Key Concept

Igneous rocks are classified according to their origin, texture,

and mineral composition.

Igneous rock is rock that forms from magma or lava.

Igneous rock may form on or below Earth’s surface.

Where igneous rock forms is its origin.

Extrusive rock is igneous rock that forms on

Earth’s surface, when lava cools.

Lava cools quickly, forming small crystals. As a result,

extrusive rock has a smooth texture. The most common

extrusive rock is basalt.

Spectacular organlike basalt columns at Svartifoss, Skaftafell National Park, Iceland

Intrusive rock is igneous rock that forms below Earth’s surface, when

magma cools.

Magma cools slowly, forming big crystals. As a result, intrusive rock

has a rough texture. The most common intrusive

rock is granite.

At an elevation of 14,505 ft., Mt. Whitney reaches the highest peak in the conterminous United States, and like much of the Sierra Nevada, it consists of Cretaceous granodiorite, a rock intermediate in composition between granite and diorite.

Igneous rocks differ in how much silica they

contain. Silica is a material formed from oxygen and silicon.

From left to right, these rocks are arranged in order of decreasing silica content: granite, diorite, and gabbro.

Low-silica rocks, such as basalt, are dark-colored

rocks.

High-silica rocks, such as granite, are light-colored rocks.

Why do intrusive rocks have a rough texture?

a. Intrusive rocks have big crystals.b. Intrusive rocks form quickly.c. Intrusive rocks form from lava.

1. Label each circle in the Venn diagram with the kind of igneous rock it describes.

a. b.

Rock Rock

Forms on the surface

Forms when magma or lava cools

Forms under the surface

extrusive intrusive

If an igneous rock is light-colored it probably

contains a lot of ?

silica

Key Concept:

People throughout history have used

igneous rock for tools and building materials.

Igneous rocks have several uses. Examples of useful igneous rocks

are granite, pumice, and obsidian.

Granite is very hard and dense. Granite has long been used for buildings

and bridges. Today, granite is also used for curbstones and kitchen

counters.

Pumice is very rough. Pumice is used for

cleaning and polishing.

Obsidian is sharp and smooth like glass. Obsidian was used by ancient native Americans to make knives

and other sharp tools.

a. A rock used for curbstones and kitchen counters is b. A rock used for cleaning and polishing is c. A rock used for making

knives and other sharp tools is

granite

pumice

obsidian