Geologists are scientists who study Earth. They want to

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1 What Is Inside Earth? Figure 1 Over time, the Grand Canyon in Arizona was carved out by the flowing water of the Colorado River. We can see that Earth s surface is constantly changing. But what is happening deeper inside Earth? How do Earth s three main layers differ from one another? How does heat move through Earth s interior? VOCABULARY geologist crust continent mantle core outer core inner core conduction convection convection current Geologists are scientists who study Earth. They want to know about the materials that Earth is made of and how Earth has changed during its history. Geologists cannot drill deep enough to study Earth s interior. The deepest well ever drilled is only about 12 kilometers (7.5 miles) deep. But the distance to Earth s center is more than 6,300 kilometers (almost 4,000 miles). So geologists use other ways to collect information about what it is like inside Earth. Scientists get clues about Earth s interior by studying the lava of volcanoes, which comes from deep inside Earth. Other evidence about Earth s interior comes from studying certain rocks at Earth s surface. These rocks actually formed deep inside Earth. Over time, they were pushed up closer to the surface and exposed when the rock above them was worn away. Geologists also study earthquakes. The energy waves produced by earthquakes move differently through different materials. By measuring the speed of earthquake waves and the paths they take, geologists have learned much about the different materials that make up Earth. Geologists have learned that Earth is made up of three main layers: the crust, the mantle, and the core. Earth s outermost layer is the crust. The crust covers Earth s entire surface and varies in thickness from 6 kilometers (about 4 miles) to 90 kilometers (about 56 miles). The crust is thinnest under the oceans and thickest under the continents, or large land masses. The crust is composed of rock with a thin layer of soil over it. Crust on the ocean floor is made mostly of the rock basalt. Continental crust is mostly granite.

2 Under Earth s crust is the mantle, which is deep inside Earth reaches the surface mainly made of rock that contains iron and magnesium. The mantle is Earth s thickest layer and it convection. through two processes: conduction and Inakes up most of Earth s mass. The upper part Conduction is the transfer of heat energy of the mantle is rigid. The lower part of the through solid matter as one particle strikes mantle flows very slowly. another. Heat moves through Earth s core and Earth s center, called the core, is made crust by conduction. The same process is at mostly of the metals iron and nickel. The work when heat from a stove burner is transferred to a pot on the stove. outer core is liquid and flows slowly. The inner core is solid. Convection is the transfer of heat energy The density of the materials inside Earth by flowing material such as a liquid or gas. Heat increases with depth. The core and the lower moves through Earth s mantle by convection. mantle are made of heavier materials than the As matter in the lower mantle is heated, it layers above them. They are also under great expands, becomes less dense, and is pushed pressure from the weight of the rock above. So upward by the cooler, denser material around the core and the lower mantle have a greater it. As the matter rises, it cools and becomes density than the upper mantle and crust. more dense. As its density increases, the matter The temperature of the materials inside sinks back down. Then it is heated again Earth also increases with depth. The inner core and the cycle repeats. This cycling of heated is the hottest layer of Earth. Scientists think the matter due to density differences is called a inner core may be as hot as the surface of the convection current. Some scientists say that Sun! Heat energy moves from warmer to cooler convection occurs throughout the mantle. Others areas, so heat moves outward from the core. suggest that this important process takes place The movement of heat energy from one place only in the upper mantle. Convection currents to another is called heat transfer. Heat from cause dramatic changes on Earth s surface. Earth s Layers Figure 2 Earth s rock layers get denser and hotter with depth. In the mantle, heated rock moves slowly in circular paths called convection currents. crust 6-90 km (4-56 mi)thick up to 440 C (825 F) mantle 2,900 km (1,800 mi} thick 1,500 C (2,800 F) 2,200 km (1,367 mi) thick 3,800 C (6,900 F) 1,250 km (777 mi) thick more than 5,000 C (9,000 F)

3 How Has Earth s Surface Changed Over Time? Breakup of Pangaea B. 135 million years ago What evidence supported the theory of continental drift? How does plate tectonics differ from the original theory of continental drift? Why is sea-floor spreading important? fossil VOCABULARY Pangaea continental drift lithosphere plate asthenosphere plate tectonics sonar mid-ocean ridge rift magma trench subduction sea-floor spreading Figure 3 This series of maps shows the probable positions of the continents following the breakup of the supercontinent Pangaea, more than 200 million years ago. Theory of Continental Drift I n 1915 German scientist Alfred Wegener proposed a theory about Earth s continents. He noticed that the coastlines of South America and Africa seem to fit together like puzzle pieces. Wegener recognized that the types of rocks in South America match those in Africa. He also found that fossils -- the remains, imprints, or traces of living things--in the rocks on both continents were identical. Based on this evidence, Wegener concluded that all the continents were once joined in a single, huge land mass. Wegener called the giant continent Pangaea, meaning "all lands." He believed that Pangaea split apart long ago to form today s continents. The continents then slid or drifted across the globe to their present positions (Figure 3). The idea that continents move from one part of Earth to another is called continental drift.

4 Plate Tectonics Wegener s theory of continental drift lacked a convincing explanation of how or why the continents moved. Wegener had suggested that the continents moved on their own. He thought they "plowed" through the ocean floor. New technology later gave scientists a tool to learn how the continents really moved. Earth s crust and the very top part of the upper mantle make up a region called the lithosphere. The lithosphere is the brittle outer layer of Earth. The lithosphere is solid, but it is not an immovable shell. Scientists discovered that the lithosphere is made up of moving sections, or plates. Earth has about a dozen large plates and several smaller plates (Figure 4). Plates can be made up of both contb nental crust and oceanic crust. Underneath the lithosphere is a region of upper mantle known as the asthenosphere. This zone of the upper mantle, though solid, is hot enough to bend and change shape like putty. Convection currents from deep inside Earth can cause the asthenosphere to flow slowly. As the asthenosphere moves, it carries the plates of the lithosphere. As a result, the sizes, shapes, and positions of Earth s continents and oceans are always changing. Some plates have moved thousands of miles over millions of years. Earth s plates continue to move today, at rates of a few centimeters a year. The theory that explains how and why plates move is called plate tectonics. ~Earth s Moving Plates Key Figure 4 The positions of Earth s continents are still changing. Plate movements are causing the Pacific Ocean to get smaller and the Atlantic Ocean to get bigger. How do you think Earth will look 65 million years from now?

5 Figure 5 The recycling of crust that occurs in sea-floor spreading causes oceanic crust to be billions of years younger than continental crust. Sea-Floor Spreading How did scientists arrive at the idea of plate tectonics? When Wegener proposed continental drift, little was known about the ocean floor. Then, in the 1950s, scientists began to map the ocean floor using sonar technology. In sonar mapping, ships bounce sound waves off the ocean bottom. The time between sending the sound and receiving its echo is used to calculate the depth of the ocean at that point. The sonar maps showed a long, narrow chain of mountains rising from the ocean floor. This underwater mountain range is called the mid-ocean ridge. A valley called a rift runs along the crest of the mid-ocean ridge. Scientists discovered that large amounts of heat were flowing from the rift. That discovery led scientists to conclude that the mid-ocean ridge formed above huge cracks in the oceanic crust. In the early 1960s, geologist Harry Hess suggested that new crest forms at the midocean ridge. Scientists found that melted rock, or magma, from the mantle rises out of cracks in the rift. The magma hardens and forms new crust, which piles high to form tire ridges. As more magma rises, it pushes the older crust away on both sides. Hess also suggested that old oceanic crust moves from the ridge toward underwater trenches. Trenches are narrow, deep regions of the ocean floor at the edges of plates. At trenches, old oceanic crust is carried down into the mantle as one plate is forced under another. This sliding of one plate under another is called subduction. The subducted crust sinks into the mantle and melts. This process of oceanic crust being created at the mid-ocean ridge, moving sideways away from the ridge, and plunging into the mantle at trenches is known as sea-floor spreading. By the late 1960s, discoveries about sea-floor spreading led scientists to develop and accept plate tectonic theory. Figure 6 An underwater mountain range winds around Earth for about 70,000 kilometers (43,500 miles).