Theory of Plate Tectonics Lecture 4

Review Wegener - continental drift hypothesis Continental puzzle Fossil records Matching mountain ranges Ancient climate Main objection to Wegener's proposal Inability to provide a mechanism Theory of Plate tectonics 4 scientific developments Ruggedness and youth of ocean floor Earthquakes and volcanoes active along ocean mountain ranges and trenches Repeated changes of earth’s magnetic field Spreading of the ocean floor hypothesis

Review Plate boundaries Divergent Oceanic and continental Convergent Oceanic-continental  Continental volcanic arcs Oceanic-oceanic  Volcanic island arcs Continental-continental  Mountain ranges - Himalayas Transform Aid in crustal movement

Theory of Plate Tectonics- Paleomagnetism  Paleomagnetism is probably the most persuasive evidence supporting the theory of plate tectonics

Theory of Plate tectonics - Evidence from ocean drilling  Seafloor spreading  Some of the most convincing evidence confirming seafloor spreading  Evidence from drilling directly into ocean-floor sediment  Age of deepest sediments - furthest away from boundary

Theory of Plate Tectonics - Hot spots and mantle plumes  Hot spots - caused by rising plumes of mantle material  Volcanoes - Hawaiian islands over hot spot  The further from Hawaii the older the island  Pacific plate moves over hot spot - volcanic mountains form  Evidence plates moved

What drives plate motion  Driving mechanism of plate tectonics Several mechanisms contribute to plate motion Scientists generally agree that convection occurring in the mantle is the basic driving force Warm less dense material rises Cooler, denser material sinks Movement of the plates and mantle is driven by the unequal distribution of Earth’s heat Heat is generated by radioactive decay of elements such as Uranium found in the mantle and crust

Several models to explain plate movement have been proposed Figure Model 1 - Slab-pull and slab-push model Descending oceanic crust pulls the plate - downward arm of convective flow Elevated ridge system pushes the plate

Several models to explain plate movement have been proposed Model 2 - Plate-mantle convection Mantle plumes extend from mantle-core boundary and cause convection within the mantle 2 different models for plate-mantle convection see the following figures

Whole-mantle convection Cold oceanic crust descends into the mantle Hot mantle plumes transport heat toward the surface

Deep-layer model Earth’s heat causes these layers of convection to slowly swell and shrink in complex patterns Some material from the lower layer flows upward as mantle plumes

Plate movement We still have a lot to learn We do know this: The unequal distribution of heat within the Earth causes the thermal convection in the mantle that ultimately drives plate motion

Measuring plate motion Figure  Using space-age technology to directly measure the relative motion of plates  Very Long Baseline Interferometry (VLBI)  large numbers of time difference measurements from distant sources observed with a global network of antennas over a period of time  Global Positioning System (GPS)

A possible view of the world 50 million years from now Figure Areas west of the San Andreas Fault slide northward past the North American plate Africa collides with Eurasia, closing the Mediterranean and initiating mountain building Australia and new Guinea are on a collision course with Asia