1. The Changing Earth Chapter 9: Plate Tectonics

2. Review Earth Has Several Layers

3. Earth is made up of materials with different densities.  Theory: Earth began as a spinning mass of rocks and dust ~4.5 billion years ago.  Collisions with comets and asteroids, pressure of Earth’s gravity, and radioactivity produce enough heat to melt materials inside Earth.  Dense material sank to the Earth’s center, less dense material moved toward the surface, forming Earth’s layers.

6. Earth’s layers have different properties. Earth’s Layers LayerCompositionTemperatureThickness Inner Coresolid metals7000-8000°C2400 km diameter Outer Coreliquid metals4400-6100 °C2300 km thick Mantleheated rock870-4400 °C2900 km thick CrustCooler rock0-700 °C6-70 km thick

7. Continued  Earth’s crust and the top of the mantle form the lithosphere.  The lithosphere sits on a layer of hotter, softer rock in the upper mantle called the asthenosphere.

8. The lithosphere is made up of many plates.

10. Continued  The lithosphere is split into large and small slabs of rocks called tectonic plates, which fit together like a jigsaw puzzle.  Most large plates contain both continental and oceanic crust.  Tectonic plates provide evidence of continental movement that supports the theory of Pangaea “all land”.

11. Plates Move Apart

13. Section 9.1 Continental Drift: Positions Change Over Time.

14. Continents join together and split apart.  Alfred Wegener proposed the theory of continental drift in early 1900’s  All continents were once joined and gradually moved apart (still moving).  Fossils, studies of ancient climates, and rock formations provide evidence.  Africa and Brazil have matching rock formations indicating they were once joined.

15. Wegner’s theorized that all the continents were once a single landmass. See Page 16

17. Wegner used fossil evidence to support his continental drift theory.

23. The theory of plate tectonics explains how plates and their continents move.  Wegener’s theory and sea floor maps were used to develop the theory of plate tectonics.  Along spreading centers in the sea floor, melted rock rises through cracks, cools, and forms new crust that builds up mid-ocean ridges.

24. Continued: Mid Ocean Ridge at Spreading Center

25. Continued  Old crust gets pushed aside and the sea floor slowly spreads apart.  Earth DOESN’T get larger because oceanic crust is destroyed along deep-ocean trenches, where the oceanic plates sink into the asthenosphere.

26. See Page 18

28. Tectonic plates have different boundaries.  Plate boundary: where the edges of two plates meet  Divergent Boundary: occurs where plates move apart, usually found in the ocean  Convergent Boundary: occurs when plates push together.  Transform Boundary: occur when plates scrape past each other.

29. Divergent Boundary Convergent Boundary Transform Boundary

30. The sea floor spreads apart at divergent boundaries.  Mid-ocean ridges and rift valleys occur at divergent boundaries in the ocean.  Mid-ocean ridges form the longest mountain ranges on earth.  Most contain a rift valley along their center.

32. Continued  Earth’s magnetic poles have switched places several times.

33. These 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.

34. Continents split apart at divergent boundaries.  Divergent 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.

35. Great Rift Valley, Africa

37. 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.  Can be used to measure plate movement because it generally stays in one place as the tectonic plate above it moves.  Can provide a fixed point for measuring the speed and direction of plate movements.

39. The Hawaiian islands are located in the middle of the Pacific Plate. The largest island, Hawaii, is still over the hot spot.

40. When 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.

41. Plates Converge or Scrape Past Each Other.

42. Tectonic plates push together and form three types of convergent boundaries. Continental- continental collision: Two continental plates collide, crumpling and folding the rock between them. Mountains could form.

43. Continued Oceanic-oceanic subductions: two oceanic plates collide and the older, denser plate sinks beneath the top plate, forming deep-ocean trenches and island arcs. Mariana trench is 11 km deep!

44. Continued Oceanic-continental subductions: an oceanic plate sinks beneath a continental plate, forming a deep-ocean trench and volcanic coastal mountains.

46. 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.

47. The San Andreas Fault is a transform boundary and moves about 1 inch per year.

48. Boundaries are formed when tectonic plates move. The direction of the movement determines the type of boundary.

49. Comparing Boundaries Divergent *plates move apart *in ocean and on land *produce mid-ocean ridges, rift valleys, volcanoes, earthquakes Transform *plates move past each other in opposite directions *in ocean and on land Convergent C-C Collision *crumples and folds crust * produces mountains, earthquakes Convergent O-O Subduction *older, denser plate sinks *produce deep-ocean trenches, island arcs Convergent O-C Subduction *oceanic plate sinks under continental plate * forms deep-ocean trench, volcanic coastal mountains

50. The 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.