Chapter 4: Igneous Rocks (Rocks From Melted Stuff)

Magma

melted rock underground

Lava

melted rock at the surface

Igneous rock

When melted rock cools and hardens

Hot chocolate (liquid) with marshmallows (crystals + gas)

As it cools, it solidifies and the sugar crystals lock together

Two places rocks form

• Intrusive (plutonic)

• Extrusive (volcanic)

Intrusive (plutonic)

cools slowly underground → big crystals (like granite)

Extrusive (volcanic)

cools fast at the surface → tiny crystals or glass (like basalt, obsidian)

What controls how the magma behaves?

More or less silica (Silica = SiO₂)

More silica (felsic)

thick/sticky, light color, explosive

Less silica (mafic)

runny, dark color, gentle

Igneous rock composition

• Felsic

• Intermediate

• Mafic

• Ultramafic

Felsic

(light) granite / rhyolite

Intermediate

(gray) diorite / andesite

Mafic

(dark) gabbro / basalt

Ultramafic

(greenish) peridotite

Texture

how crystals look = how fast it cooled

Igneous Textures

• Aphanitic

• Phaneritic

• Porphyritic

• Glassy

• Vesicular

• Pyroclastic

Aphanitic (fine)

fast (lava)

Phaneritic (coarse)

slow (magma underground)

Porphyritic

two-step cooling (big crystals in a fine background)

Glassy

quenched instantly (obsidian)

Vesicular

full of gas bubbles (pumice/scoria)

Pyroclastic

ash & chunks stuck together (tuff)

How magma forms

• Decompression

• Add water

• Add heat

Decompression

pressure off → melts – like opening a soda

Add water (volatiles)

lowers melting point – common at subduction zones

Add heat (heat transfer)

hot magma warms surrounding rock

How magma changes

• Bowen’s series

• Crystal settling

• Assimilation

• Mixing

Partial melting

Only some minerals melt → melt is more silica-rich than the original rock

Mantle (ultramafic) partially melts

basaltic magma

Continental crust partially melts

granitic magma

Intrusions

shapes magma makes underground

Dike

vertical wall

Sill

horizontal sheet (“window sill”)

Pluton

frozen magma blob

Batholith

a bunch of plutons merged (huge)