Unit 3: Plate Tectonics

Pre-write In the first box of your notes, answer the following question as best as you can (without using notes or book): What are the lithosphere and asthenosphere? What roles do they play in the processes of plate tectonics?

Lithosphere and Asthenosphere Lithosphere is the rigid outermost layer made of crust and uppermost mantle. The lithosphere is the "plate" of the plate tectonic theory. Asthenosphere is part of the mantle that flows, a characteristic called plastic behavior. The flow of the asthenosphere is part of mantle convection, which plays an important role in moving lithospheric plates.

Who proposed theory of continental drift? Alfred Wegener in 1912 Wegener used the fit of the continents, the distribution of fossils, a similar sequence of rocks at numerous locations, ancient climates, and the apparent wandering of the Earth's polar regions to support his idea. Hypothesized that all of the present-day continents were once part of a single supercontinent called Pangaea

His hypothesis was controversial and not widely accepted until the 1950s, when numerous discoveries such as  palaeomagnetism provided strong support for continental drift, and thereby a substantial basis for today's model of plate tectonics.[1][2]

Pangea Animation- lets go back in time

Evidence of Continental Drift Fossils of the same species were found on several different continents. Wegener proposed that the species dispersed when the continents were connected and later carried to their present positions as the continents drifted.

Fossil Evidence - Movement

Who described seafloor spreading (lateral movement of crust)? Harry H Hess at Princeton in the early 1960’s Proposed that there are convection cells within the Earth’s mantle driven by heat from the Earth’s core and natural radioactivity. Spreading centers= where new crust is formed above rising magma (volcanic ridges) Subduction zones= areas of descending older crustal material (deep steep sided trenches) located at the edge of the ocean, where it meets continents

Mid-oceanic ridge seafloor spreading

Types of Plate Motion The ways that plates interact depend on their relative motion and whether oceanic or continental crust is at the edge of the lithospheric plate. Plates move: Geologists call these: away from divergent toward convergent or slide past or transform each other plate boundaries.

Divergent Plate Boundary Lithospheric plates move away from each other. Ex: The mid-Atlantic Ridge, a topographically high area near the middle of the Atlantic Ocean

Convergent Plate Boundary Lithospheric plates move toward each other. Ex: The west margin of the South American continent, where the oceanic Nazca Plate is pushed toward and beneath the continental portion of the South American Plate.

Transform Plate Boundary Plates slide past each other. Ex: The San Andreas fault in California is an example of a transform plate boundary, where the Pacific Plate slides past the North American Plate.

Earthquakes and volcanoes, evidence of unrest in the Earth, help locate the edges of plates. Earthquakes are distributed in narrow, linear belts that circle the Earth. "Ring of Fire“- circle of volcanoes around Pacific. Geologists believe that areas of intense geologic activity, indicated by earthquakes, volcanoes, and/or mountain building, mark the boundaries between lithospheric plates.

Which plate are we on? We are on The Pacific Plate - is mostly oceanic lithosphere with only a small slice of continental lithosphere in southern California and Baja Mexico. The other side of the fault- North American Plate - east of the San Andreas Fault

POST-write In the last box add any additional detail or clarification that you now may have to the Pre-write question: What are the lithosphere and asthenosphere? What roles do they play in plate tectonics?