Chapter 3, Section 2Igneous Rocks

Wednesday, December 2, 2009

Pages 78 -- 81

Objective

• Identify the characteristics used to classify igneous rocks.

Vocabulary Words

• Extrusive rock

• Intrusive rock

• Porphyritic rock

Characteristics

• Igneous rocks are any rocks that form from magma or lava.– Comes from the latin word “fire”.

• Most igneous rocks made of mineral crystals.

• The are classified according to their origin, texture, and mineral composition.

Extrusive Rocks

• Formed from lava that is cooled above the surface.

• Most common is Basalt.– Basalt forms much

of the crust including the crust beneath the ocean floor.

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Intrusive Rocks

• Formed when magma hardened beneath the Earth’s surface.

• Granite is the most abundant.– Part of the Earth’s

crust that makes up the continents.

– Granite forms the core of many mountain ranges.

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Texture

• Depends on the size and shape of its mineral crystals.

• May be fine-grained, coarse-grained, glassy or porphyritic.– Rapid cooling forms fine-grained

rocks.– Slow cooling forms coarse-grained

rocks.

Porphyritic Texture

• A rock with large crystals scattered on a background of much smaller crystals.

• Formed when intrusive rocks are cooled in 2 stages.– Change in cooling rate

due to movement of the magma.

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Mineral Composition• Most of the Earth’s

minerals contain silica.– Material formed from

oxygen and silicon.– Amount of silica affects the

type of rock they form.– Low in silica: Dark-

colored rocks (Basalt)– High in silica: Light-

colored rocks (Granite)

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Basalt

Granite

Uses of Igneous Rocks

• Many igneous rocks are hard, dense, and durable.– Used for tools and

building materials

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Homework

• Workbook 3.2 (due 12/4)

• Vocabulary quiz 3.2 (on 12/9)