8th Grade Dynamic Earth (Mod E) U4L2: Plate Tectonics

Evidence for Continental Drift Puzzle-like fit of continents. Similar fossils on widely separated continents. ex. Mesosaurus (swimming reptile) Glossopteris (fern tree)

Climate clues Remains of warm-weather plants in Arctic areas. Glacial deposits in tropical areas. Rock clues Similar rock structures on different continents

- Drifting Continents

Pangea History 245 mya (million years ago) – Pangea formed – ocean Panthalassa around it 200 mya – Pangea breaks into Laurasia (becomes N. America and Eurasia) and Gondowana (one part S. America & Africa, one part Antarctica, Australia and India)

Pangea History (con’t) 150 mya – Africa & South America break apart. India, Australia & Antarctic separate India breaks away from Austrailia and moves toward Eurasia 50 mya – India collides with Eurasia, starts Himalayan Mountains

How could continents drift? Pangaea animation - http://www.classzone.com/books/earth_science/terc/content/visualizations/es0806/es0806page01.cfm

Continental Drift Hypothesis Continents have moved slowly to their current locations. Proposed by Alfred Wegner in 1912. He said all continents were once connected into one large landmass called Pangaea (meaning “all land”). Glencoe Video 297AA on Pangaea https://www.youtube.com/watch?v=T1-cES1Ekto – The posthumous triumph of Alfred Wegener

Wegner’s hypothesis of continental drift was rejected by most scientists of his time because he could not explain how or why continents had moved. After his death advances in technology resulted in new ideas that supported his hypothesis.

Mapping the Ocean Floor Scientists during World War II introduced the use of sound waves to detect submarines. In 1940’s and 1950’s scientists used sound waves on ships to map large areas of the ocean floor.

Using sound waves scientists discovered a system of underwater mountain ranges called the Mid-ocean Ridges. Do Video clip Seafloor spreading (P)

Seafloor Spreading Suggested by Harry Hess in 1960’s to explain the ridges. Hot, less dense material below the Earth's crust rises upward to the surface at Mid-ocean Ridges. Then it flows sideways, carrying seafloor away from the ridge in both directions. Seafloor spreading interactive - http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_seafloorspreading.html Really good seafloor spreading animation – http://www.absorblearning.com/media/attachment.action?quick=12n&att=2771 Midocean ridge photo - http://www.windows2universe.org/earth/interior/seafloor_spreading_interactive.html

Evidence for Seafloor Spreading: 1968 – Glomar Challenger, a research ship, drilled into the seafloor to obtain rock samples. Discovered that the youngest rock is closest to the Mid-Ocean Ridge. Note: The submersibles also found exotic life.

Magnetic alignment of rocks, in alternating strips that run parallel to ridges, indicates reversals in Earth’s magnetic field. Video – Magnetic Reversals and Sea Floor Spreading - https://www.youtube.com/watch?v=BCzCmldiaWQ

- Sea-Floor Spreading

Theory of Plate Tectonics Earth’s crust & part of the upper mantle are broken into sections called plates which move around on the plastic-like mantle (asthenosphere). Plates are made of sections containing both seafloor and continents. Tectonic Activities: * earthquakes * volcanoes * mountain building

Types of Plate Boundaries: . Divergent * 2 plates move apart * ex. 1) Mid-Atlantic Ridge 2) Great Rift Valley in Eastern Africa (formed Red Sea)

2. Convergent 2 plates collide 3 types of convergent: 1) Oceanic-continental plates * denser oceanic plate sinks under continental plate forming subduction zone and volcanic arc. (area where plate descends (line of volcanoes on continent) into mantle & melts) Ex. Cascade Range Andes Mt. (SA)

Two Converging Oceanic Plates * One plate bends and slides under the other plate forming a subduction zone. * Result is a deep sea trench and an ex. Marianna Trench island arc of volcanoes. ex. Japanese Islands

- The Theory of Plate Tectonics

Two converging continental plates * Since both continental plates are less dense than the asthenosphere, usually no subduction occurs. The plates collide, crumble up, and form mountains. ex. Himalayan Mts. (India)

Transform * Plates slide past each other in opposite direction or in the same direction at different rates of movement. * ex. San Andreas Fault (CA) p. 284

Cause of Plate Tectonics: Convection current inside Earth. The cycle of heating, rising, cooling, and sinking of material inside Earth (mantle) is thought to be the force behind plate tectonics.

What type of tectonic activity occurs at each type of boundary? Divergent = earthquake, volcano (Mid-ocean ridge Transform = earthquake Convergent Ocean-continent = earthquake, volcano in form of volcanic arc Ocean-Ocean = earthquake, volcano in the form of an island arc Continent-continent = earthquake, mountain