Plate Tectonics, Continental Drift, and Seafloor Spreading

Lesson in Earth and Life Science

1. Plate Tectonics

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3. Lesson 1: Continental Drift Theory

4. Continental Drift Theory proposed by Alfred Wegener in 1915 in his book, The Origin of Continents and Oceans A long time ago, the continents formed a single land mass, called Pangaea, which broke into Laurasia and Gondwana and then into several continents and drifted to their current positions. Wegener used observations and concepts from the fields of geography, geology, biology, and paleontology to support his hypothesis.

5. The Fit of Continental Shorelines Wegener viewed the apparent fit of the continents along their coastlines. The cartogapher Abraham Ortelius suggested in 1596 the possibility that the continents had drifted apart to their current positions. Almost as soon as the continents were mapped, it was noticed that South America fits neatly into the corner of Africa. This fit is even better if you include the shallow underwater shelf which extends around their coastlines.

6. Distribution of Glacial Sediments Wegener plotted the locations of the sediments and rocks that were formed during the last glaciation of the late Paleozoic. He found similar striations in the southern parts of South America, Africa, India, Australia, and Antarctica. These sediments were unlikely formed in their current locations, and the current orientation of these striations indicate that the glaciers moved from the ocean toward the land, which is very unlikely.

7. Paleoclimate If the southern part of Pangaea is located in the polar region as indicated by the presence of glaciers, then the rest of the supercontinent extended to the equator and to the Northern Hemisphere. Coal deposits were found in areas that presently do not have a tropical climate.

8. Distribution of Fossils Snider and Pelligrini gathered fossil evidence in 1859, which showed that fossil distributions made more sense if the continents were once joined together into a supercontinent later known as Pangaea. The fossil distributions now join up like a jigsaw puzzle. In the 19th century the naturalist Alfred Russell Wallace had studied the differences in living flora and fauna across the 35 km of water between the islands of Bali and Lombok in the East Indies. He concluded that the differences between the two sides of what became known as the Wallace line must be because the islands had moved over geological time from a position where they were far apart.

9. Distribution of Rocks Wegener was able to recognize the distinct assembly of rocks occurring on either side of the Atlantic Ocean. Pangaea was reconstructed based on the type, age, and sequence of the layering of rocks Example: The mountain belts in the eastern side of Africa and those in the western side of North America match as if they were formed together.

10. Lesson 2: Seafloor Spreading

11. Sea f loor Spreading A geologic process in which the lithosphere split apart The process by which molten material adds new oceanic crust to the ocean floor

12. Mid- o cean Ridge The undersea mountain chain where new ocean floor is produced; a divergent plate boundary

13. S onar is used to map the ocean floor Sonar bounces sound waves off underwater objects and then records the echoes of these sound waves The time it takes for the echo to arrive indicates the distance to the object

14. Harry Hammond Hess He is a professor at Princeton University. In the 1960s, he used sonar and examined the maps of mid-ocean ridges during WWII. He proposed that ocean floors move like conveyor belts, carrying the continents with them.

15. Seafloor Spreading Theory Hot, less dense material below the Earth’s crust rises toward the mid-ocean ridge The material flows sideways, carrying the seafloor away from the ridge and creates a crack in the crust The magma flows out of the cracks, cools down and becomes the new seafloor Overtime, the new oceanic crust pushes the old oceanic crust far from the ridge until the old oceanic crust is subducted The age, thickness, and density of oceanic crust increase as it moves away from the mid-ocean ridge

16. E vidence of seafloor spreading found in the 1960s Evidence from molten material Evidence from magnetic stripes Evidence from drilling samples

17. A. Evidence From Molten Material Alvin’s crew found strange rocks shaped like pillows or like toothpaste squeezed from a tube Such rocks can form only when molten material hardens quickly after erupting under water The presence of these rocks showed that molten material has erupted repeatedly from cracks along the central valley of the mid-ocean ridge. http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html

18. B. Evidence From Magnetic Stripes Most basalt magmas contain abundant molten iron. As magma starts to harden into rock, iron-rich minerals solidify first. Their crystals are pulled into alignment by the Earth’s magnetic field, just like a compass needle is pulled towards magnetic north. Once the basalt cools completely into solid rock, the alignment of the iron minerals is fixed. Thus, basalts preserve a permanent record of the strength and direction, or polarity, of the planet’s magnetic field at the time the rocks were formed.

19. B. Evidence From Magnetic Stripes When geologists studied the polarity of ancient rocks, they were stunned to discover that in many of them, iron minerals were aligned toward the south magnetic pole, not the north. Scientists have concluded that the Earth’s magnetic field has reversed itself again and again throughout the ages.

20. B. Evidence From Magnetic Stripes 1. Based on the diagram, how many times has the Earth's magnetic field reversed during the past four million years? 2. Approximately when did the current interval of normal polarity begin? 3. If there had been compasses four million years ago, which direction would compass needles have pointed? 4. Why are the magnetic stripes on the seafloor parallel to and symmetrical across the mid-ocean ridge?

21. Additional Information Polarity reversal takes thousands of years to complete and occurs when the magnetic field is weakened by about 90% to a threshold level. The measured strength of the magnetic field has dropped by 5%-10% in the last 150 years, and less precise readings suggest it may have weakened by 25%-50% over the last 5,000 years.

22. C. Evidence From Drilling Samples When scientists sampled the rocks, they found that the further away from the ridge the rocks were the older they were . Y ounger rocks were always found in the center of the ridges

23. Subduction at Deep- o cean Trenches

24. Deep- o cean Trench A deep valley along the ocean floor through which oceanic crust slowly sinks towards the mantle

25. Subduction The process by which oceanic crust sinks through a deep-ocean trench and back into the mantle; a convergent plate boundary

26. Guide For Reading: What happens to the ocean floor at deep ocean trenches? At deep-ocean trenches, two plates collide causing the denser of the two plates to dive back to the mantle. This process is known as subduction. Over tens of million of years, this material melts back into molten material and may rise again as new oceanic crust.

27. Guide For Reading: What is the process of seafloor spreading? At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. Over tens of millions of years, the process continues until the oldest ocean floor collides with the continental crust The more dense oceanic crust subducts (sinks) back into the mantle at a deep-ocean trench

28. Subduction and Earth’s Oceans

29. Subduction in the Pacific Ocean Subduction in the Pacific Ocean is occurring at a greater rate than sea-floor is expanding This is caused by the large amount of trenches

30. Subduction in the Atlantic The Atlantic Ocean is expanding at a greater rate than subducting This is because of the low number of trenches in the Atlantic Over time the entire ocean gets larger and pushes against the continents

31. Additional Information Seafloor spreading and subduction maintains the shape and diameter of the Earth Seafloor spreading creates new crust, and subduction destroys old crust

32. Lesson 3: Plate Tectonic Theory

33. What Are Tectonic Plates? A plate is a large, rigid slab of solid rock. Plates are formed from the lithosphere: the crust and the upper part of the mantle. The plates “float” on the slowly flowing asthenosphere: the lower part of the mantle. The plates include both the land and ocean floor. The Mohoriovicic discontinuity or Moho is the boundary between the crust and the mantle.

34. Major Plates African plate Antarctic plate Eurasian plate Indo-Australian plate North American plate Pacific plate South American plate

35. What Drives Plate Tectonics? The slow movement of hot, softened mantle lies below rigid plates. The hot, softened rock in the mantle moves in a circular manner in a convection flow – the heated, molten rock rises to the surface, spreads, and begins to cool, and then sinks back down to be reheated and rises again.

36. Movement of the Plates Over Time

37. Different Types of Boundaries

38. Different Types of Boundaries Convergent boundaries come together Places where crust is destroyed as one plate dives under another Divergent boundaries spread apart Places where new crust is generated as the plates pull away from each other New crust is created from magma pushing up from the mantle Transform boundaries slide against each other Places where crust is neither produced nor destroyed as the plates slide horizontally past each other

39. Oceanic-Continental Convergence The oceanic plate subducts under the continental plate because it has lower density.  The oceanic Nazca Plate is being subducted under the continental part of the South American Plate. The South American Plate is being lifted up, creating the Andes mountains. Strong, destructive earthquakes and rapid uplift of mountain ranges are common in this region. These earthquakes are often accompanied by uplift of the land by as much as a few meters. Mount Saint Helens is along the subduction zone of the Juan de Fuca plate (an oceanic plate) and the North American plate (a continental plate).

40. Oceanic-Oceanic Convergence When two oceanic plates converge, one is usually subducted under the other. An older oceanic plate is colder, therefore more dense and less buoyant, and will subduct under a younger, hotter, less dense, and more buoyant oceanic plate. In the process, a trench is formed. The deepest trenches in the oceans are along oceanic-oceanic subduction zones (i.e., the Marianas Trench in the Pacific, which is deeper than Mt. Everest is high). Subduction in oceanic-oceanic plate convergence can result in the formation of volcanoes. Examples of oceanic-oceanic convergence are the arcuate chains of islands in the southwest Pacific, Japan, and the Aleutian Islands.

41. Continental-Continental Convergence When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision between the Indian and Eurasian plates has pushed up the Himalayas and the Tibetan Plateau.

42. Continental-Continental Convergence 50 million years ago, the Indian Plate collided into the Eurasian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalaya and the Tibetan Plateau to their present heights. The Himalayas form the highest continental mountains in the world.

43. Divergence Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. The Mid-Atlantic Ridge is a divergent boundary. Sea-floor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the ocean that exists today.

44. Divergence Iceland is splitting along the Mid-Atlantic Ridge between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. In East Africa, spreading processes have already torn Saudi Arabia away from the rest of the African continent, forming the Red Sea. The actively splitting African Plate and the Arabian Plate meet in what geologists call a triple junction, where the Red Sea meets the Gulf of Aden.

45. Transform The zone between two plates that slide past one another is called a transform-fault boundary, or transform boundary. These large faults connect two spreading centers or connect trenches. Most transform faults are found on the ocean floor.

46. Transform The San Andreas Fault is one of the few transform faults exposed on land. It connects the East Pacific Rise, a divergent boundary to the south, with the Juan de Fuca Ridge, a divergent boundary to the north. Most earthquakes in California are caused by the accumulation and release of strain as the two plates slide past each other.

47. Consequences of Plate Tectonics Earthquakes and volcanic activity are linked to plate tectonic processes. The Ring of Fire is the most seismically and volcanically active zone in the world.

48. Consequences of Plate Tectonics The San Andreas Fault – a transform fault Aerial view of the area around Thingvellir, Iceland, showing a fissure zone (in shadow) that is an on-land exposure of the Mid-Atlantic Ridge. Right of the fissure, the North American Plate is pulling westward away from the Eurasian Plate (left of fissure).

49. The Aleutian Islands, an island arc The 1980 eruption of Mount Saint Helens Consequences of Plate Tectonics

50. The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form towering peaks of the Andes, as seen here in the Pachapaqui mining area in Peru. Helicopter view (in February 1994) of the active lava lake within the summit crater of 'Erta 'Ale (Ethiopia), one of the active volcanoes in the East African Rift Zone. Consequences of Plate Tectonics