Chapter 22: Earth s Interior

Transcription

1 Chapter 22: Earth s Interior Vocabulary: Geologists Uniformitarianism Silicates Crust Mantle Lithosphere Asthenosphere Mesosphere Core Rock Inorganic Streak Luster Hardness Fracture Cleavage Igneous Rock Magma Lava Intrusive Rock Extrusive Rock Sediment Sedimentary Rock Clastic Rocks Foliated Rocks Rock Cycle Metamorphic Rock Plate Tectonics Pangaea Continental Drift Mid-Ocean Ridge Subduction Trench Earthquake Divergent Boundary Stress Fault Fold Sea-floor Spreading Focus Epicenter P waves Convergent Boundary Surface Waves S waves Seismograph Transform Boundary Seismic Waves Volcano Magna Chamber Pipe Vent Crater Caldera Hot spot Shield Volcano Cinder Volcano Batholith Volcanic Neck Composite Volcano Objectives: Describe the structure and layers of the interior of the Earth. Apply concepts of Geology to describe how the Earth has changed over time. Define and identify different types of minerals by their properties. Classify rock by comparing and contrasting the properties of different rocks. Explain how rocks are continually changing into different types of rock. Use the Theory of Plate Tectonics to describe how Earth s surface is changing. Describe how tectonic plates move and identify the types of plate boundaries. Explain how tectonic plates move and the effects of tectonic plate movement. Describe the cause Earthquakes, how they are measured, and identify where most Earthquakes occur. Explain how Volcanos form and where most Volcanos are located. Discuss the difference in magma and why some volcanic eruptions are quiet and others explosive. 1

2 Key Terms: Geologist: Uniformitarianism: Lithosphere: Asthenosphere: Mesosphere: Magma: Sediment: Rock Cycle: Plate Tectonics: 2

3 Continental Drift: Subduction: Divergent Boundary: Convergent Boundary: Transform Boundary: Seismic Waves: Stress Fault: P Waves: S Waves: 3

4 Section 1: Earth s Structure Geology is the study of planet Earth, including its composition and structure. Geologists are scientists who study Earth and the processes that shape Earth over time. Geologists study two types of forces that change Earth s surface. o Constructive forces shape the surface by building up mountains and other land areas. o Destructive forces slowly wear away mountains and, eventually, every other feature on Earth s surface. James Hutton developed the principle of uniformitarianism. Uniformitarianism is the idea that geologic processes that operate today also operated in the past. Ancient rocks can be understood by observing present-day geologic processes. Features such as mountains and canyons result from geologic processes that work very slowly over long periods of time. 4

5 Earth can be divided into three main layers the crust, mantle, and core based on the materials that make up each layer. Physical conditions in Earth s interior vary from layer to layer. Temperature and pressure in Earth s interior increase with depth. As the temperature and pressure increase, the properties of the materials inside Earth also change. The Crust The rocky outer layer of Earth is the crust. Like an eggshell, Earth s crust is thin. Much of the crust is made up of silicates, rocks made of compounds of silicon and oxygen, which often contain metals such as aluminum, iron, or calcium. There are two different types of crust. Continental crust, the rock that makes up the continents, consists mainly of less-dense rocks such as granite. Continental crust averages about 40 kilometers in thickness, although it ranges in thickness from about 8 to 75 kilometers. Oceanic crust is the rock that makes up the ocean floor. Oceanic crust is composed mostly of dense rocks like basalt. Oceanic crust is about 7 kilometers thick on average, and so is much thinner than continental crust. 5

6 The Mantle Beneath the crust is the mantle, a thick layer of hot but solid rock. The mantle extends about 2850 kilometers, from beneath the crust to the top of the core. Pressure and temperature increase with depth in the mantle. The mantle is composed mainly of silicates. It is rich in iron and magnesium, and so is denser throughout than the crust. Geologists divide the mantle into three layers. The lithosphere is a layer of relatively cool, rigid rock that includes the uppermost part of the mantle and the crust. The asthenosphere is a layer of softer, weaker rock that can flow slowly. The stronger lower part of the mantle is called the mesosphere. The stiffer rock of the mesosphere extends all to the upper surface of Earth s core. 6

7 The Core Beneath the mantle is the core, a large sphere of metal that occupies Earth s center. Scientists think that the core is composed mostly of iron, with lesser amounts of nickel and some lighter elements. Within the core, pressure increases greatly with depth. At Earth s center, the pressure is estimated to be 3.6 million times the pressure at Earth s surface! The core is divided into two parts the outer core and the inner core. In the outer core, high temperatures keep the metal liquid. As Earth rotates, the flowing iron of the outer core produces an electric current and creates Earth s magnetic field. 7

8 In the inner core, the pressure is very high. As a result, the inner core is solid, even though the temperature at the center of Earth is estimated to be about 5500 C. Section 2: Minerals A mineral is a naturally occurring, inorganic solid with a crystal structure and a characteristic chemical composition. A rock is a solid combination of minerals or mineral materials. Minerals are inorganic, meaning that living things did not produce them and they occur naturally. Geologists don t classify coal as a mineral because coal was created from plant remains. Materials like brick and concrete are not considered minerals either. 8

9 Each mineral is a unique substance with its own chemical composition and crystal structure. For most minerals, the proportion of elements can vary slightly while the mineral still retains a similar set of characteristics. Within each mineral crystal, the chemical composition is nearly constant. Minerals are the building blocks of rocks. Only a fraction of the nearly 4000 known minerals are common. These common minerals are called the rockforming minerals. Each particle within a rock is a separate mineral crystal. A crystal is a solid in which atoms are arranged in a regular repeating pattern. The Properties of Minerals The properties by which minerals can be identified include their crystal structure, color, streak, luster, density, hardness, fracture, and cleavage. 9

10 Crystal Structure In each type of mineral, the atoms are arranged in a particular geometric shape, or crystal structure. Each mineral always has the same crystal structure. The size of a mineral s crystals can vary. Color Some minerals can be identified by a characteristic color. Pyrite, also known as fool s gold, is always golden in color. Crystals of pure sulfur are always yellow. But a mineral s color can often be deceptive, because slight changes in composition can cause significant changes in a mineral s color. Streak The color of a mineral s powder is known as its streak. A mineral s streak can be found by scraping the mineral on a piece of unglazed porcelain called a streak plate. The color of a mineral s streak is not always the same as the color of the mineral itself. 10

11 Luster Luster is the way in which a mineral s surface reflects light. A rough, crumbly surface is said to be an earthy luster. Other terms that describe luster include silky, pearly, and vitreous (glassy.) Galena and pyrite have a metallic luster. Sulfur has a resinous-to-greasy luster. Density The density of a mineral depends on its chemical composition. In general, minerals made up of elements with higher atomic masses have higher densities than minerals made up of atoms with lower atomic masses. Hardness The atoms of minerals are held together by chemical bonds of different kinds and strengths. Hardness is the resistance of a mineral to scratching. 11

12 A hard mineral can scratch a softer mineral. The hardness of minerals is ranked on a scale from 1 to 10, called Mohs hardness scale. Diamond is the hardest mineral, with a hardness of 10. Talc is one of the softest minerals, with a hardness of 1. Fracture and Cleavage The fracture of a mineral is how the mineral breaks. Fracture is determined by the crystalline structure of the mineral and the bonds between the atoms in the crystals. Cleavage is a type of fracture in which the mineral tends to split along regular, well-defined planes where the bonds are weakest. Mica and graphite form sheets. Each sheet contains chemical bonds that are very strong. The sheets are held together with weak bonds, so they can easily be peeled apart from each other. 12

13 Other Properties Some minerals have unusual properties. Acids dissolve calcite easily. Magnetite is strongly attracted by a magnet. Fluorescent minerals like fluorite give off visible light when they are held under an ultraviolet light. Section 3: Rocks and the Rock Cycle Classifying Rocks Rocks are classified into three major groups igneous, sedimentary, and metamorphic based on how they form. The size, shape, and arrangement of the crystals and other particles that make up a rock give the rock its texture. A rock s texture reveals what the rock is made from and how and where it formed. 13

14 Igneous Rock Igneous rock forms when molten material cools and solidifies either inside Earth or at the surface. An igneous rock is a rock that forms from magma. Magma is a mixture of molten rock and gases, including water vapor, which forms underground. Magma that flows out of volcanoes is called lava An igneous rock that forms underground from hardened magma is called an intrusive rock. An igneous rock that forms at Earth s surface is called an extrusive rock. Extrusive rocks and intrusive rocks have differences in texture caused by differences in how the rocks formed. Intrusive rocks cool slowly underground, allowing their crystals to grow large. Large crystals give intrusive rocks a coarse-grained texture. Extrusive igneous rocks cool very quickly at the surface. Their crystals do not grow much before the rock cools. This gives extrusive rocks a fine-grained texture. Sedimentary Rock Geologists classify sedimentary rocks into three main groups according to how they form: clastic rocks, chemical rocks, and organic rocks. 14

15 Sediment consists of small, solid pieces of material that comes from rocks or living organisms. The process of weathering breaks down rock at Earth s surface, turning it into smaller pieces. Minerals dissolved in water are also sediment. Clastic Rock Sedimentary rocks that form from the broken fragments of other rocks are called clastic rocks. The fragments that make up clastic rocks are usually held together by cement. Chemical Rock Chemical sedimentary rocks form when minerals precipitate out of solution. Organic Rock Rainwater dissolves many minerals on the land. These dissolved minerals are then carried into the ocean. As water evaporates from the ocean surface, the concentration increases until the minerals precipitate. Some rocks form as the result of organic processes. Marine animals extract calcium carbonate from ocean water to form their shells and skeletons. The shells and skeletons sink to the ocean floor. The fragments compact and cement together, forming limestone. 15

16 Metamorphic Rock Metamorphic rock forms when a rock is transformed by heat, pressure, or chemical reactions. Most metamorphic rocks form under high temperatures and pressures deep underground. Heat deep inside Earth allows the minerals to recrystallize, and small crystals to enlarge. Chemical changes occur, and new minerals may replace the original minerals. The Rock Cycle The rock cycle is a series of processes in which rocks continuously change from one type to another. Constructive forces form new igneous rock. Destructive forces break down rock, forming sediment. 16

17 Section 4: Plate Tectonics The theory of plate tectonics explains the formation and movement of Earth s plates. Wegener hypothesized that the continents were once joined in single supercontinent, which then broke into pieces that moved apart. a Plate tectonics is the theory that pieces of Earth s lithosphere, called plates, move about slowly on top of the asthenosphere. According to Wegener s hypothesis, the continents move slowly across Earth s surface in a process called continental drift. In 1912, Alfred Wegener proposed a hypothesis of continental drift to explain these puzzling observations. Wegener called the ancient supercontinent Pangaea. 17

18 The Mid-Ocean Ridge When scientists mapped the ocean floor, they found a chain of underwater mountains which they called the mid-ocean ridge. It forms the world s longest mountain chain. Formation of Oceanic Crust Sea-floor spreading is the process by which new oceanic crust is created at mid-ocean ridges as older crust moves away. The mid-ocean ridge is a huge crack where magma pushes upward. The parts of the ocean floor on both sides of the central valley are moving apart. Magma from the mantle wells up and solidifies to form new oceanic crust. 18

19 Subduction of Oceanic Plates As sea-floor spreading occurs, old oceanic plates sink into the mantle in the process of subduction. Subduction zones are near the edges of oceanic plates. As a plate sinks through a subduction zone, it bends, forming a depression in the ocean floor called a trench. Evidence for Sea-floor Spreading Scientists discovered patterns of parallel magnetic stripes that were identical on the two sides of the mid-ocean ridge. Earth s magnetic field has reversed itself many times. The magnetic field causes rock crystals to line up in a certain way before the rock solidifies. Stripes show that new ocean floor was added to both sides of the mid-ocean ridge. 19

20 Section 5: Earthquakes An earthquake is a movement of Earth s lithosphere that occurs when rocks in the lithosphere suddenly shift, releasing stored energy. The energy released during an earthquake is carried by vibrations called seismic waves. A tsunami is a large sea wave generated by an underwater earthquake, volcano, or landslide. Stress in Earth s Crust As tectonic plates move, they cause stress in the crust, which in turn produces faults and folds. Stress is a force that squeezes rocks together, stretches or pulls them apart, or pushes them in different directions. Earthquakes happen because of the ways that plate movements affect the lithosphere. 20

21 The forces of plate movement cause deformation, or changes in the shape or volume of a mass of rock. A fault is a break in a mass of rock along which movement occurs. The two slabs of rock on either side of a fault move in relation to each other. Many faults occur along plate boundaries. A fold is a bend in layers of rock. Folds form where rocks are squeezed together but do not break. Rocks tend to fold rather than break when they are under high temperature or pressure. Earthquakes occur because stress forces have exceeded the strength of rock. The location beneath Earth s surface where an earthquake begins is called the focus, also known as the hypocenter. The location on Earth s surface directly above the focus is called the epicenter. Seismic waves move out in all directions from the focus. Types of Seismic Waves Earthquakes produce three main types of seismic waves: P waves, S waves, and surface waves. P waves are longitudinal waves similar to sound waves. As longitudinal waves move through a material, particles vibrate in the direction of the waves motion. P waves compress and expand the ground like an accordion. P waves are the fastest seismic waves. P waves can travel through both solids and liquids. 21

22 S waves are transverse waves, like light and other electromagnetic radiation. S waves cause particles to vibrate at right angles to the direction the waves move. Unlike P waves, S waves cannot travel through liquids. Surface waves are waves that develop when seismic waves reach Earth s surface. Surface waves move more slowly than P waves and S waves. They usually produce larger ground movements and greater damage. Some surface waves are transverse waves, and others have a rolling motion at Earth s surface that is similar to ocean waves. Measuring Earthquakes To measure earthquakes and pinpoint their epicenters, geologists record seismic waves using seismographs. A device that can detect and record seismic waves is called a seismograph. 22

23 The Richter scale rates earthquakes based on measurements of the times and amplitudes of seismic waves by certain seismographs. Most earthquakes are concentrated along plate boundaries, where many faults are found. Scientists have mapped Earth s interior, analyzing how seismic waves move through its layers. Wave speeds and paths are affected by the temperature, composition, and density of the rocks they pass through. Seismic waves interacting with boundaries between different kinds of rock are reflected, refracted, or diffracted. Geologists infer that Earth s outer core is liquid because S waves cannot pass through it. They can also tell that the core is mostly iron because P waves travel through it at a speed that matches laboratory experiments on iron. 23

24 Section 6: Volcanoes A volcano is a mountain that forms when magma reaches the surface. Volcanoes can result from several different geological processes and can take a variety of forms. Under certain conditions, small amounts of mantle rock can melt, forming liquid magma. The magma rises upward through the crust, erupting at the surface as a volcano. The process that leads to a volcanic eruption begins deep inside Earth. Magma rises because it is less dense than the solid rock around and above it. How a Volcano Erupts Magma is under pressure and contains dissolved gases, including carbon dioxide and water vapor. 24

25 Lower pressure near the surface allows the gases in magma to expand rapidly. An eruption occurs when the gases bubble out through a crack in the crust, propelling magma to the surface. Structure of a Volcano Before an eruption, magma often collects in a pocket called a magma chamber. Magma slowly accumulates in the magma chamber until enough pressure builds up to start an eruption. Then, magma rises to the surface in a narrow, vertical channel called a pipe. An opening in the ground where magma escapes to the surface is called a vent. Often there is one central vent at the top of a volcano. Sometimes there are other vents that open along a volcano s side. At the top of the central vent in most volcanoes is a bowl-shaped pit called a crater. After an eruption, a volcano s magma chamber and main vent may empty of magma, creating a hollow shell. If this shell collapses inward, it creates a huge depression, called a caldera, at the top of the volcano. 25

26 Volcanoes erupt explosively or quietly, depending on the characteristics of the magma. Quiet Eruptions Volcanoes that have very hot, low-silica magma generally erupt quietly. In a quiet eruption, lava erupts in a stream of low-viscosity lava, called a lava flow. Lava flows from a quiet eruption can travel for great distances. Explosive Eruptions High-silica magma produces explosive eruptions. Thick magma can clog a volcanic pipe, causing enormous pressure to build up. When the volcano finally explodes, lava and hot gases are hurled outward. Most volcanoes occur along plate boundaries or at hot spots in the crust. 26

27 Volcanoes often form along a converging plate boundary where an oceanic plate is subducted into the mantle. As it sinks through the mantle, the plate causes melting. Magma forms and rises to the surface. Volcanoes also form along a diverging plate boundary where magma rises to fill the gap between two separating plates. Some volcanoes occur at hot spots, regions where hot rock extends from deep within the mantle to the surface. The three major types of volcano are shield volcanoes, cinder cones, and composite volcanoes. A quiet eruption of low-viscosity lava produces a wide, flat volcano called a shield volcano. If an eruption is entirely ash and cinders, the result will be a small, steep-sided volcano called a cinder cone. A volcano that forms from explosive eruptions that produce a combination of lava and ash is called a composite volcano. 27

28 What landforms are formed from lava and magma? Igneous features formed by magma include batholiths, sills, dikes, and volcanic necks. 28