1 00:00:02,890 --> 00:00:09,806 Earth, a 4.5- Billion-year-old planet, still evolving. 2 00:00:09,806 --> 00:00:17,763 As continents shift and clash, volcanoes erupt and glaciers grow and recede, 3 00:00:17,763 --> 00:00:22,514 the earth's crust is carved in numerous and fascinating ways, 4 00:00:22,514 --> 00:00:26,721 leaving a trail of geological mysteries behind. 5 00:00:30,387 --> 00:00:31,846 In this episode, 6 00:00:31,846 --> 00:00:37,387 the 450-million-year-old geological history of New York City is explored. 7 00:00:37,387 --> 00:00:41,302 A metropolis pockmarked with strange rocks, 8 00:00:41,302 --> 00:00:44,427 haunted by footprints of ancient giant reptiles, 9 00:00:44,427 --> 00:00:49,968 and lined with a vast curtain of solidified lava. 10 00:00:51,259 --> 00:00:53,468 Scientists investigate the evidence 11 00:00:53,468 --> 00:00:59,301 for fiery volcanoes, massive floods and ice sheets 12 00:00:59,301 --> 00:01:02,717 four times as high as the Empire State building. 13 00:01:02,717 --> 00:01:08,591 The clues to understanding New York City's geological past provides a window 14 00:01:08,591 --> 00:01:11,758 into the formation of the earth itself. 15 00:01:24,339 --> 00:01:28,630 The investigation into New York City's geological history begins here, 16 00:01:28,630 --> 00:01:31,714 with Manhattan's rocky outcrops. 17 00:01:33,380 --> 00:01:36,921 These rocks are clues to how the land was made 18 00:01:36,921 --> 00:01:41,837 and how its geology helped it become a dense, thriving, pulsating city. 19 00:01:43,837 --> 00:01:46,545 They're scattered all over Manhattan, 20 00:01:46,545 --> 00:01:48,878 poking through the surface of parks 21 00:01:48,878 --> 00:01:51,794 and through the concrete between the buildings. 22 00:01:51,794 --> 00:01:57,835 Some, squashed between two apartment blocks, are the size of a whale. 23 00:01:57,835 --> 00:02:01,959 They are the extraordinary survivors of ancient times. 24 00:02:06,667 --> 00:02:10,209 Most importantly, they are the surface tips of the bedrock 25 00:02:10,209 --> 00:02:14,042 in which Manhattan's buildings are anchored. 26 00:02:15,292 --> 00:02:22,124 Gigantic skyscrapers stand in two clusters, in downtown and midtown. 27 00:02:22,124 --> 00:02:26,664 In the section between, the buildings are lower. 28 00:02:26,664 --> 00:02:30,331 The clues to the shape of Manhattan's familiar skyline 29 00:02:30,331 --> 00:02:33,789 are the rocks beneath the surface. 30 00:02:36,663 --> 00:02:41,830 A leading expert on the rocks in New York is geologist Charles Merguerian. 31 00:02:41,830 --> 00:02:44,913 The entire history of the development of the earth's crust 32 00:02:44,913 --> 00:02:46,871 is emblazoned in the rocks beneath us. 33 00:02:46,871 --> 00:02:50,537 The rocks here in New York City harbour an ancestry 34 00:02:50,537 --> 00:02:53,703 that dates back over a billion years of time. 35 00:02:53,703 --> 00:03:00,661 Merguerian is searching for evidence to show how the city's bedrock was made. 36 00:03:00,661 --> 00:03:05,202 At Inwood Hill Park in Upper Manhattan, he's found an extremely hard piece 37 00:03:05,202 --> 00:03:08,576 of the bedrock known as Manhattan schist. 38 00:03:08,576 --> 00:03:12,409 To the untrained eye, it's just a piece of rock, 39 00:03:12,409 --> 00:03:16,492 but to Merguerian, this is his first clue. 40 00:03:16,492 --> 00:03:18,533 The rocks that we're looking at right here 41 00:03:18,533 --> 00:03:20,367 are rocks of the Manhattan schist formation 42 00:03:20,367 --> 00:03:23,575 and the... these rocks are very severely deformed, 43 00:03:23,575 --> 00:03:27,241 and the structures here in this rock is a structure that comes up like this, 44 00:03:27,241 --> 00:03:31,115 bends around and comes back down on itself as such, 45 00:03:31,115 --> 00:03:35,657 and in three-dimensional view, it's a structure that looks something like this. 46 00:03:35,657 --> 00:03:39,698 A very, very tight fold with a plunge towards the south here. 47 00:03:39,698 --> 00:03:42,989 These are rocks that were very, very strongly deformed 48 00:03:42,989 --> 00:03:45,113 over protracted periods of time. 49 00:03:45,113 --> 00:03:49,238 And it's the same bedrock that occurs over much of New York City. 50 00:03:53,279 --> 00:03:57,237 This tight fold in the rock suggests New York's bedrock 51 00:03:57,237 --> 00:03:59,487 was formed under great pressure. 52 00:03:59,487 --> 00:04:06,403 To confirm this hunch, Merguerian takes a sample to the lab for detailed analysis. 53 00:04:06,403 --> 00:04:13,777 Radiometric dating proves this rock is about 450 million years old. 54 00:04:15,360 --> 00:04:19,318 But the rock has even greater secrets to tell. 55 00:04:20,818 --> 00:04:23,317 It contains a kaleidoscope of minerals, 56 00:04:23,317 --> 00:04:27,067 which opens a window into the ancient world. 57 00:04:27,067 --> 00:04:31,067 To me, minerals are like the instrument cluster in your car, 58 00:04:31,067 --> 00:04:33,567 they tell you everything about how your car is running. 59 00:04:35,274 --> 00:04:40,316 Merguerian uses a microscope with polarised light to view the minerals. 60 00:04:40,316 --> 00:04:43,691 The examination tells us the former depth regime, 61 00:04:43,691 --> 00:04:46,939 how deep the rocks were, they tell you the age of the rocks, 62 00:04:46,939 --> 00:04:48,898 they tell you everything you want to know 63 00:04:48,898 --> 00:04:50,731 about the development of the earth's crust. 64 00:04:51,731 --> 00:04:56,689 What's striking about these samples is that the minerals inside are elongated. 65 00:04:56,689 --> 00:05:02,980 It is a clue that these rocks must once have been crushed by massive forces. 66 00:05:02,980 --> 00:05:05,562 And the colours support this theory. 67 00:05:05,562 --> 00:05:11,520 Under the polarised light, the sample from Inwood Hill Park shows up blue. 68 00:05:11,520 --> 00:05:17,394 This comes from a mineral called kyanite, which forms at great depths. 69 00:05:17,394 --> 00:05:20,477 It's conclusive evidence that this rock 70 00:05:20,477 --> 00:05:24,144 was compressed deep under the surface. 71 00:05:24,144 --> 00:05:28,185 Rocks forged at these depths are much harder, 72 00:05:28,185 --> 00:05:31,685 ideal for a city's foundations. 73 00:05:31,685 --> 00:05:36,476 But what gigantic weight was on top? 74 00:05:36,476 --> 00:05:40,183 Merguerian believes there is only one answer. 75 00:05:40,183 --> 00:05:42,932 The rock was once buried under the crushing weight 76 00:05:42,932 --> 00:05:46,307 of a chain of massive mountains. 77 00:05:46,307 --> 00:05:49,890 The minerals that we find in the bedrock units of New York City 78 00:05:49,890 --> 00:05:53,223 tell us that the rocks of New York City were formerly buried 79 00:05:53,223 --> 00:05:57,473 when they were formed, under very high pressures, 80 00:05:57,473 --> 00:06:00,056 and that those high pressures indicate 81 00:06:00,056 --> 00:06:04,597 that these rocks formerly were produced at depths of 20 to 25 miles, 82 00:06:04,597 --> 00:06:08,847 and probably the mountains were as high as the Alps are today. 83 00:06:11,930 --> 00:06:17,512 But even the most impressive mountain chains can't survive the ravages of time. 84 00:06:17,512 --> 00:06:19,887 The Rocky Mountains, for example. 85 00:06:19,887 --> 00:06:24,261 Millions of years ago, they soared nearly six miles into the sky. 86 00:06:24,261 --> 00:06:28,510 Today, erosion has halved their size. 87 00:06:32,177 --> 00:06:35,301 The same process happened in New York. 88 00:06:35,301 --> 00:06:40,010 Rain, wind and ice wore the ancient mountains almost flat. 89 00:06:40,010 --> 00:06:43,634 But the microscopic crystals found in the rock in Manhattan 90 00:06:43,634 --> 00:06:47,550 testify that they existed in the past. 91 00:06:50,592 --> 00:06:53,091 How did the mountains form? 92 00:06:53,091 --> 00:06:56,841 The answer lies in the way the earth's crust moves. 93 00:06:59,507 --> 00:07:05,007 A network of interlocking individual pieces makes up the Earth's surface. 94 00:07:05,007 --> 00:07:08,048 Geologists call them tectonic plates. 95 00:07:08,048 --> 00:07:15,796 Over millions of years, they collide and break apart to form different continents. 96 00:07:15,796 --> 00:07:21,671 450 million years ago, the Earth's surface looked completely different. 97 00:07:21,671 --> 00:07:23,295 North America was much further 98 00:07:23,295 --> 00:07:24,921 to the south. 99 00:07:24,921 --> 00:07:27,920 MERGUERIAN: North America was tilted 90 degrees clockwise 100 00:07:27,920 --> 00:07:29,712 from its present orientation 101 00:07:29,712 --> 00:07:31,795 and it was straddling the equator. 102 00:07:31,795 --> 00:07:33,753 As such, the climate was tropical, 103 00:07:33,753 --> 00:07:38,377 the east coast of North America was really experiencing Club Med conditions. 104 00:07:40,586 --> 00:07:42,502 The weather may have been awesome, 105 00:07:42,502 --> 00:07:46,543 but the ancient East Coast was heading for trouble. 106 00:07:49,668 --> 00:07:52,084 The plate beneath it was moving. 107 00:07:52,084 --> 00:07:56,958 The East Coast was on a collision course with ancient West Africa. 108 00:07:56,958 --> 00:08:02,208 450 million years ago, they collided. 109 00:08:02,208 --> 00:08:05,499 The impact unleashed geological chaos. 110 00:08:05,499 --> 00:08:09,291 Under intense compression, the land was forced upwards 111 00:08:09,291 --> 00:08:12,082 to form a soaring range of mountains. 112 00:08:12,082 --> 00:08:15,331 The collision that took place is the most fundamental 113 00:08:15,331 --> 00:08:17,873 and impressive mountain-building event 114 00:08:17,873 --> 00:08:20,622 to affect the east coast of North America. 115 00:08:23,581 --> 00:08:27,122 Today, all that remains are their stumps, 116 00:08:27,122 --> 00:08:32,204 stumps that form the bedrock of modern-day New York. 117 00:08:32,204 --> 00:08:36,287 The collision that built up the ancient mountains also folded the bedrock 118 00:08:36,287 --> 00:08:38,828 into dips and rises. 119 00:08:38,828 --> 00:08:44,536 These folds are responsible for the shape of Manhattan's skyline. 120 00:08:47,036 --> 00:08:52,994 The city boasts two clusters of skyscrapers in downtown and midtown. 121 00:08:52,994 --> 00:08:58,243 Here, the hard bedrock that formed deep underground was forced up. 122 00:08:58,243 --> 00:09:01,992 It is now close to the surface and provides solid anchorage 123 00:09:01,992 --> 00:09:04,992 for the high-rise buildings. 124 00:09:04,992 --> 00:09:09,241 In the dip in the middle the rock was folded down. 125 00:09:09,241 --> 00:09:14,158 The area is filled with loose sediments, less suitable for skyscrapers. 126 00:09:14,158 --> 00:09:16,366 MERGUERIAN: When the bedrock is at the Earth's surface 127 00:09:16,366 --> 00:09:18,074 where it's actually exposed, 128 00:09:18,074 --> 00:09:19,990 then it's pretty easy to build tall buildings 129 00:09:19,990 --> 00:09:22,865 'cause you can root them directly into solid rock. 130 00:09:22,865 --> 00:09:27,698 However, in areas where the bedrock is deep and covered by glacial sediment, 131 00:09:27,698 --> 00:09:30,364 in those cases, it's very difficult to build tall buildings 132 00:09:30,364 --> 00:09:35,072 because you need to root those buildings either into solid rock 133 00:09:35,072 --> 00:09:39,779 or build concrete abutments called caissons that can support tall buildings. 134 00:09:41,696 --> 00:09:45,403 New York's deep history is beginning to take shape. 135 00:09:46,695 --> 00:09:49,153 Building up from tiny crystals in the rock, 136 00:09:49,153 --> 00:09:52,903 scientists revealed how New York's bedrock was formed 137 00:09:52,903 --> 00:09:57,195 under the crushing weight of a massive ancient mountain range. 138 00:09:57,195 --> 00:09:59,860 The result was hard Manhattan schist, 139 00:09:59,860 --> 00:10:04,901 a perfect foundation for the city's skyscrapers. 140 00:10:04,901 --> 00:10:08,193 But New York City still had a long way to go. 141 00:10:10,276 --> 00:10:13,318 The colliding plates created an enormous landmass - 142 00:10:13,318 --> 00:10:17,025 the last great supercontinent, called Pangaea. 143 00:10:17,025 --> 00:10:21,358 New York was now trapped in the centre... 144 00:10:21,358 --> 00:10:25,482 ...but somehow it made it back to the coast. 145 00:10:25,482 --> 00:10:30,107 Hidden beyond the city's streets is evidence of huge volcanic eruptions, 146 00:10:30,107 --> 00:10:35,148 mass extinctions and continents torn into pieces. 147 00:10:35,148 --> 00:10:37,481 Clues that could explain 148 00:10:37,481 --> 00:10:42,022 how New York became one of the world's great maritime cities. 149 00:10:48,147 --> 00:10:49,980 Investigators are piecing together 150 00:10:49,980 --> 00:10:53,562 how New York City's unique geology was formed. 151 00:10:55,562 --> 00:10:59,020 Much of its early success as a trade and commerce centre 152 00:10:59,020 --> 00:11:03,686 is owed to its deep-water harbour and its location at the coast. 153 00:11:06,561 --> 00:11:10,686 But 450 million years ago, things were different. 154 00:11:10,686 --> 00:11:14,268 The area of New York City was landlocked, 155 00:11:14,268 --> 00:11:17,602 embedded in the heart of a huge supercontinent. 156 00:11:17,602 --> 00:11:20,601 How did it get to the coast? 157 00:11:20,601 --> 00:11:25,850 The investigation fast-forwards 250 million years. 158 00:11:25,850 --> 00:11:30,808 In a quarry in New Jersey, 25 miles northeast of Manhattan, 159 00:11:30,808 --> 00:11:35,182 paleontologist Paul Olson unearths the first of a string of clues 160 00:11:35,182 --> 00:11:39,932 that could explain how New York City reached the coast. 161 00:11:39,932 --> 00:11:42,848 A giant fossilised footprint. 162 00:11:42,848 --> 00:11:46,014 This is the footprint, actually the mud that filled in the footprint, 163 00:11:46,014 --> 00:11:51,388 of a four-footed crocodile relative that was the dominant carnivore 164 00:11:51,388 --> 00:11:53,514 during the late Triassic. 165 00:11:53,514 --> 00:11:56,722 You can see the toes here have little pads on them 166 00:11:56,722 --> 00:12:00,513 and here's the handprint, and these animals would have been, 167 00:12:00,513 --> 00:12:03,054 in this case, about the size of a modest crocodile, 168 00:12:03,054 --> 00:12:09,428 but some of them became much, much larger, the size even of a T. Rex. 169 00:12:09,428 --> 00:12:12,553 (ROARS) 170 00:12:12,553 --> 00:12:16,178 The footprints are from a huge crocodile-like creature 171 00:12:16,178 --> 00:12:17,719 called Postosuchus. 172 00:12:17,719 --> 00:12:22,718 It first appeared on the Earth around 230 million years ago. 173 00:12:22,718 --> 00:12:28,134 Then, some 30 million years later, its footprints suddenly vanished. 174 00:12:28,134 --> 00:12:31,093 But Postosuchus wasn't alone. 175 00:12:31,093 --> 00:12:35,551 Half of all land animals perished at the same time. 176 00:12:35,551 --> 00:12:38,050 The fossil evidence proves it to be 177 00:12:38,050 --> 00:12:41,882 one of the biggest mass extinctions ever recorded. 178 00:12:41,882 --> 00:12:43,882 The evidence for this mass extinction 179 00:12:43,882 --> 00:12:47,882 is that we have lots and lots of fossils right in this area. 180 00:12:47,882 --> 00:12:52,048 And what you see is especially in the... in the reptile footprints, 181 00:12:52,048 --> 00:12:54,381 you see one group of forms, 182 00:12:54,381 --> 00:12:58,339 the forms that are related to crocodilians, disappear. 183 00:13:00,714 --> 00:13:06,296 Whatever caused the mass extinction must have been a catastrophic event. 184 00:13:06,296 --> 00:13:10,296 Olsen had a hunch that the mass extinction was somehow related 185 00:13:10,296 --> 00:13:13,463 to New York's return to the coast. 186 00:13:14,962 --> 00:13:19,712 The ancient area of New York sat on a line of great weakness, 187 00:13:19,712 --> 00:13:22,795 the plate boundary where two continents joined 188 00:13:22,795 --> 00:13:25,627 to form the supercontinent Pangaea. 189 00:13:25,627 --> 00:13:30,835 And it was unstable, prone to earthquakes and volcanoes. 190 00:13:30,835 --> 00:13:34,918 Olsen's quest - to find the evidence for the natural disaster 191 00:13:34,918 --> 00:13:38,667 that finished off Postosuchus 200 million years ago. 192 00:13:41,001 --> 00:13:44,001 A band of dark rock above the footprints 193 00:13:44,001 --> 00:13:46,875 in the New Jersey quarry caught his eye. 194 00:13:46,875 --> 00:13:52,541 It was basalt, the smoking gun Olson was looking for. 195 00:13:54,291 --> 00:13:57,332 Basalt is a volcanic rock. 196 00:13:57,332 --> 00:14:03,248 It forms when hot lava erupts onto the surface and cools. 197 00:14:03,248 --> 00:14:08,164 Did the volcanoes that forged this basalt trigger the mass extinction 198 00:14:08,164 --> 00:14:11,830 and also rip Pangaea apart? 199 00:14:11,830 --> 00:14:16,080 On its own, the evidence at the quarry was unconvincing. 200 00:14:16,080 --> 00:14:19,829 The layer of basalt is only a few feet thick. 201 00:14:22,454 --> 00:14:26,287 To prove mass lava flows caused this global catastrophe, 202 00:14:26,287 --> 00:14:29,870 scientists needed corroborative evidence. 203 00:14:33,286 --> 00:14:34,994 High above the Hudson River, 204 00:14:34,994 --> 00:14:38,035 geologist Matt Gorring follows another lead. 205 00:14:38,035 --> 00:14:40,951 He's studying the Palisades, 206 00:14:40,951 --> 00:14:44,577 a dramatic geologic feature that hugs the Hudson River, 207 00:14:44,577 --> 00:14:50,825 beginning across mid Manhattan and running into northeast New Jersey. 208 00:14:50,825 --> 00:14:53,659 They too are made of basaltic rock, 209 00:14:53,659 --> 00:14:58,408 the same rock implicated in the mass extinction of land animals. 210 00:14:58,408 --> 00:15:03,116 But the Palisades are on an altogether different scale. 211 00:15:03,116 --> 00:15:10,448 The Palisades are a sheet of basaltic magma, about 1,000 feet thick, 212 00:15:10,448 --> 00:15:12,490 it's about 40 miles long, 213 00:15:12,490 --> 00:15:14,156 so it's a very prominent set of cliffs 214 00:15:14,156 --> 00:15:17,531 that run all the way up the west side of the Hudson River. 215 00:15:19,281 --> 00:15:23,155 Here is proof of massive volcanic activity. 216 00:15:24,613 --> 00:15:28,030 Hot lava flooded out of ruptures in the Earth's crust 217 00:15:28,030 --> 00:15:32,279 and covered ancient North America in a mile-deep sheet. 218 00:15:33,737 --> 00:15:36,195 The lava cracked as it cooled. 219 00:15:37,320 --> 00:15:41,653 The vertical ruptures formed regular pencil-shaped columns. 220 00:15:43,194 --> 00:15:46,569 These distinctive rock formations have been known to geologists 221 00:15:46,569 --> 00:15:48,736 since the 19th century. 222 00:15:51,485 --> 00:15:55,193 Intriguingly, they appear on both sides of the Atlantic, 223 00:15:55,193 --> 00:16:00,734 in North and South America, Europe and Africa. 224 00:16:00,734 --> 00:16:04,608 Geologists suspected that their presence pointed to the spot 225 00:16:04,608 --> 00:16:09,566 where Africa and Europe separated from America. 226 00:16:09,566 --> 00:16:14,649 But that was just an unproven theory, until the 1950s, 227 00:16:14,649 --> 00:16:18,066 when scientists developed a revolutionary technique 228 00:16:18,066 --> 00:16:20,315 called paleomagnetism. 229 00:16:20,315 --> 00:16:24,898 Now, they could study the magnetic properties of rocks. 230 00:16:26,564 --> 00:16:31,438 Many rocks, including basalt, have a distinctive magnetic signature, 231 00:16:31,438 --> 00:16:34,354 formed as the rock is born. 232 00:16:34,354 --> 00:16:37,980 Tiny crystals inside the rock act like compass needles. 233 00:16:37,980 --> 00:16:41,270 When the magma that forms the rock is fluid, 234 00:16:41,270 --> 00:16:45,603 the crystals align to the Earth's magnetic field, pointing north. 235 00:16:48,145 --> 00:16:51,436 As the rock solidifies, the crystals freeze, 236 00:16:51,436 --> 00:16:55,478 forever locked in that fixed magnetic alignment. 237 00:16:55,478 --> 00:16:58,727 As continents move and the rocks travel, 238 00:16:58,727 --> 00:17:03,393 the crystals end up pointing in a different direction than north. 239 00:17:03,393 --> 00:17:08,976 Gorring and his team investigate the magnetic signature of the Palisades. 240 00:17:12,392 --> 00:17:17,016 To get a sample, they bore into the rock with a water-cooled drill. 241 00:17:21,516 --> 00:17:26,890 They measure the exact orientation of the crystals today with a compass. 242 00:17:26,890 --> 00:17:29,765 When they offset this reading with magnetic north, 243 00:17:29,765 --> 00:17:33,556 they can calculate the original location of the rock. 244 00:17:33,556 --> 00:17:36,556 Uh... 14. 245 00:17:36,556 --> 00:17:39,347 One of the useful things that you can do with this rock 246 00:17:39,347 --> 00:17:42,764 is you can take it back in the lab and measure its magnetic orientation, 247 00:17:42,764 --> 00:17:46,555 and that magnetic orientation will be when this rock crystallised 248 00:17:46,555 --> 00:17:48,512 or solidified 200 million years ago. 249 00:17:48,512 --> 00:17:51,429 So this rock would have minerals that would be pointing 250 00:17:51,429 --> 00:17:54,596 in some other direction other than north today. 251 00:17:57,386 --> 00:18:01,136 When scientists compared the magnetic orientation of the Palisades 252 00:18:01,136 --> 00:18:04,094 with the other basalt outcrops around the Atlantic, 253 00:18:04,094 --> 00:18:08,135 they discovered they formed at approximately the same latitude. 254 00:18:08,135 --> 00:18:11,385 Not only did the rocks have the same age, 255 00:18:11,385 --> 00:18:14,926 they were also born at the same location. 256 00:18:14,926 --> 00:18:20,759 For example, 200 million years ago, New York and Morocco were neighbours. 257 00:18:25,759 --> 00:18:29,174 The geologists had all the proof they needed. 258 00:18:29,174 --> 00:18:32,840 They could now confidently piece together what happened. 259 00:18:35,174 --> 00:18:39,840 It began with a global volcanic disaster. 260 00:18:39,840 --> 00:18:41,881 About 200 million years ago, 261 00:18:41,881 --> 00:18:44,923 North America and Africa began to pull apart from each other. 262 00:18:46,423 --> 00:18:48,630 There were gigantic lava outpourings. 263 00:18:48,630 --> 00:18:54,004 These lava flows erupted along very long cracks in the Earth's crust, 264 00:18:54,004 --> 00:18:55,963 that would have produced fountains of lava 265 00:18:55,963 --> 00:19:00,337 extending thousands and thousands of feet into the atmosphere. 266 00:19:02,254 --> 00:19:05,211 They covered an almost inconceivably large area, 267 00:19:05,211 --> 00:19:08,628 roughly four million square miles, 268 00:19:08,628 --> 00:19:13,419 from southwestern France to southwestern Brazil, 269 00:19:13,419 --> 00:19:16,252 from New York to central Mali in Africa. 270 00:19:16,252 --> 00:19:19,919 This area was covered in ponded lava flows 271 00:19:19,919 --> 00:19:23,918 that in some places ended up being nearly a mile thick. 272 00:19:27,501 --> 00:19:31,167 Volcanic eruptions led to soaring temperatures. 273 00:19:31,167 --> 00:19:36,833 Half of the plants and animals died. Postosuchus didn't stand a chance. 274 00:19:40,291 --> 00:19:43,457 As enormous forces tore Pangaea apart, 275 00:19:43,457 --> 00:19:47,332 a giant sea formed between the separating land masses - 276 00:19:47,332 --> 00:19:50,081 the Atlantic Ocean. 277 00:19:50,081 --> 00:19:54,123 The city of New York was now at the coast. 278 00:19:58,997 --> 00:20:02,955 200-million-year-old footprints beneath a layer of basalt 279 00:20:02,955 --> 00:20:06,663 and the Palisades towering above the Hudson 280 00:20:06,663 --> 00:20:08,954 provide evidence that Pangaea split apart 281 00:20:08,954 --> 00:20:12,120 to create the east coast of North America. 282 00:20:13,454 --> 00:20:17,120 But the story of New York City was far from over. 283 00:20:17,120 --> 00:20:21,536 After being built by fire, the region was about to be overcome 284 00:20:21,536 --> 00:20:24,869 by another destructive force. 285 00:20:29,493 --> 00:20:34,951 Scientists are piecing together the story of New York's violent geological past. 286 00:20:36,867 --> 00:20:40,909 250 million years ago, the Atlantic Ocean opened up, 287 00:20:40,909 --> 00:20:44,867 leaving the area of New York on the coast. 288 00:20:44,867 --> 00:20:49,116 But the maritime city still had a long way to go. 289 00:20:54,199 --> 00:20:57,073 There was no deep Hudson River channel. 290 00:20:57,073 --> 00:21:00,906 It was nothing but a small stream. 291 00:21:02,531 --> 00:21:06,406 What forces transformed it into the wide river 292 00:21:06,406 --> 00:21:09,739 capable of carrying heavy freighters far inland? 293 00:21:13,322 --> 00:21:18,196 A clue to how the Hudson Valley was created is the strange boulders 294 00:21:18,196 --> 00:21:21,862 that are scattered throughout Manhattan's Central Park. 295 00:21:21,862 --> 00:21:24,987 Some of them weigh several tons. 296 00:21:24,987 --> 00:21:31,444 But they're strangers to these parts, totally unlike the surrounding rocks. 297 00:21:33,569 --> 00:21:37,527 Geologist Charles Merguerian investigates where they came from. 298 00:21:39,402 --> 00:21:42,276 This boulder is a boulder from the Palisade sheet 299 00:21:42,276 --> 00:21:44,568 on the other side of the Hudson in New Jersey. 300 00:21:44,568 --> 00:21:46,942 You can see it's very nicely polished. 301 00:21:46,942 --> 00:21:50,109 Compositionally, it's totally different than the surrounding bedrock, 302 00:21:50,109 --> 00:21:51,650 which is Manhattan schist, 303 00:21:51,650 --> 00:21:54,233 and the Manhattan schist here is very rich in mica. 304 00:21:54,233 --> 00:21:57,525 This rock has no light-coloured mica in it whatsoever. 305 00:21:57,525 --> 00:22:02,024 The Palisade sheet is located to the west and north of us. 306 00:22:05,816 --> 00:22:10,356 The Palisades run for 40 miles along the Hudson River. 307 00:22:10,356 --> 00:22:14,564 Something immensely powerful must have moved the boulders 308 00:22:14,564 --> 00:22:16,563 such a great distance. 309 00:22:18,522 --> 00:22:22,896 Merguerian knows the answer is ice. 310 00:22:25,646 --> 00:22:30,188 Scientists have noticed a similar phenomenon 4,000 miles away 311 00:22:30,188 --> 00:22:32,145 in the Swiss Alps. 312 00:22:33,729 --> 00:22:37,395 As huge glaciers grind their way across the landscape, 313 00:22:37,395 --> 00:22:42,186 they gouge out lumps of rock and carry them along in the base of the ice. 314 00:22:43,228 --> 00:22:49,227 These rocks act like sandpaper and carve out deep scratches. 315 00:22:49,227 --> 00:22:53,227 When the ice melts, it leaves the boulders behind. 316 00:22:53,227 --> 00:22:59,434 Merguerian is convinced the same thing happened in Central Park. 317 00:22:59,434 --> 00:23:01,892 Ice moved the Palisade boulders 318 00:23:01,892 --> 00:23:05,766 and carved out grooves in the bedrock underneath. 319 00:23:05,766 --> 00:23:10,558 This bedrock exposure in Central Park shows the profound effects of glaciation 320 00:23:10,558 --> 00:23:13,599 in the form of these spectacular glacial grooves 321 00:23:13,599 --> 00:23:18,306 that move up the outcrop and show this pattern 322 00:23:18,306 --> 00:23:20,722 where glaciers grabbed huge boulders 323 00:23:20,722 --> 00:23:25,222 and those huge boulders acted like tools to produce these scratches. 324 00:23:28,139 --> 00:23:32,221 To Merguerian, the rocks in Central Park are compelling evidence 325 00:23:32,221 --> 00:23:35,346 that New York once was covered in ice. 326 00:23:36,721 --> 00:23:40,262 Over millions of years, growing and receding ice 327 00:23:40,262 --> 00:23:44,304 has repeatedly turned North America into a frozen wilderness. 328 00:23:48,428 --> 00:23:53,219 But the grooves on the rocks in New York don't tell the full story. 329 00:23:53,219 --> 00:23:58,926 The destruction caused by the ice points to a gigantic glacial event 330 00:23:58,926 --> 00:24:02,218 that would dwarf the future metropolis. 331 00:24:04,676 --> 00:24:07,384 To find out the extent of the ice, 332 00:24:07,384 --> 00:24:09,092 Merguerian travelled to Bear Mountain, 333 00:24:09,092 --> 00:24:12,466 some 50 miles north of New York City. 334 00:24:15,508 --> 00:24:18,383 Once again, the clue was in the rocks. 335 00:24:19,424 --> 00:24:23,757 He found glacial marks similar to those in Central Park. 336 00:24:25,299 --> 00:24:27,923 What we're looking at here are chatter marks. 337 00:24:27,923 --> 00:24:32,131 Chatter marks are very diagnostic features of glacial erosion, 338 00:24:32,131 --> 00:24:37,089 they're produced by boulders embedded in the base of a thick sheet of glacial ice. 339 00:24:37,089 --> 00:24:40,380 Those boulders impinged on this bedrock surface, 340 00:24:40,380 --> 00:24:45,088 polishing it, smoothing it off and then plucking pieces of rock off 341 00:24:45,088 --> 00:24:50,045 as the glacial ice moved over with the boulders embedded in the base. 342 00:24:51,671 --> 00:24:55,503 The gouges in the rocks could mean just one thing. 343 00:24:55,503 --> 00:24:59,377 The glacier must have been thousands of feet thick. 344 00:24:59,377 --> 00:25:01,794 In this case, although we're standing at an elevation 345 00:25:01,794 --> 00:25:04,544 of about 1,280 feet above sea level, 346 00:25:04,544 --> 00:25:09,002 the glacial ice sheet covered Bear Mountain as if it weren't even there. 347 00:25:11,376 --> 00:25:15,126 Scientists have found identical chatter marks on nearby peaks 348 00:25:15,126 --> 00:25:17,709 up to a mile above sea level. 349 00:25:17,709 --> 00:25:20,376 It was unmistakable proof. 350 00:25:20,376 --> 00:25:26,249 A glacier at least one mile thick ground its way across these mountains. 351 00:25:28,333 --> 00:25:32,873 Did this same ice sheet also plough through Central Park? 352 00:25:32,873 --> 00:25:36,665 Another nearby rock face provided the answer. 353 00:25:36,665 --> 00:25:38,790 The feature that we're looking at here 354 00:25:38,790 --> 00:25:41,456 are a series of sub-parallel glacial scratches and grooves 355 00:25:41,456 --> 00:25:45,873 and these are, again, are produced by the glacial ice sheet 356 00:25:45,873 --> 00:25:49,413 dragging boulders across this very durable granite surface, 357 00:25:49,413 --> 00:25:51,539 it's kind of polished the surface. 358 00:25:51,539 --> 00:25:55,621 And, in addition, it's produced these rather subtle but... but... but obvious, 359 00:25:55,621 --> 00:25:59,329 when the lighting is right, striae or grooves in the bedrock. 360 00:25:59,329 --> 00:26:01,995 Now if we measure the orientation of these... 361 00:26:07,120 --> 00:26:13,535 ...these... these come out about north 20 degrees west, 362 00:26:13,535 --> 00:26:18,202 just about identical in orientation to the striae that we measured at Central Park. 363 00:26:20,618 --> 00:26:24,784 It's significant evidence that the ice sheet that covered Bear Mountain 364 00:26:24,784 --> 00:26:28,450 also flowed over the surface of Central Park. 365 00:26:28,450 --> 00:26:33,200 Proof that New York City was covered by a glacier four times higher 366 00:26:33,200 --> 00:26:35,949 than the Empire State Building. 367 00:26:35,949 --> 00:26:37,782 MERGUERIAN: Just imagine glacial ice, 368 00:26:37,782 --> 00:26:40,865 a huge thick ice sheet over a mile thick, 369 00:26:40,865 --> 00:26:44,198 exerting tremendous pressure on the surface 370 00:26:44,198 --> 00:26:48,031 and sculpting the surface into the landscape that we see today. 371 00:26:50,406 --> 00:26:54,239 The ice sheet's crushing weight bulldozed everything in its path 372 00:26:54,239 --> 00:26:58,280 and cut through the remains of the ancient mountains. 373 00:26:58,280 --> 00:27:00,613 Before the ice arrived, 374 00:27:00,613 --> 00:27:04,238 the waters of the Hudson River had gently cut down through the landscape 375 00:27:04,238 --> 00:27:06,154 to form a V-shaped valley. 376 00:27:07,612 --> 00:27:11,279 But a mile-thick glacier takes no prisoners. 377 00:27:11,279 --> 00:27:15,528 It gouged out the sides and the bottom of the river valley 378 00:27:15,528 --> 00:27:18,028 and turned it into a U-shaped riverbed. 379 00:27:20,194 --> 00:27:24,152 The Hudson River was now navigable for big ships. 380 00:27:26,443 --> 00:27:30,568 The picture of modern-day New York was almost complete. 381 00:27:32,609 --> 00:27:36,400 Long grooves in the rocks in Central Park showed scientists 382 00:27:36,400 --> 00:27:41,150 that a vast ice sheet flowed over the eroded remains of these mountains. 383 00:27:41,150 --> 00:27:45,066 And marks and glacial striations on Bear Mountain 384 00:27:45,066 --> 00:27:48,232 proved this ice sheet was at least four times higher 385 00:27:48,232 --> 00:27:49,899 than the Empire State Building. 386 00:27:53,024 --> 00:27:54,190 When the ice melted, 387 00:27:54,190 --> 00:27:58,939 it left a vast ridge of debris blocking the Hudson River from the Atlantic. 388 00:28:00,147 --> 00:28:05,522 The final challenge for geologists was to find out how the ridge was destroyed 389 00:28:05,522 --> 00:28:09,605 and how New York's harbour opened to the oceans. 390 00:28:18,396 --> 00:28:23,270 New York today boasts one of the largest natural harbours in the world. 391 00:28:23,270 --> 00:28:26,728 But it wasn't always that way. 392 00:28:26,728 --> 00:28:33,060 Towards the end of the last Ice Age, the port's wide entrance was blocked. 393 00:28:33,060 --> 00:28:40,310 16,000 years ago, the melting glaciers left behind a 220-foot-high wall of debris. 394 00:28:40,310 --> 00:28:43,142 The ridge stretched from Long Island to Staten Island 395 00:28:43,142 --> 00:28:45,725 and forced the Hudson River through a narrow, 396 00:28:45,725 --> 00:28:47,892 more westerly course to the ocean. 397 00:28:50,142 --> 00:28:53,808 What powerful forces destroyed this rock jam? 398 00:28:53,808 --> 00:28:58,099 The prime suspect was a flash flood. 399 00:29:01,349 --> 00:29:06,306 But scientists needed evidence to prove that such a flood had happened. 400 00:29:07,306 --> 00:29:11,055 In the 1960s, fishermen made an unexpected find 401 00:29:11,055 --> 00:29:13,389 at the mouth of the Hudson River. 402 00:29:13,389 --> 00:29:19,555 They dredged up a giant mammoth tusk from the depths of the sea floor. 403 00:29:19,555 --> 00:29:22,222 (TRUMPETS) 404 00:29:25,179 --> 00:29:29,304 Herds of these giant beasts roamed the plains of North America 405 00:29:29,304 --> 00:29:34,136 before they became extinct at the end of the last Ice Age. 406 00:29:38,636 --> 00:29:42,594 Finding the odd mammoth tusk here and there is not so surprising. 407 00:29:42,594 --> 00:29:45,177 But since the initial discovery, 408 00:29:45,177 --> 00:29:48,385 scientists have found hundreds more tusks and bones 409 00:29:48,385 --> 00:29:51,510 in the mouth of the Hudson River. 410 00:29:51,510 --> 00:29:55,676 It was as though a violent torrent swept the mammoths away 411 00:29:55,676 --> 00:29:58,050 and dumped their remains off the coast. 412 00:29:58,050 --> 00:30:01,925 And there were more clues nearby. 413 00:30:04,592 --> 00:30:09,549 Huge boulders resting on the sandy sea floor, 414 00:30:09,549 --> 00:30:13,174 some of them as big as cars. 415 00:30:13,174 --> 00:30:16,631 The boulders must have been part of the ancient moraine 416 00:30:16,631 --> 00:30:20,255 that once ran between Long Island and Staten Island. 417 00:30:22,006 --> 00:30:27,713 Geologist David Franzi knows that only a raging torrent could have shifted them. 418 00:30:29,046 --> 00:30:31,504 Based on the size of the boulders that we see here, 419 00:30:31,504 --> 00:30:34,296 we know that that flood must have discharged 420 00:30:34,296 --> 00:30:38,378 on the order of 1.5 million cubic feet per second. 421 00:30:39,712 --> 00:30:43,586 That's three times larger than the largest Mississippi River flood ever recorded. 422 00:30:44,836 --> 00:30:48,461 And all that water had to come from somewhere. 423 00:30:48,461 --> 00:30:51,544 Scientists began looking for the source of this flood, 424 00:30:51,544 --> 00:30:57,876 a flood powerful enough to transport huge boulders all the way to the sea. 425 00:30:57,876 --> 00:31:01,168 Rocks to a geologist are like pages in a history book. 426 00:31:01,168 --> 00:31:05,750 For us, erosion oftentimes rips some of the pages out of our history book, 427 00:31:05,750 --> 00:31:09,541 so it's the job of the geologist to put together a fragmentary record 428 00:31:09,541 --> 00:31:12,916 into a coherent history of the events that happened in the past. 429 00:31:14,708 --> 00:31:18,624 300 miles north of the city, in upstate New York, 430 00:31:18,624 --> 00:31:20,915 Franzi tracked what might have been the flood's path 431 00:31:20,915 --> 00:31:22,457 to an unusual grove of trees 432 00:31:22,457 --> 00:31:24,165 on Covey Hill 433 00:31:24,165 --> 00:31:25,998 in the Adirondack Mountains. 434 00:31:28,706 --> 00:31:31,997 The trees are jack pines. 435 00:31:31,997 --> 00:31:36,288 They are rare in this area, where the soil is usually fertile and deep. 436 00:31:38,330 --> 00:31:40,704 But on Covey Hill, their presence shows 437 00:31:40,704 --> 00:31:44,955 that there is no more than a few inches of soil on top of the bedrock. 438 00:31:46,204 --> 00:31:48,079 The jack pine is essentially rooted 439 00:31:48,079 --> 00:31:50,245 right on the top of a rock's surface here. 440 00:31:50,245 --> 00:31:53,953 This is a bare sandstone surface, very little mineral soil, 441 00:31:53,953 --> 00:31:58,244 and it's subject to prolonged periods of dryness during the summertime. 442 00:31:58,244 --> 00:32:01,202 Jack pine's adaptations make it able to survive here 443 00:32:01,202 --> 00:32:03,160 where no other tree species can. 444 00:32:05,118 --> 00:32:07,660 What happened to the soil? 445 00:32:10,118 --> 00:32:12,076 The jack pines continue to grow 446 00:32:12,076 --> 00:32:16,034 at the entrance of a long gorge over 300 feet wide. 447 00:32:17,450 --> 00:32:21,575 Usually, gorges like this are cut down over thousands of years, 448 00:32:21,575 --> 00:32:25,991 but in this case, the missing topsoil points to a sudden flood event. 449 00:32:25,991 --> 00:32:31,407 A raging torrent must have ripped away the soil and cut deep into the rock. 450 00:32:36,698 --> 00:32:40,198 In a helicopter, Franzi follows the gorge west. 451 00:32:42,781 --> 00:32:45,613 Eventually, it opens into a vast, empty basin 452 00:32:45,613 --> 00:32:50,863 located next to one of the Great Lakes, Lake Ontario. 453 00:32:50,863 --> 00:32:54,362 It doesn't take much of a stretch of the imagination here 454 00:32:54,362 --> 00:32:57,154 to imagine this valley filled with water 455 00:32:57,154 --> 00:33:00,945 and then with these hills poking up through as islands. 456 00:33:02,570 --> 00:33:09,527 16,000 years ago, this basin was filled with two billion cubic miles of water - 457 00:33:09,527 --> 00:33:12,777 a huge lake geologists call Lake Iroquois. 458 00:33:12,777 --> 00:33:16,485 It formed at the end of the last Ice Age. 459 00:33:19,651 --> 00:33:24,693 As glaciers receded, the melt waters slowly filled up the lake. 460 00:33:24,693 --> 00:33:27,900 The ice dams holding the waters weakened. 461 00:33:27,900 --> 00:33:34,690 Eventually, the dams collapsed, causing sudden and devastating flash floods. 462 00:33:34,690 --> 00:33:38,274 Lake level dropped on the order of 70 feet 463 00:33:38,274 --> 00:33:41,315 and about 160 cubic miles of water 464 00:33:41,315 --> 00:33:44,273 were released into the Champlain Valley, catastrophically. 465 00:33:44,273 --> 00:33:48,189 That floodwater would have coursed down the Champlain Valley, 466 00:33:48,189 --> 00:33:51,647 through the Hudson Valley and ultimately out into the Atlantic Ocean. 467 00:33:52,689 --> 00:33:57,480 The torrent raced towards New York City, 300 miles to the south, 468 00:33:57,480 --> 00:34:00,479 then took the straightest course to the sea. 469 00:34:00,479 --> 00:34:04,020 The floodwaters smashed into the ancient moraine, 470 00:34:04,020 --> 00:34:08,896 the huge pile of debris blocking the direct exit of the Hudson River. 471 00:34:13,561 --> 00:34:17,102 The bridge we see behind me spans the channel that was cut 472 00:34:17,102 --> 00:34:18,644 by the flood event. 473 00:34:18,644 --> 00:34:21,602 When the flood wave came through, it was of sufficient intensity 474 00:34:21,602 --> 00:34:25,851 to over-top the dam and very rapidly cut the channel. 475 00:34:28,392 --> 00:34:31,642 The gap that was created by the flood still exists. 476 00:34:31,642 --> 00:34:35,684 It's now a tidal strait called the Narrows. 477 00:34:37,183 --> 00:34:41,766 Today, the gap is spanned by the Verrazano Bridge. 478 00:34:41,766 --> 00:34:45,641 The channel is deep enough for even the biggest ocean-going ships. 479 00:34:45,641 --> 00:34:50,474 It's the most important entrance to New York City's harbour. 480 00:34:54,265 --> 00:34:57,722 Mammoth tusks and huge boulders at the mouth of the river 481 00:34:57,722 --> 00:35:01,347 showed scientists that there was a torrent big enough to blast a hole 482 00:35:01,347 --> 00:35:03,555 through the ancient moraine. 483 00:35:03,555 --> 00:35:09,055 And a channel leading towards the Great Lakes revealed the source of the flood. 484 00:35:09,055 --> 00:35:11,805 It was this flood that created the Narrows 485 00:35:11,805 --> 00:35:16,303 and gave New York a wide entrance to its port. 486 00:35:16,303 --> 00:35:21,095 A unique geology laid down the foundations for New York City. 487 00:35:24,845 --> 00:35:28,594 But the same forces that constructed it may also have sown the seeds 488 00:35:28,594 --> 00:35:30,927 for New York's destruction. 489 00:35:37,676 --> 00:35:44,342 Scientists have pieced together the half-billion-year history of New York City. 490 00:35:44,342 --> 00:35:50,341 Huge mountains, volcanic eruptions and glacial ice shaped the area. 491 00:35:50,341 --> 00:35:53,799 But New York's story doesn't end here. 492 00:35:56,091 --> 00:36:00,382 The geology that created one of the greatest cities on Earth 493 00:36:00,382 --> 00:36:03,714 also has the potential to destroy it. 494 00:36:05,631 --> 00:36:09,131 Experts have been studying the potential threat to the city. 495 00:36:09,131 --> 00:36:11,714 We're standing here in Lower Manhattan 496 00:36:11,714 --> 00:36:13,047 on one of our major thoroughfares, 497 00:36:13,047 --> 00:36:14,089 Canal Street. 498 00:36:14,089 --> 00:36:16,547 And it's important, because in 1821, 499 00:36:16,547 --> 00:36:22,879 a Category 2 hurricane raised the water level at the Battery 13 feet in one hour 500 00:36:22,879 --> 00:36:26,379 and, literally, the Hudson River met the East River 501 00:36:26,379 --> 00:36:29,712 and Canal Street was covered by water 502 00:36:29,712 --> 00:36:34,877 and Manhattan was actually two islands for three hours, until the water receded. 503 00:36:37,669 --> 00:36:42,085 New York City is vulnerable because of its position on the coast. 504 00:36:44,710 --> 00:36:50,584 Long Island stretches northeast at a right angle from the New Jersey shore. 505 00:36:50,584 --> 00:36:55,000 New York City is nestled behind the western end of Long Island. 506 00:36:55,000 --> 00:36:56,833 Normally, the island protects the city 507 00:36:56,833 --> 00:36:58,333 from the sea, 508 00:36:58,333 --> 00:37:02,125 but when hurricanes threaten, the opposite is true. 509 00:37:02,125 --> 00:37:05,998 Long Island becomes a dangerous liability. 510 00:37:08,207 --> 00:37:12,331 Hurricanes racing north along the beaches of the Atlantic coast 511 00:37:12,331 --> 00:37:15,498 pile up huge bulges of water in front of them. 512 00:37:15,498 --> 00:37:17,956 They're called storm surges. 513 00:37:18,998 --> 00:37:21,580 Hitting the right-angled junction at Long Island, 514 00:37:21,580 --> 00:37:24,830 the winds funnel the storm surge in through the Narrows, 515 00:37:24,830 --> 00:37:28,330 the gap between Long Island and New Jersey. 516 00:37:28,330 --> 00:37:32,496 This is the place, at the actual apex of the right angle in New York 517 00:37:32,496 --> 00:37:37,328 where all the water being pushed by a hurricane would be concentrated. 518 00:37:37,328 --> 00:37:39,870 And in the distance is the Verrazano Bridge, 519 00:37:39,870 --> 00:37:43,828 and all that water is gonna go through the passage we call the Narrows 520 00:37:43,828 --> 00:37:46,535 and it's gonna be accelerated towards New York City, 521 00:37:46,535 --> 00:37:50,161 where it will rise to abnormal heights. 522 00:37:55,535 --> 00:37:58,409 Experts believe that in the United States, 523 00:37:58,409 --> 00:38:03,159 New York is the third most vulnerable city after Miami and New Orleans 524 00:38:03,159 --> 00:38:05,200 to a hurricane disaster. 525 00:38:06,242 --> 00:38:10,491 If it was hit today, the consequences would be serious. 526 00:38:10,491 --> 00:38:15,741 New York City is hit by hurricanes only infrequently. 527 00:38:15,741 --> 00:38:21,324 Like, in 1821 and in 1893 and in 1938. 528 00:38:21,324 --> 00:38:24,156 However, the point is that the hurricane 529 00:38:24,156 --> 00:38:28,698 that will eventually hit New York City again will be catastrophic, 530 00:38:28,698 --> 00:38:31,780 and what is going to happen when the utilities are knocked out? 531 00:38:31,780 --> 00:38:35,655 What is going to happen when salt water reaches into the subways 532 00:38:35,655 --> 00:38:38,446 and ruins the electrical system? 533 00:38:38,446 --> 00:38:40,737 We're talking about unbelievable amounts of money 534 00:38:40,737 --> 00:38:42,904 to restore the infrastructure. 535 00:38:42,904 --> 00:38:46,279 We're talking about setbacks and delays in commerce 536 00:38:46,279 --> 00:38:51,653 and banking and transportation, a catastrophe that's never been seen. 537 00:38:54,320 --> 00:38:58,485 Storm surges are not the only threat to New York's future. 538 00:39:00,569 --> 00:39:06,526 Earthquakes are also part of the vast geological forces that shape this area. 539 00:39:06,526 --> 00:39:08,693 They are still at work today. 540 00:39:10,984 --> 00:39:14,817 Some could change the city in an instant. 541 00:39:17,150 --> 00:39:20,525 November 4th 1884. 542 00:39:20,525 --> 00:39:25,232 New York City was shaken by an earthquake that lasted ten seconds. 543 00:39:25,232 --> 00:39:29,190 The Brooklyn Bridge swayed and people panicked. 544 00:39:29,190 --> 00:39:34,523 The earthquake showed 5.5 on the Richter scale. 545 00:39:35,523 --> 00:39:40,855 January 17th 2001, New York City was struck again. 546 00:39:42,397 --> 00:39:46,854 This time the quake was relatively small, only 2.4, 547 00:39:46,854 --> 00:39:51,563 but it struck right under 125th Street. 548 00:39:51,563 --> 00:39:55,479 The earthquake in 2001 is the first earthquake 549 00:39:55,479 --> 00:39:59,436 that we could confidently locate in Manhattan, 550 00:39:59,436 --> 00:40:04,145 that's its claim to fame, it was felt widely. 551 00:40:14,393 --> 00:40:18,934 It's impossible to study the cause of the quakes at the surface. 552 00:40:20,559 --> 00:40:23,892 The evidence is buried beneath the city. 553 00:40:26,642 --> 00:40:29,058 Deep within New York's bedrock, 554 00:40:29,058 --> 00:40:34,058 seismologist Leonardo Seeber studies the cause of these quakes. 555 00:40:35,057 --> 00:40:36,599 (HORN BLARES) 556 00:40:38,932 --> 00:40:43,556 In a subway tunnel 100 feet beneath the bedrock under the East River, 557 00:40:43,556 --> 00:40:45,764 there is a ready-made laboratory. 558 00:40:45,764 --> 00:40:50,222 Here, Seeber can study the rocks up close and personal. 559 00:40:52,305 --> 00:40:56,054 It is the same bedrock Manhattan is built on. 560 00:40:56,054 --> 00:41:00,179 But Seeber fears it isn't as solid as once was thought. 561 00:41:01,304 --> 00:41:07,179 New York area is considered a seismic zone, 562 00:41:07,179 --> 00:41:11,094 meaning there is a cluster of known earthquakes 563 00:41:11,094 --> 00:41:12,886 that have occurred in this area. 564 00:41:12,886 --> 00:41:19,093 So we are, as geologists, very eager to discover which faults are responsible 565 00:41:19,093 --> 00:41:20,593 for these earthquakes. 566 00:41:22,010 --> 00:41:24,717 The majority of earthquakes occur at the boundaries 567 00:41:24,717 --> 00:41:27,842 between separate sections of the Earth's crust, 568 00:41:27,842 --> 00:41:31,675 the tectonic plates on which the continents sit. 569 00:41:31,675 --> 00:41:35,175 But New York's earthquakes are different. 570 00:41:35,175 --> 00:41:38,382 The city is firmly in the middle of a tectonic plate, 571 00:41:38,382 --> 00:41:41,466 halfway between the mid-Atlantic ridge to the east 572 00:41:41,466 --> 00:41:44,341 and the San Andreas Fault to the west. 573 00:41:47,091 --> 00:41:50,548 Seeber is anxious to discover what's going on. 574 00:41:51,798 --> 00:41:56,423 As he examines the walls of the tunnel, he comes across a possible clue. 575 00:41:57,797 --> 00:42:01,547 The long fractures in the rock are fault lines that formed 576 00:42:01,547 --> 00:42:03,880 when pressure built up. 577 00:42:03,880 --> 00:42:08,046 As the tension was released, the rock cracked and shifted. 578 00:42:08,046 --> 00:42:11,837 This is felt on the surface as an earthquake. 579 00:42:11,837 --> 00:42:14,379 SEEBER: This is an example of a very small fault, 580 00:42:14,379 --> 00:42:20,712 but it's a fault that probably did generate some small earthquakes. 581 00:42:20,712 --> 00:42:22,836 When one of these faults generates an earthquake, 582 00:42:22,836 --> 00:42:29,585 we think that perhaps other faults of the same family can generate earthquakes. 583 00:42:31,501 --> 00:42:34,043 These faults in New York's bedrock are evidence 584 00:42:34,043 --> 00:42:38,501 that the area was hit by earthquakes in the past. 585 00:42:38,501 --> 00:42:44,667 But Seeber has no way of knowing if the faults are still active and dangerous. 586 00:42:48,333 --> 00:42:51,124 If a large earthquake hit New York today, 587 00:42:51,124 --> 00:42:54,457 the consequences would be catastrophic. 588 00:42:55,582 --> 00:42:57,623 A large proportion of New York City buildings 589 00:42:57,623 --> 00:43:00,582 are simply not built to withstand earthquake shaking. 590 00:43:00,582 --> 00:43:03,665 We worry about transportation tunnels, 591 00:43:03,665 --> 00:43:06,456 in particular, tunnels that traverse rivers 592 00:43:06,456 --> 00:43:09,789 where parts of the tunnels are rooted in solid rock 593 00:43:09,789 --> 00:43:12,080 and other parts are resting on soft sediment. 594 00:43:12,080 --> 00:43:14,330 The oscillation of these two different materials 595 00:43:14,330 --> 00:43:17,079 could cause severe cracking and fracturing. 596 00:43:17,079 --> 00:43:20,413 The infrastructure would be severely damaged, 597 00:43:20,413 --> 00:43:25,245 it would take tens of years to repair the damage caused by such a large event. 598 00:43:26,829 --> 00:43:29,745 With the evidence geologists have collected, 599 00:43:29,745 --> 00:43:34,119 the story of the creation of New York City can now be told. 600 00:43:35,202 --> 00:43:40,494 The city's bedrock was formed under a chain of mountains over a mile high. 601 00:43:40,494 --> 00:43:44,785 Volcanoes and lava fields over millions of square miles 602 00:43:44,785 --> 00:43:50,617 split up the ancient supercontinent and created the east coast of North America. 603 00:43:50,617 --> 00:43:54,366 Glacial ice, four times as high as the Empire State building, 604 00:43:54,366 --> 00:43:56,991 carved out the deep Hudson River. 605 00:43:56,991 --> 00:44:00,324 A catastrophic flash flood broke through the moraine 606 00:44:00,324 --> 00:44:05,532 to form the Narrows and opened up New York City's harbour to the oceans. 607 00:44:09,864 --> 00:44:12,406 Looking ahead to the distant future, 608 00:44:12,406 --> 00:44:15,989 geologists see more challenging times for the city. 609 00:44:17,031 --> 00:44:18,905 In 40,000 years, 610 00:44:18,905 --> 00:44:22,905 they predict this region will be engulfed by another ice sheet. 611 00:44:23,905 --> 00:44:29,571 And in 250 million years, the Atlantic will start to shrink again. 612 00:44:29,571 --> 00:44:34,654 Europe and Africa will eventually crash back into the American coast. 613 00:44:34,654 --> 00:44:38,028 The fossilised remains of the once great city of New York 614 00:44:38,028 --> 00:44:43,194 will become just another layer of rock in a vast new mountain range. 615 00:44:43,194 --> 00:44:48,486 A footnote in the immense, ever-changing story of planet Earth.