Plate boundaries Crust: chemical layer, more Si rich Lithosphere (crust plus upper mantle, rigid). Only upper part really rigid, lower part creeps, but travels with the rest as plate. Temperature at base of lithosphere ~ 1200°C Asthenosphere: deforms readily by creep, stepwise increase in density Upper mantle (lower lithosphere and asthenosphere): chemical, more Fe and Mg-rich Lower mantle: smooth increase in density and seismic velocity, also deforms by creep, but much more slowly because of greater density Two layer convection? Tectonics due to heat loss via convection 1 2 Wilson Cycle N. Australian Margin East African Rift W. North America Red Sea Atlantic Ocean Himalaya History of the Atlantic 700 Ma 600 Ma 550 Ma handout 500 Ma 430 Ma 360 Ma 260 Ma Modes of Continent Formation 3 Last word on timing of continent formation History of the Pacific History of the Pacific 4 History of the Pacific History of the Pacific History of the Pacific 5 Accreted Terranes Plates and mantle heat flow Plates and mantle heat flow 6 Plates and mantle heat flow Hotspots and mantle heat flow Hotspot - a volcanic upwelling whose position is not correlated with respect to plate motion. Pseudo 1D upwelling Hotspots and mantle heat flow Tracks/age progress 7 Hotspots and mantle heat flow Tracks/age progress Hotspots and plate motion Change in direction of plate motion Hotspot Distribution 1) None on South America 2) Many on Africa a) Africa have the highest average topography (~900 m) b) Only continent with active rifting c) Africa has moved the least among the continents (~200 Ma in the same place) d) No subduction zones - ringed by ridges 3) Many hotspots on the mid-Atlantic ridge. Are continents split by hotspots? 8 Hotspot Distribution Many hotspots on the mid-Atlantic ridge. Are continents split by hotspots? Age vs. Thickness of Oceanic Lithosphere Age vs. Depth of the Ocean Ocean depth increases as square root of either age or distance from spreading center 9 Oceanic plateaus and flood basalts Oceanic plateaus and flood basalts Hot spots only important for last 400 Mya Non-tectonic form of heat loss. More important on other planet that lack tectonics. Is Earth in transition to a non-tectonic style of heat loss? 10.