Lesson Objectives

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

Introduction

Igneous rocks form from the cooling of molten magma into rocks 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

Rock beneath 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 existing rock that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust.
The abundance of elements found in Earth’s crust and in magma. The remaining 1.5% is made up of many elements that are present in tiny quantities. || Element || Symbol || percent ||
Oxygen
O
46.6%
Silicon
Si
27.7%
Aluminum
Al
8.1%
Iron
Fe
5.0%
Calcium
Ca
3.6%
Sodium
Na
2.8%
Potassium
K
2.6%
Magnesium
Mg
2.1%
Total

98.5%
(Source: http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust)
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 to create magma. 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, 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.
Here’s a fractional crystallization animation: http://authors.ck12.org/wiki/index.php/HS_Earth_Science_Multimedia.
Bowen’s Reaction Series indicates the temperatures at which minerals melt or crystallize. 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.
external image images?id=325744
To see a diagram illustrating Bowen's Reaction Series, visit this website: http://csmres.jmu.edu/geollab/Fichter/RockMin/RockMin.html.
This excellent video that explains Bowen’s Reaction Series in detail: http://www.youtube.com/watch?v=en6ihAM9fe8.
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. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form and so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see Figure below for an example).
Close up of granite.
Close up of granite.

Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), biotite (black, platy).
Igneous rocks make up most of the rocks on Earth. Most igneous rock are buried below the surface and are covered with sedimentary rock, or are 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.
external image images?id=309222
California
Igneous rocks are called extrusive when they cool and solidify above the surface, usually from a volcano, so they are also called volcanic (illustrated in Figure below).
Extrusive igneous rocks form after lava cools above the surface.
Extrusive igneous rocks form after lava cools above the surface.

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).
Basalt rocks in the South Pacific Ocean.
Basalt rocks in the South Pacific Ocean.

Cooled lava forms basalt with no visible crystals. Why are there no visible crystals?
What does the andesite photo in the lesson Types of Rocks (refer to Figure above) indicate about how that magma cooled? The rock has large crystals set within a matrix of tiny crystals. In this case, a magma cooled enough to form some crystals and then erupted so that the rest of the lava cooled rapidly. This is called porphyritic texture.
Cooling rate and gas content create other textures (see Figures below, Figures below and Figures below for examples of different textures).
Obsidian rock.
Obsidian rock.

Obsidian is lava that cools so rapidly crystals do not form, creating natural glass.
Pumic rock.
Pumic rock.

Pumice contains holes where gas bubbles were trapped in the molten lava, creating
Basalt rocks on floor of ocean.
Basalt rocks on floor of ocean.

The most common extrusive igneous rock is basalt because it makes up most of the seafloor, These are examples of basalt below the South Pacific Ocean.

Igneous Rock Classification

Igneous rocks are classified by their composition, felsic to ultramafic. The characteristics and example minerals in each type are included in the Table below.
Composition
Color
Density
Minerals
Felsic
Light
Low
Quartz, orthoclase feldspar
Intermediate
Intermediate
Intermediate
Plagioclase feldspar, biotite, amphibole
Mafic
Dark
High
Olivine, pyroxene
Ultramafic
Very dark
Very high
Olivine

Type
Amount of Silica
Extrusive
Intrusive
Ultramafic
<45%
Komatiite
Peridotite
Mafic
45-52%
Basalt
Gabbro
Intermediate
52-63%
Andesite
Diorite
Intermediate-Felsic
63-69%
Dacite
Granodiorite
Felsic
>69% SiO2
Rhyolite
Granite
Some of the rocks in the Table above were pictured earlier in this chapter. Look back at them and then, 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:
Four different rocks.
Four different rocks.

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.
A granite bear statue.
A granite bear statue.

Granite is an igneous rock used commonly in statues and building materials.
Pumice is commonly used as an abrasive, as shown in Figure above. 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 art work and is still used for art today (see Figure below).
An Egyptian vase made out of diorite.
An Egyptian vase made out of diorite.

This diorite vase was made by ancient Egyptians about 3600 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 these differences?
  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 cools on the seafloor and what will be the size of its crystals?

Vocabulary

volcanic rocks Rocks that originate in a volcano or volcanic feature. vesicular Igneous rock texture with holes that indicate the presence of gas bubbles in the magma. ultramafic A type of igneous rock that contains more than 90% mafic minerals. porphyritic Igneous rock texture in which visible crystals are found in a matrix of tiny crystals. partial melting The melting of some, but not all, of the minerals in a rock, depending on temperature. mafic A type of igneous rock that is made mostly of dense, dark minerals, such as olivine and pyroxene. intrusive Igneous rocks that form inside the Earth from slowly cooling magma. intermediate A type of igneous rock that is in between felsic and mafic. fractional crystallization The crystallization of a fraction of the minerals in magma depending on temperature. felsic A type of igneous rock that is made mostly of light minerals such as quartz and feldspar. extrusive Igneous rocks that form at Earth’s surface from rapidly cooling lava. Bowen’s Reaction Series The order in which minerals undergo partial melting or fractional crystallization, which depends on temperature and the composition of the mineral.

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?