Chapter 5: Igneous Rocks

BIG Idea Igneous rocks were the first rocks to form as Earth cooled from a molten mass to the crystalline rocks of the early crust.

The word ‘Igneous’ means:

of or like fire [Latin ignis fire]

What are igneous rocks?

IGNEOUS rocks are…rocks that form when molten material cools and hardens either below or on Earth’s surface.

How do we define MOLTEN?

melted changed into liquid form by

heat glowing ‘red-hot’

MAGMA: THE PARENT TO ALL ROCKS

Molten rock beneath the surface of the Earth (forms underground)

LAVA: molten rock that flows out ON to Earth’s surface

Igneous rocks are classified based on:

1. mineral composition2. crystal size3. texture

Two (2) Types of Igneous Rocks:

1. EXTRUSIVE: RAPID (fast) COOLING LAVA ON THE EARTH’S SURFACE

small or no crystals fine-grained or glassy textures

2. INTRUSIVE: SLOW COOLING MAGMA DEEP WITHIN THE EARTH’S CRUST

larger crystals coarser texture

TEXTURE What is the ‘texture’ of an igneous rock?

the size, shape, and arrangement of the crystals

1. EXTRUSIVE: SMALL (or no) CRYSTALS - FINE GRAIN (fast

cooling, no time to grow)

EXTRUSIVE rocks can have air pockets due to fast cooling…

Can you name this rock? Hints:

it’s the only rock that will float on water, and

it’s used for pedicures…

Pumice!

This texture is described as ‘vesicular’

Gas pockets (holes) make the rock look ‘spongy’

pumice vesicular

basalt

scoria

Obsidian: cooled too quickly for crystals to form

Also known as ‘volcanic glass’

2. INTRUSIVE: LARGE WELL-DEVELOPED CRYSTALS = COARSE GRAIN TEXTURE (slow cooling, a lot of time to grow)

PORPHYRITIC: LARGE AND SMALL CRYSTALS

Magma cooled in two stages

COMPOSITION OF IGNEOUS ROCKS

1. FELSIC: high silica (Si

and O) and aluminum (Al) content

light color low density

Rhyolite

2. MAFIC: high iron (Fe)

and magnesium (Mg) content

dark color high density

Basalt

3. INTERMEDIATE: medium color medium

density

Diorite

Lava Plateaus