Putting Things Together. Plate Tectonics & Earth History

Transcription

1 Putting Things Together Plate Tectonics & Earth History

2 Ideas of Importance The Earth is Hot The Heat is released through earthquakes and volcanoes. Earthquakes and volcanoes occur along linear belts. Certain types of earthquakes occur with certain types of volcanoes. The interior of the earth contains a several layers. The upper most layer of the earth is called the lithosphere and it is solid. Beneath the lithosphere lies the semi-molten & densee asthenosphere. Oceanic lithosphere and continental lithosphere have different densities.

3 Plate Tectonics (1968) The unifying concept in Geological Sciences. The earth is made up of large plates that move around the earth. A plate is defined as a single chunk of lithosphere that moves in more or less the same direction. How do we distinguish a plate boundary?

4 Layers and Physical Properties Note that the continental lithosphere is thicker and lighter than the oceanic lithosphere. Therefore it floats higher in the asthenosphere.

5 Reverse Faults/Andesitic Volcanism

6 Putting them together Oceanic crust is thinner and denser and will be pushed under the continental crust. As it gets deeper it begins to warm up. As it warms up the oceanic crust melts (basalt) and rises up through continental crust (granite). The resultant product of Basalt+Granite= Andesite or Diorite.

7 Key Point #1 Reverse Faults and Andesitic Volcanoes tend to occur where oceanic lithosphere is pushed beneath continental lithosphere. This process is called SUBDUCTION and the relationship between the oceanic and continental lithosphere is called CONVERGENCE

8 What Happens Here? Hint: Think about density and age.

9 Deep Sea Trenches

10 Last Example Hint: This Presentation Style

11 Continental Collision=Orogeny Orogeny=Mountain Building. Example, Himalayas are the result of the Indian Continent colliding with the Asian Continent.

12 Summing Up Convergent Boundaries form where plates move together. This can be: Ocean-Ocean Convergence Continent-Ocean Convergence Continent-Continent Convergence

13 What about when things move apart? In the simplest case, two pieces of oceanic crust break apart and hot asthenosphere pours out of the crack. These are basaltic volcanoes seen at mid-ocean ridges. We see normal-type faulting in this situation. Of course, continents also rip apart (Rio Grande Rift-North America) and the East African Rift in Ethiopia, Kenya and Tanzania are other examples. DIVERGENT BOUNDARIES

14 Sometimes, Plates Slide Past Each Other Transform Boundaries

15 The Mystery of the Transform Boundary Ridge Fault Ridge Geologists in the 1960 s documented earthquakes and faults on the floor of the Atlantic Ocean (and others as well). The situation they encountered is shown in this figure. What type of Fault is this? Map View

16 Right Lateral? Ridge Focal Mechanism Maps indicated right-lateral motion. This was a mystery to geologists until someone put the whole picture together. Fault Ridge

17 The Whole Picture The key to the mystery: Ridge Fault Ridge (a) Ridges formed first and formed offset due to the Earth s curavture. (b) Basalt erupting from the ridges caused spreading of the ocean floor in the direction shown. (c) Fault formed to accommodate this motion.

18 Ridge Ridge Defining the System Two Plates separated by both a divergent plate margin and a transform plate margin! Plate A Plate B Fault

19 Transform Boundaries 3 Types of Transform Boundaries Ridge-Ridge (example given) Ridge-Trench Trench-Trench Plate Picture on a Ridge-Ridge is as follows

20 A whole Earth Picture

21 Summary Slide The Outer part (lithosphere-crustasthenosphere) of the Earth is a dynamic system. New oceanic material is created at Divergent Plate Boundaries Old Oceanic material is destroyed at Convergent Plate Boundaries. Transform Plate Boundaries accommodate motions between plates.

22 What this means The outer part of the Earth moves around through time. This is called continental drift. How can we tell where a continent was located in ancient times? What causes the motion of the continents?

23 Evidence for Continental Drift The idea that continents have moved through time is not new, but it was only recently supported by evidence. Alfred Wegener (German Meteorologist) proposed an idea in the early 1900 s. It was based on the following observations: Matching coastlines (e.g. South America and Africa) Glacial Deposits on the Equator made no sense-- but if the continents moved from the poles to the equator Matching fossils such as land-based dinosaurs across the Atlantic South America Africa

24 Problems In Wegener s Day, the oceans were thought to be much older than the continents. Wegener had no mechanism to propel continents through this old ocean. Idea was ridiculed until we began to study the ocean floor.

25 WW II-Submarines War required the Navy to map the ocean floor with great precision. The Mid-Ocean ridge was discovered and the volcanic activity hinted that the oceans might be much younger.

26 Harry Hess & Geopoetry In the 1960 s Harry Hess (Princeton University) suggested that the mid-ocean ridges were the sites of new crust formation and that the Atlantic Ocean was spreading apart slowly. He called the idea SEA FLOOR SPREADING

27 Vine, Mathews and Morley In the early 1960 s, two groups of researchers (Vine and Mathews) and Morley independently provided evidence that Hess was corrrect. We are going to discuss this evidence shortly..

28 The Clincher for Drifting Continents If we want to demonstrate drift of oceans and continents then. Three Items are important: Rocks that are old Rocks that record how old they are. Rocks that record where they were. We know that some rocks maintain a memory of age via radioactivity. What about the where?

29 PaleoMagnetism Paleomagnetism is the study of fossil magnetism in rocks. This magnetism allows us to tell where continents were during times past. Review of the Magnetic Field of the Earth

30 Earth s Magnetic Field Main Features 1. The Field is generated by the liquid motion of the outer core about the solid Fe inner core. 2. The field is nearly dipolar (bar magnet) 3. The earth s field is a vector having 3-D direction and strength 4. The magnetic field reverses.

31 Generation of the Main Field The Inner Core and Outer core both contain significant Fe. When Fe is in motion, it will generate a magnetic field. Convection of the fluid in the outer core generates the main field.

32 Dipole Field The Earth can be thought of as a huge bar magnet within the core. Bar magnets have a N pole and a south pole.

33 3-D Vector Field The earth s magnetic field has both a strength and a direction. The directional information is what we are after since it is the information that will tell us WHERE a continent was in the past.

34 Declination/Inclination Declination: On a compass it is the angle between you and the north pole. Inclination: This is the angle the field makes with the surface of the earth. It is horizontal at the equator and vertical at the poles.

35 Reversals of the Field It has been known for a long time now that the magnetic field of the earth reverses through time. N pole becomes south pole. Compasses that now point N will point S.

36 Reversal and Declination In a NORMAL FIELD a compass will point to the North magnetic pole. In a REVERSE FIELD a compass will point to the South Magnetic field. N S N S

37 Reversal & Inclination NORMAL REVERSE

38 More on Inclination 90 N 60 N 30 N 0 N 30 S 60 S 90 S NORMAL FIELD Arrows represent inclination w/respect to the surface of the earth at a range of latitudes! 90 N 60 N 30 N 0 N 30 S 60 S 90 S REVERSE FIELD

39 Field Reversals The Earth s Magnetic Field flips randomly from normal to reverse. The last reversal was ~780,000 years ago. Note: The field (not the earth) flips.

40 Rock Magnets We will get back to the field and its utility in paleomagnetism shortly. If we are to use paleomagnetism, then we must have some way to record this in the rocks. Minerals called magnetite and hematite have cool properties as shown.

41 Thermal Remanence Earth s Field Earth s Field Magnetites will align themselves with the magnetic field at the time they formed. Once they remember, they don t forget!! Blob of Magma `1200 C Igneous Rock < 575 C

42 Detrital Remanence Earth s Field Turbulent As hematite (rust) particles settle through the water column, they align themselves with the earth s field. Calm Ocean Floor

43 Sea Floor Spreading and Magnetic Reversals As new basaltic material erupts, it records the direction the magnetic field at the time of eruption. This is useful.

44 Tape Recorder A N N N B R R R R Mid-Ocean Ridge erupts new basalt with Normal polarity and pushes the older basalt to the side.

45 Remove Today s A N N B R R R R

46 Remove Reversed A N N B R R

47 Remove Normal A B R R

48 Remove ALL A B

49 Magnetic Reversals Important Note: Magnetic Reversals act as a tape recorder on the ocean floor and allow us to run the tape backward to establish relationships between continents. Magnetic stripes do not move the plates, they record the motion. Oldest ocean floor is ~180 Ma.

50 Paleomagnetic Reconstructions Paleomagnetism allows us to reconstruct continents beyond 200 Ma. Magnetic minerals record the location of the rock when it formed. We measure this and use the information to locate continents.

51 Quick Review Latitude Lines: Run E-W, but denote North-South position. Longitude Lines: Run N-S, but denote East-West position Latitude Lines Run E-W, but denote North-South Location Longitude Lines Run N-S, but denote East-West Location

52 Declination When rocks are magnetized, their declinations point either North (Normal Field) or South (Reverse Field). Magnetization is locked in the rock so that if the continents rotate, so will the declination. We use this to determine how to orient a continent.

53 Reconstruction 2 N In a normal field, a rock forming today would have a declination pointing north.

54 Reconstruction 3 N Suppose: (a) We find a 200 million year old rock. (b) Declination is due west (c) Field was normal when the rock formed.

55 Reconstruction 4 N This tells us that the continent has rotated 90 degrees west since the rock formed.

56 Inclination The inclination in a rock tells us where (latitudinally) the rock was located when it formed. The formula is tan (Inclination)= 2 * tan(latitude)

57 Inc lination Latitude

58 Reconstruction 5 N Suppose our inclination is 0, then our formula yields 0 latitude. We have now fully reconstructed our continent. One small problem with this!

59 Reconstruction 6 NORTH POLE All of these are correct! We have no control on longitude as we go back in time and must use other tools in addition to paleomagnetism.

60 Summary We have documented that plates can, do and have moved through geologic time. We can document this motion using magnetism and geochronology. What causes the motion?

61 HEAT!! (Thermal Convection) CC CC Hot Stuff Rises Cold Stuff Sinks HEATSOURCE

62 Inside the Earth Convection Cell MANTLE HEAT

63 Plate Driving Forces RIDGE PUSH Ridge Push- Effective in the region of the ridge (gravitational energy) Trench Pull Trench Pull- Effective across the entire plate (gravitational energy) MANTLE HEAT Thermal Convection-Heat released from within.