12/3/2014. Plate Tectonics: A Scientific Revolution Unfolds Earth Science, 13e Chapter 7. Continental drift: an idea before its time

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1 Plate Tectonics: A Scientific Revolution Unfolds Earth Science, 13e Chapter 7 Stanley C. Hatfield Southwestern Illinois College Continental drift: an idea before its time Alfred Wegener First proposed hypothesis, 1915 Continental drift hypothesis Supercontinent called Pangaea began breaking apart about 200 million years ago Continents drifted to present positions Pangaea approximately 200 million years ago 1

2 Continental drift: an idea before its time Wegener s continental drift hypothesis 4 Evidences used by Wegener Fit of South America and Africa Fossils match across the seas Rock types and structures match Ancient climates (ice and glaciers) Main objection to Wegener s proposal was its inability to provide a mechanism Similar mountain ranges on different continents Plate tectonics: the new paradigm More encompassing than continental drift Associated with Earth s rigid outer shell Called the lithosphere Consists of several plates Plates are moving slowly Largest plate is the Pacific plate Plates are mostly beneath the ocean 2

3 Plate tectonics: the new paradigm Asthenosphere Exists beneath the lithosphere Hotter and weaker than lithosphere Allows for motion of lithosphere Plate boundaries All major interactions among plates occur along their boundaries Plate tectonics: the new paradigm Plate boundaries 3. Types of plate boundaries 1. Divergent plate boundaries (constructive margins) Two plates move apart Mantle material upwells to create new seafloor Divergent boundaries are located along oceanic ridges 3

4 Video clip: 4.4a divergent boundary formation Make notes on how the oceanic-ridge was/is thought to have formed The East African rift a divergent boundary on land Plate tectonics: Plate boundaries Types of plate boundaries 2. Convergent plate boundaries (destructive margins) A. Plates collide, an ocean trench forms and lithosphere is subducted into the mantle 4

5 Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries 2. Convergent plate boundaries (destructive margins) A. Oceanic-continental convergence Denser oceanic slab sinks into the asthenosphere Pockets of magma develop and rise Continental volcanic arc forms Examples include the Andes, Cascades, and the Sierra-Nevadan system An oceanic-continental convergent plate boundary Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries 2. Convergent plate boundaries (destructive margins) B. Oceanic-oceanic convergence Two oceanic slabs converge and one descends beneath the other Often forms volcanoes on the ocean floor Volcanic island arc forms as volcanoes emerge from the sea Examples include the Aleutian, Mariana, and Tonga islands 5

6 An oceanic-oceanic convergent plate boundary An oceanic-oceanic convergent plate boundary Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries 3. Convergent plate boundaries (destructive margins) 3. Continental-continental convergence When subducting plates contain continental material, two continents collide Can produce new mountain ranges such as the Himalayas 6

7 The collision of India and Asia produced the Himalayas Video clip 4.4b Converg margins - India and Eurasia Make notes on what created the Himalayan mountain range The collision of India and Asia produced the Himalayas 7

8 Plate tectonics: Plate boundaries Types of plate boundaries 3. Transform fault boundaries Plates slide past one another No new crust is created or destroyed Transform faults Most join two segments of a mid-ocean ridge Aid the movement of oceanic crustal material Video clip: 4.4c Plate boundary features Make notes on where earthquakes, volcanoes occur in regards to plate boundaries and which types Testing the plate tectonics model Evidence: A. From ocean drilling Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment Age of deepest sediments (using oxygen isotopes in Foraminifera shells, and potassium-argon dating) Thickness of ocean-floor sediments verifies seafloor spreading more on this 8

9 Let s talk sediment! Sediment is simply rocks, dirt and other material that has been eroded by the continents and deposited on the ocean floor Deep sea drilling and seismic surveys have revealed information about Percent the of Total thickness of Percent of ocean Region sediment: Continental shelves Continental slopes Ocean Area Volume of Marine Sediments Average Thickness 9% 15% 2.5 km (1.6 mi) 6% 41% 9 km (5.6 mi) Continental rises 6% 31% 8 km (5 mi) Deep-ocean floor 78% 13% 0.6 km (0.4 mi) Why is 78% of our ocean only covered by 600 meters of sediment? 9

10 Ocean crust density is ~2.9g/cm 3 Currently 7 BILLION km 3 of continental crust -remember these numbers -haven t shown all of them A closer look with the math: Average deep ocean sediment is ~0.4 mile thick Over the ENTIRE OCEAN (shallow and deep) is ~0.56 mile (~3000 ft) Volume of sediment 0.56 mile x million mile 2 (area of worldwide ocean) = 2.3 g 77 million mile 3 of sediment (volume) 1 cm 1 cm 1 cm Average density of sediment is 2.30 g/cm 3 (2.9 g/cm 3?) : So converting we get 10.7 billion tons/mile 3 So mass of ocean sediment (density x volume) is: ~820 billion million tons Pearson Education, Inc. 10

11 Where am I going with this We just saw the mass of ocean sediment was ~820 billion million tons Continents above sea level have: ~30.4 million cubic miles and a mass of about 383 million billion tons. If all this mass eroded into the oceans, that s only about half of what is currently there!!! In another way, the erosion of 2x our present continental mass would be needed to produce today's mass of ocean sediments! Putting into perspective If put into railroad freight cars, each 11 tons We would need 2.5 billion train cars! Extend to the moon and back 34 times! If traveling at 60 miles per hour, it would take 32 years to pass you. 80 train cars of sediment per second are being added to the ocean! Does this rate fit evolutionary thinking? Already said continents have mass of ~383 million billion tons If oceans are receiving 27.5 billion tons a year, then: 383 million billion tons 27.5 billion tons per year = 14 million years. But evolution says continents have existed for 1 billion years Continents should have eroded 70 times! 11

12 This professor has his bachelors and masters in geology. Is he correct? Maybe, may be not. Although he works for ICR and not for NOAA this is definitely interesting research. Just to show you another perspective take it or leave it Testing the plate tectonics model B. Hot spots and mantle plumes Caused by rising plumes of mantle material Volcanoes can form over them (Hawaiian Island chain) The Hawaiian Islands have formed over a hot spot 12

13 Testing the plate tectonics model C. Paleomagnetism Ancient magnetism preserved in rocks Paleomagnetic records show Polar wandering (evidence that continents moved) Earth s magnetic field reversals Recorded in rocks as they form at oceanic ridges Paleomagnetic reversals recorded by basalt flows Measuring plate motion Measuring plate motion By using hot spot tracks like those of the Hawaiian Island Using space-age technology to directly measure the relative motion of plates Very Long Baseline Interferometry (VLBI) Global Positioning System (GPS) 13

14 Directions and rates of plate motions What drives plate motion Driving mechanism of plate tectonics No one model explains all facets of plate tectonics Much more to be studied! Earth s heat is the driving force Several models have been proposed Slab-pull and ridge-push model Descending oceanic crust pulls the plate Elevated ridge system pushes the plate Several mechanisms contribute to plate motion 14

15 What drives plate motion Several models have been proposed Plate-mantle convection Mantle plumes extend from mantle-core boundary and cause convection within the mantle Models 1. Layering at 660 kilometers 2. Whole-mantle convection Layering at 660 km Whole-mantle convection 15

16 Video clip: 4.4d convection tectonics How does the earth s interior heat cause plates to move? End of Chapter 7 HW: Plate Boundaries Coloring Page HW: p. 218 #1,2 p. 221 #1-3 p. 222 #1,2 16