Topic 2-Lesson 1

1. Topic 2-Lesson 1 Plate Tectonics-The Birth of a Theory

2. The Theory of Plate Tectonics If we look at a globe carefully, most of the continents seem to fit together like a puzzle. The West African coastline seems to fit nicely into the east coast of South America. The fit is even more striking when the submerged continental shelves are compared rather than the coastlines.

3. The Theory of Continental Drift Alfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a singe protocontinent which he called Pangea (meaning ‘All lands’) and over time they have drifted apart.

4. The Theory of Continental Drift Wegener’s Hypothesis lacked a geological mechanism to explain how the continents could drift across the Earth’s surface as he had proposed. In 1929 Arthur Holmes elaborated one of Wegener’s many hypothesis:

5. The Theory of Continental Drift Arthur Holmes stated that the mantle undergoes thermal convection. And that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents.

6. The Theory of Continental Drift Not until the 1960’s did Holmes’ idea receive any attention. Greater understanding of the ocean floor and the discoveries of features like mid-ocean ridges, geomagnetic anomalies parallel to the mid-ocean ridges, and the association of island arcs and oceanic trenches occurring together and near continental margins, suggested convection might indeed be at work.

7. The Theory of Continental Drift To understand this theory better we need to look at the geological processes taking place and the evidence which supports it. We’ll start by looking at the crust.

8. Earth’s Crust The crust covers the mantle and is the Earth’s hard outer shell, the surface we are living on . Compared to the other layers the crust is much thinner.

9. Earth’s Crust The crust is made up of solid material but this material is not the same everywhere. There is an oceanic crust and a continental crust.

10. Oceanic Crust This crust is below the ocean and is 6-11 km thick. The rocks of the oceanic crust are very young compared with rocks of the continental crust; not older than 200 million years. The main rock type is basalt and the average density is 3g/cm^3

11. Continental Crust Continental crust is the part of Earth’s crust not covered by water. This is much thicker than oceanic crust with an average of about 30-40km and a maximum of 70km. Continental crust is much older, some rocks are 4.1 billion years old. Continental crust consists of igneous rocks such as Granite. The average density is 2.7g/cm^3

12. Video • Plate Tectonics Video

13. Homework • Read pages 122-125 Prelim Spotlight Text • Update vocab list • Compete DOT Point 2.1 pg 89

14. Topic 2-Lesson 2 Evidence for Continental Drift

15. Plate Tectonics The Theory of Plate Tectonics builds on Wegener’s Theory of Continental Drift. This theory states that the Earth’s crust or Lithosphere is broken up into “tectonic plates” each of which are moving as a result of convection currents in the mantle.

16. Plate Tectonics • The plate tectonics theory became established during the 1960’s and is now regarded as the unifying theory of geology. This means that all geological processes can be explained and understood in terms of this model.

17. Convection Currents • Convection currents are in all unevenly heated fluids. If we look at the diagram below of a jug of water being heated by a flame, the water at the bottom of the beakeris heated the most. It expands, becomes less dense and rises to the surface.

18. Convection Currents • Water from besides the point of heating replaces the rising water. As hot water moves to the surface, it looses heat and is pushed sideways by the rising water below. This cooler surface water has a higher density and so sinks to the bottom where the cycle starts again.

19. Convection Currents • Convection currents are driven by the effects of gravity on different densities within a fluid.

20. Convection Currents • The mantle isn’t as fluid as water, it’s only partially molten and able to flow slowly over time. Magma rises above the hottest parts of the mantle, strikes the overlying crust and them moves sideways. As it moves sideways it looses heat, becomes more dense and sinks back into the mantle

21. Convection Currents • It’s the sideways motion of the convection current in the mantle that is believed to be the driving force behind continental drift.

22. Evidence for Continental Drift • Despite the fact that Wegener collected a variety of evidence to support this theory, it wasn’t widely accepted until exploration of the ocean floor found a number of features that could only be explained by continental drift. The evidence includes: • Continental Shapes • Cross-continental geological formations • Glacial deposits • Palaeoclimates • Apparent polar wandering • Magnetic Reversals • Direct Measurement • We are going to look at some of the evidence today and the rest next lesson.

23. Continental Shapes • For centuries scholars have commented on how perfectly South America fits with Africa. This jigsaw-type fit is not apparent with the shapes of other continents however the mapping of the sea floor tells another story. When using the entire continental shape rather than just the dry land component, the jigsaw fit occurs with many continents.

24. Cross-continental geological formations • The position of some geological features on widely separated continents align perfectly when the continents are placed together. Various features such as folded mountains, sedimentary deposits and fossils act like parts of a picture printed onto blank jigsaw pieces.

25. Glacial Deposits • We know that during ice ages, ice and glaciers spread outward over large regions from the poles. These giant glaciers leave deep scratches in rocks that can today be easily identified and dated. During the Palaeozoic glaciation 300mya, rocks far away from the poles near the equator have such marks on them.

26. Palaeoclimates (ancient climates) • Numerous types of rocks and minerals can only be created under specific climactic conditions. We know coal is formed from organic materials deposited in warm, swampy areas. Antarctica has coal deposits! What does this tell us about it’s location over time?

27. Homework • Read pages 125-127 Prelim Spotlight Text • Complete DOT Point 5.1 pg 103 • Update vocab list

28. Topic 2-Lesson 2 continued Evidence for Continental Drift

29. Review • In the last lesson we looked at the following evidence which supports continential drift: • Continental Shapes • Cross-continental geological formations • Glacial Deposits • Palaeoclimates • Today we are going to look at • Apparent polar wondering • Magnetic reversals • Age of ocean rocks and sediments • Direct measurement

30. Apparent Polar Wandering When lava erupts onto the surface, the iron minerals within it, such as magnetite, align themselves with the Earth’s magnetic field in the same way as a compass.

31. Apparent Polar Wandering Once the lava cools and solidifies, the minerals are locked into position. By analysing the alignment of magnetic minerals within lava, geologists are able to determine the directions of the North and South poles when the lava erupted.

32. Apparent Polar Wandering In addition to the magnetic bearings, the distance at which the lava formed from either pole can also be estimated by the angle of tilt of these magnetic minerals. But How???

33. Apparent Polar Wandering If you hold a hand compass vertically to the ground, it tells us the angle of the Earths magnetic field.

34. Apparent Polar Wandering At the equator, the compass needle will be parallel with the ground. This is how the minerals would align themselves.

35. Apparent Polar Wandering At the magnetic South Pole the south end of the compass needle will point straight at the ground.

36. Apparent Polar Wandering Midway between the equator and the poles, the compass needle will point down at a 45-degree angle

37. Apparent Polar Wandering If lava erupted today in Sydney, the south end of the magnetic minerals (compass needle) would point down at 34 degrees.

38. Apparent Polar Wandering By measuring the angle of the iron minerals formed in rocks, scientists are able to determine both the direction of the Pole and the distance from the pole when the mineral solidified.

39. Magnetic Reversals For reasons not fully understood, the polarity of Earth’s magnetic field reverses every few million years. In other words, the magnetic North Pole becomes the magnetic South Pole and vice versa.

40. Magnetic Reversals Such magnetic reversals are believed to be caused by the changes in the circulation patterns of molten iron in the Earth’s outer core.

41. Magnetic Reversals This flipping is rapid and the locations of the poles of the globe stay much the same. If this were to happen today, the north end of our compass would point towards Antarctica. A series of magnetic reversals were first identified in lave fields on land. Using radiometric dating scientists were able to measure dates for the reversals. But why does this matter?

42. Magnetic Reversals The importance of magnetic reversals to continental drift can be seen when examining the polarity of rocks across mid ocean ridges.

43. Magnetic Reversals The pattern of magnetic reversals on one side of oceanic ridge is a mirror image of that on the other. This can only be explained if the crust is being pulled apart at these ridges.

44. Magnetic Reversals As the crust breaks open in the central valley of an oceanic ridge, lava erupts, forming a long and narrow plug. Further spreading at the ridge will cause the old plug to tear down the middle and fresh lava to create a new plug. If Earth’s magnetic field reversed between these eruptions, the different lava flows can be identified.

45. Magnetic Reversals Think about it: If new crust is being created at mid-ocean ridges and the planet isn’t growing in size. What’s happening to the old crust??

46. Age of Ocean Rocks and Sediments If new crust is continuously formed at oceanic ridges and is then pulled apart sideways, rocks should get older the further they are from the ridge.

47. Age of Ocean Rocks and Sediments Radioisotope dating has shown this to be the case at all oceanic ridges. Ocean basins formed at ridges are only 200 million years old.

48. Age of Ocean Rocks and Sediments Organic debris and other sediments rain down onto the deep ocean floor at a steady rate. The fact that little or no sediment covers rocks at the ridge, while sediment layers get thicker as you move away from it. This is further evidence that ocean rocks increase with age away from the ridge.

49. Direct Measurement • With today's technologies such as computers, laser measurement and satellite remote sensing, the slow movement of the Earth’s plates can be measured directly. By calculating the distance between two points on ether side of a plate boundary, the rate and direction of relative movement can be determined.

50. Evidence of Continental Drift • The scientific community finally accepted Wegner’s theory because of all the evidence. It was unfortunate that this didn’t happen until 30 years after his death.