Ch. 6.2. Igneous Rocks

1. Intrusive igneous rock: formed from the cooling and solidification of magma beneath Earth’s surface

2. Extrusive igneous rock: formed from the cooling and solidification of lava at Earth’s surface

The Formation of Magma

• 3 factors influence rock melting:- temperature - pressure- presence of fluids in the rock (such as

water)

Partial Melting• Minerals with lower melting points are the first

minerals to melt.

• As the temperature increases and as other minerals melt, the magma’s composition changes.

The Formation of Magma

Chapter 6

Fractional Crystallization

• Minerals that have the highest freezing points crystallize first.

• When magma cools, the cooling process is the reverse of the process of partial melting.

•

The Formation of rocks

Chapter 6

A. Intrusive Igneous Rock Structures

• Batholiths: intrusive formations that spread over a large area

• Stocks: similar to batholiths but smaller

• Sills: layers of rock parallel to the layers of rock that surround it.

• Dykes: new layers cutting across existing layers.

B. Extrusive Igneous Rock Structures

– Volcano: a vent through which magma, gases, or volcanic ash is expelled.

– Lava flows, lava plateaus, and tuff layers

Igneous RockIgneous Rock

MagmaMagma

MeltingMelting SolidifyingSolidifying

Partial MeltingPartial Melting Fractional Crystallization Fractional Crystallization

TextureTexture

IntrusiveIntrusive ExtrusiveExtrusive

a. Coarse-Grained • Rocks cooled slowly, have large

mineral crystals

b. Fine-Grained • rocks cooled fast, have small grains.

Textures of Igneous Rocks:

c. porphyritic texture: magma cooled slowly at first, but then cools more rapidly as the magma nears or reaches Earth’s surface– large crystals together with smaller

crystals.

d. vesicular texture

– Contains bubbles of gases

e. glassy texture: highly viscous magma cooled very rapidly

– No large crystals

Composition of Igneous Rock

1. Felsic Rock: - rich in feldspars and silica- light in color.

• Ex: granite, rhyolite, obsidian, and pumice.

2. Mafic Rock- rich in magnesium and iron,

dark in color.

• Example: basalt and gabbro.

3. Intermediate Rocks: • proportions of silica between

felsic and mafic. • Examples: diorite and

andesite.

Intrusive vs. extrusive igneous rock