Plate Tectonics Plate boundaries 1)The plate tectonic system 2)A theory is born 3) Early evidence for continental drift 4) Continental drift and paleomagnetism 6)History and future of plate motions system Plate boundaries define the major lithospheric plates. Blue: Seafloor spreading; Green: Transform faulting; Red: Subduction See 3-14 b Plate boundaries Plate boundaries Transform (shear) Divergent (constructive) Convergent (destructive) Divergent (constructive) See Fig. 3-14a Earthquake occurrence coincides with plate boundaries, reflecting the energy released during subduction, seafloor spreading or plates sliding past each other (transform). Plates slide past each other. New material is added to the plates at spreading center Lithospheric plates are destroyed when they converge at trenches. See Fig. 3-20 and Table 3-3
Transform (shear) Transform boundary Transform faults and fracture zones. In transform faults the plate movement is opposite, at fracture zones it is in the same direction. Transform faults are always between two mid-ocean ridges. Earthquakes are frequent but generally shallow. Fig.4-18 Divergent plate boundaries Divergent plate boundaries Oceanic type Mid-ocean ridges form a continuous mountain chain winding around the globe through all ocean basins. Continental type They resemble the seams of a baseball.
The mid-atlantic ridge surfaces in Iceland. New volcanic material fills the rift valley as plates are pulled apart. Rifts opening by the divergence of the Arabian plate from the African Plate formed the Gulf of Suez and Gulf of Aqaba. Rift opening by the divergence of Baja California from the Mexican mainland creates the Gulf of California. 4. Growth 5) of Plate ocean basins boundaries The breakup of Pangaea: A) Upwarping, stretching Molten basalt moves through the cracks of the lithosphere B) Formation of a rift valley. Two continents are separated by a valley that downdrops and eventually floods Oceanic rises and ridges 2-3 cm/yr C. Linear sea. The high density basalt sinks below sea level, a new ocean basin forms. 16 cm/yr D. After millions of years, the rift becomes Fig. 3-24 See movie! the new mid-ocean ridge. Continental margins form. 3-26
ocean-ocean Island arc trench system ocean-continent Continental arc trench system Andes mountains (Peru-Chile trench), Cascade mountains (Juan de Fuca plate subduction) continent-continent Convergent boundaries Japan (Japan trench) Mariana (Mariana's trench) Himalayas, Alps, Appalachians Convergent boundaries Example of ocean-continent convergent boundary is the subduction of the Juan de Fuca plate off Washington unter the North-American Plate Trench Fig. 3-28 Eruption of Mount St. Helens 1980. Tectonic features off the coast of Washington and the Cascadia Mountain range, including convergent, divergent and transform boundaries 4) Continental drift and paleomagnetism Fig. 3-30 Fig. 3-12 Age of sea-floor measured from magnetic reversals and deep sea drilling.
Example of ocean-ocean convergent plate boundary. Peru-Chile trench and the Andes mountains (continental arc). Change in elevation across a distance of 200 km is more than 15,000 m. Fig. 4-12 Open ocean convergence zones are called trenches Marianas trench Plate Tectonics 1)The plate tectonic system 2)A theory is born 3) Early evidence for continental drift 4) Continental drift and paleomagnetism 6)History and future of plate motions system 6) History and future of plate motions 750 Ma Late Proterozoic supercontinent Rodinia 458 Ma Middle Ordovician Gondwana forms 390 Ma Early Devonian Euramerica forms 237 Ma Early Triassic supercontinent Pangaea 195 Ma Early Jurassic Breakup of Pangaea 152 Ma Late Cretaceous N and S Atlantic opened Future plate motions see Oc webpage and movie! See Fig. 3-38!
There are likely two convection cells in the mantle, separated by a boundary at about 700 km depth, a whole mantle convection and a shallow convection. There are likely two convection cells in the mantle, separated by a boundary at about 700 km depth, a whole mantle convection and a shallow convection. Hotspots Hawaiian islands and Emperor seamounts result from the movement of the Pacific plate over the Hawaiian hotspot. See Fig. 3-32 One support of deep mantle convection are plumes Hotspots of lava that might originate at the core-mantle boundary and that give rise to island chains such as Hawaii. Fig. 4.23
Hot spots are plumes originating from the core-mantle boundary See Fig. 3-31 Distribution of Hot Spots Summary: Modern supporting evidence Geomagnetism Earth s magnetic field reversals are recorded parallel to mid-ocean ridges Earthquakes Deep earthquakes line up along plate boundaries Deep Sea Ocean Drilling Confirmed ocean floor age increases away from mid-ocean ridge Hot spots Linear island chains in direction of plate motion Modern measurements Using satellites to measure current movement of continents