GEOL1340 UNIT 3

IGNEOUS ROCKS

Igneous rocks

Rocks formed through the freezing or crystallization of a melt (magma or lava)

What is a Rock?

A naturally occurring, coherent aggregate of one or more minerals

Coherent Mechanism

Held together by interlocking crystalline grains (formed during crystallization) or clastic grains (bound by natural cement and pressure)

The 3 Major Rock Families

Igneous, Sedimentary, and Metamorphic

Igneous rocks

Solidify directly from molten rock (magma/lava)

Sedimentary rocks

Form at or near the surface via weathering, erosion, deposition, and lithification of clasts or chemical precipitates

Metamorphic rocks

Formed when pre-existing rocks recrystallize under heat, pressure, and fluid activity without fully melting

Exhumation

The combined process of tectonic uplift and erosion of overlying rock that exposes deep intrusive/metamorphic rocks at Earth's surface

Magma

Molten rock located underground (at depth)

Lava

Molten rock that has emerged onto Earth's surface

Magma Composition components

Composed of silica frameworks ($SiO_4^{4-}$), metal cations ($Fe^{2+}, Mg^{2+}, Na^+, K^+$), dissolved volatiles ($H_2O, CO_2, SO_2$), and solids/xenoliths

Decompression Melting

Occurs when pressure decreases while temperature remains high as mantle rock rises (dominant at divergent boundaries, mid-ocean ridges, and rifts)

Flux Melting

Occurs when water ($H_2O$) and other volatiles are introduced into the hot mantle, lowering the rock's melting temperature (dominant at subduction zones/convergent boundaries)

Heat-Transfer Melting

Occurs when hot magma ascending from depth melts surrounding lower crustal rock

Felsic Magma Silica Content

Highest ($>66\%$) silica content with light colors (Quartz, K-Feldspar); intrusive is Granite, extrusive is Rhyolite

Intermediate Magma Silica Content

Medium ($52\text{--}66\%$) silica content with salt-and-pepper/grey color (Plagioclase, Amphibole); intrusive is Diorite, extrusive is Andesite

Mafic Magma Silica Content

Low ($45\text{--}52\%$) silica content with dark green to black color (Pyroxene, Ca-Plagioclase); intrusive is Gabbro, extrusive is Basalt

Ultramafic Magma Silica Content

Lowest ($<45\%$) silica content with very dark/green color (Olivine-rich); intrusive is Peridotite, extrusive is Komatiite

Bowen's Reaction Series high temperature end

Olivine & Ca-rich Plagioclase crystallize first at high temperature ($\sim1300^\circ\text{C}$)

Bowen's Reaction Series low temperature end

Quartz, Muscovite, & K-Feldspar crystallize last at low temperature ($\sim750^\circ\text{C}$)

Phaneritic (Coarse-Grained) texture

Large, visible crystals indicating slow cooling underground (Intrusive/Plutonic)

Aphanitic (Fine-Grained) texture

Microscopic crystals ($<1\text{ mm}$) indicating rapid cooling at or near the surface (Extrusive/Volcanic)

Porphyritic texture

Mixed texture featuring large crystals (phenocrysts) suspended within a fine-grained matrix (groundmass), indicating a two-stage cooling history

Glassy / Vitric texture

No crystalline structure at all, formed by extremely rapid (instant) cooling (e.g., Obsidian)

Obsidian

Volcanic glass that is felsic in composition, exhibits conchoidal fracture, and cools very rapidly at the surface

Vesicular texture

Contains holes (vesicles) formed by escaping gas bubbles trapped in cooling lava (e.g., Vesicular Basalt, Pumice)

Contact boundary

The distinct physical boundary formed between a new intrusive igneous body and the pre-existing host/country rock surrounding it

Dike

A tabular, sheet-like intrusion that cuts vertically across (discordant) host rock layers

Sill

A tabular, sheet-like intrusion that injects horizontally between (concordant) parallel host rock layers

Batholith

An immense, irregular intrusive pluton with a surface area $>100\text{ km}^2$ (e.g., Coast Mountains Batholith)

Stock

A pluton similar to a batholith but smaller ($<100\text{ km}^2$)

Volcanic Neck / Pipe

The central conduit of a volcano that filled with magma and solidified; resistant neck remains after erosion (e.g., Shiprock, New Mexico)

Explosive Eruptions

Driven by high viscosity (felsic/intermediate magma) + high trapped gas content

Effusive (Quiet) Eruptions

Low viscosity (mafic/basaltic magma) lets gas escape easily, creating flowing lava rivers

Tephra pyroclastic debris sizes

Categorized by size into Ash ($<2\text{ mm}$), Lapilli/Cinders ($2\text{--}64\text{ mm}$), and Bombs/Blocks ($>64\text{ mm}$)

Pillow Basalts

Formed when mafic lava erupts underwater (e.g., at mid-ocean ridges) and is quenched instantly into rounded, pillow-shaped blobs

Flood Basalts / Superplumes

Enormous accumulations of basaltic lava covering thousands of square kilometers caused by mantle superplumes/hot spots

Lahars

Destructive volcanic mudflows formed when volcanic ash mixes with melted glacier ice or heavy rain

Crater

Small top-depression formed by outward explosions of rocks from a volcano

Caldera

Massive sinkhole-like depression formed when a volcano collapses inward into its empty magma chamber