Plate Tectonics

1. Plate Tectonics Chapter 10 – Section 1

2. Ch. 10-1: Continental Drift • 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. Wegener’s Hypothesis • Early map makers noticed that the edges of the continents looked like puzzle pieces that may fit together. • Alfred Wegener was a German scientist and in 1912 he proposed the continental drift theory. • He hypothesized that all the continents were once part of a single landmass that he called a supercontinent. • 250 million years ago they began to break up and drift to where they are now found today • Speculated that mountain ranges were areas of Earth’s crust that were crumpled up as the land masses moved

4. Evidence for Wegener’s Theory • Fossil Evidence – fossils of certain animals were found on multiple continents. These animals could not swim and there was no evidence of land bridges. • Evidence from Rock Formations – Mountain ranges of similar age and composition extended across several continents. If lined up, they would have been part of the same chain • Climatic Evidence – Plant and debris showed evidence of glaciers in now tropical areas. Also, subtropical swamps once covered areas that are now much colder

5. Missing Mechanisms • Wegerner’s theory did not take off during his lifetime because he was unable to explain how the continents actually drifted. • He proposed that continents plowed through the rock on the ocean floor, but that was easily shown not to be true. • Wegener died in 1930 still trying to find a plausible explanation for continental drift.

6. Mid-Ocean Ridges • In 1947 the evidence needed to support Wegener’s theory came to light when a group of scientists began studying the Mid-Atlantic Ridge. • Mid-ocean ridges are undersea mountain ranges w/ a steep, narrow valley running through the center. • They noticed that the sediment covering the ocean floor was thicker the further they travelled from the ridge. • They also found that the closer to the ridge, the younger the sediment and actual rock layers were. • Figure 5, p. 242

7. Sea-Floor Spreading • Figure 6, p. 243 • In the late 1950’s, Harry Hess, a geologist, proposed that at the center of a valley there was a rift, or crack, that would form as the two edges of the ridge pulled apart. • Magma would then rise to the surface and fill the crack, cooling into new rock • The ridge would move apart again and a new crack would form, magma would rise and cool, newer rock formed again. • Robert Dietz named this process sea-floor spreading • Evidence to support this theory came in the form of paleomagnetism in the mid 1960s.

8. Paleomagnetism • Figure 7 , p. 244 • Paleomagnetism is the residual magnetism of rock as it solidifies from iron-rich minerals in magma. • Magnetic reversals – using patterns of normal polarity and reversed polarity to form the geomagnetic reversal time scale. • Figure 8, p. 245 • Magnetic symmetry: matching regions of polarity on either side of the rift valley.

9. Wegener Redeemed • The reversal patterns of the ocean floor also matched rocks found on land and supported the continental drift theory. • Now that scientists could see the mechanism behind the theory, they supported Wegener’s original idea.

10. Plate Tectonics Chapter 10 – Section 2

11. Ch. 10-2 The Theory of Plate Tectonics • Summarize the theory of plate tectonics. • Identify and describe the three types of plate boundaries. • List and describe three causes of plate movement.

12. Plate Tectonics • The theory that explains why and how continents move. The study of the formation of features in Earth’s crust. • Lithosphere is the crust or thin outer shell of Earth. • It is broken into several blocks called tectonic plates, figure 1, p. 247 • Aesthenosphere is a layer of “plastic” rock that moves much like silly putty and allows the tectonic plates to move across it. • Oceanic crust is very dense rock that is rich in Mg and Fe • Continental crust is less dense rock and is rich in Si • Tectonic plates may be one or both types of crust

13. Tectonic Activity • 15 major plates have been identified – Figure 2, p. 248 • Earthquakes – sudden movements or shifts in plates along the boundaries trigger earthquakes. Plotting locations of quakes can help locate plate boundaries • Volcanoes – locations of these also help locate plate boundaries. • Example: Pacific Ring of Fire

14. Types of Plate Boundaries • Divergent – 2 plates are pulling away from each other. Figure 3, p. 249 • Forms mid-ocean ridges and rift valleys • Convergent – 2 plates are colliding. Figure 4, p. 250 • Three types of collisions can occur at convergent boundaries • Oceanic & Continental lithospheres collide & oceanic lithosphere subducts. Deep ocean trenches form at subduction zones. • 2 continental plates collide, edges crumple and lift up forming mountain chains • 2 oceanic plates collide, one subducts, magma may rise and form an island arc.

15. Types of Boundaries Cont’d • Tranform boundaries – 2 plates slide past each other horizontally. Figure 5, p. 251 • Transform boundaries do not produce magma, but they do form fault lines or fracture zones. • Short segments of mid-ocean ridge can be connected by transform boundaries forming a fracture zone • Table 1, p. 251 summarizes all 3 types of plate boundaries and movements that occur at each.

16. Causes of Plate Motion • Force behind plate movement is not fully understood, but most believe convection to play a part. • The cycle is called a convection cell – Figure 6, p. 252 • Mantle convection may occur due to energy generated in Earth’s core and radioactivity w/in the mantle. • Convection currents may drag along the bottom of the plates, pulling/pushing them across the surface of the aesthenosphere.

17. Causes of Plate Motion • Ridge Push – Newly formed rock at a mid-ocean ridge is less dense. As it cools it begins to become more dense, it sinks, and it pushes against the existing plate Figure 7, p. 253 • Slab Pull – In an area of a subduction zone, one are of lithosphere is being pulled below another. As the plate begins its downward movement, the rest of the plate is pulled along with it. Figure 8, p. 254.

18. Plate Tectonics Chapter 10 – Section 3

19. Ch. 10-3: The Changing Continents • 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.

20. Reshaping Earth’s Crust • The continents are constantly changing in shape and size. They always have been and always will. • Cratons – large areas of stable rocks that are older than 540 millions years old. • Shields – Rocks within the cratons that have been exposed at Earth’s surface. • Rifting and Continental Reduction – Figure 1, p. 255 • Rifting is the breaking apart of continents • May be due to a build-up of heat in Earth’s interior, as Si is an insulator in continental crust

21. Terranes & Continental Growth • Continents may also grow by gaining new material, such as terranes • Terranes are pieces of lithosphere that have unique geologic histories compared to the surrounding lithosphere. • Figure 2, p. 256 show terranes becoming part of a continent at convergent boundaries, a process called accretion. • Types of materials that form terranes may include sea-mounts, atolls, or continental crust (mountain chains)

22. Effects of Continental Change • Changes in climate • Location in relation to poles or equator • Location in relation to oceans, mountains, or other continents • Evidence shows that even Africa’s Sahara desert was once covered by glaciers. • Changes in life – Figure 3, p. 257 • New species can form due to rifting, separating an existing species • Isolation can allow for unique adaptations to occur

23. The Supercontinent Cycle • The process by which supercontinent form and break apart • Convergent plate boundaries collide, heat builds up underneath continental plates, and they break apart once again. Figure 4, p. 258-259 • Pangaea formed 300 million years ago, Panthalassa was the single large ocean that surrounded it. • 250 million years ago Pangaea broke apart into Laurasia and Gondwanaland, further rifts and collisions brought continents to current locations.

24. Geography of the Future • Figure 5, p. 260 • Continued plate movement at current pace will cause Africa to collide with Eurasia, closing the Mediterranean Sea • A new ocean will form as east Africa breaks from the rest of Africa. North & South America will move east across the Atlantic Ocean, causing it to close as the continents collide with Africa. • Part of North America will shift to where Alaska sits today, LA will sit north of where San Francisco currently is • In 250 million years the continents will form the next supercontinent • How might climates change??