Download - Chapter 5: Igneous Rocks
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