Chapter 4 – Igneous Rocks and Intrusive Activity

Magma

molten rock located beneath the surface that may contain dissolved gases and suspended crystals

Lava

When magma reaches the surface

Components of Magma

• Liquid (melt)

• Solids

• Volatile

Components of Magma: Liquid (melt)

Composed mainly of silicate ions (Si and O)

Components of Magma: Solids

Crystals of silicate minerals already formed

Components of Magma: Volatiles

Dissolved gases like H₂O, CO₂, and SO₂, which vaporize near the surface as pressure drops

From Magma to Rock

Cooling of magma causes ions to arrange into crystalline structures, forming minerals

Igneous rock

When fully crystallized, this solid mass of interlocking silicate minerals becomes

Intrusive (Plutonic)

rocks form below the surface (slow cooling → large crystals)

Extrusive (Volcanic)

rocks form at or near the surface (rapid cooling → small crystals)

Four Major Magma Types

Felsic, Intermediate, Mafic, Ultramafic

Felsic SiO₂ Content

66–76%

Felsic Typical Minerals

Quartz, Feldspar, Muscovite

Felsic Color

Light

Felsic Density

Low

Intermediate SiO₂ Content

52–66%

Intermediate Typical Minerals

Amphibole, Plagioclase

Intermediate Color

Gray

Intermediate Density

Medium

Mafic SiO₂ Content

45–52%

Mafic Typical Minerals

Pyroxene, Olivine

Mafic Color

Dark

Mafic Density

High

Ultramafic SiO₂ Content

38–45%

Ultramafic Typical Minerals

Olivine, Pyroxene

Ultramafic Color

Greenish

Ultramafic Density

Very High

Silica Viscosity

High silica = thick (felsic); Low silica = fluid (mafic)

Silica Melting Temperature

Increases with decreasing silica

Silica Eruption Style

Felsic → explosive; Mafic → gentle

Texture

• size, shape, and arrangement of crystals

• Indicates cooling rate and environment of formation.

Igneous Textures

Aphanitic (fine-grained), Phaneritic (coarse-grained), Porphyritic, Glassy, Vesicular, Pyroclastic

Aphanitic (fine-grained) Cooling Rate

Rapid

Aphanitic (fine-grained) Setting

Extrusive

Aphanitic (fine-grained) Example

Basalt

Phaneritic (coarse-grained) Cooling Rate

Slow

Phaneritic (coarse-grained) Setting

Intrusive

Phaneritic (coarse-grained) Example

Granite

Porphyritic Cooling Rate

Two-stage cooling

Porphyritic Setting

Mixed

Porphyritic Example

Andesite

Glassy Cooling Rate

Instant cooling

Glassy Setting

Surface (lava quenched)

Glassy Example

Obsidian

Vesicular Cooling Rate

Gassy lava

Vesicular Setting

Extrusive

Vesicular Example

Pumice, Scoria

Pyroclastic Cooling Rate

Explosive eruption

Pyroclastic Setting

Extrusive

Pyroclastic Example

Tuff, Volcanic breccia

Naming Igneous Rocks

Based on composition + texture

Felsic Rocks

Granite (intrusive) / Rhyolite (extrusive)

Intermediate Rocks

Diorite / Andesite

Mafic Rocks

Gabbro / Basalt

Ultramafic Rocks

Peridotite / Komatiite

Pyroclastic Rocks

Composed of volcanic fragments (ash, pumice, lapilli) cemented together

Origin of Magma

Magma originates when solid rock in the crust and mantle melts due to

1. Decompression Melting

2. Addition of Volatiles

3. Heat Transfer

Origin of Magma: Decompression Melting

• Occurs at mid-ocean ridges where rising mantle rock experiences lower pressure.

• “Depressurization can trigger partial melting”

Origin of Magma: Addition of Volatiles

Water and CO₂ lower the melting point of rock (common at subduction zones)

Origin of Magma: Heat Transfer

Rising magma melts surrounding crustal rock by conduction

How Magmas Evolve

• Bowen’s Reaction Series

• Magmatic Differentiation

• Crystal Settling

• Assimilation

• Magma Mixing

Bowen’s Reaction Series

• Describes the sequence of mineral crystallization from cooling magma.

• Early minerals (olivine, pyroxene) form at high T°, later ones (quartz, feldspar) at low T°

Magmatic Differentiation

Separation of minerals during cooling changes magma composition

Crystal Settling

Denser minerals sink, removing Mg & Fe from remaining melt

Assimilation

Incorporation of surrounding rock into magma

Magma Mixing

Two magmas of different compositions combine

Partial Melting

Only some minerals melt → melt richer in silica than source rock

Mantle peridotite (ultramafic)

partially melts → basaltic magma (mafic)

Crustal rocks (felsic)

partially melt → granitic magma (felsic)

Intrusive Igneous Activity

• Intrusive Settings

• Plutons and Batholiths

• Erosion & Exposure

Intrusive Settings

• Magma cools slowly at depth; forms plutons, dikes, and sills.

• “Sills are horizontal intrusions (like a window sill), while dikes are vertical or steeply inclined fractures filled with magma.”

Pluton

Solidified magma chamber

Batholith

Huge (>100 km²) composite body formed from multiple plutons (e.g., Sierra Nevada, Yosemite’s Half Dome)

Erosion & Exposure

Over time, erosion reveals resistant granite batholiths (e.g., Enchanted Rock, TX, part of the Llano Uplift)

Enchanted Rock (Llano Uplift, TX)

Felsic, intrusive granite dome (batholith) formed ~1.1 billion years ago during the Grenville orogeny

Mount Rushmore

Carved from a felsic intrusive igneous rock—granite

Half Dome (Yosemite)

Exposed magma chamber formed ~150 million years ago

Crystallization

Ions arrange into minerals as magma cools

Composition

Determined by silica content (felsic → mafic)