Section 5.1: What are igneous rocks? Section 5.2: Classification of Igneous Rocks Igneous Rocks 5

Objectives Summarize igneous rock formation. Describe the composition of magma. Identify the factors that affect how rocks melt and crystallize. What are igneous rocks? Section 5.1

Review Vocabulary silicate: mineral that contains silicon and oxygen, and usually one or more other elements What are igneous rocks? Igneous rocks are the rocks that form when molten material cools and crystallizes. Section 5.1

New Vocabulary lava igneous rock partial melting Bowen’s reaction series fractional crystallization What are igneous rocks? Section 5.1

Igneous Rock Formation Lava is magma that flows out onto Earth’s surface. Igneous rocks form when lava or magma cools and minerals crystallize. What are igneous rocks? Section 5.1

Igneous Rock Formation The type of igneous rock that forms depends on the composition of the magma, which is often a slushy mix of molten rock, dissolved gases, and mineral crystals. Composition of magma What are igneous rocks? Section 5.1

Igneous Rock Formation Magma is classified as basaltic, andesitic, or rhyolitic, based on the amount of silica it contains. Silica affects melting temperature and impacts how quickly magma flows. What are igneous rocks? Section 5.1 Composition of magma

What are igneous rocks? Section 5.1 Please click the image above to view the interactive table.

Magma can be formed either by melting of Earth’s crust or by melting within the mantle. Magma formation Igneous Rock Formation What are igneous rocks? Section 5.1

What are igneous rocks? Section 5.1 The four main factors involved in the formation of magma are temperature, pressure, water content, and mineral content. Magma formation Igneous Rock Formation

What are igneous rocks? Section 5.1 Temperature generally increases with depth in Earth’s crust. This temperature increase is known as the geothermal gradient. Magma formation Igneous Rock Formation

What are igneous rocks? Section 5.1 Pressure also increases with depth as a result of the weight of overlying rock. The increased pressure on a rock also increases the rock’s melting point. Magma formation Igneous Rock Formation

What are igneous rocks? Section 5.1 Rocks and minerals often contain small percentages of water, which changes the melting point of the rocks. As water content increases, the melting point decreases. Magma formation Igneous Rock Formation

What are igneous rocks? Section 5.1 Rocks melt at lower or higher temperatures because their component minerals have lower or higher melting points. In general, rocks that are rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon. Magma formation Igneous Rock Formation

What are igneous rocks? Section 5.1 The process whereby some minerals melt at relatively low temperatures while other minerals remain solid is called partial melting. Partial melting Igneous Rock Formation

What are igneous rocks? Section 5.1 If temperatures are not high enough to melt the entire rock, the resulting magma will have a different composition than that of the original rock. Partial melting Igneous Rock Formation

What are igneous rocks? Section 5.1 Canadian geologist N. L. Bowen demonstrated that as magma cools and crystallizes, minerals form in predictable patterns in a process now known as the Bowen’s reaction series. Bowen’s Reaction Series

What are igneous rocks? Section 5.1 Bowen discovered two main patterns, or branches, of crystallization. Bowen’s Reaction Series

The left branch of Bowen’s reaction series represents the iron-rich minerals, which undergo abrupt changes as magma cools and crystallizes. Iron-rich minerals Bowen’s Reaction Series What are igneous rocks? Section 5.1

The right branch of Bowen’s reaction series represents the plagioclase feldspars, which undergo a continuous change of composition as magma cools. Feldspars Bowen’s Reaction Series What are igneous rocks? Section 5.1

When magma cools, the first minerals that crystallize are the last minerals that melted during partial melting. This process is called fractional crystallization. Fractional Crystallization What are igneous rocks? Section 5.1

In the basaltic intrusion of the Palisade Sills in the Hudson River valley of New York and New Jersey, small crystals formed in the chill zone as the outer areas of the intrusion cooled more quickly than the interior. Visualizing Fractional Crystallization and Crystal Settling What are igneous rocks? Section 5.1

What are igneous rocks? Section 5.1 Please click the image above to view the video.

Under certain conditions, newly formed crystals can be removed from magma, ending the chemical reactions between the magma and the minerals. Fractional Crystallization What are igneous rocks? Section 5.1

Objectives Classify different types and textures of igneous rocks. Recognize the effects of cooling rates on the grain sizes in igneous rocks. Describe some uses of igneous rocks. Classification of Igneous Rocks Section 5.2

fractional crystallization: a sequential process during which early formed crystals are removed from the melt and do not react with the remaining magma Review Vocabulary Classification of igneous rocks is based on mineral composition, crystal size, and texture. Classification of Igneous Rocks Section 5.2

New Vocabulary intrusive rock extrusive rock basaltic rock granitic rock texture Classification of Igneous Rocks Section 5.2 porphyritic texture vesicular texture pegmatite kimberlite

Mineral Composition of Igneous Rocks When magma cools and crystallizes below Earth’s surface, intrusive rocks form. Magma that cools and crystallizes on Earth’s surface forms extrusive rocks. Classification of Igneous Rocks Section 5.2

Mineral Composition of Igneous Rocks Igneous rocks are classified by their mineral compositions. Basaltic rocks are dark-colored, have lower silica contents, and contain mostly plagioclase and pyroxene. Classification of Igneous Rocks Section 5.2

Classification of Igneous Rocks Section 5.2 Mineral Composition of Igneous Rocks Granitic rocks are light-colored, have high silica contents, and contain mostly quartz, potassium feldspar, and plagioclase feldspar.

Rocks that have a composition of minerals that is somewhere between basaltic and granitic are called intermediate rocks. A fourth category, called ultrabasic, contains the rock peridotite. Ultrabasic rocks contain only iron-rich minerals such as olivine and pyroxene and are always dark. Classification of Igneous Rocks Section 5.2 Mineral Composition of Igneous Rocks

Classification of Igneous Rocks Section 5.2 Mineral Composition of Igneous Rocks Rock type can be determined by estimating relative percentages of minerals in the rocks.

Texture Texture refers to the size, shape, and distribution of the crystals or grains that make up a rock. Classification of Igneous Rocks Section 5.2

Classification of Igneous Rocks Section 5.2 When lava flows on Earth’s surface, it cools quickly and there is not enough time for large crystals to form. The resulting extrusive igneous rocks have crystals so small that they cannot be seen without magnification. Crystal size and cooling rates Texture

Classification of Igneous Rocks Section 5.2 Crystal size and cooling rates Texture When lava cools so quickly that crystals do not form, volcanic glass is the result. When magma cools slowly below Earth’s surface, there is sufficient time for large crystals to form. Thus, intrusive rocks can have crystals larger than 1 cm.

Classification of Igneous Rocks Section 5.2 A porphyritic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals. Porphyritic rocks Texture

Classification of Igneous Rocks Section 5.2 The spongy appearance that results from gas bubbles trapped in lava is called vesicular texture. Vesicular rocks Texture

Classification of Igneous Rocks Section 5.2 Thin Sections Scientists use a special microscope, called a petrographic microscope, to help them identify minerals by examining the mineral grains in sections of rock thin enough for light to pass through.

Classification of Igneous Rocks Section 5.2 Igneous Rocks as Resources Valuable ore deposits often occur within igneous intrusions. At other times, they may occur as veins in the rocks surrounding intrusions. Veins

Classification of Igneous Rocks Section 5.2 Igneous Rocks as Resources Veins of extremely large-grained minerals that can contain ores are called pegmatites. Pegmatites

Diamonds and other minerals that can form only under very high pressure are found in rare, ultrabasic rocks known as kimberlites. Classification of Igneous Rocks Section 5.2 Igneous Rocks as Resources Kimberlites

Classification of Igneous Rocks Section 5.2 The interlocking grain textures and the resistance to weathering of many of the minerals present in igneous rocks make them especially useful as building materials. Igneous Rocks as Resources Igneous rocks in construction

Chapter Resource Menu Section Questions Chapter Assessment Questions Standardized Test Practice Earth Science Online Glencoe Earth Science Transparencies Image Bank Vocabulary Animations Click on a hyperlink to view the corresponding feature. Study Guide Igneous Rocks 5

Key Concepts Section 5.1 What are igneous rocks? Igneous rocks are the rocks that form when molten material cools and crystallizes. Magma consists of molten rock, dissolved gases, and mineral crystals. Magma is classified as basaltic, andesitic, or rhyolitic, based on the amount of silica it contains. Study Guide 5

Different minerals melt and crystallize at different temperatures. Bowen’s reaction series defines the order in which minerals crystallize from magma. Section 5.1 What are igneous rocks? 5 Study Guide Key Concepts

Classification of igneous rocks is based on mineral composition, crystal size, and texture. Classification of igneous rocks is based on three main characteristics. The rate of cooling determines crystal size. Section 5.2 Classification of Igneous Rocks 5 Study Guide Key Concepts

Ores often occur in pegmatites. Diamonds occur in kimberlites. Some igneous rocks are used as building materials because of their strength, durability, and beauty. Section 5.2 Classification of Igneous Rocks 5 Study Guide Key Concepts

a. true b. false The presence of water decreases the melting temperature of rock. 5.1 Section Questions Igneous Rocks 5

a. iron b. potassium c. silica d. water Which component has the highest abundance in most of the magmas that form on Earth? 5.1 Section Questions Igneous Rocks 5

a. The crystals grew quickly. b. The crystals were partly dissolved. c. The magma had high gas content. d. The magma’s composition changed. Crystals of plagioclase feldspar in an igneous rock are visibly zoned. Which explanation explains this observation? 5.1 Section Questions Igneous Rocks 5

Which rock is the intrusive equivalent of basalt? a. granite b. diorite c. gabbro d. obsidian 5.2 Section Questions Igneous Rocks 5

In which igneous rock should a person prospect for diamonds? a. granite b. kimberlite c. basalt d. pegmatite 5.2 Section Questions Igneous Rocks 5

What information about an igneous rock might a person learn from a description of its texture? 5.2 Section Questions Igneous Rocks 5 Answer: A description of the texture of an igneous rock may provide information about the depths at which a rock was formed, and how quickly the magma cooled.

According to Bowen’s reaction series, which mineral crystallizes at the highest temperature? a. olivine b. amphibole c. quartz d. potassium feldspar Chapter Assessment Questions Igneous Rocks 5

Which texture indicates that an igneous rock cooled very quickly? a. vesicular b. coarse-grained c. glassy d. spongy Chapter Assessment Questions Igneous Rocks 5

A student is examining thin sections of a gabbro and a granite. Which mineral should the student identify in the gabbro but probably not identify in the granite? a. quartz b. potassium feldspar c. muscovite mica d. olivine Chapter Assessment Questions Igneous Rocks 5

Magmas of several different compositions have erupted from the same volcano. How can this be explained by the concept of fractional crystallization? Chapter Assessment Questions Igneous Rocks 5

Chapter Assessment Questions Igneous Rocks 5 Answer: Fractional crystallization is one way that different magmas can be produced at the same volcano. If magma is injected beneath the volcano and begins to crystallize, the composition of the remaining magma will change as the crystals form. The magma may then erupt from the volcano at any time.

Why are igneous rocks sometimes used as building stones? Answer: Igneous rocks are hard and often durable. They can endure the outdoor environment on buildings or in statues. Many igneous rocks consist of crystals that have attractive colors, shapes, and arrangements. When cut and polished, these rocks make attractive floors, walls, and countertops. Chapter Assessment Questions Igneous Rocks 5

An igneous rock consists entirely of olivine. Which composition does the rock have? a. granitic b. intermediate c. basaltic d. ultrabasic Standardized Test Practice Igneous Rocks 5

The igneous rock pumice has many holes. What do these holes represent? a. dissolved crystals b. gas pockets c. pores between grains d. fractures in the rock Standardized Test Practice Igneous Rocks 5

The graph shows Earth’s geothermal gradient. In which of the following depth ranges does temperature increase most rapidly? a. 0 to 1000 meters b. 1000 to 2000 meters c. 3000 to 4000 meters d. 4000 to 5000 meters Standardized Test Practice Igneous Rocks 5

a. rhyolite b. granite c. diorite d. gabbro Which rock formed from lava that cooled on Earth’s surface? Standardized Test Practice Igneous Rocks 5

Hot, water-rich fluids sometimes form during the last stages of magma crystallization. Why are these fluids important? Standardized Test Practice Igneous Rocks 5

Answer: The fluids often include rare and valuable elements that have very low concentrations in Earth’s crust as a whole. Watery fluids sometimes form metal-rich veins or other metal-rich deposits that include these valuable elements. Standardized Test Practice Igneous Rocks 5

Igneous Rocks 5 Glencoe Earth Science Transparencies

Igneous Rocks 5 Image Bank

Section 5.1 Vocabulary lava igneous rock partial melting Bowen’s reaction series fractional crystallization Igneous Rocks 5

Section 5.2 Vocabulary Igneous Rocks 5 intrusive rock extrusive rock basaltic rock granitic rock texture porphyritic texture vesicular texture pegmatite kimberlite

Types of Magma Visualizing Fractional Crystallization and Crystal Settling Animations 5