1	Plate Tectonics
2	Continental Drift In the early 1900’s, Alfred Wegener suggested Continental Drift Alfred noted that several continents had the same rocks and fossils
3	Continental Drift Hypothesis Supercontinent called Pangea began breaking apart about 200 million years ago Continents "drifted" to present positions Evidence used in support of continental drift hypothesis Fit of the continents Fossil evidence Rock type and structural similarities E.g., Matching mountain ranges Paleoclimatic evidence
4	Continental Drift Hypothesis Received skepticism Lack of a mechanism to move the continents across the globe No evidence that ocean floor had been disturbed by motion of continents through them Most evidence was from southern continents, so American geologists not very familiar with it Continental drift and the scientific method A few scientists considered Wegener’s ideas plausible and continued the search for new data
5	Fit of the Continents
6	Fossil Evidence
7	Matching Mountain Ranges
8	Movement of Plates Over Time
9	Plate Tectonics Through additional tests and studies, in 1968, the plate tectonic theory unfolded Plate tectonics identifies 7 major plates on surface of Earth Pacific, North American, South American, African, Eurasian, Indo-Australian, Antarctic Many smaller plates, total of about 24
10	Plate Tectonics Several plates include both continental and oceanic crust Plates move at a rate of a few cm per year Rates have been verified using measurements from global positioning systems (GPS) and lasers Movement of plates creates earthquakes, volcanoes, and builds mountains Driving force - unequal distribution of heat within the earth Mantle convection: heated rock within the earth rises and goes toward the surface where it cools, becomes more dense and sinks again
11	Earth’s Tectonic Plates
12	Plate Tectonics 3 types of plate boundaries Divergent Plates moving away from each other Creates sea floor spreading and rifts Convergent Plates colliding Forms subduction zones or mountain building Transform boundaries Plates slide past each other horizontally no production or destruction of lithosphere
13	Divergent Plate boundaries Constructive plate margins - seafloor spreading Plates move away from the ridge, the gap is filled with molten rock form the asthenosphere The rock cools and forms new sea floor This process creates new ocean crust-produced the Atlantic Ocean Oceanic lithosphere elevated at ridge Young crust is hot and buoyant Increases in density and thickness with age Cooling and addition of sediment on top Examples of recently formed divergent plate boundaries Red Sea, Gulf of California
14	Divergent Plate Boundaries
15	Divergent Plate boundaries Continental Rifts Extension of crust can cause landmass to break Hot rock from below pushes on the crust and creates tensional cracks The hot rock spreads laterally and the broken crust is pulled apart As the crust is pulled apart, large slabs of cooled crust sink generating a rift Further spreading generates a sea and eventually, an ocean basin
16	Development of Ocean Basin from Rift Valley within a Continent
17	Convergent Plate boundaries Destructive plate margins New seafloor being created at ridges, but size of earth not changing, so lithosphere must be consumed somewhere 3 types of convergent plate boundaries Oceanic-Continental (subduction occurs) Oceanic-Oceanic (subduction occurs) Continental-Continental (mountain building, no subduction)
18	Convergent Plate Boundaries Oceanic-continental convergence The more dense oceanic crust is pushed into the asthenosphere, usually at a 45° angle or greater This is called a subduction zone The oceanic plate melt in the higher temperature of the asthenosphere Remaining magma eventually rises to the surface and can form volcanic eruptions Leads to formation of volcanic arcs: The Cascade Range
19	Convergent Boundaries
20	Oceanic-Continental Plate Convergence
21	Convergent Plate boundaries Oceanic-Oceanic Plate Convergence Similar to oceanic-continental boundary where one plate goes beneath the other Volcanic island arcs formed Tonga, Mariana, Aleutian Young island arcs have thin crust but thicken with age (Japan)
22	Oceanic-Oceanic Plate Convergence
23	Convergent Plate boundaries Continental-Continental Plate Convergence No subduction Continental crust too light to sink into mantle Primarily mountain building Himalayas (India-Asia), Alps, Appalachains, Urals
24	Continental-Continental Plate Convergence
25	Transform Plate Boundaries Transform faults No lithosphere is created or destroyed 2 plates slide past each other Transform faults parallel direction of movement Transport oceanic crust to deep ocean trenches Mendocino fault connects Juan De Fuca Ridge to Cascade subduction zone San Andreas transform fault cuts continental crust Connects spreading center in Gulf of California to Cascade subduction zone
26	Transform Faults Bounding Pacific Plate
27	Importance of Plate Tectonics Unifying theory for earth sciences Helped to explain several major problems in geology Relationship of continents and ocean basins Distribution of earthquakes, volcanoes, and mountain belts Past distribution of animals and plants Distribution of mineral and oil deposits and where to explore for new deposits