Lesson Objectives

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

Vocabulary

• Bowen’s Reaction Series
• extrusive
• felsic
• fractional crystallization
• intermediate
• intrusive
• mafic
• partial melting
• pluton
• porphyritic
• ultramafic
• vesicular
• volcanic rock

Introduction

Igneous rocks form from the cooling and hardening of molten magma in many different environments. These rocks are identified by their composition and texture. More than 700 different types of igneous rocks are known.

Magma Composition

The rock beneath the Earth’s surface is sometimes heated to high enough temperatures that it melts to create magma. Different magmas have different composition and contain whatever elements were in the rock that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust. Table below lists the abundance of elements found in the Earth’s crust and in magma. The remaining 1.5% is made up of many other elements that are present in tiny quantities.

Whether rock melts to create magma depends on:

• Temperature: Temperature increases with depth, so melting is more likely to occur at greater depths.
• Pressure: Pressure increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures.
• Water: The addition of water changes the melting point of rock. As the amount of water increases, the melting point decreases.
• Rock composition: Minerals melt at different temperatures, so the temperature must be high enough to melt at least some minerals in the rock. The first mineral to melt from a rock will be quartz (if present) and the last will be olivine (if present).

The different geologic settings that produce varying conditions under which rocks melt will be discussed in the “Plate Tectonics” chapter.

As a rock heats up, the minerals that melt at the lowest temperatures will melt first. Partial melting occurs when the temperature on a rock is high enough to melt only some of the minerals in the rock. The minerals that will melt will be those that melt at lower temperatures. Fractional crystallization is the opposite of partial melting. This process describes the crystallization of different minerals as magma cools.

Bowen’s Reaction Series indicates the temperatures at which minerals melt or crystallize (Figure below). An understanding of the way atoms join together to form minerals leads to an understanding of how different igneous rocks form. Bowen’s Reaction Series also explains why some minerals are always found together and some are never found together.

Bowen’s Reaction Series.

To see a diagram illustrating Bowen’s Reaction Series, visit this website.

If the liquid separates from the solids at any time in partial melting or fractional crystallization, the chemical composition of the liquid and solid will be different. When that liquid crystallizes, the resulting igneous rock will have a different composition from the parent rock.

Intrusive and Extrusive Igneous Rocks

Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see Figure below for an example).

Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and biotite (black, platy).

Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought igneous rocks to the surface. Figure below shows a landscape in California’s Sierra Nevada made of granite that has been raised to create mountains.

California’s Sierra Nevada is intrusive igneous rock exposed at Earth’s surface.

Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks (Figure below).

Extrusive igneous rocks form after lava cools above the surface.

Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (Figure below).

Cooled lava forms basalt with no visible crystals. Why are there no visible crystals?

Some volcanic rocks have a mixed texture. A rock such as an andesite may have large crystals set within a matrix of tiny crystals. In this case, the magma cooled enough to form some crystals before erupting. Once erupted, the rest of the lava cooled rapidly. This is called porphyritic texture.

Cooling rate and gas content create other textures (see Figure below for examples of different textures). Lavas that cool extremely rapidly may have a glassy texture. Those with many holes from gas bubbles have a vesicular texture.

Different cooling rate and gas content resulted in these different textures.

Igneous Rock Classification

Igneous rocks are classified by their composition, from felsic to ultramafic. The characteristics and example minerals in each type are included in Table below.

Some of the rocks in the Table above were pictured earlier in this chapter. Look back at them and, using what you know about the size of crystals in extrusive and intrusive rocks and the composition of felsic and mafic rocks, identify the rocks in the following photos in Figure below:

These are photos of A) rhyolite, B) gabbro, C) peridotite, and D) komatiite.

Uses of Igneous Rocks

Igneous rocks have a wide variety of uses. One important use is as stone for buildings and statues. Granite is used for both of these purposes and is popular for kitchen countertops (Figure below).

Granite is an igneous rock used commonly in statues and building materials.

Pumice, shown in Figure above, is commonly used as an abrasive. Pumice is used to smooth skin or scrape up grime around the house. When pumice is placed into giant washing machines with newly manufactured jeans and tumbled, the result is “stone-washed” jeans. Ground up pumice stone is sometimes added to toothpaste to act as an abrasive material to scrub teeth.

Peridotite is sometimes mined for peridot, a type of olivine that is used in jewelry. Diorite was used extensively by ancient civilizations for vases and other decorative artwork and is still used for art today (Figure below).

This diorite statue was made in approximately 2120 BC.

Lesson Summary

• Igneous rocks form either when they cool very slowly deep within the Earth (intrusive) or when magma cools rapidly at the Earth’s surface (extrusive).
• Rock may melt to create magma if temperature increases, pressure decreases, or water is added. Different minerals melt at different temperatures.
• Igneous rocks are classified on their composition and grain size, which indicates whether they are intrusive or extrusive.

Review Questions

1. What is the visible difference between an intrusive and an extrusive igneous rock?
2. How does the difference in the way intrusive and extrusive rocks form lead to the differences in how the rocks appear?
3. What causes solid rocks to melt?
4. How are partial melting and fractional crystallization the same and different from each other?
5. How are igneous rocks classified?
6. Describe two ways granite is different from basalt.
7. List three common uses of igneous rocks.
8. Occasionally, igneous rocks contain both large crystals and tiny mineral crystals. Propose a way that both sizes of crystals might have formed in the rock.
9. How do you imagine an igneous rock will cool on the seafloor, and what will be the size of its crystals?

Further Reading / Supplemental Links

• A way to learn about the three rock types and some of the rocks within each type.

Points to Consider

• Are igneous rocks forming right now?
• Why don’t all igneous rocks with the same composition have the same name?
• Could an igneous rock cool at two different rates? What would the crystals in such a rock look like?