Continental Drift: An Idea Before Its Time Alfred Wegener (1880 1930) Continental drift hypothesis: The world's continents are in motion and have been drifting apart into different configurations over geologic time. Proposed that the continents were at one time joined together to form the supercontinent of Pangaea "universal land" 2013 Pearson Education, Inc.
Evidence for Continental Drift Wegener used evidence from many disciplines to support his hypothesis: Jigsaw fit of the continents Fossil evidence Matching rock types Structural similarities in mountain chains on different continents Paleoclimatic evidence 2013 Pearson Education, Inc.
Rock Correlation Across Seas
Rock Correlation Across Seas
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Paleoclimatic evidence
A Mechanism for Continental Drift Despite evidence to support continental drift, Wegener could not explain how the continents moved. Without a suitable explanation, Wegener's ideas were dismissed. Detailed mapping of the seafloor revealed: Huge mountain ranges in the middle of ocean basins Deep trenches alongside some continental margins So, the deepest parts of the ocean are near the continents, and out in the middle of the ocean, the water is relatively shallow.
Earth s deep-ocean trenches and ridges (rises)
Distribution of the oceanic ridge system
Harry Hess's hypothesis of seafloor spreading provided the mechanism for continental drift. The seafloor is not permanent; it is constantly being renewed. Mid-ocean ridges are sites of new lithosphere formation. Oceanic trenches are sites of lithosphere destruction (subduction). 2013 Pearson Education, Inc.
Seafloor Spreading Is Supported by Magnetic Studies of the Ocean Floor Lava erupted at the mid-ocean ridges is rich in iron. Magnetite crystals in lava align themselves to Earth's magnetic field. Earth's magnetic poles flip the north and south poles exchange positions. This is known as magnetic reversal. 2013 Pearson Education, Inc.
Seafloor Spreading Is Supported by Magnetic Studies of the Ocean Floor The seafloor holds a record of Earth's magnetic field at the time the rocks of the seafloor cooled. The magnetic record appears as parallel, zebralike stripes on both sides of mid-ocean ridges. The age of the ocean floor and the rate of seafloor spreading could be determined 2013 Pearson Education, Inc.
New crust generated here! *
Seafloor Spreading and Magnetization
Seafloor spreading hypothesis Hot, molten rock moves up from Earth s interior emerging along a rift Outflow in both directions creates new rocks Drilling evidence Thin sediment layers near ridge, becoming thicker toward continents Older fossils and rocks near continents, becoming younger near rifts
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Plate Tectonics: A Modern Version of an Old Idea Plate tectonics is the unifying theory that explains the dramatic, changing surface features of the Earth in terms of the shifting of tectonic plates. 2013 Pearson Education, Inc.
Features of Tectonic Plates Plates are sections of Earth's strong, rigid outer layer the lithosphere. Because plates are composed of lithosphere, they consist of the uppermost mantle and oceanic or continental crust. Plates overlie the weaker asthenosphere. They ride along with the asthenosphere as it flows. 2013 Pearson Education, Inc.
Earth is divided into a dozen or so major lithospheric plates as well as a few smaller ones. Plates are in motion and continually changing in shape and size. The largest plate is the Pacific Plate. Several plates include an entire continent plus a large area of seafloor. 2013 Pearson Education, Inc.
Speed of Tectonic Plates Earth's plates move in different directions and at different speeds. Continental plates tend to move slowly. Oceanic plates tend to move faster. 2013 Pearson Education, Inc.
Plate Boundaries Interactions between plates occur along plate boundaries. The creation and destruction of lithosphere occur along plate boundaries. Earthquakes, volcanoes, and mountains occur along plate boundaries and sometimes along former plate boundaries. 2013 Pearson Education, Inc.
Plate Tectonics: Three Types of Plate Boundaries Divergent plate boundaries Magma generation and lithosphere formation Convergent plate boundaries Magma generation and lithosphere destruction Transform fault boundaries No magma generation, no formation or destruction of lithosphere 2013 Pearson Education, Inc.
Divergent Boundary Features Plates move away from one another. As plates move apart, the asthenosphere rises and partially melts to form lava. New crust is formed as lava fills in the gaps between plates. In the ocean, there is seafloor spreading. Mid-ocean ridge On land, continents tear apart. Rift valley Shallow earthquakes occur. 2013 Pearson Education, Inc.
Continental rifting
Convergent Boundary Features Plates move toward each other. Oceanic crust is destroyed when plates converge at a subduction zone. Continental crust is deformed when plates collide. Deep earthquakes occur at subduction zones. 2013 Pearson Education, Inc.
Three possibilities 1. Converging continental and oceanic plates 2. Converging oceanic plates 3. Converging continental plates
Types of Convergent Boundaries Oceanic oceanic convergence When two oceanic plates converge, the older and denser plate descends beneath the other plate. As the denser plate descends, partial melting of mantle rock generates magma and volcanoes. If the volcanoes emerge as islands, a volcanic island arc is formed (examples: Japan, Aleutian islands, Tonga islands). 2013 Pearson Education, Inc.
Oceanic continental convergence The denser oceanic slab sinks into the asthenosphere. As the denser plate descends, partial melting of mantle rock generates magma. The mountains produced by volcanic activity from subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades). 2013 Pearson Education, Inc.
Continental continental convergence Continued subduction can bring two continents together. The less dense, buoyant continental lithosphere does not subduct. The result is a collision between two continental blocks. The process produces mountains (Himalayas, Alps, Appalachians). 2013 Pearson Education, Inc.
Deformed and thickened granitic crust
Transform-Fault Boundaries Plates slide past one another and no new lithosphere is created or destroyed. Most transform faults join two segments of a mid-ocean ridge. Transform faults are oriented perpendicular to mid-ocean ridges. Permits plates to move from offset ridge segments Shallow but strong earthquakes occur.
Transform faults
Transform Faults, Plate Margins
Transform-Fault Boundaries Most transform-fault boundaries are located within ocean basins. A few transform-fault boundaries, such as the infamous San Andreas Fault, cut through continental crust. 2013 Pearson Education, Inc.
http://www.youtube.com/watch?v=ugcded4xvd4 2013 Pearson Education, Inc.
http://media.hhmi.org/biointeracti ve/earthviewer_web/earthviewer.h tml
Plate Boundary Features
Plate Puzzle Activity Give one piece of the puzzle to each student or pair of students. Tell them that they will be responsible for their puzzle piece placing it in the right position to form the world map and determining the piece s motion with respect to surrounding pieces. Have the students assemble the map (similar to putting together a jigsaw puzzle) on the floor or on a large table. Once the plates are assembled, have the students note the vectors on the plates. The numbers along the arrows indicate the plate s speed (in mm/y). Some plates have multiple vectors. This shows that different portions of the plate are moving at different speeds in slightly different directions.
Plate Puzzle Activity After a brief time for discussion, ask a few students to explain what their plate s motion is and how it is interacting with adjacent plates. Tell them that as the plates move, they build up stress and that at a certain point the stress is released, causing an earthquake. You may also teach them how plate movement builds mountain ranges and volcanoes. Note that volcanoes and earthquake occurrences are accumulated along plate boundaries. Also, you can point out specific landmarks such as the Ring of Fire and mountain ranges such as the Himalayas.
Plate Puzzle Activity To discuss specific plate movement, here is further information: Convergence: the South American plate and the Nazca Plate, the Western Pacific, India and Asia. Note that convergence occurs when two plates are moving in almost opposite directions (for example, South America and Nazca), or when two plates are moving in nearly the same direction but the plate that is following is moving faster (for example, the Pacific Plate and the Philippine Plate). These two types of motions that result in convergence could be modeled with two parallel lines of students representing the edges of two plates. In the first type of convergence, the students face each other and walk slowly forward until collision. In the second type of convergence, the students face the same direction and walk slowly forward, with the second line of students walking faster until colliding with the first line.
Plate Puzzle Activity These two types of motions that result in convergence could be modeled with two parallel lines of students representing the edges of two plates. In the first type of convergence, the students face each other and walk slowly forward until collision. In the second type of convergence, the students face the same direction and walk slowly forward, with the second line of students walking faster until colliding with the first line.
Plate Puzzle Activity To discuss specific plate movement, here is further information: Divergence; for example: the Mid Atlantic Ridge (note Iceland on two sides of the ridge) or the East Pacific Rise. Transform [this one is more difficult]; for example: the San Andreas fault in California, New Zealand (the Alpine fault), and the transform faults along the southern boundary of the Nazca Plate.
Exploring Plate Boundaries with Seismic Data Activity Use the IRIS Earthquake Browser www.iris.edu/ieb Explore current seismicity of the planet Explore the earthquake patterns at convergent and divergent plate boundaries Identify geomorphological features associated with plate boundaries
Exploring Plate Boundaries with Seismic Data Activity Questions Do you have a good sense of where plate boundaries are? If not, how can you change the settings to give you a better idea of where plate boundaries may be? How many large earthquakes ( 7.0 magnitude) have occurred so far this year? Where do the deepest earthquakes ( 300 km) tend to occur? Switch to a satellite view of the world. Do earthquake patterns correlate with any visible features? If so, describe the correlations.
Exploring Plate Boundaries with Seismic Data Apply knowledge to identify a developing plate boundary Identify and describe earthquake pattern in East Africa Use evidence (seismicity and geomorphology) to determine the type of boundary in East Africa and Southern Alaska Make predictions about future geomorphology in the region
Exploring Plate Boundaries with Seismic Data Questions Which countries are the most seismically active? Do earthquakes in East Africa tend to be deep or shallow? Of the types of boundaries investigated earlier, which one best explains the pattern you see in East Africa? In satellite view, do you see any features consistent with the type of boundary you proposed in the previous question? If so, identify them. If not, why do you think that is? Based on the type of plate boundary you have discovered in this region, what do you think will happen in this region of the world over time? What large-scale features do you expect to see developing in the future?