A Layered Earth

1. A Layered Earth

2. A Layered Earth Chemical Properties • Crust (oceanic crust versus continental crust) • Mantle • Core

3. A Layered Earth Chemical Properties • Oceanic Crust • Thin • Basalt • Dark-colored (MAFIC) • Composed of: oxygen, silicon, magnesium, iron

4. A Layered Earth Chemical Properties • Continental crust • Thick • Granite • Light-colored (FELSIC) • Composed of: oxygen, silicon and aluminum

5. A Layered Earth Chemical Properties • Mantle • Thickest layer, ~68% of Earth’s mass, 83% of volume • Layer beneath the crust • Thought to be composed of: oxygen, iron, magnesium • and silicon

6. A Layered Earth Chemical Properties • Core (outer and inner core) • Dense, inner layer • Composed of: iron and nickel

7. A Layered Earth Physical Properties • Lithosphere • Asthenosphere • Mantle • Core

8. A Layered Earth Physical Properties

9. A Layered Earth Physical Properties • Lithosphere (lithos = rock) • Earth’s cool, rigid outer layer • 100-200 km (60-125 miles thick) • Composed of: oceanic (2.9 g/cm3) • continental crust (2.7 g/cm3) • uppermost part of mantle (cool and rigid)

10. A Layered Earth Physical Properties • Asthenosphere (asthenes = weak) • Hot, partially melted upper mantle below the lithosphere • Extending to a depth of 350-650 km (220-400 miles) • Capable of flow • 3.3 g/cm3

11. A Layered Earth Physical Properties • Mantle • Extends to the core • Hot and dense, flows much more slowly • Lower mantle (4.5 g/cm3)

12. A Layered Earth Physical Properties • Core • Outer Core = dense, viscous liquid • Inner Core = Solid, max density of 16 g/cm3

13. Earth Structure and Plate tectonics • Earthquakes and earthquake waves • The layers of the Earth • Continental drift • Plate boundaries • The confirmation of plate tectonics

15. Earthquakes and earthquake waves • Seismic waves: Low-frequency pulses of energy generated • by the forces that cause earthquakes. Seismic waves can • spread rapidly through Earth in all directions and then return • to the surface. • Seismic waves tell us about the Earth’s interior • Recorded by seismographs

16. Earthquakes and earthquake waves • The Earth is not homogeneous in composition • Seismic waves speed up with depth because the Earth’s • density increases with depth • Geothermal gradient • Global distribution and current quakes

17. A Layered Earth • Internal Heat • Main source of heat within the Earth is caused by: • Radioactive decay: a process that generates heat when • unstable forms of elements are transformed into new • elements. • Conduction and Convection

18. A Layered Earth • Isostatic Equilibrium: • Less dense crust floats on top of the denser and • deformable rocks of the mantle • Concept of “floating” crust in gravitational balance is • called isostacy • If weight is added or removed from the crust, isostatic • adjustment will take place as the crust subsides or • rebounds

19. A Layered Earth • Isostacy examples • Large regions of Earth’s continents are held above • sea level by isostatic equilibrium, a process analogous • to a ship floating in water

20. A Layered Earth Buoyancy: the ability of an object to float in a fluid by displacing a volume of that fluid equal in weight to the floating object’s own weight Faults: fractures along a plane of weakness

21. Continental Drift

22. Continental drift: An idea before its time • Alfred Wegener • First proposed his continental drift hypothesis in 1915 • Published The Origin of Continents and Oceans • Continental drift hypothesis • Supercontinent called Pangaea began breaking apart about 200 million years ago

23. Pangaea approximately 200 million years ago

24. Continental drift: An idea before its time • Continental drift hypothesis • Continents "drifted" to present positions • Evidence used in support of continental drift hypothesis • Fit of the continents • Fossil evidence • Rock type and structural similarities • Paleoclimatic evidence

25. The great debate • Objections to the continental drift hypothesis • Inability to provide a mechanism capable of moving continents across the globe • Wegner suggested that continents broke through the ocean crust, much like ice breakers cut through ice

26. Wegener’s matching of mountain ranges on different continents

27. Paleoclimatic evidence for Continental Drift

28. The great debate • Continental drift and the scientific method • Wegner’s hypothesis was correct in principle, but contained incorrect details • For any scientific viewpoint to gain wide acceptance, supporting evidence from all realms of science must be found • A few scientists considered Wegner’s ideas plausible and continued the search

29. Plate tectonics: A modern version of an old idea • Much more encompassing theory than continental drift • The composite of a variety of ideas that explain the observed motion of Earth’s lithosphere through the mechanisms of subduction and seafloor spreading

30. Plate tectonics: A modern version of an old idea • Earth’s major plates • Associated with Earth's strong, rigid outer layer • Known as the lithosphere • Consists of uppermost mantle and overlying crust • Overlies a weaker region in the mantle called the asthenosphere

31. Plate tectonics: A modern version of an old idea • Earth’s major plates • Seven major lithospheric plates • Plates are in motion and continually changing in shape and size • Largest plate is the Pacific plate • Several plates include an entire continent plus a large area of seafloor

32. Earth Structure and Plate tectonics

33. Plate tectonics: A modern version of an old idea • Plate boundaries • All major interactions among individual plates occur along their boundaries • Types of plate boundaries • Divergent plate boundaries (constructive margins) • Convergent plate boundaries (destructive margins) • Transform fault boundaries (conservative margins)

34. Plate Boundaries

35. Plate tectonics: A modern version of an old idea • Plate boundaries • Each plate is bounded by a combination of the three types of boundaries • New plate boundaries can be created in response to changes in the forces acting on these rigid slabs

36. Plate tectonics: A modern version of an old idea • Earth’s major plates • Plates move relative to each other at a very slow but continuous rate • Average about 5 centimeters (2 inches) per year • Cooler, denser slabs of oceanic lithosphere descend into the mantle

37. Divergent Plate Boundaries Mid-Atlantic Ridge System DIVERGENT BOUNDARY

38. Divergent plate boundaries • Most are located along the crests of oceanic ridges and can be thought of as constructive plate margins • Oceanic ridges and seafloor spreading • Along well-developed divergent plate boundaries, the seafloor is elevated forming oceanic ridges

39. Divergent plate boundaries • Oceanic ridges and seafloor spreading • Seafloor spreading occurs along the oceanic ridge system • Spreading rates and ridge topography • Ridge systems exhibit topographic differences • Topographic differences are controlled by spreading rates

40. Divergent Plate Boundaries Oceanic ridges and seafloor spreading • Longest topographic feature on Earth’s surface • Represent 20% of Earth’s surface, 1000-4000 km wide • Create new seafloor! • Typical spreading rates ~5 cm (2 inches) per year • No ocean floor is older than the Jurassic! (180 my)

41. Divergent boundaries are located mainly along oceanic ridges

42. Seafloor Spreading • Most convincing evidence from ocean drilling • Age of deepest sediments • Youngest sediments are near the ridges • Older sediments are far away from the ridges • Confirms seafloor-spreading hypothesis • Ages show that the ocean basins are geologically young

43. Seafloor Spreading

44. Divergent plate boundaries • Spreading rates and ridge topography • Topographic differences are controlled by spreading rates • At slow spreading rates (1-5 centimeters per year), a prominent rift valley develops along the ridge crest that is wide (30 to 50 km) and deep (1500-3000 meters) • At intermediate spreading rates (5-9 centi-meters per year), rift valleys that develop are shallow with subdued topography

45. Divergent plate boundaries • Spreading rates and ridge topography • Topographic differences are controlled by spreading rates • At spreading rates greater than 9 centimeters per year no median rift valley develops and these areas are usually narrow and exten-sively faulted • Continental rifts • Splits landmasses into two or more smaller segments

46. Divergent plate boundaries • Continental rifts • Examples include the East African rifts valleys and the Rhine Valley in northern Europe • Produced by extensional forces acting on the lithospheric plates • Not all rift valleys develop into full-fledged spreading centers

47. The East African rift – a divergent boundary on land

48. Convergent Plate Boundaries CASCADE RANGE Mt. Hood

49. Convergent Plate Boundaries • Types: • oceanic-continental (Andes) • oceanic-oceanic (Aleutian trench) • continental-continental (Himalayas) p. 52

50. Convergent plate boundaries • Older portions of oceanic plates are returned to the mantle in these destruc-tive plate margins • Surface expression of the descending plate is an ocean trench • Called subduction zones • Average angle at which oceanic litho-sphere descends into the mantle is about 45