2A. DRIFTING CONTINENTSAlfred Wegner proposed the Theory of Continental Drift in early 1900’s..Wegner’s theorized that all the continents were once a single landmass about 245 million years ago that gradually drifted apart (are still drifting).Called this supercontinent Pangea which is Greek for “all Earth”The land masses Split again – Laurasia & Gondwana 180 million years ago
4BUT>>>>>Wegner couldn’t explain why the plates moved 2. Evidence for PangaeaContinents fit together like puzzle piecesFossils of the same plants and animals are found on different continents**climate evidence supports thisMountains that appear on one coastline had similar formations and rocks on opposite coastlinesBUT>>>>>Wegner couldn’t explain why the plates moved
83. How did plates move then? 1950’s-mapping of the seafloor found Mid-ocean Ridges-huge underwater mountain chains found in every ocean around the Eartha. They formed along cracks in the crust where molten rock pushed up and out in a process called Sea Floor Spreadingyounger rock found closer to ridge than on continents.Earth DOESN’T get larger because oceanic crust is destroyed along deep-ocean trenches, where the oceanic plates sink into the asthenosphereIf this is true, why isn’t the Earth getting bigger?
104. How do plates move then?Due to tremendous heat, rock in the asthenosphere is like hot taffyThis allows plates to ride on top of hot, flowing rock.Plates move because heat is being released from deep inside the earth.Convection currents causes hot material to rise and expand (plates diverge) and cooler material to sink and contract (plates converge).https://www.youtube.com/watch?feature=player_embedded&v=ryrXAGY1dmE
14See Page 35Boundaries are formed when tectonic plates move. The direction of the movement determines the type of boundary.
151. Divergent Boundary: plates move apart See Page 23Usually found in the oceanMid-ocean ridges and rift valleys occurMid-ocean ridges form the longest mountain ranges on earth.Most contain a rift valley along their center.
17Divergent boundaries on continents produce rift valleys. Magma rises through cracks and forms volcanoes.As rift valleys grow wider, continents split apart.If the valley continues to widen, the thinned floor sinks below sea level.It may fill with water to form a sea or lake.
18Continuedc. Earth’s magnetic poles have switched places several times at divergent boundaries.See Page 24
19See Page 24These magnetic reversals are caused by changes in Earth’s magnetic fields. Bands of rock record periods of magnetic reversals. As molten material cools, magnetic minerals line up with the magnetic field. When it hardens, the minerals act like tiny compass needles.
20d. Hot spots can be used to track plate movements. Hot Spot: an area of volcanic activity that develops above where magma rises in a plume from the mantle.They don’t move but plates doA fixed point to measure speed and direction of plates
21The Hawaiian islands are located in the middle of the Pacific Plate The Hawaiian islands are located in the middle of the Pacific Plate. The largest island, Hawaii, is still over the hot spot.
22See Page 28When the plate moves on, it carries the first volcano away from the hot spot. Heat from the mantle plume will then melt the rock at a new site, forming a new volcano.
23They form three types of convergent boundaries. 2.Convergent Boundary: plates push together.They form three types of convergent boundaries.One plate is subducted or pushed below the lighter plate
24a. Continental-Continental 2 continental plates collide crumpling and folding the rock between them. Mountains could form.
25ContinuedOceanic-oceanic subductions: two oceanic plates collide and the older, denser plate sinks beneath the top plate, forming deep-ocean trenches and island arcs.
26ContinuedOceanic-continental subductions: an oceanic plate sinks beneath a continental plate, forming a deep-ocean trench and volcanic coastal mountains.
283. Tectonic plates scrape past each other at transform boundaries. Two plates move past each other in opposite directions.No crust is formed or destroyed.Occurs on the sea floor and on land.
29The San Andreas Fault is a transform boundary and moves about 1 inch per year.
30Comparing Boundaries Divergent Transform Convergent C-C Collision *plates move apart *in ocean and on land *produce mid-ocean ridges, rift valleys, volcanoes, earthquakesTransform*plates move past each other in opposite directions *in ocean and on landConvergentC-C Collision*crumples and folds crust * produces mountains, earthquakesO-O Subduction*older, denser plate sinks *produce deep-ocean trenches, island arcsO-C Subduction*oceanic plate sinks under continental plate * forms deep-ocean trench, volcanic coastal mountains
31The theory of plate tectonics helps geologists today. The plate tectonics theory enables geologists to understand how Earth’s continents and ocean basins formed.Helps scientists predict earthquakes and volcanic activity.