4.2 Igneous Rocks

Lesson Objectives

• Describe how igneous rocks are formed.
• Describe the properties of some common types of igneous rocks.
• Relate some common uses of igneous rocks.

Vocabulary

• extrusive
• intrusive

Introduction

Most of the Earth is made of igneous rock. The entire mantle is igneous rock, as are some areas of the crust. One of the most common igneous rocks is granite (Figure below). Many mountain ranges are made of granite. People use granite for countertops, buildings, monuments and statues. Pumice is also an igneous rock. Perhaps you have used a pumice stone to smooth your skin. Pumice stones are put into giant washing machines with new jeans and tumbled around. The result is stone-washed jeans!

This life-size elephant is carved from granite.

Forming Crystals

Igneous rocks form when magma cools and forms crystals. These rocks can form at Earth’s surface or deep underground. Figure below shows a landscape in California’s Sierra Nevada that consists entirely of granite.

The Sierra Nevada of California are composed mainly of granite. These rocks are beautifully exposed in the Yosemite Valley.

Intrusive igneous rocks cool and form into crystals beneath the surface. Deep in the Earth, magma cools slowly. Slow cooling gives large crystals a chance to form. Intrusive igneous rocks have relatively large crystals that are easy to see. Granite is the most common intrusive igneous rock. Figure below shows four types of intrusive rocks.

(A) This granite has more plagioclase feldspar than many granites. (B) Diorite has more dark-colored minerals than granite. (C) Gabbro. (D) Peridotite is an intrusive igneous rock with olivine and other mafic minerals.

Extrusive igneous rocks form above the surface. The lava cools quickly as it pours out onto the surface (Figure below). Extrusive igneous rocks cool much more rapidly than intrusive rocks. They have smaller crystals, since the rapid cooling time does not allow time for large crystals to form. Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form a glass, such as obsidian. Others, such as pumice, contain holes where gas bubbles were trapped in the lava. The holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt. It is the rock that makes up the ocean floor. Figure below shows four types of extrusive igneous rocks.

(A) Lava cools to form extrusive igneous rock. The rocks here are basalts. (B) The strange rock formations of Chiricahua National Monument in Arizona are formed of the extrusive igneous rock rhyolite.

(A) This rhyolite is light colored. Few minerals are visible to the naked eye. (B) Andesite is darker than rhyolite. (C) Since basalt crystals are too small to see, the rock looks dark all over. (D) Komatiite is a very rare ultramafic rock. This rock is derived from the mantle.

Composition

Igneous rocks are grouped by the size of their crystals and the minerals they contain. The minerals in igneous rocks are grouped into families. Some contain mostly lighter colored minerals, some have a combination of light and dark minerals, and some have mostly darker minerals. The combination of minerals is determined by the composition of the magma. Magmas that produce lighter colored minerals are higher in silica. These create rocks such as granite and rhyolite. Darker colored minerals are found in rocks such as gabbro and basalt.

There are actually more than 700 different types of igneous rocks. Diorite is extremely hard and is commonly used for art. It was used extensively by ancient civilizations for vases and other decorative art work (Figure below).

This sarcophagus is housed at the Vatican Museum. The rock is the igneous extrusive rock porphyry. Porphyry has large crystals because the magma began to cool slowly, then erupted.

Lesson Summary

• Igneous rocks form either when they cool very slowly deep within the Earth or when magma cools rapidly at the Earth's surface.
• Composition of the magma will determine the minerals that will crystallize forming different types of igneous rocks.

Lesson Review Questions

Recall

1. What is the difference between an intrusive and an extrusive igneous rock?

2. List three common uses of igneous rocks.

Apply Concepts

3. Why do extrusive igneous rocks usually have smaller crystals than intrusive igneous rocks?

4. How are igneous rocks classified?

Think Critically

5. Occasionally, igneous rocks will contain both large crystals and tiny mineral crystals. Propose a way that both these sizes of crystals might have formed in the rock.

6. Why is the ocean floor more likely to have extrusive rocks than intrusive rocks?

Points to Consider

• Do you think igneous rocks could form where you live?
• Would all igneous rocks with the same composition have the same name? Explain why they might not.
• Could an igneous rock cool at two different rates? What would the crystals in such a rock look like?