Section 1: Continental Drift

Transcription

1 Plate Tectonics Section 1 Section 1: Continental Drift Preview Key Ideas Wegener s Hypothesis Sea-Floor Spreading Paleomagnetism Wegener Redeemed Continental Drift (Pangaea)

2 Plate Tectonics Section 1 Key Ideas Summarize Wegener s hypothesis of continental drift. Describe the process of sea-floor spreading. Identify how paleomagnetism provides support for the idea of sea-floor spreading. Explain how sea-floor spreading provides a mechanism for continental drift.

3 Plate Tectonics Section 1 Wegener s Hypothesis continental drift the hypothesis that a single large landmass broke up into smaller landmasses to form continents, which then drifted to their present location. As people studied continental coastlines on maps, they noticed that the continents looked as though they would fit together like parts of a giant jigsaw puzzle. The hypothesis of continental drift was first proposed by German scientist Alfred Wegener in Wegener used several different types of evidence to support his hypothesis.

4 Plate Tectonics Section 1 Wegener s Hypothesis, continued Fossil Evidence Fossils of the same plants and animals could be found in areas of continents that had once been connected. Evidence from Rock Formations Ages and types of rocks in the coastal regions of widely separated areas matched closely. Climactic Evidence Changes in climatic patterns suggested the continents had not always been located where they are now.

5 Plate Tectonics Section 1 Wegener s Hypothesis, continued Similar rock formations and fossil evidence supported Wegener s hypothesis.

6 Plate Tectonics Section 1 Wegener s Hypothesis, continued Missing Mechanisms Despite the evidence that supported his hypothesis, Wegener s ideas were strongly opposed. Other scientists of the time rejected the mechanism proposed by Wegener to explain how continents moved. Wegener suggested that the continents moved by plowing through the rock of the ocean floor. This idea was shown to be physically impossible. Wegener spent the rest of his life searching for a mechanism for the movement of continents.

7 Plate Tectonics Section 1 Wegener s Hypothesis, continued Reading Check Why did many scientists reject Wegener s hypothesis of continental drift? Many scientists rejected Wegener s hypothesis because the mechanism that Wegener suggested was easily disproved by geologic evidence.

8 Plate Tectonics Section 1 Wegener s Hypothesis, continued Mid-Ocean Ridges mid-ocean ridge a long, undersea mountain chain that has a steep, narrow valley at its center, that forms as magma rises from the asthenosphere, and that creates new oceanic lithosphere (sea floor) as tectonic plates move apart The evidence that Wegener needed to support his hypothesis was discovered nearly two decades after his death.

9 Plate Tectonics Section 1 Wegener s Hypothesis, continued Mid-Ocean Ridges In 1947, a group of scientists set out to map the Mid- Atlantic Ridge. While studying the Mid-Atlantic Ridge, scientists noticed two surprising trends. 1. The sediment that covers the sea floor is thinner closer to a ridge than it is farther from the ridge 2. The ocean floor is very young. While rocks on land are as much as 4 billion years old, none of the oceanic rocks are more than 200 million years old.

10 Plate Tectonics Section 1 Wegener s Hypothesis, continued Mid-Ocean Ridges, continued Rocks closer to a mid-ocean ridge are younger than rocks farther from the ridge. Rocks closer to the ridge are covered with less sediment than rocks farther from the ridge.

11 Plate Tectonics Section 1 Sea-Floor Spreading In the late 1950s, geologist Harry Hess proposed that the valley at the center of a mid-ocean ridge was a crack, or rift, in Earth s crust. As the ocean floor moves away from the ridge, molten rock, or magma, rises to fill the crack. sea-floor spreading the process by which new oceanic lithosphere (sea floor) forms as magma rises to Earth s surface and solidifies at a mid-ocean ridge Hess suggested that if the sea floor is moving, the continents might be moving also. He suggested this might be the mechanism that Wegener was searching for.

12 Plate Tectonics Section 1 Sea-Floor Spreading, continued As the ocean floor spreads apart, magma rises to fill the rift and then cools to form new rock.

13 Plate Tectonics Section 1 Sea-Floor Spreading, continued Reading Check How does new sea floor form? New sea floor forms as magma rises to fill the rift that forms when the ocean floor moves away from a midocean ridge.

14 Plate Tectonics Section 1 Paleomagnetism As magma solidifies to form rock, iron-rich minerals in the magma align with Earth s magnetic field. When the rock hardens, the magnetic orientation of the minerals becomes permanent. paleomagnetism the study of the alignment of magnetic minerals in rock, specifically as it relates to the reversal of Earth s magnetic poles; also the magnetic properties that rock acquires during formation

15 Plate Tectonics Section 1 Paleomagnetism, continued Magnetic Reversals Scientists have discovered rocks whose magnetic orientations point opposite of Earth s current magnetic field. Rocks with magnetic fields that point north, or normal polarity, are classified in the same time interval. Rocks with magnetic fields that point south, or reverse polarity, also fall into specific time intervals. When scientists placed these periods of normal and reversed polarity in chronological order, they discovered a pattern of alternating normal and reversed polarity in the rocks. Scientists used this pattern to create the geomagnetic reversal time scale.

16 Plate Tectonics Section 1 Paleomagnetism, continued Magnetic Symmetry Scientists discovered a striped magnetic pattern on the ocean floor on each side of a mid-ocean ridge. The pattern on one side of the ridge is a mirror image of the pattern on the other side. When drawn on a map, these patterns match the geomagnetic reversal time scale. The symmetry of magnetic patterns and the symmetry of ages of sea-floor rocks supoprts Hess s idea of sea-floor spreading.

17 Plate Tectonics Section 1 Paleomagnetism, continued Magnetic Symmetry The pattern of magnetic symmetry and age of rock formation indicate that new rock forms at the center of a ridge and then move away from the center in opposite directions.

18 Plate Tectonics Section 1 Paleomagnetism, continued Reading Check How are magnetic patterns in sea-floor rock evidence of sea-floor spreading? The symmetrical magnetic patterns in sea-floor rocks show that rocks formed at one place (at a ridge) and then broke apart and moved away from the center in opposite directions.

19 Plate Tectonics Section 1 Wegener Redeemed Reversal patterns on the sea floor could also be found on land. The reversals in land rocks also matched the geomagnetic reversal time scale. Because the same pattern appears in rocks of the same ages on both land and the sea floor, scientists agreed that the magnetic patterns showed change over time. The idea of sea-floor spreading provides a way for the continents to move over the Earth s surface. Sea-floor spreading was the mechanism that verified Wegener s hypothesis of continental drift.

20 Plate Tectonics Section 1 Continental Drift (Pangaea) Click below to watch the Visual Concept.

21 Plate Tectonics Section 2 Section 2: The Theory of Plate Tectonics Preview Key Ideas How Continents Move Tectonic Plates Types of Plate Boundaries Causes of Plate Motion Plate Tectonics

22 Plate Tectonics Section 2 Key Ideas Summarize the theory of plate tectonics. Identify and describe the three types of plate boundaries. List and describe three causes of plate movement.

23 Plate Tectonics Section 2 How Continents Move plate tectonics the theory that explains how large pieces of the lithosphere, called plates, move and change shape. lithosphere the solid, outer layer of Earth that consists of the crust and the rigid upper part of the mantle. asthenosphere the solid, plastic layer of the mantle beneath the lithosphere; made of mantle rock that flows very slowly, which allows tectonic plates to move on top of it.

24 Plate Tectonics Section 2 How Continents Move, continued The lithosphere forms the thin outer shell of Earth and is broken into several blocks, or tectonic plates. The tectonic plates ride on the asthenosphere in much the same way that blocks of wood float on water. Tectonic plates can include continental crust, oceanic crust, or both. Continents and oceans are carried along on the moving tectonic plates in the same way that passengers are carried by a bus.

25 Plate Tectonics Section 2 Tectonic Plates Scientists have identified about 15 major tectonic plates and many smaller plates. Plates are often bordered by major surface features, such as mountain ranges or deep trenches in the oceans. Scientists identify plate boundaries primarily by studying data from earthquakes. The locations of volcanoes can also help identify the locations of plate boundaries.

26 Plate Tectonics Section 2 Tectonic Plates, continued The boundaries of tectonic plates do not always match the outlines of continents.

27 Plate Tectonics Section 2 Tectonic Plates, continued Reading Check How do scientists identify locations of plate boundaries? Scientists use the locations of earthquakes, volcanoes, trenches, and mid-ocean ridges to outline tectonic plates.

28 Plate Tectonics Section 2 Types of Plate Boundaries Tectonic plate boundaries may be in the middle of the ocean floor, around the edges of continents, or even within continents. The three types of plate boundaries are divergent boundaries, convergent boundaries, and transform boundaries. Each plate boundary is associated with a characteristic type of geologic activity.

29 Plate Tectonics Section 2 Types of Plate Boundaries, continued Divergent Boundaries divergent boundary the boundary between tectonic plates that are moving away from each other Magma rises to the surface and cools to form the warm, light rock that sits higher than the surrounding sea floor because it is less dense. The narrow valley that forms where the plates separate is a rift valley.

30 Plate Tectonics Section 2 Types of Plate Boundaries, continued Convergent Boundaries convergent boundary the boundary between tectonic plates that are colliding When oceanic lithosphere collides with continental lithosphere, the denser oceanic lithosphere subducts, or sinks under the less dense continental lithosphere. The region along this plate boundary is called a subduction zone.

31 Plate Tectonics Section 2 Types of Plate Boundaries, continued Reading Check Describe the three types of collisions that happen at convergent boundaries. Collisions at convergent boundaries can happen between two oceanic plates, between two continental plates, or between one oceanic plate and one continental plate.

32 Plate Tectonics Section 2 Types of Plate Boundaries, continued Convergent Boundaries, continued When two plates that are made of continental lithosphere collide, the colliding edges crumple and thicken, which cause uplift that forms large mountain ranges. When two plates that are made of oceanic lithosphere collide, one plate subducts under the other plate, and a deep-ocean trench forms. A chain of volcanic islands, called an island arc, may form.

33 Plate Tectonics Section 2 Types of Plate Boundaries, continued Transform Boundaries transform boundary the boundary between tectonic plates that are sliding past each other horizontally Plate edges at a transform boundary scrape against each other in a series of sudden spurts of motions that are felt as earthquakes. Short segments of a mid-ocean ridge are connected by transform boundaries called fracture zones.

34 Plate Tectonics Section 2 Causes of Plate Motion The movement of tectonic plates is part of the mantle convection system. Convection is the movement of heated material due to differences in density that are caused by differences in temperatures. The convection process can be modeled by boiling water in a pot on the stove. As the water at the bottom of the pot is heated, the water at the bottom expands and becomes less dense than the cooler water above it. The cooler, denser water sinks, and the warmer water rises to the surface to create a cycle called a convection cell.

35 Plate Tectonics Section 2 Causes of Plate Motion, continued Mantle Convection Scientists think that Earth is also a convecting system. Energy generated by Earth s core and radioactivity within the mantle heat the mantle material. This heated material rises through the cooler, denser material around it. As the hot material rises, the cooler, denser material flows away from the hot material and sinks into the mantle to replace the rising material. As the mantle material moves, the overlying tectonic plates move along with it.

36 Plate Tectonics Section 2 Causes of Plate Motion, continued Ridge Push Newly formed rock at a mid-ocean ridge is warmer and less dense than older rock nearby. As the newer rock cools and becomes denser, it begins to slide down the slope between the lithosphere and asthenosphere. The sliding rock exerts a force called a ridge push, which pushes the rest of the plate away from the midocean ridge.

37 Plate Tectonics Section 2 Causes of Plate Motion, continued

38 Plate Tectonics Section 2 Causes of Plate Motion, continued Reading Check How may density differences in the rock at a mid-ocean ridge help to drive plate motions? When denser lithosphere sinks into the asthenosphere, the asthenosphere must move out of the way. As the asthenosphere moves, it drags or pushes on other parts of the lithosphere, which causes movement.

39 Plate Tectonics Section 2 Causes of Plate Motion, continued Slab Pull Where the lithosphere cools and becomes dense enough, it begins to subduct into the asthenosphere. As the leading edge of the plate sinks, it pulls the rest of the plate along behind it. The force exerted by the sinking plate is called slab pull. In general, plates that are subducting move faster than plates that are not subducting. This evidence indicates that the downward pull of the subducting lithosphere is a strong driving force for tectonic plate motion.

40 Plate Tectonics Section 2 Causes of Plate Motion, continued

41 Plate Tectonics Section 3 Section 3: The Changing Continents Preview Key Ideas Reshaping Earth s Crust Effects of Continental Change The Supercontinent Cycle Maps in Action

42 Plate Tectonics Section 3 Key Ideas Identify how movements of tectonic plates change Earth s surface. Summarize how movements of tectonic plates have influenced climates and life on Earth. Describe the supercontinent cycle.

43 Plate Tectonics Section 3 Reshaping Earth s Crust Slow movements of tectonic plates change the size and shape of the continents over millions of years. All of the continents that exist today contain large areas of stable rock, called cratons, that are older than 540 million years. Rocks within the cratons that have been exposed at Earth s surface are called shields. One way that continents change shape is by breaking apart. rifting the process by which Earth s crust breaks apart; can occur within continental crust or oceanic crust

44 Plate Tectonics Section 3 Reshaping Earth s Crust, continued Continents change not only by breaking apart but also by gaining material. Most continents consist of cratons surrounded by a patchwork of terranes. terrane a piece of lithosphere that has a unique geologic history and that may be part of a larger piece of lithosphere, such as a continent When a tectonic plate carrying a terrane subducts under a plate made of continental crust, the terrane is scraped off of the subducting plate and becomes part of the continent. The process in which a terrane becomes part of a continent is called accretion.

45 Plate Tectonics Section 3 Reshaping Earth s Crust, continued Reading Check Describe the process of accretion. As a plate subducts beneath another plate, islands and other land features on the subducting plate are scraped off the subducting plate and become part of the overriding plate.

46 Plate Tectonics Section 3 Effects of Continental Change Modern climates are a result of past movements of tectonic plates. When continents move, the flow of air and moisture around the globe changes and causes climates to change. Geologic evidence shows that ice once covered most of Earth s continental surfaces. As continents began to drift around the globe, however, global temperatures changed and much of the ice sheet melted. As continents rift or as mountains form, populations of organisms are separated. When populations are separated, new species may evolve from existing species.

47 Plate Tectonics Section 3 The Supercontinent Cycle Using evidence from many scientific fields, scientists can construct a general picture of continental change throughout time. Scientists think that, at several times in the past, the continents were arranged into large landmasses called supercontinents. Supercontinents broke apart to form smaller continents that moved around the globe. Eventually, the smaller continents joined again to form another supercontinent. supercontinent cycle the process by which supercontinents form and break apart over millions of years

48 Plate Tectonics Section 3 The Supercontinent Cycle, continued Why Supercontinents Form The movement of plates toward convergent boundaries causes continents to collide. Because neither continent subducts beneath the other, the plate boundary becomes inactive, and a new convergent boundary forms. Over time, all of the continents collide to form a supercontinent. As heat from Earth s interior builds up under the supercontinent, rifts form in the supercontinent. The supercontinent breaks apart, and plates carrying separate continents move around the globe.

49 Plate Tectonics Section 3 The Supercontinent Cycle, continued Formation of Pangaea Pangaea the supercontinent that formed 300 million years ago and that began to break up 200 million years ago Several mountain ranges, such as the Appalachian Mountains and the Ural Mountains formed during the collisions that created Pangaea. A body of water called the Tethys Sea cut into the eastern edge of Pangaea. Panthalassa the single, large ocean that covered Earth s surface during the time the supercontinent Pangaea existed

50 Plate Tectonics Section 3 The Supercontinent Cycle, continued

51 Plate Tectonics Section 3 The Supercontinent Cycle, continued

52 Plate Tectonics Section 3 The Supercontinent Cycle, continued Breakup of Pangaea About 200 million years ago (during the Mesozoic Era), Pangaea began to break into two continents-laurasia and Gondwanaland. Laurasia drifted northward, rotated, and broke up to form North America and Eurasia. Gondwanaland also broke into two continents. One broke apart to become Africa and South America. The other separated to form India, Australia, and Antarctica.

53 Plate Tectonics Section 3 The Supercontinent Cycle, continued

54 Plate Tectonics Section 3 The Supercontinent Cycle, continued The Modern Continents Slowly, the continents moved into their present positions. As the continents drifted, they collided with terranes and other continents. Mountain ranges, such as the Rocky Mountains, the Andes, and the Alps, formed. Tectonic plate motion also caused new oceans to open up and caused others to close.

55 Plate Tectonics Section 3 The Supercontinent Cycle, continued

56 Plate Tectonics Section 3 The Supercontinent Cycle, continued Reading Check What modern continents formed from Gondwanaland? The continents Africa, South America, Antarctica, and Australia formed from Gondwanaland. The subcontinent of India was also part of Gondwanaland.

57 Plate Tectonics Section 3 The Supercontinent Cycle, continued Geography of the Future As tectonic plates continue to move, Earth s geography will change dramatically. Scientists predict that in 250 million years, the continents will come together again to form a new supercontinent.