Unit 11: Plate Tectonics A. Alfred Wegner 1. Continental drift hypothesis a. single supercontinent called Pangaea b. 200 million years ago Pangaea (all land) began to break up and started drifting to their present positions 2. Evidences a. Matching fossils - Mesosaurus fossils limited to South America and southern Africa b. Rock Types and Structures - several mountain belts that end at one coastline, reappear on a landmass across the ocean 3. Hypothesis rejected a. Wegener could not describe a mechanism that was capable of moving the continents across the globe B. Theory of Plate Tectonics 1. The theory that proposes that Earth s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself 2. Lithosphere rigid outer layer of Earth, including the crust and upper mantle a. divided into segments called plates seven major plates b. largest = Pacific plate 3. Move about 5 cm/yr (about as fast as your fingernail)
Types of Plate Boundaries C. Divergent boundaries - when two plates move apart 1. results in upwelling of material from the mantle to create new seafloor 2. Oceanic ridge seafloor that is elevated along well developed divergent boundaries 3. Rift valleys deep faulted structures found along the axes of some segments 4. Seafloor spreading The process in which the ocean floor is extended when two plates move apart, forming a crack where magma can rise to the surface, cooling and forming new crust. Ex: East African Rift valley D. Convergent boundaries - when two plates move together 1. results in a subduction zone - when one oceanic plate is forced down into the mantle beneath a second plate 2. creates an ocean trench 3. 3 types of convergent boundaries a. Oceanic-Continental - Ocean plate (more dense) sinks down under continental plate i. Can eventually cause volcanic eruptions ii. Ex. Andes b. Oceanic- Oceanic - One descends beneath the other i. Can cause volcanoes to form on ocean floor ii. Activity will eventually build a chain of volcanic structures that become islands called volcanic island arc iii. Ex. Aleutian Islands off the shore of Alaska
c. Continental-Continental - Plates collide i. Causes formation of complex mountains ii. Ex. Himalayas iii. Urbanizing mountainsides leads to rosion, landslides, and water pollution E. Transform fault boundaries - two plates grind past each other without the production or destruction of lithosphere 1. can cause earthquake activity 2. Ex. San Andreas fault zone F. Evidence for Plate Tectonics 1. Paleomagnetism - the study of the Earth s magnetic field in the past used as evidence for seafloor spreading a. Normal Polarity when rocks show the same magnetism as the present magnetic field b. Reverse polarity rocks that show the opposite magnetism c. the discovery of strips of alternating polarity, while lie as mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading
2. Earthquake patterns 3. Ocean drilling 4. Hot spots - a concentration of heat in the mantle capable of producing magma, which rises to Earth s surface a. pacific plate moves over a hot spot, producing the Hawaiian Islands b. supports the idea that the plates move over Earth s surface G. Mechanisms of Plate Motion 1. Convection warm, less dense material rises and cooler, denser material sinks 2. Convection Currents transfer of thermal energy from warmer regions of magma below the crust to cooler regions a. convection occurring in the mantle is the basic driving force for plate movement
3. Slab-Pull - occurs because old oceanic crust, sinks into the asthenosphere and pulls the trailing lithosphere along a. thought to be the primary downward arm of convective flow in the mantle 4. Ridge-Push - results from the elevated position of the oceanic ridge system a. causes oceanic lithosphere to slide down the sides f the oceanic ridge b. less important than slab-pull 5. Mantle convection - mantle plumes are masses of hotter-than-normal mantle material that ascend toward the surface a. the unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion