1 00:00:01,010 --> 00:00:12,279 Once upon a time, South America lived harmoniously alongside Africa 2 00:00:12,279 --> 00:00:15,699 until a crack in the earth drove the two continents apart. 3 00:00:16,559 --> 00:00:19,940 This breakup began about 200 million years ago 4 00:00:19,940 --> 00:00:23,839 during the separation of the supercontinent known as Pangea. 5 00:00:24,539 --> 00:00:28,039 Their proximity back then explains why the same plant fossils 6 00:00:28,039 --> 00:00:30,300 and reptile fossils like the Mesosaurus 7 00:00:30,300 --> 00:00:34,960 can be found on the South American east coast and African west coast. 8 00:00:35,520 --> 00:00:39,179 However, this evidence does not account for how the continents moved apart. 9 00:00:39,820 --> 00:00:43,200 For that, we'll need to take a close look at the Earth below our feet. 10 00:00:43,780 --> 00:00:45,100 Though you may not realize it, 11 00:00:45,299 --> 00:00:48,259 the ground below you is traveling across the Earth 12 00:00:48,259 --> 00:00:50,820 at a rate of about 10 centimeters per year, 13 00:00:51,280 --> 00:00:53,500 or the speed at which your fingernails grow. 14 00:00:54,259 --> 00:00:55,960 This is due to plate tectonics, 15 00:00:56,320 --> 00:00:58,740 or the large-scale movement of Earth's continents. 16 00:00:59,079 --> 00:01:02,820 The motion occurs within the top two layers of the Earth's mantle, 17 00:01:02,820 --> 00:01:05,299 the lithosphere and asthenosphere. 18 00:01:06,040 --> 00:01:09,459 The lithosphere, which includes the crust and uppermost mantle, 19 00:01:09,920 --> 00:01:11,379 comprises the land around you. 20 00:01:11,900 --> 00:01:13,920 Beneath the lithosphere is the asthenosphere, 21 00:01:14,359 --> 00:01:17,879 the highly viscous but solid rock portion of the upper mantle. 22 00:01:18,579 --> 00:01:22,019 It's between 80 and 200 kilometers below the Earth's surface. 23 00:01:22,640 --> 00:01:26,319 While the asthenosphere wraps around the Earth's core as one connected region, 24 00:01:26,739 --> 00:01:30,219 the lithosphere is separated on top into tectonic plates. 25 00:01:30,980 --> 00:01:33,120 There are seven primary tectonic plates 26 00:01:33,120 --> 00:01:35,480 that compose the shape of the planet we know today. 27 00:01:36,180 --> 00:01:38,019 Like the other smaller tectonic plates, 28 00:01:38,420 --> 00:01:41,180 the primary plates are about 100 kilometers thick 29 00:01:41,180 --> 00:01:43,719 and are composed of one or two layers, 30 00:01:44,099 --> 00:01:46,280 continental crust and oceanic crust. 31 00:01:47,239 --> 00:01:49,239 Continental crust forms the continents 32 00:01:49,239 --> 00:01:51,980 and areas of shallow water close to their shores, 33 00:01:52,480 --> 00:01:55,420 whereas oceanic crust forms the ocean basins. 34 00:01:55,599 --> 00:01:58,659 The transition from the granitic continental crust 35 00:01:58,659 --> 00:02:00,799 to the basaltic oceanic crust 36 00:02:00,799 --> 00:02:03,159 occurs beyond the continental shelf, 37 00:02:03,500 --> 00:02:05,620 in which the shore suddenly slopes down 38 00:02:05,620 --> 00:02:06,659 towards the ocean floor. 39 00:02:07,519 --> 00:02:09,319 The South American plate is an example 40 00:02:09,319 --> 00:02:11,639 of a tectonic plate made of two crusts, 41 00:02:11,860 --> 00:02:13,639 the continent we know from today's map 42 00:02:13,639 --> 00:02:16,800 and a large region of the Atlantic Ocean around it. 43 00:02:17,699 --> 00:02:19,419 Collectively comprising the lithosphere, 44 00:02:19,900 --> 00:02:21,879 these plates are brittler and stiffer 45 00:02:21,879 --> 00:02:23,759 than the heated, malleable layer 46 00:02:23,759 --> 00:02:25,280 of the asthenosphere below. 47 00:02:25,939 --> 00:02:27,759 Because of this, the tectonic plates 48 00:02:27,759 --> 00:02:31,580 Floats float on top of this layer independently of one another. 49 00:02:31,580 --> 00:02:34,560 The speed and direction in which these tectonic plates move 50 00:02:34,560 --> 00:02:38,080 depends on the temperature and pressure of the asthenosphere below. 51 00:02:38,080 --> 00:02:42,219 Scientists are still trying to nail down the driving forces behind this movement, 52 00:02:42,219 --> 00:02:44,879 with some theories pointing towards mantle convection, 53 00:02:44,879 --> 00:02:49,879 while others are examining the influence of the Earth's rotation and gravitational pull. 54 00:02:49,879 --> 00:02:51,979 Though the mechanics have not been sorted out, 55 00:02:51,979 --> 00:02:56,060 the scientific community agrees that our tectonic plates are moving, 56 00:02:56,060 --> 00:02:58,620 than they have been for billions of years. 57 00:02:58,620 --> 00:03:00,599 Because these plates move independently, 58 00:03:00,599 --> 00:03:04,280 a fair amount of pushing and pulling between the plates occurs. 59 00:03:04,280 --> 00:03:07,139 The first type of interaction is a divergent boundary 60 00:03:07,139 --> 00:03:09,960 in which two plates move away from one another. 61 00:03:09,960 --> 00:03:14,939 We see this in the Mid-Atlantic Ridge between South America and Africa. 62 00:03:14,939 --> 00:03:17,620 The next interaction is when two plates collide, 63 00:03:17,620 --> 00:03:19,699 known as a convergent boundary. 64 00:03:19,699 --> 00:03:22,060 In this instance, the land is pushed upward 65 00:03:22,060 --> 00:03:24,719 to form large mountain ranges like the Himalayas. 66 00:03:24,719 --> 00:03:28,319 In fact, the Indian plate is still colliding with the Eurasian plate, 67 00:03:28,319 --> 00:03:31,759 which is why Mount Everest grows one centimeter per year. 68 00:03:31,759 --> 00:03:34,060 Finally, there's the transform boundaries, 69 00:03:34,060 --> 00:03:36,479 where two plates scrape past one another. 70 00:03:36,479 --> 00:03:39,620 The grinding of the transform boundary leads to many earthquakes, 71 00:03:39,620 --> 00:03:44,280 which is what happens in the 810-mile-long San Andreas Fault. 72 00:03:44,280 --> 00:03:46,060 The moving Earth is unstoppable, 73 00:03:46,060 --> 00:03:49,879 and while a shift of 10 centimeters per year may not seem like a lot, 74 00:03:49,879 --> 00:03:53,699 over millions of years, our planet will continue to dramatically change. 75 00:03:53,699 --> 00:03:57,699 Mountains will rise, shorelines will recede, islands will pop up. 76 00:03:57,699 --> 00:04:01,699 In fact, one projected map shows the cities of Los Angeles and San Francisco 77 00:04:01,699 --> 00:04:05,520 on top of each other. 78 00:04:05,520 --> 00:04:08,520 Maybe South America and Africa will come together again, too. 79 00:04:08,520 --> 00:04:10,520 Only time will tell.