Continental Drift & Plate Tectonics
Alfred Wegener, a German scientist, proposed the hypothesis of CONTINENTAL DRIFT, in 1912. Hypothesis stated: All Earth s continents were once a single landmass (Pangaea) and then slowly drifted apart. Pangaea began to split 200 million years ago.
Evidence supporting Wegener s Hypothesis 1. Fossils: a reptile, Mesosaurus, was found both in Eastern South America and Western Africa only. 2. Climate changes: fossils of tropical plants have been found on Greenland, indicating it might have once been close to the equator. 3. Geology: Rocks found on one continent match up with rocks on another continent, like a puzzle. Ex. Brazil to Western Africa.
From Continental Drift came the... THEORY OF PLATE TECTONICS In the mid-1900 s, scientists proved that the Earth was made up of 18 moving plates. Each plate is made of the crust and the top part of the mantle called the LITHOSPHERE. The plates move on a layer of hot, molten rock in the rest of the mantle called the ASTHENOSPHERE.
http://ircamera.as.arizona.edu/natsci102/natsci102/lectures/earth.htm http://curriculum.kcdistancelearning.com/courses/envscix-apu10/a/unit05/apes_5.c.4.html
What causes plates to move? Convection Currents transfer heat energy from the inner core to the crust. The rocks within the asthenosphere rise toward the crust when heated, because they are less dense. Rocks begin to cool and sink back again into the asthenosphere, as they become more dense. Old crust is being destroyed at the same rate new crust is formed = Earth stays the same size.*
divergent boundaries Spreading centers are where 2 plates are moving away from each other. New crust is formed here, as magma rises from the asthenosphere. Results on Oceanic Crust: Mid-ocean ridges (Oceanic Crust splits apart) 1. Mid-ocean Ridges: : longest chains of mountains on Earth a. Mid-Atlantic Ridge 11,000 Km from Iceland to Antarctica; where the North & South American Plates are moving away from the Eurasian & African Plates.
divergent boundaries Results on Continental Crust: Rift Valleys (hot magma pushes up on Continental Crust, and the crust cracks as it stretches apart) As the plates continue to pull apart, the valley floor sinks and becomes thinner. Once the valley floor is below sea level, the valley fills with water. Ex. Red Sea created by Arabian & African plates moving apart. Great Rift Valley of Africa is being created as the African Plate splits apart. Iceland is being split into 2 islands from the Mid-Atlantic Ridge.
3 Types of Convergent Boundaries 1. Continental-Continental Collision occurs where 2 plates carrying continental crust push together. Both plates are the same density, so neither will sink beneath the other. This causes the plates to crumple and fold. Continue folding creates mountains. European Alps result of collision between African & European Plates. Mt. Everest (29,035 ft) Among the Himalayas where the Indian & European Plates collided.
Convergent Boundary SUBDUCTION 2. Oceanic Oceanic Subduction: This occurs when an older oceanic plate sinks (subducts) below another younger oceanic plate. The older plate is pulled back into the asthenosphere because it is colder and denser. Results: 1. Deep-ocean trenches Deepest places in the oceans; Thinnest part of the crust. 2. Island Arcs Chains of volcanic islands that form above deep ocean trenches.
Convergent Boundary SUBDUCTION 3. Oceanic Continental Subduction: This occurs when an older oceanic plate sinks (subducts) below another younger Continental plate. The older plate is pulled back into the asthenosphere because it is colder and denser. Results: 1. Deep-ocean trenches Deepest places in the oceans; Thinnest part of the crust. 2. Volcanic Mountains Chains of volcanic mountains that form along the edge of the continent.
Transform Boundaries Crust is neither created or destroyed. Two plates scrape past each other as they move in opposite directions. San Andreas Fault line runs from the Gulf of California through San Francisco. At the rate of movement, LA will be part of San Francisco in 10 million years.
HOT SPOTS Hot spots are stationary areas where material from deep within the mantle rises and then melts, forming magma. They can be in the middle of a plate or along a boundary. Volcanos form above the hot spot when magma erupts through the crust and reaches the surface. Ex: Hawaiian Islands and Yellowstone Caldera