Chapter 5 “Igneous Rock”

Objectives

• Describe how the cooling rate of magma & lava affects the texture of igneous rocks.

• Classify igneous rocks according to their mineral composition.

• Describe a number of identifiable rock structures.

2 types of Igneous Rocks

• Intrusive– Created inside the earth

• Extrusive– Created on top of the ground

Intrusive Igneous Rocks

• Magma cools slowly underground

• Creates large, well-developed crystals

• Example:– Granite

Extrusive Igneous Rocks

• Created when lava cools rapidly

• Examples:– Basalt (oceans)

– Obsidian (glass)

– Pumice (has gas bubbles)

What happens if the outside cools rapidly – but the inside cools slowly?

• Creates a rock with a porphyritic texture.

• Small crystals outside– Quickly formed

• Larger crystals embedded inside– Formed slowly

Mineral Composition of

Igneous Rocks

• #1 Felsic– High in silica (SiO)

– Examples:

– Feldspar, quartz, mica

– Granite, Rhyolite, Obsidian

• #2 Intermediate• #3 Mafic

Mineral Composition of

Igneous Rocks (cont.)

• #2 Intermediate– Medium colored

(grays/browns/reds)

– Pink Feldspar

– Diorite, Andesite

Mineral Composition of

Igneous Rocks (cont.)

• #3 Mafic• Rich in Magnesium & Iron• Heavy & dense• Olivine, Hornblende• Basalt, Gabbro

Igneous Rock Structures

• Intrusions– Formed inside the earth

• Extrusions– Formed on the surface

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Igneous Intrusions

• Batholiths– Means “deep rock”– Large igneous

formations• Must be over 100 km2

• The continents are underlain by large batholiths

• Similar, smaller areas are called “stocks”

Intrusions (cont.)

• Laccolith– Means “layer of rock”

– Found in groups

– Bubbles of magma that have cooled underground

– Usually mushroom shaped

Intrusions (cont.)

• Sill– A flat layer of

solidified magma

• Dikes– Vertical columns of

solidified magma

Igneous Extrusions• Volcanic Neck

– What’s left of an ancient volcano

– May also include exposed fingers of hardened lava known as dikes

Igneous Extrusions (cont.)

• Lava flows– Flat layers of solidified lava

Igneous Extrusions (cont.)

• Lava plateau– Wide, flat areas of hardened lava

Assignments:• Complete Chapter 4 interactive notes for Homework

Groups

Intrusive VS Extrusive

Felsic

Intermediate

Mafic