Intrusive igneous rocks form when magma cools within existing rocks in Earth’s crust.

Extrusive igneous rocks form when magma cools on Earth’s surface, where they have been “extruded.”

The two most obvious textural features of an igneous rock are the size of its mineral grains and how the mineral grains are packed together.

• Intrusive rocks are coarse-grained. Magma that solidifies in the crust cools slowly and has sufficient time to

form large mineral grains.

Extrusive rocks are fine-grained. Magma that solidifies on the surface usually cools rapidly, allowing

insufficient time for large crystals to grow.Coarse-grained igneous rock is called a phanerite (from the Greek word meaning visible).Igneous rock that contains unusually large mineral grains (2cm or larger) is called a pegmatite.Fine-grained igneous rock is called an aphanite (from the Greek word meaning invisible).

- Factors: cooling rate and viscosity

Granite RhyoliteFigure 4.7 A

DioriteAndesite

Figure 4.7 B

Gabbro Basalt

Figure 4.7 C

• The isolated large grains are phenocrysts.

Glassy rocks.Extrusive igneous rocks that are largely or whollyglassy are called obsidian.They display a distinctiveconchoidal fracture (smooth, curved surface).

Once the texture of an igneous rock is determined, its name will depend on its mineral assemblage. All common igneous rocks consist largely of:Quartz. FeldsparMicaAmphibole.Pyroxene.Olivine.

The overall lightness or darkness of a rock is a valuable indicator of its makeup.FELSIC Light-colored rocks are:

Quartz. Feldspar. Muscovite.

MAFIC Dark-colored rocks are:Biotite. Amphibole. Pyroxene.

Felsic Intermediate Mafic

Granite Diorite Gabbro

Rhyolite Andesite Basalt

(Porphyritic)

Intr

usi

veE

xtru

sive

Figure 5.7Figure 5.7

All bodies of intrusive igneous rock, regardless of shape or size, are called plutons, after Pluto, the Greek god of the underworld.

Plutons are given special names depending on their shapes and sizes.

Figure 4.11

Tephra can be converted into pyroclastic rock in two ways:Through the addition of a cementing agent, such as

quartz or calcite, introduced by groundwater.Through the welding of hot, glassy, ash particles.

Welded tuff.

A magma of a given composition can crystallize into many different kinds of igneous rock.

Solidifying magma forms several different minerals which start to crystallize from the cooling magma at different temperatures.

Crystal-melt separation can occur in a number of ways:• Compression can squeeze melt out of a crystal-melt

mixture. • Dense, early crystallized minerals may sink to the

bottom of a magma chamber, thereby forming a solid mineral layer covered by melt.

However a separation occurs, the compositional changes it causes are called magmatic differentiation by fractional crystallization.

Canadian-born scientist N. L. Bowen (1887-1956) first recognized the importance of magmatic differentiation by fractional crystallization.

Bowen argued that a single magma could crystallize into both basalt and rhyolite because of fractional crystallization.

1200

oC

1000

o

Two series of minerals formed during crystallization of magma

Intrus. Extrus.

Gabbro Basalt

Diorite Andesite

Granite Rhyolite

Low Silica Magma

High Silica Magma

Fra

me-

D

oubl

e

S

ingl

e

Isol

ated

wor

k

S

heet

Cha

in

Cha

in

Temperature ofCrystallization

750

o

Figure 4.19 A

Figure 4.19 B

The solid olivine reacts with silica in the melt to form a more silica-rich mineral, pyroxene.

The pyroxene in turn can react to form amphibole.

Amphibole can react to form biotite. Such a series of reactions is called a

discontinuous reaction series.

Figure 4.20

Figure 4.21