Earth's Dynamic Interior - PDF

6.1 Earth’s Dynamic Interior Earth’s magnetism, which causes the auroras, is evidence of Earth’s dynamic interior. FIGURE 1: Edmond Halley CAN YOU EXPLAIN IT? Halley knew that his model was hypothetical and might not be completely correct, but he thought that it could explain some important obse...

6.1 Earth’s Dynamic Interior Earth’s magnetism, which causes the auroras, is evidence of Earth’s dynamic interior. FIGURE 1: Edmond Halley CAN YOU EXPLAIN IT? Halley knew that his model was hypothetical and might not be completely correct, but he thought that it could explain some important observations, calculations, and inferences about Earth’s properties. His model was based on information and evidence available at the time, but it was also based on some nonscientific beliefs that many people at the time accepted. The idea that Earth is hollow was not new—many people thought that there were vast caverns that extended deep into Earth’s interior. Gather Evidence Record evidence about the 308 Unit 6 Plate Tectonics Image Credits: (t) ©SurangaSL/Shutterstock, (b) ©Erich Lessing/Art Resource, NY In the late 1600s, English astronomer Edmond Halley presented a model of Earth’s structure. Halley originally claimed that Earth was composed of an outer rocky shell and an inner rocky sphere, separated by a wide gap filled with glowing air. EXPLORATION 1 Evidence of Structure and Composition The structure and composition of Earth’s surface are relatively easy to determine through direct observations and sampling. On land and at sea, we can conduct field investigations and collect rocks and sediment. FIGURE 2: Earth’s interior is divided into three distinct layers based on composition. crust 12 km 32 km mantle crust ea_cnlese812840_210a MASTER ART core upper mantle At 12 261 meters deep, the Kola Superdeep Borehole in northwestern Russia is the deepest borehole on Earth. On land, we can investigate quarries, mines, and road cuts. The investigation of the ocean floor is more difficult, but it can be done with ships and sampling equipment such as drills and dredges. Investigating Earth’s deep interior, however, is more difficult. One way scientists have tried to get a better understanding of Earth’s interior is through drilling. The deepest hole ever drilled, the Kola Superdeep Borehole, is 12 261 meters deep. Twelve kilometers below Earth’s surface seems deep, but it is only one-third of the way through the continental crust and only 1/500th of the total distance to Earth’s center. Drilling through the deep crust and into the upper mantle would be expensive and difficult. However, scientists from the International Ocean Discovery Program continue their attempt to drill to the mantle. If successful, it will provide important information about the composition, temperature, pressure, and other properties of the crust and uppermost mantle. But scientists still won’t be able to sample the lower mantle or the core. To understand the composition and structure of Earth’s interior as a whole, and to understand how it changes over time, scientists gather and analyze direct evidence, such as drilling samples, along with indirect evidence from energy released during earthquakes, gravity measurements, laboratory experiments, and chemical analysis of small fragments of mantle rock found within other rock. Lesson 1 Earth’s Dynamic Interior 309 Models of Earth’s Layers FIGURE 3: Models of the Solid Earth Earth’s interior can be divided into distinct layers based on chemical and physical properties. crust lithosphere asthenosphere mesosphere mantle compositional model structural model outer core core inner core Scientists have synthesized the results of investigations to develop models that summarize our understanding of Earth’s interior. The two most basic models are the compositional model and the structural model. Earth’s compositional layers—crust, mantle, and core—are distinguished by their chemical composition, the minerals and rock they are made of. Its structural layers—lithosphere, asthenosphere, mesosphere, outer core, and inner core—are distinguished by physical properties such as temperature, physical state, and whether the layers flow or behave rigidly. The models shown in Figure 3 are simple and appear to show that each layer is homogenous, the same throughout, and that the boundaries between the layers are sharp and even. In reality, Earth’s interior is much more complicated. The exact composition and the physical properties of the rock vary from place to place. While some boundaries are relatively distinct and smooth, others are more blurry or uneven. Geologists who study Earth’s interior have developed more detailed models that show additional layers and sublayers. 310 Unit 6 Plate Tectonics Outer Layers The outermost compositional layer of Earth is the crust. The crust along with part of the mantle below make up Earth’s outermost structural layer, the lithosphere. FIGURE 4: Earth’s lithosphere, which includes the crust and uppermost mantle, is made of rigid silicate rock that increases in density and temperature with depth. lithosphere asthenosphere continental crust upper mantle Our understanding of the lithosphere is based on evidence from rocks and rock formations, remote sensing of subsurface features, and laboratory experiments. We can, for example, study granites, gabbros, and other rocks exposed by erosion to infer the composition, temperature, pressure, and structure of the deep crust. Scientists can examine the rocks, mineral crystals, and gases that result from volcanic eruptions to gather evidence of the chemical composition and physical properties of the crust and mantle below. Finally, we can use data from earthquakes to infer physical properties and boundaries within the lithosphere. Seismic waves—waves caused by earthquakes—reflect off boundaries and refract—change speed and direction—when they move from one material into another. One good example of this occurs in a zone relatively near Earth’s surface where seismic waves refract, which indicates a transition between two layers. This zone marks the boundary between the crust and mantle and is called the Mohorovičić discontinuity, or the Moho for short. 0 100 Depth (km) oceanic crust Explain What types of investigations are required to develop a complete and accurate model of the crust and lithosphere? Why is it important to draw conclusions based on many different observations and measurements, and types of investigations? Collaborate Look at the diagram of the lithosphere. Which of the major features shown can be studied directly? Which can be understood only through indirect observations or interpretations of other forms of data? Lesson 1 Earth’s Dynamic Interior 311 Middle Layers Earth’s average density is about 5.5 g/cm 3. Since the crust has an average density of less than 3.0 g/cm3, there must be materials deeper within Earth that are much denser than those on the surface. Most of those materials—about 83% of Earth’s volume—are in the middle compositional layer of Earth known as the mantle. The mantle is denser than the crust because it is made of a higher proportion of heavier elements and because it is compressed by the weight of the crust above. The mantle can be divided into three structural layers based on their physical properties. The part of the mantle just above the core is called the mesosphere. FIGURE 5: Earth’s mantle is composed of solid iron- and magnesium-rich silicate rocks, and increases in density and temperature with depth. crust lithosphere asthenosphere mantle Analyze If you were planning an investigation to drill into Earth’s mantle, where do you think would be the best place to drill? Why? mesosphere Evidence from Rocks Because Earth’s mantle is so far underground, it is almost impossible to sample directly. Most of our understanding of the mantle comes from inferences based on the rocks we can examine on the surface, geophysical measurements, and laboratory experiments. For example, the basaltic lava that erupts on the ocean floor is thought to be composed of mantle rock. Geologists use the chemical and physical properties of basalt to infer what the composition of the mantle must be. This approach to determine the characteristics of the mantle is consistent with estimates of the density of the mantle based on measurements of Earth’s gravity and the way Earth moves in space. In a few places on Earth, large sections of the oceanic lithosphere have been pushed up on land. At the base of these structures are rocks called peridotite, which are similar to basalt but have more iron and magnesium and less silica. Geologists think these are actual samples of Earth’s mantle. Evidence from Earthquakes Most of our understanding of the details of Earth’s structure—whether layers are solid or liquid, how hot they are, and where the boundaries between layers are—comes from analyzing data from earthquakes. 312 Unit 6 Plate Tectonics FIGURE 6: Seismic Waves P-waves expand and contract material so that it moves along the same direction as the wave is moving. S-waves shear or bend material so that it moves perpendicular to the direction of the wave. undisturbed medium compressions S-wave P-wave amplitude wavelength rarefactions When an earthquake occurs, masses of rock bend and then break and snap back suddenly. This causes waves of energy called seismic waves to move out in all directions. Two main types of seismic waves move through Earth’s interior: P-waves and S-waves. The behavior of seismic waves depends on the properties of the material that the waves move through. Both P-waves and S-waves refract as they move from one material to another. In general, they move faster through denser, more rigid material, and slower through less dense or less rigid material. They also reflect off of different layers. Scientists analyze the behavior of seismic waves—how they change speed and direction—to infer the density and composition of rocks, the thickness of rock layers, and the physical state of the layers. For example, the fact that the velocity of seismic waves increases with depth in the lithosphere provides evidence that density increases with depth. We know where the Moho is because there is a strong refraction of seismic waves in this zone. Because the velocity of the waves decreases suddenly, scientists infer that the lithosphere ends and the next-lower layer, the asthenosphere, begins. From this evidence, scientists infer that rocks in the asthenosphere are not rigid, but are plastic, or very bendable, like putty or clay, and close to their melting point. Data Analysis 14 14 12 12 10 10 8 8 6 6 4 4 2 2 0 0 Analyze Describe and compare the velocities of P- and S-waves with depth in Earth’s mantle. What do the gradual changes show? What do the sudden changes show? 0 4000 5000 6000 3000 Depth (km) P-waves S-waves density asthenosphere mesosphere outer core inner core 1000 lithosphere Density (g/cm3) Velocity of earthquake waves (m/s) FIGURE 7: Velocity of P-waves (Vp) and S-waves (Vs) with depth. 2000 Lesson 1 Earth’s Dynamic Interior 313 Explain Is it possible to estimate Earth’s average composition by analyzing rocks on Earth’s surface? Explain why or why not. Evidence from Outer Space To estimate the composition of the mantle and other interior layers, we need to have a good idea of the proportion of different elements that make up the planet as a whole. If Earth were homogeneous, or the same all the way through, we could do this easily by analyzing surface rock. But other evidence tells us that it isn’t homogenous. Earth is separated into layers of different composition. Fortunately, there are a few rocks on Earth’s surface that provide good evidence for Earth’s composition and for the composition of the mantle and core: meteorites. Meteorites are rock that fall to Earth from space. Some appear to be unchanged since the solar system formed nearly 4.6 billion years ago, and thus provide information about the materials that formed Earth at the time of the solar system’s beginning. Geologists think that Earth’s average composition is about the same as the composition of these meteorites. Other meteorites appear to be broken pieces of crust, mantle, and core of other bodies in space— such as the moon, Mars, and large asteroids—and provide additional evidence for the layering of Earth’s interior. FIGURE 8: Meteorites provide evidence of Earth’s composition. Many stony meteorites are similar to rock on Earth’s crust but appear to be unchanged since they formed 4.56 billion years ago. b Geologists think that iron meteorites are broken pieces of the cores of small bodies Image Credits: (cl) ©Tom McHugh/Science Source; (cr) ©Mark Williamson/Science Source; (bl) ©NASA/JPL-Caltech a 314 Unit 6 Plate Tectonics Image Credits: ©Max Alexander/Science Source Inner Layers There is no direct evidence for the composition and physical properties of core, which includes Earth's innermost layers. There are no pieces of the core on the surface, and it is physically impossible to drill 2900 km below Earth’s surface to collect a sample. However, scientists can use similar techniques for inferring the characteristics of the innermost layers as they do for the mantle. Lesson 1 Earth’s Dynamic Interior 315 FIGURE 11: Earth’s core is composed primarily of iron and nickel. outer core Seismic data show that there are two internal layers: an outer core that is liquid and an inner core that is solid. Earth’s mass is greater than it would be if Earth were composed entirely from materials found at Earth’s surface. We know this because of Earth’s gravitational interaction with the sun. Scientists infer from this that Earth must have a core made of a very dense material, such as an iron-nickel alloy. The composition of meteorites supports the idea that there is a large amount of iron in Earth and that it is concentrated in the core. Experiments to figure out at what temperature and pressure different iron alloys crystallize help us infer what the exact composition of the core is and how its composition changes with depth. core Analyze Look back at the graph showing how seismic wave velocity changes with depth. What is the evidence that Earth’s outer core is liquid? inner core FIGURE 12: The existence of the aurora and and the behavior of a compass needle both provide information about Earth’s deep interior. a at the two images. Do research to find out what these two phenomena have in common. What do they both tell us about Earth? b Compass needle Earth’s magnetic field is additional evidence supporting the claim that Earth has an iron-rich core. Earth’s magnetic field is similar to that of a bar magnet: One pole is located near the North Pole and the other near the South Pole. We know this because of measurements made using compasses and other instruments, such as magnetometers. We can infer from mapping thousands of measurements around the globe that the magnetic field originates from deep within Earth, not somewhere on the surface. We can conclude that Earth has had a magnetic field for a long time because evidence is recorded in ancient rocks more than 4 billion years old. It makes sense that the core is made of a material such as iron that is a good electrical conductor. You can think of the outer core as working something like an electromagnet. The flow of liquid iron in the outer core generates an electric current that in turn produces a magnetic field. As we will see in the next section, observations that the magnetic field changes over time are further evidence that the outer core is liquid. Explain Scientists conduct a variety of investigations to understand Earth’s composition and structure. Summarize the different types of investigations described in this lesson and provide at least one example of how each is used to understand Earth’s interior. 316 Unit 6 Plate Tectonics Image Credits: (l) ©SurangaSL/Shutterstock; (r) ©Four sided triangle/Alamy Analyze Look carefully Aurora