5 — Volcanic block

Solid rock fragment larger than 64 mm ejected during an eruption.

M5 — Shield volcano

Broad, gently sloping volcano built mainly from fluid basaltic lava flows; Hawaii is the classic example.

M5 — Composite (stratovolcano)

Large cone built from alternating lava flows and pyroclastic deposits; commonly andesitic and capable of explosive eruptions.

M5 — Cinder cone

Small, steep volcano built mostly of pyroclastic fragments; magma is often basaltic.

M5 — Lava dome

Steep mound or plug formed by very viscous, commonly rhyolitic lava that piles up near the vent.

M5 — Crater

Summit depression less than about 1 km across in the course material.

M5 — Caldera

Large collapse depression formed when a major eruption partly empties the magma chamber and the roof collapses.

M5 — Crater Lake

A caldera in Oregon formed after collapse of Mount Mazama; later filled with water.

M5 — Volcanic neck

Erosion-resistant magma solidified in a volcanic conduit and left standing after the surrounding cone erodes.

M5 — Shiprock, New Mexico

Classic example of a volcanic neck, with radiating dikes.

M5 — Volcanic pipe

A conduit that carries magma from the mantle toward the surface.

M5 — Diatreme

Volcanic pipe formed by explosive gas-driven eruption and later filled with broken rock fragments.

M5 — Kimberlite pipe

A volcanic pipe that can bring diamonds and mantle material from great depths to the surface.

M5 — Supervolcano

In the course material, a volcano with VEI 8 that has erupted more than 1,000 km³ of material.

M5 — Flood basalt

Huge, widespread basaltic lava flows produced by low-viscosity lava erupting from fissures.

M5 — Fissure eruption

Lava erupts from long fractures rather than a single central vent.

M5 — Divergent boundary volcanism

Decompression melting produces large volumes of basaltic magma at mid-ocean ridges.

M5 — Convergent boundary volcanism

Water/volatiles from a subducting plate trigger melting and produce volcanic arcs, often with explosive volcanoes.

M5 — Ring of Fire

Volcanic belt around the Pacific largely associated with convergent plate boundaries and subduction.

M5 — Hot spot

Localized intraplate volcanism above a mantle plume; the plate moves over the relatively fixed source.

M5 — Hawaii

Oceanic hot spot producing basaltic shield volcanoes.

M5 — Yellowstone

Continental hot spot associated with silica-rich magma, calderas, and past supereruptions.

M5 — Pyroclastic flow

Fast-moving, extremely hot mixture of ash, gas, and rock fragments that moves downslope.

M5 — Tiltmeter

Instrument that measures small changes in ground tilt caused by volcanic deformation.

M5 — GPS volcano monitoring

Measures movement and deformation of a volcano's surface.

M5 — Gas monitoring

Tracks changes in volcanic gas release that can help indicate rising magma and possible unrest.