Plattentektonik

1. Plattentektonik Institut für Geowissenschaften Universität Potsdam

2. Übersicht zur Vorlesung

3. Plattentektonik Ozeane Kontinente 3 Typen von Plattengrenzen

5. Earth’s Plates

6. Divergent boundaries are located mainly along oceanic ridges

7. 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

8. Ridge morphology • Faster spreading ridges are characterized by • more volcanism • smoother topography - less faulting • fewer moderate earthquakes • Slower spreading ridges are characterized by • less volcanism • rough topography - more extension by faulting • more moderate sized earthquakes The differences are related to temperature.

9. 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 cm per year), rift valleys that develop are shallow with subdued topography

10. 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 extensively faulted • Continental rifts • Splits landmasses into two or more smaller segments

11. 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

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

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

14. Convergent plate boundaries • Although all have the same basic charac-teristics, they are highly variable features • Types of convergent boundaries • Oceanic-continental convergence • Denser oceanic slab sinks into the asthenosphere

15. Convergent plate boundaries • Types of convergent boundaries • Oceanic-continental convergence • As the plate descends, partial melting of mantle rock generates magmas having a basaltic or, occasionally andesitic composition • Mountains produced in part by volcanic activity associated with subduction of oceanic lithosphere are called continentalvolcanic arcs (Andes and Cascades)

16. An oceanic-continental convergent plate boundary

17. Convergent plate boundaries • Types of convergent boundaries • Oceanic-oceanic convergence • When two oceanic slabs converge, one descends beneath the other • Often forms volcanoes on the ocean floor • If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)

18. An oceanic-oceanic convergent plate boundary

19. Convergent plate boundaries • Types of convergent boundaries • Continental-continental convergence • Continued subduction can bring two continents together • Less dense, buoyant continental lithosphere does not subduct • Result is a collision between two continental blocks • Process produces mountains (Himalayas, Alps, Appalachians)

20. A continental-continental convergent plate boundary

21. The collision of India and Asia produced the Himalayas

22. Transform fault boundaries • The third type of plate boundary • Plates slide past one another and no new lithosphere is created or destroyed • Transform faults • Most join two segments of a mid-ocean ridge as parts of prominent linear breaks in the oceanic crust known as fracture zones

23. Transform fault boundaries

24. East Pacific Rise west of Costa Rica

25. Transform fault boundaries • Transform faults • A few (the San Andreas fault and the Alpine fault of New Zealand) cut through continental crust

26. Transform Margin

27. Testing the plate tectonics model • Paleomagnetism • Ancient magnetism preserved in rocks at the time of their formation • Magnetized minerals in rocks • Show the direction to Earth’s magnetic poles • Provide a means of determining their latitude of origin

28. Dip of needle = inclination • When a rock cools below the Curie point, the magnetization direction is locked in • We can determine the “paleolatitude” • Also used in archeology

30. Paleomagnetism • The measurement of remnant magnetism can provide information important information about where a rock may have come from. • Measuring a paleomagnetic direction: • An individual lava flow may not record an “average” pole (secular variation), so samples from a series of flows may be taken • Oriented (azimuth and dip) rock cores separated by up to a few meters are drilled (using non-magnetic equipment). • If the rock has been tilted since its formation, this has to be measured. • The magnetization direction is measured (by measuring all three axis of the core) using a very sensitive magnetometer. • The direction, which is relative to the cylinder is calculated with respect to north and the vertical. • The magnetization direction is plotted on a stereonet.

31. Paleomagnetism • Magnetic inclination varies from vertical in the center to horizontal at the circumference. • Declination is the angle around the circle clockwise from north. • Downward magnetizations (positive inclination) are plotted as open circle. Negative magnetizations are plotted as solid circles. • Plot mean direction and 95% confidence interval (95% probability of containing the true direction). From Mussett and Khan, 2000

32. Magnetostratigraphy • By measuring the polarity of magnetization of a rock of know age (radiometric data, sediment on ocean floor above basement) we can build up a magnetic polarity timescale. • At even smaller scales we can examine secular variation within a series of lava flow (assuming a high resolution series of flows). • If these flows are historic, we could probably date them. • If they are very old, we could use the pattern of secular variation to correlate between outcrops. • Archeological applications – dating ancient fireplaces. • The resultant magnetic timescale can be used to date sediments and the seafloor by the recognition of distinctive reversal patterns. From Mussett and Khan, 2000

33. Geomagnetic Reversals • The first comprehensive magnetic study was carried out off the Pacific coast of North America. • Researchers discovered alternating strips of high- and low- intensity magnetism. • In 1963 Vine and Matthews demonstrated that stripes of high intensity magnetism formed when the Earth’s magnetic field was in the present direction, and stripes of low intensity magnetism formed when the Earth’s magnetic field was in the reversed direction.

34. A scientific revolution begins • During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor From SeaBeam operators manual From http://www.navsource.org/archives/09/09570302.jpg

35. A scientific revolution begins • An Extensive oceanic ridge system was discovered. • Part of this system is the Mid-Atlantic Ridge. • A central valley shows us that tensional forces are pulling the ocean crust apart at the ridge crest. • High heat flow. • Volcanism.

36. A scientific revolution begins • Deep earthquakes showed that tectonic activity was taking place beneath the deep trenches. • Flat topped seamounts were discovered hundreds of meters below sea level. • Dredges of rocks from the seafloor did not recover any rocks older than 180 million years old. • Sediment thickness on the seafloor was much less than expected (the seafloor being younger than expected).

37. Testing the plate tectonics model • Paleomagnetism • Polar wandering • The apparent movement of the magnetic poles illustrated in magnetized rocks indicates that the continents have moved • Polar wandering curves for North America and Europe have similar paths but are separated by about 24 of longitude • Different paths can be reconciled if the continents are place next to one another

38. Apparent polar-wandering paths for Eurasia and North America

39. Testing the plate tectonics model • Magnetic reversals and seafloor spreading • Earth's magnetic field periodically reverses polarity – the north magnetic pole becomes the south magnetic pole, and vice versa • Dates when the polarity of Earth’s magnetism changed were determined from lava flows

40. Testing the plate tectonics model • Magnetic reversals and seafloor spreading • Geomagnetic reversals are recorded in the ocean crust • In 1963 the discovery of magnetic stripes in the ocean crust near ridge crests wastied to the concept of seafloor spreading

41. Paleomagnetic reversals recorded by basalt at mid-ocean ridges

42. Inpretation of magnetic anomalies from ship-track wiggles, (Barckhausen et al. 2001).

43. Testing the plate tectonics model • Magnetic reversals and seafloor spreading • Paleomagnetism (evidence of past magnetism recorded in the rocks) was themost convincing evidence set forth to support the concept of seafloor spreading • The Pacific has a faster spreading rate than the Atlantic

44. Testing the plate tectonics model • Plate tectonics and earthquakes • Plate tectonics model accounts for the global distribution of earthquakes • Absence of deep-focus earthquakes along the oceanic ridge is consistent with plate tectonics theory • Deep-focus earthquakes are closely associated with subduction zones • The pattern of earthquakes along a trench provides a method for tracking the plate's descent

45. Deep-focus earthquakes occur along convergent boundaries

46. Earthquake foci in the vicinity of the Japan trench

47. Testing the plate tectonics model • Evidence from ocean drilling • Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment • Age of deepest sediments • Thickness of ocean-floor sediments verifies seafloor spreading

48. Testing the plate tectonics model • Hot spots • Caused by rising plumes of mantle material • Volcanoes can form over them (Hawaiian Island chain) • Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary

49. The Hawaiian Islands have formed over a stationary hot spot

50. Measuring plate motions • A number of methods have been em-ployed to establish the direction and rate of plate motion • Volcanic chains • Paleomagnetism • Very Long Baseline Interferometry (VLBI) • Global Positioning System (GPS)