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11A_3 – Slide 1 Topic 11A: Plate Tectonics, Part III Online Lecture: Types of Plate Boundaries ○ Boundaries: – Divergent – Convergent – Transform Faults ○ Hotspots Why do the Plates Move? 11A_3 – Slide 2 ○ rising magma (lava) forces the plates apart ○ friction with the convection cells in the mantle drag the plates away from MOR ○ The higher density, cold side of the plate is pulling its end of the plate down. (“Slab Pull”) Continental Lithosphere 3.10 g/cm3 Old Oceanic Lithosphere 3.28 g/cm3 Upper Mantle 3.25 g/cm3 Plate motion has been observed using satellites & instruments on the ground over the last few decades: a few inches per year, on average, consistent with sediment layer thickness, radiometric dating, etc. 11A_3 – Slide 3 The MOR: Where Plates Move Apart I ○ plates move away from the MOR “Diverging” – lava comes up & cools: adding to plate (gets larger) ○ sink down a bit into the mantle as they get older, because their density gets higher: – cooler (contracts / shrinks) This is why the MOR is higher than the rest of the ocean floor. – more and more sediments pile up on top of the plates, making them heavier Higher Density Ends Sink Lower Density Ends Rise Calcareous Ooze Red Clay MOR Ocean Crust Cr Ocean ust Ocean Crust Origin of a MOR feature 11A_3 – Slide 4 #1 Notice that there are no trenches next to either continent; continental lithosphere can be pushed by oceanic lithosphere (they can be part of the same plate). #2 #3 #4 Continents Splitting: Where Plates Move Apart II 11A_3 – Slide 5 Red Sea ○ Convection cells can cause continental lithosphere to split (“rift”): – basalt lava comes up in the gap, making new oceanic crust – eventually the gap could become an entire ocean, like the Atlantic ri Af ia ud Sa abia Ar Iceland ca Subduction Zones: Where Plates Come Together I Oceanic Crust meets Oceanic Crust Volcanic Islands “Converge” Mid-Ocean Ridge Volcanic Mountains Trench Continental Lithosphere Ocean Litho. s Cell Convection Cell e ak Older crust more dense than Convection Younger crust Basalt more dense than Granite qu th Ea rt h Continental Crust meets Oceanic Crust r Ea qu ak es Trench 11A_3 – Slide 6 Continental Collisions: Where Plates Come Together II 11A_3 – Slide 7 ○ When 2 plates with continental lithosphere come together, the continental crust does not dive down, because its density is lower than the density of the mantle. – The material has to go somewhere, so it goes both up and down, thickening the crust: results in rising mountains when the plates move towards each other during earthquakes #1 #2 Structure of the Mid-Ocean Ridge: Transform Faults earthquakes noes volca The Mid-Ocean Ridge (MOR) is “broken” into “pieces.” volca noes earthquakes “Gaps” / “Jumps” are “Transform Faults” earthquakes noes volca Mid-Ocean Ridge Transform Fault “Side-swipe” Car Car 11A_3 – Slide 8 Lithosphere Mantle Trench 11A_3 – Slide 9 The San Andreas Fault We have earthquakes in southern California because we live along the mid-ocean ridge! n Sa s ea dr An Fa t ul Where plates run into and grind against one another, they are “squeezed” or “bent.” The pressure to move builds up, eventually overcomes friction, and the plate(s) “slip” forward and “snap” back into shape (an earthquake happens). 11A_3 – Slide 10 Hotspots ○ Hotspots are special places where magma from deep inside the Earth is rising up, melting the bottom of a plate. – lava keeps coming up, forming a volcano Hotspots can also occur under a continent (“Old Faithful” in Yellowstone Nat. Park) or at the midocean ridge (Iceland). IslandSeamount Chain 11A_3 – Slide 11 Hotspots & Island-Seamount Chains ○ The plate is moving, so the volcano gets carried off the hotspot. ○ More volcanoes grow over the hotspot, then get carried off: – creates a “chain ” of islands & seamounts – best example are the islands of Hawaii (part of a chain reaching all the way to Alaska) 1 2 3 4 5 Hawaiian Islands Hotspots: From Islands to Seamounts 11A_3 – Slide 12 ○Where Volcano stops growing when it moves off the hotspot, do you think the hotspot is in because more lava the Hawaiianno Islands? Whycomes there? up ○ Volcano begins to sink, Which direction is the Pacific because it cools: plate moving? How can you tell? – contracts = becomes smaller – more dense = volcano & the plate sink into the mantle a bit (do not float as high anymore) ○ Evidence for sinking islands: – flat-topped seamounts (eroded by waves) – coral atolls Plate Motion #1 Mantle Plume Corals Grow Upward