Igneous Rocks


Igneous Rocks

Igneous Rocks

Chapter 4

Section 2

• Magma is molten rock

and becomes solid

when it cools.

• Magma comes from

deep below the earth’s



Magma (cont’d)

• It’s depths are ranging from near the surface

to about 150 km below the earth’s surface.

• Since magma is less dense than the solid

rock it’s forced upwards toward the surface.

• When magma reaches the surface it flows

out from volcanoes, then it becomes lava.

Intrusive Rocks

Rocks that form from


• Intrusive = interior

• Intrusive rocks are

only found at the

surface after layers of

rock and soil have

been removed by

weathering and


• Slow cooling = large

mineral grains

• Extrusive rocks are

rocks that are formed

as lava cools on the

surface of the earth.

• exposure to air and

water, cools lava


• quick cooling = small

mineral grains.

Extrusive Rocks

Volcanic Glass

Examples of Volcanic Glass

Volcanic Glass

Pumice Obsidian Scoria

Igneous Rock

Basaltic Granitic Andesitic

Classifying Igneous Rocks

• Basaltic Rock

– Dense, dark colored

– Magma is rich in iron and magnesium

– Poor in silica

– Lava is fluid and flows freely

– Black beaches of Hawaii

• Granitic Rocks

– Light-colored

– Lower denisty than basaltic rocks

– Magma is thick and stiff

– Contains lots of silica but low in iron and


– Volcanoes can be explosive – built up gas


• Andesitic Rocks

– Mineral composition between granitic and

basaltic rocks

– Volcanoes can erupt violently

– Volcanoes form around Pacific Rim

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