Plate Tectonics. Lesson 2. Continental Drift. Chapter 7 OUR DYNAMIC PLANET

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1 Chapter 7 OUR DYNAMIC PLANET Lesson 2 Plate Tectonics Main Idea: Moving plates cause the Earth s surface to change very slowly over millions of years, resulting in the current positions of the continents and in geological features such as mountain ranges and deep sea trenches. Next Generation Science Standards --Performance Expectations ESS2-2 Construct an explanation based on evidence for how geosciences processes have changed Earth s surface at varying times and spatial scales. ESS2-3 Analyze and interpret data on the distribution of fossils and rocks, continental shapes and seafloor structures to provide evidence of the past plate motions. What you ll learn Describe the different plate boundaries Compare and contrast between the theory of continental drift and the theory of plate tectonics Synthesize evidence and proof of Plate Tectonics Why is it important? Plate tectonics has changed the surface of the Earth gradually over millions of years, and is the key to how life has changed on this planet. Continental Drift Alfred Wegener, a German climatologist and geophysicist was trying to figure out how the Earth s surface could change over long periods of time. "Doesn't the east coast of South America fit exactly against the west coast of Africa, as if they had once been joined?" wrote Wegener to his future wife in December "This is an idea I'll have to pursue." He was not the first to make this observation, but he was the first to propose the hypothesis of continental drift, which stated the continents, were not fixed, but had drifted to their current locations through geologic time. Many things change over time...but it was hard to believe the continents could move! Many scientists of Wegener s time thought the idea was ludicrous. Wegener tried to prove his theory in spite of the ridicule he received from others in the scientific community. He died without proving his theory, in November 1930, while on his fourth expedition to Greenland. Figure 1. Alfred Wegener believed the continents moved across the oceans, because he found much evidence that suggested it. He was unable to prove his hypothesis. Vocabulary plate tectonics mid ocean ridge plate boundary continental drift seafloor spreading convergent boundary divergent boundary transform boundary Pangaea subduction ocean trench 1

2 Wegener had much compelling evidence to support his claim, which we will explore. Later, other scientists added to Wegener s work with new discoveries. Wegener s Ideas and Evidence In his book The Origin of Continents and Oceans in 1912, Wegener suggested that continents once formed a single supercontinent, he called Pangaea (pan JEE uh) which means all land. About 200 million years ago, Pangaea broke into two land masses he called Laurasia and Gondwanaland. Later these two land masses broke into the continents of North America, Eurasia, South America, Africa, Australia and Antarctica. He proposed the continents slowly plowed through the ocean floor driven by the spin of the Earth, and that over millions of years slowly drifted to their present locations. Physicists and geologist of the time disagreed with Wegener s explanation, pointing out continental drift was not necessary to explain many of his observations. Let s look as some of evidence Wegener found. Wegener cited several pieces of evidence to support his explanation in addition to the observation that the continents fit together like a jigsaw puzzle Figure 2. Others had also noticed this puzzle like fit, but Wegener also cited evidence in fossils, rocks and glacial deposits. Figure 2 Wegener proposed the hypothesis of continental drift, while he was alive he could not prove how the continents moved, but he did find evidence which supported his idea. 250 million years ago 150 million years ago 100 million years ago Earth today 2

3 Evidence from Fossils Wegener cited fossils from three ancient reptiles (Lystrosaurus, Cynognathus and Mesosaurus) and a plant (Glossopteris) had been found in parts of Africa, South America, India, Australia and Antarctica. These organisms would not have been able to cross the vast oceans which currently separate these continents; this suggested the continents they lived on were once connected. In addition, fossils of Glossopteris and Lystrosaurus have been found in Antarctica, a place too cold for these organisms to survive. This suggests that Antarctica drifted from a warmer region to its current colder climate. Fossils of other warm weather plants were also found on the island of Spitsbergen in the Arctic Ocean. To explain this, Wegener hypothesized that Spitsbergen drifted from tropical regions to the arctic. Wegener also used continental drift to explain evidence of glaciers found in temperate and tropical areas. Glacial deposits and rock surfaces scoured and polished by glaciers are found in South America, Africa, India, and Australia. This shows that parts of these continents were covered with glaciers in the past. How could you explain why glacial deposits are found in areas where no glaciers exist today? Wegener thought that these continents were connected and partly covered with ice near Earth s South Pole long ago. Figure 3 The location of fossils on several continents provided evidence for Wegener s hypothesis of Continental Drift. 3

4 Evidence from Rocks Parts of Africa and South America contain rocks of the same age and type, see Figure 4. If these continents were once joined, similar rock layers would continue across their borders. Mountain ranges and mineral deposits across today s continents would also line up in a similar way. These rocks and minerals suggest that the continents drifted apart. Parts of the Appalachian Mountains of the eastern United States are similar to those found in Greenland and Western Europe. If you were to study rocks from eastern South America and western Arica you would find other rock structures that are also similar. Other rock evidence is found in coal deposits found in Antarctica and North America. Coal is developed from decaying tropical plants. Tropical plants are found near the equator. Today however, neither Antarctica nor North America are near the equator. For coal to be found on these continents Antarctica and North America must have at one time been at the equator. So, North America must have moved north from the equator to its current location, and Antarctica must have moved south. All of this evidence supported Wegener s idea that the continents were once connected in the past. Without evidence of how the continents could move, continental drift remained a hypothesis. Figure 4 Rock evidence supports the hypothesis of continental drift. 4

5 New Technology Reveals New Evidence Mapping the Ocean Floor Harry Hess was a geologist and Navy submarine commander during World War II. Part of his job, during the 1940 s and 1950 s, was to study the ocean floor. In 1946, using sound waves on moving ships he and his men mapped large areas of the ocean floor in detail. Sound waves echoed off the ocean bottom the longer the sound waves took to return to the ship, the deeper the water was. Using sound waves, they discovered an underwater system of ridges, or mountains, and valleys like those found on the continents. In some of these underwater ridges are rather long rift valleys where volcanic eruptions and earthquakes occur from time to time. Some of these volcanoes actually are visible above the ocean surface. In the Atlantic, the Pacific, and in other oceans around the world, a system of ridges, called the mid-ocean ridges, is present. Others at this time were also investigating oceans, finding similar mountain ranges, such as the Mid-Atlantic range that runs from the Atlantic to the southwest of Africa. These ridges, it was later discovered, run through much of the ocean floor. Where did these ridges come from? Figure 5 Harry Hess studied the deepest parts of the ocean floor while serving as a submarine commander. The Great Global Rift In 1953, American physicists Maurice Ewing and Bruce Heezen discovered that through this underwater mountain range ran a deep canyon. In some places the canyon, called the Great Global Rift came very close to land. The rift appeared to be breaks in the earth's crust, but perfectly fitted breaks. The rift outlined chunks of the earth's crust, which were named 5

6 tectonic plates. Six large and several smaller plates make up the surface of the globe. Most of the world's earthquakes and volcanoes occur at the plates' edges. The large plate containing most of the Pacific Rim accounts for 80 percent of the earthquake energy of the planet. Hess Proposes Sea-Floor Spreading The discovery of the Great Global Rift in the 1950s inspired Hess to look back at his data from years before working on the submarine. Hess proposed that hot, less dense material below Earth s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions, as seen in Figure 6. As the seafloor spreads apart, magma is forced upward and flows from the cracks. It becomes solid as it cools and forms new seafloor. As new seafloor moves away from the mid-ocean ridge, it cools, contracts, and becomes denser. This denser, colder seafloor sinks, helping to form the ridge. In 1962, he added that as the magma oozed up between the plates in the Great Global Rift, it would cool in the ocean water. As a result, the cooling magma would expand and push the plates on either side of it apart. This way, the Atlantic Ocean would get wider but the coastlines of the landmasses would not change dramatically. The continents would move gradually, but over millions of years, the small movements would add up to great distances. Figure 6 The movement of the continents are a result of sea-floor spreading. Hess proved Wegener's basic idea right and explained the mechanism that broke the oncejoined continents into the seven with which we are familiar. The continents are attached to the plates and do not move independently of them. But the plates themselves shift and change shape, carrying the continents along. 6

7 Evidence for Spreading In 1968, scientists aboard the research ship Glomar Challenger began gathering information about the rocks on the seafloor. Glomar Challenger was equipped with a drilling rig that allowed scientists to drill into the seafloor to obtain rock samples. Scientists found that the youngest rocks are located at the mid-ocean ridges, see Figure 7. The ages of the rocks become increasingly older in samples obtained farther from the ridges, adding to the evidence for seafloor spreading. Figure 7 Newest rocks are located on the ridge, ocean floor gets older as it moves away from the Magnetic Clues Earth s magnetic field has a north and a south pole. Magnetic lines of force leave Earth near the South Pole and enter Earth near the North Pole. During a magnetic reversal, the lines of magnetic force run the opposite way. Scientists have determined that Earth s magnetic field has reversed itself many times in the past. These reversals occur over intervals of thousands or even millions of years. The reversals are recorded in rocks forming along mid-ocean ridges. Figure 8 Changes in the Earth s magnetic field are preserved in the rock that forms on both sides of mid-atlantic ridges. Magnetic Time Scale Iron-bearing minerals, such as magnetite, that are found in the rocks of the seafloor can record Earth s magnetic field direction when they form. Whenever Earth s magnetic field reverses, newly forming iron minerals will record the magnetic reversal. 7

8 Using a sensing device called a magnetometer (mag nuh TAH muh tur) to detect magnetic fields, we find that rocks on the ocean floor show periods of magnetic reversal. The magnetic alignment in the rocks reverses back and forth over time in strips parallel to the mid-ocean ridges, as shown in Figure 8. A strong magnetic reading is recorded when the polarity of a rock is the same as the polarity of Earth s magnetic field today. Because of this, normal polarities in rocks show up as large peaks. This discovery provided strong support that seafloor spreading was indeed occurring. The magnetic reversals showed that new rock was being formed at the mid-ocean ridges. This also helped to explain how the crust and continents could move, something Wegener couldn t do. Development of a Unifying Theory A unifying theory ties other theories together, and that is just what John Tuzo Wilson ( ) a Canadian geophysicist was able to do in the 1960 s. He proposed the lithosphere (crust and uppermost mantle) is broken into separate sections called plates. He combined information from continental drift and sea floor spreading, into a single unifying theory, which explains much about the history of the Earth. This theory is called Plate Tectonics. Plate tectonics not only explains what the Earth was like in the past, it also explains how and why it has changed and how it continues to change. We now know that plate tectonics is responsible for forming mountain ranges, rift valleys and for earthquakes and volcanoes. In 1963, Tuzo Wilson proposed that plates might move over fixed hotspots in the mantle, forming volcanic island chains like Hawaii. In 1965, he followed this discovery with the idea of a third type of plate boundary - transform faults. Also known as a conservative plate boundaries, these faults slip horizontally, connecting oceanic ridges (divergent boundaries) to ocean trenches (convergent boundaries). Transform faults were the missing piece in the puzzle of plate tectonic theory. They allowed for plates to slide past each other without any oceanic crust being created or destroyed. The most famous example is the San Andreas Fault between the North American and Pacific plates. Plate Boundaries The plates are curved like the Earth s surface and slowly move along their boundaries in different ways and at different speeds. Most move only a few centimeters a year (about as fast as your fingernails grow). The plate edges cannot be seen the way coastlines and continents can. Most plates contain parts of the ocean floor, so the boundaries are below the oceans. When plates move, they can interact in several ways. They can move toward each other and converge, or collide. They also can pull apart or slide alongside one another. When the plates interact, the result of their movement is seen at the plate boundaries, as in Figure 9. 8

9 What are the general ways that plates interact? Movement along any plate boundary means that changes must happen at other boundaries. What is happening to the Atlantic Ocean floor between the North American and African Plates? Compare this with what is happening along the western margin of South America. Figure 9 This diagram shows the major plates of the lithosphere, their direction of movement, and the type of boundary between them (examine the key for movement direction). Divergent boundaries Figure 10 Divergent boundary in the ocean. When plates are moving away from each other, or pulling apart, we call this a divergent boundary. The word divergent means moving away from each other at a common point. You learned about divergent boundaries when you read about seafloor spreading. In the Atlantic Ocean, the North American Plate is moving away from the Eurasian and the African Plates as shown in Figure 10. That divergent boundary is called the Mid-Atlantic Ridge, which is slowly becoming wider due to seafloor spreading. 9

10 Mountain range. This mountain building of the ocean floor occurs when convection currents rise in the mantle under the oceanic crust and create magma where two tectonic plates meet at a divergent boundary. Figure 11 Continental divergent boundary The Great Rift Valley in eastern Africa might become a divergent plate boundary. There, a valley has formed where a continental plate is being pulled apart. Figure 11 is a side view of what a rift valley might look like and shows how the hot material rises up where plates separate. Convergent boundaries As new crust is added in one location, it disappears below the surface at another. The disappearance of crust occurs at a convergent boundary, a boundary where two plates move towards each other. The word convergent means coming together. When plates converge, one plate (the heavier/ denser plate) is pulled underneath the lighter plate. When one plate is pulled under the other it is called subduction. As the plate moves lower it contacts the hot mantle and melts, beginning a new journey through the rock cycle. Oceanic-Continental Figure 12 Volcanoes and deep sea trenches created as ocean crust is subducted and melts. When an oceanic plate converges with a less dense continental plate, the denser oceanic plate sinks under the continental plate. The area where an oceanic plate subducts, or goes down, into the mantle is called a subduction zone. Some volcanoes form above subduction zones. Figures 12 and 13 shows how this type of convergent boundary creates a deep-sea trench where one plate bends and sinks beneath the other. 10

11 High temperatures cause rock to melt around the subducting slab as it goes under the other plate. The newly formed magma is forced upward along these plate boundaries, forming volcanoes. The Andes mountain range of South America contains many volcanoes. They were formed at the convergent boundary of the Nazca and the South American Plates. Figure 13 Geographic and geologic effects of ocean-continental convergent boundary. Oceanic-Oceanic A subduction zone also can form where two oceanic plates converge. In this case, the colder, older, denser oceanic plate bends and sinks down into the mantle. The Mariana Islands in the western Pacific are a chain of volcanic islands formed where two oceanic plates collide, see Figure 14. Here the Mariana Islands are formed when the large Pacific plate is suducted under the Philippine plate. Figure 14 Shows the Mariana Islands near Japan were created by collision of two oceanic plates. 11

12 Continental-Continental Usually, no subduction occurs when continental plates collide, as shown in Figure 15. Because both of these plates are less dense than the material in the asthenosphere, the two plates collide and fold/crumple up, forming mountain ranges. Earthquakes are common at these convergent boundaries. However, volcanoes do not form because there is no subduction. The Himalayas in Asia (where the Indo-Australian Plate collides with the Eurasian Plate) started forming about 50 million years ago, and continue to form today. The Appalachian Mountains in the U.S. formed during plate collision that resulted in the creation of Pangaea about 480 million years ago. They are very old and have been worn down by erosion over time. Figure 15 Depiction of the plate interaction that created the Himalayas (top), the Himalayas (bottom left) are still being formed; the Appalachians (bottom right) are smooth due to millions of years of erosion. 12

13 Transform boundaries The third type of plate boundary is called a transform boundary. It is also knows as a conservative plate boundary, because these faults neither create nor destroy lithospheric plates, their movement is predominantly horizontal, as the plates slide past each other. They can be on land, like the San Andreas Fault in California, Figure 16. They can be found under the ocean, hidden in the deep oceans where they form a series of short zig zags. Here the plates move in opposite directions or in the same direction at different rates. When one plate slips past another suddenly, earthquakes occur. The Pacific Plate is sliding past the North American Plate, forming the famous San Andreas Fault in California, as seen in Figure 16. The San Andreas Fault is part of a transform plate boundary. It has been the site of many earthquakes. Figure 16 The San Andreas Fault is a transform boundary. Causes of Plate Tectonics Convection inside the Earth This analogy may help you understand the Earth s convection currents. Soup that is cooking in a pan on the stove contains currents caused by an unequal distribution of heat in the pan. Hot, less dense soup is forced upward by the surrounding, cooler, denser soup. As the hot soup reaches the surface, it cools and sinks back down into the pan. This entire cycle of heating, rising, cooling, and sinking is called convection current. A version of this same process, occurring in the mantle, is thought to be the force behind plate tectonics. Scientists suggest that differences in density cause hot, plastic-like rock to be forced upward toward the surface. 13

14 Moving Mantle Material Even though Wegner wasn t able to come up with an explanation for why plates move, others were able to build on his ideas and researchers have determined the plates move due to the convection currents within the asthenosphere. It is, therefore, the transfer of heat inside Earth that provides the energy to move plates and causes many of Earth s surface features. Plate motion is directly caused by the movement of convection currents. Figure 17 Convection currents drive plate motion. Features Caused by Plate Tectonics Earth is a dynamic planet with a hot interior. This heat leads to convection, which powers the movement of plates. As the plates move, they interact. The interaction of plates produces forces that build mountains, create ocean basins, and cause volcanoes. When rocks in Earth s crust break and move, energy is released in the form of seismic waves. Humans feel this release as earthquakes. You can see some of the effects of plate tectonics in mountainous regions, where volcanoes erupt, or where landscapes have changes as a result of past earthquake or volcanic activity. 14

15 Normal Faults and Rift Valleys Tension forces, which are forces that pull apart, can stretch Earth s crust. This causes large blocks of crust to break and tilt or slide down the broken surfaces of crust. When rocks break and move along surfaces, a fault forms. Faults interrupt rock layers by moving them out of place. Entire mountain ranges can form in the process, called fault-block mountains, as shown in Figure 18. Generally, the faults that form from pull-apart forces are normal faults faults in which the rock layers above the fault move down when compared with rock layers below the fault. Rift valleys and mid-ocean ridges can form where Earth s crust separates. Examples of rift valleys are the Great Rift Valley in Africa, and the valleys that occur in the middle of mid-ocean ridges. Examples of mid-ocean ridges include the Mid-Atlantic Ridge and the East Pacific Rise. Figure 18 Fault-block mountains can form when the Earth s crust us stretched by tectonic forces. The arrows show the direction of moving blocks. Mountain and Volcanoes Compression forces squeeze objects together. Where plates come together, compression forces produce several effects. As continental plates collide, the forces that are generated cause massive folding and faulting of rock layers into mountain ranges such as the Himalaya, shown previously in Figure 15, and the Appalachian Mountains. The type of faulting produced is generally reverse faulting. Along a reverse fault, the rock layers above the fault surface move up relative to the rock layers below the fault. As you learned earlier, when two oceanic plates converge, the denser plate is forced under the other plate. Curved chains of volcanic islands called island arcs form above the sinking plate, see previous Figure 14. If an oceanic plate converges with a continental plate, the denser oceanic plate slides under the continental plate. Folding and faulting at the continental plate margin can thicken the continental crust to produce mountain ranges. Volcanoes also typically are formed at this type of convergent boundary. 15

16 Strike-Slip Faults Figure 19 At transform boundaries, two plates slide past one another without converging or diverging. The plates stick and then slide, mostly in a horizontal direction, along large strike-slip faults. In a strike-slip fault, rocks on opposite sides of the fault move in opposite directions, or in the same direction at different rates. This type of fault movement is shown in Figure 19. One such example is the San Andreas Fault. When plates move suddenly, vibrations are generated inside Earth that are felt as an earthquake. Earthquakes, volcanoes, and mountain ranges are evidence of plate motion. Plate tectonics explains how activity inside Earth can affect Earth s crust differently in different locations. You ve seen how plates have moved since Pangaea separated. Is it possible for us to measure how far plates move each year? Measuring Plate Movements Until recently, we could only indirectly check plate movement, by studying the magnetic characteristics of rocks on the seafloor or volcanoes and earthquakes. New technology now allows us to measure the amount of movement of Earth s plates to 1 cm. Using a Satellite Laser Ranging System or SLRS, scientists now directly measure plate movement by bounce laser beams from a station on one plate off a satellite, to a station on another plate measure the elapsed time measure elapsed time at a later date use the difference in elapsed times to calculate the rate of movement between the two plates. Current data shows that Hawaii is moving toward Japan at a rate of about 8.3 cm per year. Maryland is moving away from England at a rate of 1.7 cm per year. 16

17 What have you learned? Plate tectonic theory arose out of the hypothesis of continental drift proposed by Alfred Wegener. He suggested that continents once formed a single land mass that drifted apart. But without evidence of how the continents moved, it remained a hypothesis. Technological advancements allowed scientists to discover more natural phenomena in support of Wegener s idea, such as seafloor spreading, reversals of magnetic field directions, earthquake and volcanic activity in specific locations, the presence of hotspots and transform boundaries. The idea of Plate Tectonics unifies all these ideas to give a complete picture of these natural occurrences. The theory of Plate Tectonics explains the formation, movement and subduction of the Earth s plates as follows: The Earth s plastic-like layer below the lithosphere is called the asthenosphere, it has convection currents rising from the hot core to the cooler lithosphere then cycling back again and again. Earth's surface is covered by a series of crustal plates (lithosphere), which float on the asthenosphere. Cycling of the convection currents in the asthenosphere move the plates in different directions, causing changes in the Earth s surface. The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated. Changes in the Earth s surface include volcanoes, mountain ranges, and deep ocean trenches. Plates move along their boundaries in several ways. The interaction of the plates, give us the geologic features of our Earth. Tension forces cause normal faults, rift valleys, and mid-ocean ridges at divergent boundaries. At convergent boundaries, compression forces cause folding, reverse faults, and mountains. At transform boundaries, two plates slide past one another along strike-slip faults. New technology using a satellite and lasers allow scientists to measure plate movement as little as 1 cm. 17

18 Lesson Review 1. How did the theory of plate tectonics evolve? 2. What is the difference between the theory of continental drift and the theory of plate tectonics? 3. Name and describe the three primary types of plate boundaries and what landforms do they cause? 4. What natural processes occur as a result of tectonic plate movement? 5. What scientific evidence supports the theory of plate tectonics? 6. Name the types of boundaries? a. b. 7. Explain a transform boundary and give an example. 18