Plate Tectonics A. Continental Drift Theory 1. Early development 2. Alfred Wegener s mechanism B. Seafloor Spreading 1. Earthquakes and volcanoes 2. Seafloor maps and dates 3. Continental drift revisited C. Global Plate Tectonics 1. General mechanisms a. Earth s interior structure b. Isostacy c. Convective heat loss 2. The theory tested: a. Seafloor temperature b. Sediment thickness D. Plate Boundaries and Faults 1. Divergent boundaries 2. Convergent boundaries 3. Transform faults and fracture zones 4. Hot spots 5. Guyots and seamounts
Plate Tectonics A. Continental Drift Theory 1. Early development a. In the 1500 da Vinci noticed that the continents looked like they fit together like jigsaw puzzle pieces b. Other scholars, such as Francis Bacon wrote of a correspondence between coastlines, especially S. America and Africa c. In 1885 Edward Suess noted that some fossils were similar on the coasts of both continents, further supporting the idea that they had once fit together as one large land mass 2. Alfred Wegener s mechanism a. In 1912 Afred Wegener proposed the theory of continental drift: The continents were once joined in a single super-continent, he named Pangea. He hypothesized that the heavy continental crust was flung towards the equator by centrifugal forces created on a rotating earth. b. He cited four major lines of evidence to support his theory: I. Fit of the continents II. Similarities in fossils III. Glacial geology found in the tropics IV. Coal found in Antarctica c. Very few people paid attention: geologists believed that the mantle was rigid and there were no tracks in the ocean crust (a moving continent would surely leave a track) B. Seafloor Spreading 1. Earthquakes and volcanoes a. About twenty-five years later, Hugo Benioff mapped the locations of deep earthquakes in the Pacific b. Patterns of quakes suggested that the lithosphere exists in sections c. Seismological studies confirmed that the upper mantle was partially molten and NOT rigid 2. Seafloor maps and dates a. Around the same time, technology that allowed the radiometric dating of rocks was perfected b. The seafloor was dated in many spots and found to be less than 200 million years old (awfully young) c. Echo sounders developed during WWII were used to map geological features on the sea floor: found underwater mountain ranges that ran parallel to the coastline in the middle of the Atlantic 3. Continental drift revisited: the 1960's a. Sir Edward Bullard used a computer to map the continents with a shoreline that reflected sea levels 2000m deeper than today. The continent jigsaw puzzle was complete. b. Howard Hess proposed a theory to explain the mechanisms that has moved the
continents over geologic time. The theory was coined PLATE TECTONICS. C. Global Plate Tectonics 1. General mechanisms a. Earth s interior structure I. The Earth is composed of the crust, mantle, and core. II. The crust floats on the denser upper mantle (Crust = 2.7-2.9 g/cm 3 ; Upper mantle = 3.3g/cm 3 ) III. The crust and the upper mantle together are termed the LITHOSPHERE. This outer layer has broken into the major plates that are described in the theory of plate tectonics. IV. The ASTHENOSPHERE is the plastic, molten layer of the mantle directly below the lithosphere. It is hypothesized that convection cells within this layer drive plate motion. b. Isostacy is the term used to describe the buoyant equilibrium of crustal flotation. Lithospheric plates float on the asthenosphere at a depth where they displace an equal mass of asthenosphere. c. Convective heat loss theory I. Heat is created at the center of the earth through radioactive decay. Some of this heat is moved to the Earth s surface through convection in much the same way as a thunderstorm forms at the coast during the summer: Hot mantle fluids rise towards the lithosphere, where they cool, turn aside, and sink, completing the convection cell. II. It is hypothesized that these cells drive plate motion at the Earth s surface. III. New sea floor is created at mid ocean ridges and spreads outward. These spreading centers were hypothesized to correspond to the upwelling portion of the convection cells. These areas are called DIVERGENT PLATE BOUNDARIES. 2. The theory tested a. Sea floor temperature: If new ocean crust is being formed at mid ocean ridges and spreading outward, the new crust near the ridge should be warm and older crust away from the ridge should be cooler. It is. b. Sediment thickness: If new ocean crust is being formed at mid ocean ridges and spreading outward, it should be collecting more and more sediment as it moves away from the ridge and ages. Sediment thicknesses increase with increasing distance from ridges. c. Water depth: As newly formed ocean crust cools it becomes more dense and therefore occupies less volume. It also will reach isostacy with time. This line of reasoning suggests that ocean depths should becomes deeper with increasing distance from the spreading ridge. They do. d. Magnetic anomalies: Earth s magnetic field aligns minerals in fluid basalt which are frozen in place as the rock solidifies. The Earth s magnetic field switches poles periodically. The rock record on both sides of the Mid Atlantic spreading center reflects these changes.
D. Plate Boundaries and Faults 1. Divergent boundaries are also called spreading centers. They are found at junctures between two plates where new magma upwells and forms new basaltic crust. a. Divergent boundaries in the ocean are called mid ocean ridges or rises. b. Divergent boundaries can also occur in continental crust. These boundaries will eventually form new ocean basins. This is how the Atlantic Ocean was formed. An example of a modern day continental divergent boundary is the East African Rift. 2. Convergent boundaries form where plates collide. Some crust is destroyed at these boundaries. a. Ocean-ocean plate junctures = Island Arc System I. Ocean crust of one plate is subducted and heated II. The heating releases water, volatiles and low density magma which rises to the surface of the overriding plate. This creates volcanoes and may produce islands. III. An example is the Aleutians. b. Ocean-continental boundary = Continental Arc System I. Oceanic crust is always subducted because it is most dense. II. The melting basalt is mixed with some continental granite and creates andesite. III. Produce continental volcanoes and mountains IV. Examples are the Andes (from the mineral named andesite) and the Cascades. c. Continental-continental boundary = Continental Mountain System I. Both colliding plates are covered with low density continental crust. This crust floats above the subduction zone. II. The floating crust is folded and uplifted, forming mountain ranges. III. Examples are the Himalayas and the Alps. 3. Transform Boundaries: faults and fracture zones a. These faults are formed because the plates are moving over a spherical surface which causes stress fractures and earthquakes. b. Transform faults form perpendicular to ridge axes, and movement of the plates is in different directions. c. Fracture zones are inactive faults in which the two sides are moving in the same direction. Fracture zones are NOT plate boundaries. d. An example of a transform fault is the San Andreas Fault. 4. Hot spots a. Stationary sources of heat in the LOWER mantle. Speading center heat and basalt originates in the UPPER mantle. b. Hot spots occur both near the edges of plates and in the center of plates. They are not well understood. c. Because hot spots are stationary, island chains can form as oceanic plates move over them. An example of this is the Hawaiin Islands.
d. Other examples of hot spots are Iceland and the hydrothermal areas in Yellowstone National Park. 5. Guyots and seamounts a. They are formed as submarine volcanoes. b. Seamounts never reach the ocean surface and retain there original form. c. Guyots are seamounts that grew to sea level. Wave action weathered the tops of the seamounts, shaving them flat. Guyots were then re-submerged by rise in sea level.