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. Chapter 20: ROCKS AND MINERALS Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley This lecture will help you understand: • • • • • • • • • • The Geosphere is Made Up of Rocks and Minerals Minerals Mineral Properties Classification of Rock-Forming Minerals The Formation of Minerals and Rock Rock Types Igneous Rocks Sedimentary Rocks Metamorphic Rocks The Rock Cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Geosphere is Made Up of Rocks and Minerals • Just eight elements account for 98% of Earth’s mass. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Geosphere is Made Up of Rocks and Minerals • From Earth’s early molten (or nearly molten) state under the influence of gravity, the heaviest elements sank to the center of the Earth. So the core is composed of dense, iron-rich material. • Lighter elements migrated toward Earth’s surface. So the mantle is less dense than the core. • The crust is composed of even lighter, silicon- and oxygen-rich material. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Geosphere is Made Up of Rocks and Minerals Oxygen and silicon make up 75% of Earth’s crust. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Minerals • They are naturally occurring (formed naturally rather than manufactured). • They are crystalline solids. • They have an orderly and repeating arrangement of atoms, ions, and or molecules. • They have a definite chemical composition, with slight variations. • They are generally inorganic. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Minerals Minerals are classified by: • Chemical composition • Crystal structure Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Minerals Crystallization is the formation and growth of a solid from a liquid or gas — Atoms come together in specific chemical compositions and geometric arrangements. — The combination of chemical composition and arrangement of atoms in an internal structure makes each mineral unique. — Many minerals are identifiable by their crystal form. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Minerals • Some minerals can have the same composition but have a different crystal structure. — Different arrangements of the same atoms result in different minerals. • Diamond and graphite are two examples — Such minerals are called polymorphs. — With a different crystal structure, the minerals will have different properties. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties • Physical properties are an expression of chemical composition and internal crystal structure: —Crystal form —Hardness —Cleavage and fracture —Color —Density Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties: Crystal Form Crystal form—crystal shape—is the outward expression of a mineral’s internal arrangement of atoms. • Internal atomic arrangement is determined by atom/ion charge, size, and packing. • The conditions in which the crystal grows also affect crystal form. —Temperature, pressure, space for growth • Well-formed minerals are rare in nature—most minerals grow in cramped confined spaces. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties: Hardness Hardness is the resistance of a mineral to scratching. • Hardness is dependent on the strength of a mineral’s chemical bonds. • Bond strength is determined by ionic charge, atom (or ion) size, and packing. • The Mohs Scale compares the hardness of different minerals. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties: Hardness CHECK YOUR NEIGHBOR Gold is a soft mineral because: A. B. C. D. Its atoms are very tightly packed. Its atoms are large. Its atoms are tightly bonded. It has a very high melting point. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties: Hardness CHECK YOUR ANSWER Gold is a soft mineral because: A. B. C. D. Its atoms are very tightly packed. Its atoms are large. Its atoms are tightly bonded. It has a very high melting point. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Propteries: Cleavage and Fracture Cleavage is the property of a mineral to break along planes of weakness. • Planes of weakness are determined by crystal structure and bond strength. Fracture occurs in minerals where bond strength is generally the same in all directions. • Minerals that fracture do not exhibit cleavage. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties: Color • Color is an obvious feature for many minerals, but it is not reliable for mineral identification. —Chemical impurities can change a mineral’s color. —A mineral may occur in many color variations or be colorless. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties: Density Density is the ratio of a mineral’s mass to its volume. In simple terms, it is how heavy a mineral feels for its size (volume). Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties CHECK YOUR NEIGHBOR Physical properties of a mineral are predominantly related to A. B. C. D. the external conditions of temperature, pressure, and amount of space available for growth. the chemical composition and the internal arrangement of the atoms that make up the mineral. crystal form, hardness, cleavage and fracture, and density. all of the above. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties CHECK YOUR ANSWER Physical properties of a mineral are predominantly related to A. B. C. D. the external conditions of temperature, pressure, and amount of space available for growth. the chemical composition and the internal arrangement of the atoms that make up the mineral. crystal form, hardness, cleavage and fracture, and density. all of the above Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties CHECK YOUR NEIGHBOR The color of a mineral is predominantly related to A. B. C. D. its chemical composition. its internal arrangement of atoms. its luster. temperature and pressure conditions at the time of crystallization. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Mineral Properties CHECK YOUR ANSWER The color of a mineral is predominantly related to A. B. C. D. its chemical composition. its internal arrangement of atoms. its luster. temperature and pressure conditions at the time of crystallization. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Classification of Rock-Forming Minerals There are two classifications of minerals: —Silicate minerals —Nonsilicate minerals Silicate minerals make up more than 90% of the Earth’s crust. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Classification of Rock-Forming Minerals Silicate minerals are made up of silicon (Si) and oxygen (O) atoms, along with other elements (Al, Mg, Fe, Mn, and Ti). Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Classification of Rock-Forming Minerals The Silicates are divided into two groups: • Ferromagnesian silicates — Contain iron and/or magnesium — Tend to have high density and are darkly colored • Nonferromagnesian silicates — No iron or magnesium — Tend to have low density and are light in color Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Classification of Rock-Forming Minerals All silicate minerals have the same fundamental structure of atoms—the silicate tetrahedron. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Classification of Rock-Forming Minerals Nonsilicate minerals make up about 8% of Earth’s crust. • Carbonate minerals • Calcite, dolomite • Oxide minerals • Ore minerals—hematite, magnetite, chromite • Sulfide minerals • Ore minerals—pyrite, galena • Sulfate minerals • Gypsum, anhydrate • Native elements • Gold, platinum, iron Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Classification of Rock-Forming Minerals CHECK YOUR NEIGHBOR The silicates are the most common and abundant mineral group, because silicon and oxygen are A. B. C. D. the hardest elements on Earth’s surface. the two most abundant elements in the Earth’s crust. found in the mineral quartz. formed in a tetrahedral structure. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Classification of Rock-Forming Minerals CHECK YOUR ANSWER The silicates are the most common and abundant mineral group, because silicon and oxygen are A. B. C. D. the hardest elements on Earth’s surface. the two most abundant elements in the Earth’s crust. found in the mineral quartz. formed in a tetrahedral structure. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Formation of Minerals and Rock Minerals form by the process of crystallization. Minerals crystallize from two primary sources: —Magma (molten rock) —Water solutions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Formation of Minerals and Rock Minerals crystallize systematically based on their respective melting points. —The first minerals to crystallize from a magma are those with the highest melting point and the lowest amount of silica. —The last minerals to crystallize from a magma are those with lower melting points and higher amounts of silica. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Formation of Minerals and Rock Water solutions associated with later stages of crystallization from magma account for many important ore deposits. • As water solutions become chemically saturated, minerals precipitate. • Ore deposits can be deposited into cracks or into the matrix of the rock itself. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Formation of Minerals and Rock • Water solutions can precipitate chemical sediments such as carbonates and evaporites. • For chemical sediments, solubility rather than melting point determines which minerals will form first. —Low-solubility minerals precipitate first. —High-solubility minerals precipitate last. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Formation of Minerals and Rock CHECK YOUR NEIGHBOR In the crystallization of chemical sediments from a water solution, low-solubility minerals precipitate first, because A. B. C. D. they dissolve more easily. they are more susceptible to being dissolved. they are not easily dissolved. they are infinitely soluble. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Formation of Minerals and Rock CHECK YOUR ANSWER In the crystallization of chemical sediments from a water solution, low-solubility minerals precipitate first, because A. B. C. D. they dissolve more easily. they are more susceptible to being dissolved. they are not easily dissolved. they are infinitely soluble. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Rock Types A rock is an aggregate of minerals—a physical mixture. The three categories of rock reflect how they were formed: • Igneous • Formed from cooling and crystallization of magma or lava • Sedimentary • Formed from preexisting rocks subjected to weathering and erosion • Metamorphic • Formed from preexisting rock transformed by heat, pressure, or chemical fluids Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks • Igneous rocks are formed from the cooling and crystallization of magma or lava. — Magma is molten rock that forms inside Earth. — Lava is molten rock (magma) erupted at Earth’s surface. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks: Generation of Magma • Role of heat: — Temperature increases within Earth’s upper crust—the geothermal gradient—at an average rate of 30°C per kilometer. — Rocks in the lower crust and upper mantle are near their melting points. — Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may help to induce melting. — Heat is a minor player. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks: Generation of Magma • Role of pressure: — Reduced pressure lowers the melting temperature of rock. — When confining pressures drop, decompression melting occurs. • Analogies and examples: —The solid inner core —A pressure cooker Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks: Generation of Magma • Role of fluids —Fluids (primarily water) cause rocks to melt at lower temperatures. —Analogy: salt added to water causes ice to melt at a lower temperature. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks CHECK YOUR NEIGHBOR Even though the temperature at depth is hotter than rock’s melting point, the rocks at depth are solid, because A. B. C. D. they are under enormous pressure from rock above. increased pressure prevents their melting. temperature would have to be even higher to counteract the increase in pressure. all of the above. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks CHECK YOUR ANSWER Even though the temperature at depth is hotter than rock’s melting point, the rocks at depth are solid, because A. B. C. D. they are under enormous pressure from rock above. increased pressure prevents their melting. temperature would have to be even higher to counteract the increase in pressure. all of the above. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks The mineral makeup of igneous rock is dependent on the chemical composition of the magma from which it crystallizes. Three types of magma: – Basaltic – Andesitic – Granitic Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks • Melting of rock into magma occurs over a broad temperature range. – Partial melting rules. • Minerals with low melting points (high silica content) are the first to melt. • Partial melting results in a silica-enriched magma. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks Minerals crystallize systematically based on their respective melting points. • Minerals with high melting points(low silica content) are the first to crystallize. • The process of crystallization acts to enrich the remaining magma with silica. – Minerals with low melting points (high silica content) are left in the melt. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks Igneous rocks are classified according to where they are formed. • Rocks formed from magma that crystallizes at depth are termed intrusive, or plutonic rocks. • Rocks formed from lava at the surface are classified as extrusive, or volcanic rocks. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks There are three types of volcanoes: • Shield • Cinder cone • Composite Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Igneous Rocks Igneous rock that cools below Earth’s surface is called plutonic. Granite is the most common plutonic rock. Batholiths are large expanses of plutonic rock. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks Sedimentary rocks are products of 4 processes: • • • • Weathering Erosion Deposition Sedimentation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks • Weathering—the physical breakdown and chemical alteration of rock at Earth’s surface. • Two types of weathering: — Mechanical weathering—breaking and disintegration of rocks into smaller pieces. — Chemical weathering—chemical decomposition and transformation of rock into one or more new compounds. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks: Mechanical Weathering • Frost wedging—alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks. • Thermal expansion—alternate expansion and contraction due to heating and cooling. • Biological activity—disintegration resulting from plants and animals. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks: Chemical Weathering • Main producer of sediment. • Breaks down rock components and the internal structures of • minerals. • Most important • agent is water. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks: Erosion • Erosion—the physical removal of material by mobile agents such as water, wind, ice, or gravity. • Erosion does not occur in place. It involves movement. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks: Deposition Deposition — the stage in which eroded particles come to rest. Larger particles are the first to be deposited. Smaller particles are able to remain with the flow. In this way, sediments are sorted according to size as they are deposited. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks: Sedimentation • During sedimentation, sediment particles are deposited horizontally layer by layer. • As deposited sediment accumulates, it changes into sedimentary rock. • Lithification occurs in two steps: —Compaction —Cementation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks: Sedimentation • Compaction—Weight of overlying material presses down upon deeper layers. • Cementation—Compaction releases “pore water” rich in dissolved minerals. This mineralized “pore water” acts as a glue to cement sediment particles together. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks: Classifying Sedimentary Rocks • Sedimentary rock types are based on the source of the material: — Clastic rocks—transported sediment particles—bits and pieces of weathered rock (shale, sandstone, conglomerate) — Chemical rocks—sediments that were once in solution (travertine, halite, limestone) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Clastic Sedimentary Rocks classified by particle size Shale • Mud-sized particles in thin layers • Most common sedimentary rock Sandstone • Composed of sand-sized particles • Quartz is the predominant mineral Conglomerates • Composed of particles greater than 2 mm in diameter • Consists largely of rounded gravels Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Chemical Sedimentary Rocks Limestone: — Most abundant chemical rock. — Composed chiefly of the mineral calcite. — Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms). Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Chemical Sedimentary Rocks Evaporites: —Evaporation triggers the deposition of chemical precipitates. —Examples include rock salt and rock gypsum. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Chemical Sedimentary Rocks Coal: • Different from other rocks, because it is composed of organic material. • Stages in coal formation (in order): —Plant material —Peat —Lignite —Bituminous coal —Anthracite coal Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks CHECK YOUR NEIGHBOR The most characteristic feature of sedimentary rocks is A. B. C. D. they contain fossils. the lithification and cementation of sediments. the layered sequence of strata. the fusing of unconsolidated sediments into solid rock. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Sedimentary Rocks CHECK YOUR ANSWER The most characteristic feature of sedimentary rocks is A. B. C. D. they contain fossils. the lithification and cementation of sediments. the layered sequence of strata. the fusing of unconsolidated sediments into solid rock. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Metamorphic Rocks • Metamorphic rocks are produced from: —Igneous rocks —Sedimentary rocks —Other metamorphic rocks • Metamorphism occurs via recrystallization and mechanical deformation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Metamorphic Rocks Contact metamorphism: • a body of rock is intruded by magma • is typically associated with high temperatures and high water content— lots of chemical activity, not much, or no, mechanical deformation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Metamorphic Rocks Regional metamorphism: • the alteration of rock by both heat and pressure over an entire region rather than just near a contact between rock bodies. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Metamorphic Rocks Metamorphic rock is defined by appearance and mineral content: • Foliated—layered in sheets (schist, slate, gneiss) • Nonfoliated—not layered (marble and quartzite) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Metamorphic Rocks CHECK YOUR NEIGHBOR Marble is an example of: A. B. C. D. Crystalline, metamorphosed limestone. Foliated metamorphic rock. Recrystallized sandstone. Nonfoliated mica. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Metamorphic Rocks CHECK YOUR ANSWER Marble is an example of: A. B. C. D. Crystalline, metamorphosed limestone. Foliated metamorphic rock. Recrystallized sandstone. Nonfoliated mica. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The Rock Cycle Molten rock rises from the depths of Earth, cools, solidifies, and eventually returns to become magma again. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley