1 00:00:02,567 --> 00:00:09,487 Earth, a 4.5- Billion-year-old planet, still evolving. 2 00:00:09,487 --> 00:00:17,487 As continents shift and clash, volcanoes erupt and glaciers grow and recede, 3 00:00:17,487 --> 00:00:22,207 the earth's crust is carved in numerous and fascinating ways, 4 00:00:22,207 --> 00:00:26,439 leaving a trail of geological mysteries behind. 5 00:00:30,127 --> 00:00:31,567 In this episode, 6 00:00:31,567 --> 00:00:37,127 the 450-million-year-old geological history of New York City is explored. 7 00:00:37,127 --> 00:00:41,047 A metropolis pockmarked with strange rocks, 8 00:00:41,047 --> 00:00:44,167 haunted by footprints of ancient giant reptiles, 9 00:00:44,167 --> 00:00:49,719 and lined with a vast curtain of solidified lava. 10 00:00:51,007 --> 00:00:53,207 Scientists investigate the evidence 11 00:00:53,207 --> 00:00:59,047 for fiery volcanoes, massive floods and ice sheets 12 00:00:59,047 --> 00:01:02,487 four times as high as the Empire State building. 13 00:01:02,487 --> 00:01:08,367 The clues to understanding New York City's geological past provides a window 14 00:01:08,367 --> 00:01:11,518 into the formation of the earth itself. 15 00:01:24,127 --> 00:01:28,407 The investigation into New York City's geological history begins here, 16 00:01:28,407 --> 00:01:31,524 with Manhattan's rocky outcrops. 17 00:01:33,167 --> 00:01:36,727 These rocks are clues to how the land was made 18 00:01:36,727 --> 00:01:41,642 and how its geology helped it become a dense, thriving, pulsating city. 19 00:01:43,647 --> 00:01:46,367 They're scattered all over Manhattan, 20 00:01:46,367 --> 00:01:48,687 poking through the surface of parks 21 00:01:48,687 --> 00:01:51,607 and through the concrete between the buildings. 22 00:01:51,607 --> 00:01:57,647 Some, squashed between two apartment blocks, are the size of a whale. 23 00:01:57,647 --> 00:02:01,765 They are the extraordinary survivors of ancient times. 24 00:02:06,487 --> 00:02:10,047 Most importantly, they are the surface tips of the bedrock 25 00:02:10,047 --> 00:02:13,881 in which Manhattan's buildings are anchored. 26 00:02:15,127 --> 00:02:21,967 Gigantic skyscrapers stand in two clusters, in downtown and midtown. 27 00:02:21,967 --> 00:02:26,527 In the section between, the buildings are lower. 28 00:02:26,527 --> 00:02:30,167 The clues to the shape of Manhattan's familiar skyline 29 00:02:30,167 --> 00:02:33,637 are the rocks beneath the surface. 30 00:02:36,527 --> 00:02:41,687 A leading expert on the rocks in New York is geologist Charles Merguerian. 31 00:02:41,687 --> 00:02:44,767 The entire history of the development of the earth's crust 32 00:02:44,767 --> 00:02:46,727 is emblazoned in the rocks beneath us. 33 00:02:46,727 --> 00:02:50,407 The rocks here in New York City harbour an ancestry 34 00:02:50,407 --> 00:02:53,567 that dates back over a billion years of time. 35 00:02:53,567 --> 00:03:00,567 Merguerian is searching for evidence to show how the city's bedrock was made. 36 00:03:00,567 --> 00:03:05,087 At Inwood Hill Park in Upper Manhattan, he's found an extremely hard piece 37 00:03:05,087 --> 00:03:08,487 of the bedrock known as Manhattan schist. 38 00:03:08,487 --> 00:03:12,327 To the untrained eye, it's just a piece of rock, 39 00:03:12,327 --> 00:03:16,407 but to Merguerian, this is his first clue. 40 00:03:16,407 --> 00:03:18,447 The rocks that we're looking at right here 41 00:03:18,447 --> 00:03:20,287 are rocks of the Manhattan schist formation 42 00:03:20,287 --> 00:03:23,487 and the... these rocks are very severely deformed, 43 00:03:23,487 --> 00:03:27,167 and the structures here in this rock is a structure that comes up like this, 44 00:03:27,167 --> 00:03:31,047 bends around and comes back down on itself as such, 45 00:03:31,047 --> 00:03:35,567 and in three-dimensional view, it's a structure that looks something like this. 46 00:03:35,567 --> 00:03:39,647 A very, very tight fold with a plunge towards the south here. 47 00:03:39,647 --> 00:03:42,927 These are rocks that were very, very strongly deformed 48 00:03:42,927 --> 00:03:45,047 over protracted periods of time. 49 00:03:45,047 --> 00:03:49,199 And it's the same bedrock that occurs over much of New York City. 50 00:03:53,247 --> 00:03:57,207 This tight fold in the rock suggests New York's bedrock 51 00:03:57,207 --> 00:03:59,447 was formed under great pressure. 52 00:03:59,447 --> 00:04:06,367 To confirm this hunch, Merguerian takes a sample to the lab for detailed analysis. 53 00:04:06,367 --> 00:04:13,762 Radiometric dating proves this rock is about 450 million years old. 54 00:04:15,327 --> 00:04:19,286 But the rock has even greater secrets to tell. 55 00:04:20,807 --> 00:04:23,287 It contains a kaleidoscope of minerals, 56 00:04:23,287 --> 00:04:27,047 which opens a window into the ancient world. 57 00:04:27,047 --> 00:04:31,047 To me, minerals are like the instrument cluster in your car, 58 00:04:31,047 --> 00:04:33,561 they tell you everything about how your car is running. 59 00:04:35,287 --> 00:04:40,327 Merguerian uses a microscope with polarised light to view the minerals. 60 00:04:40,327 --> 00:04:43,687 The examination tells us the former depth regime, 61 00:04:43,687 --> 00:04:46,967 how deep the rocks were, they tell you the age of the rocks, 62 00:04:46,967 --> 00:04:48,927 they tell you everything you want to know 63 00:04:48,927 --> 00:04:50,724 about the development of the earth's crust. 64 00:04:51,727 --> 00:04:56,727 What's striking about these samples is that the minerals inside are elongated. 65 00:04:56,727 --> 00:05:03,007 It is a clue that these rocks must once have been crushed by massive forces. 66 00:05:03,007 --> 00:05:05,607 And the colours support this theory. 67 00:05:05,607 --> 00:05:11,567 Under the polarised light, the sample from Inwood Hill Park shows up blue. 68 00:05:11,567 --> 00:05:17,447 This comes from a mineral called kyanite, which forms at great depths. 69 00:05:17,447 --> 00:05:20,527 It's conclusive evidence that this rock 70 00:05:20,527 --> 00:05:24,207 was compressed deep under the surface. 71 00:05:24,207 --> 00:05:28,247 Rocks forged at these depths are much harder, 72 00:05:28,247 --> 00:05:31,727 ideal for a city's foundations. 73 00:05:31,727 --> 00:05:36,527 But what gigantic weight was on top? 74 00:05:36,527 --> 00:05:40,247 Merguerian believes there is only one answer. 75 00:05:40,247 --> 00:05:43,007 The rock was once buried under the crushing weight 76 00:05:43,007 --> 00:05:46,367 of a chain of massive mountains. 77 00:05:46,367 --> 00:05:49,967 The minerals that we find in the bedrock units of New York City 78 00:05:49,967 --> 00:05:53,287 tell us that the rocks of New York City were formerly buried 79 00:05:53,287 --> 00:05:57,567 when they were formed, under very high pressures, 80 00:05:57,567 --> 00:06:00,127 and that those high pressures indicate 81 00:06:00,127 --> 00:06:04,687 that these rocks formerly were produced at depths of 20 to 25 miles, 82 00:06:04,687 --> 00:06:08,965 and probably the mountains were as high as the Alps are today. 83 00:06:12,047 --> 00:06:17,607 But even the most impressive mountain chains can't survive the ravages of time. 84 00:06:17,607 --> 00:06:20,007 The Rocky Mountains, for example. 85 00:06:20,007 --> 00:06:24,367 Millions of years ago, they soared nearly six miles into the sky. 86 00:06:24,367 --> 00:06:28,645 Today, erosion has halved their size. 87 00:06:32,287 --> 00:06:35,447 The same process happened in New York. 88 00:06:35,447 --> 00:06:40,127 Rain, wind and ice wore the ancient mountains almost flat. 89 00:06:40,127 --> 00:06:43,767 But the microscopic crystals found in the rock in Manhattan 90 00:06:43,767 --> 00:06:47,680 testify that they existed in the past. 91 00:06:50,727 --> 00:06:53,247 How did the mountains form? 92 00:06:53,247 --> 00:06:57,001 The answer lies in the way the earth's crust moves. 93 00:06:59,647 --> 00:07:05,167 A network of interlocking individual pieces makes up the Earth's surface. 94 00:07:05,167 --> 00:07:08,207 Geologists call them tectonic plates. 95 00:07:08,207 --> 00:07:15,967 Over millions of years, they collide and break apart to form different continents. 96 00:07:15,967 --> 00:07:21,847 450 million years ago, the Earth's surface looked completely different. 97 00:07:21,847 --> 00:07:23,487 North America was much further 98 00:07:23,487 --> 00:07:25,087 to the south. 99 00:07:25,087 --> 00:07:28,127 MERGUERIAN: North America was tilted 90 degrees clockwise 100 00:07:28,127 --> 00:07:29,887 from its present orientation 101 00:07:29,887 --> 00:07:32,007 and it was straddling the equator. 102 00:07:32,007 --> 00:07:33,967 As such, the climate was tropical, 103 00:07:33,967 --> 00:07:38,597 the east coast of North America was really experiencing Club Med conditions. 104 00:07:40,807 --> 00:07:42,727 The weather may have been awesome, 105 00:07:42,727 --> 00:07:46,766 but the ancient East Coast was heading for trouble. 106 00:07:49,887 --> 00:07:52,287 The plate beneath it was moving. 107 00:07:52,287 --> 00:07:57,167 The East Coast was on a collision course with ancient West Africa. 108 00:07:57,167 --> 00:08:02,447 450 million years ago, they collided. 109 00:08:02,447 --> 00:08:05,727 The impact unleashed geological chaos. 110 00:08:05,727 --> 00:08:09,527 Under intense compression, the land was forced upwards 111 00:08:09,527 --> 00:08:12,327 to form a soaring range of mountains. 112 00:08:12,327 --> 00:08:15,567 The collision that took place is the most fundamental 113 00:08:15,567 --> 00:08:18,127 and impressive mountain-building event 114 00:08:18,127 --> 00:08:20,880 to affect the east coast of North America. 115 00:08:23,847 --> 00:08:27,367 Today, all that remains are their stumps, 116 00:08:27,367 --> 00:08:32,447 stumps that form the bedrock of modern-day New York. 117 00:08:32,447 --> 00:08:36,567 The collision that built up the ancient mountains also folded the bedrock 118 00:08:36,567 --> 00:08:39,087 into dips and rises. 119 00:08:39,087 --> 00:08:44,798 These folds are responsible for the shape of Manhattan's skyline. 120 00:08:47,327 --> 00:08:53,287 The city boasts two clusters of skyscrapers in downtown and midtown. 121 00:08:53,287 --> 00:08:58,527 Here, the hard bedrock that formed deep underground was forced up. 122 00:08:58,527 --> 00:09:02,287 It is now close to the surface and provides solid anchorage 123 00:09:02,287 --> 00:09:05,287 for the high-rise buildings. 124 00:09:05,287 --> 00:09:09,527 In the dip in the middle the rock was folded down. 125 00:09:09,527 --> 00:09:14,447 The area is filled with loose sediments, less suitable for skyscrapers. 126 00:09:14,447 --> 00:09:16,687 MERGUERIAN: When the bedrock is at the Earth's surface 127 00:09:16,687 --> 00:09:18,367 where it's actually exposed, 128 00:09:18,367 --> 00:09:20,287 then it's pretty easy to build tall buildings 129 00:09:20,287 --> 00:09:23,167 'cause you can root them directly into solid rock. 130 00:09:23,167 --> 00:09:28,007 However, in areas where the bedrock is deep and covered by glacial sediment, 131 00:09:28,007 --> 00:09:30,687 in those cases, it's very difficult to build tall buildings 132 00:09:30,687 --> 00:09:35,407 because you need to root those buildings either into solid rock 133 00:09:35,407 --> 00:09:40,117 or build concrete abutments called caissons that can support tall buildings. 134 00:09:42,047 --> 00:09:45,756 New York's deep history is beginning to take shape. 135 00:09:47,047 --> 00:09:49,487 Building up from tiny crystals in the rock, 136 00:09:49,487 --> 00:09:53,247 scientists revealed how New York's bedrock was formed 137 00:09:53,247 --> 00:09:57,567 under the crushing weight of a massive ancient mountain range. 138 00:09:57,567 --> 00:10:00,207 The result was hard Manhattan schist, 139 00:10:00,207 --> 00:10:05,287 a perfect foundation for the city's skyscrapers. 140 00:10:05,287 --> 00:10:08,563 But New York City still had a long way to go. 141 00:10:10,647 --> 00:10:13,687 The colliding plates created an enormous landmass - 142 00:10:13,687 --> 00:10:17,407 the last great supercontinent, called Pangaea. 143 00:10:17,407 --> 00:10:21,727 New York was now trapped in the centre... 144 00:10:21,727 --> 00:10:25,887 ...but somehow it made it back to the coast. 145 00:10:25,887 --> 00:10:30,487 Hidden beyond the city's streets is evidence of huge volcanic eruptions, 146 00:10:30,487 --> 00:10:35,527 mass extinctions and continents torn into pieces. 147 00:10:35,527 --> 00:10:37,887 Clues that could explain 148 00:10:37,887 --> 00:10:42,438 how New York became one of the world's great maritime cities. 149 00:10:47,927 --> 00:10:49,767 Investigators are piecing together 150 00:10:49,767 --> 00:10:53,362 how New York City's unique geology was formed. 151 00:10:55,327 --> 00:10:58,807 Much of its early success as a trade and commerce centre 152 00:10:58,807 --> 00:11:03,483 is owed to its deep-water harbour and its location at the coast. 153 00:11:06,367 --> 00:11:10,487 But 450 million years ago, things were different. 154 00:11:10,487 --> 00:11:14,087 The area of New York City was landlocked, 155 00:11:14,087 --> 00:11:17,407 embedded in the heart of a huge supercontinent. 156 00:11:17,407 --> 00:11:20,407 How did it get to the coast? 157 00:11:20,407 --> 00:11:25,647 The investigation fast-forwards 250 million years. 158 00:11:25,647 --> 00:11:30,647 In a quarry in New Jersey, 25 miles northeast of Manhattan, 159 00:11:30,647 --> 00:11:35,007 paleontologist Paul Olson unearths the first of a string of clues 160 00:11:35,007 --> 00:11:39,767 that could explain how New York City reached the coast. 161 00:11:39,767 --> 00:11:42,687 A giant fossilised footprint. 162 00:11:42,687 --> 00:11:45,847 This is the footprint, actually the mud that filled in the footprint, 163 00:11:45,847 --> 00:11:51,247 of a four-footed crocodile relative that was the dominant carnivore 164 00:11:51,247 --> 00:11:53,367 during the late Triassic. 165 00:11:53,367 --> 00:11:56,567 You can see the toes here have little pads on them 166 00:11:56,567 --> 00:12:00,367 and here's the handprint, and these animals would have been, 167 00:12:00,367 --> 00:12:02,927 in this case, about the size of a modest crocodile, 168 00:12:02,927 --> 00:12:09,287 but some of them became much, much larger, the size even of a T. Rex. 169 00:12:09,287 --> 00:12:12,407 (ROARS) 170 00:12:12,407 --> 00:12:16,047 The footprints are from a huge crocodile-like creature 171 00:12:16,047 --> 00:12:17,607 called Postosuchus. 172 00:12:17,607 --> 00:12:22,607 It first appeared on the Earth around 230 million years ago. 173 00:12:22,607 --> 00:12:28,007 Then, some 30 million years later, its footprints suddenly vanished. 174 00:12:28,007 --> 00:12:30,967 But Postosuchus wasn't alone. 175 00:12:30,967 --> 00:12:35,447 Half of all land animals perished at the same time. 176 00:12:35,447 --> 00:12:37,967 The fossil evidence proves it to be 177 00:12:37,967 --> 00:12:41,767 one of the biggest mass extinctions ever recorded. 178 00:12:41,767 --> 00:12:43,767 The evidence for this mass extinction 179 00:12:43,767 --> 00:12:47,807 is that we have lots and lots of fossils right in this area. 180 00:12:47,807 --> 00:12:51,967 And what you see is especially in the... in the reptile footprints, 181 00:12:51,967 --> 00:12:54,287 you see one group of forms, 182 00:12:54,287 --> 00:12:58,280 the forms that are related to crocodilians, disappear. 183 00:13:00,647 --> 00:13:06,207 Whatever caused the mass extinction must have been a catastrophic event. 184 00:13:06,207 --> 00:13:10,247 Olsen had a hunch that the mass extinction was somehow related 185 00:13:10,247 --> 00:13:13,398 to New York's return to the coast. 186 00:13:14,887 --> 00:13:19,647 The ancient area of New York sat on a line of great weakness, 187 00:13:19,647 --> 00:13:22,727 the plate boundary where two continents joined 188 00:13:22,727 --> 00:13:25,567 to form the supercontinent Pangaea. 189 00:13:25,567 --> 00:13:30,807 And it was unstable, prone to earthquakes and volcanoes. 190 00:13:30,807 --> 00:13:34,887 Olsen's quest - to find the evidence for the natural disaster 191 00:13:34,887 --> 00:13:38,641 that finished off Postosuchus 200 million years ago. 192 00:13:40,967 --> 00:13:43,967 A band of dark rock above the footprints 193 00:13:43,967 --> 00:13:46,847 in the New Jersey quarry caught his eye. 194 00:13:46,847 --> 00:13:52,524 It was basalt, the smoking gun Olson was looking for. 195 00:13:54,287 --> 00:13:57,327 Basalt is a volcanic rock. 196 00:13:57,327 --> 00:14:03,247 It forms when hot lava erupts onto the surface and cools. 197 00:14:03,247 --> 00:14:08,167 Did the volcanoes that forged this basalt trigger the mass extinction 198 00:14:08,167 --> 00:14:11,847 and also rip Pangaea apart? 199 00:14:11,847 --> 00:14:16,087 On its own, the evidence at the quarry was unconvincing. 200 00:14:16,087 --> 00:14:19,841 The layer of basalt is only a few feet thick. 201 00:14:22,487 --> 00:14:26,287 To prove mass lava flows caused this global catastrophe, 202 00:14:26,287 --> 00:14:29,882 scientists needed corroborative evidence. 203 00:14:33,327 --> 00:14:35,007 High above the Hudson River, 204 00:14:35,007 --> 00:14:38,087 geologist Matt Gorring follows another lead. 205 00:14:38,087 --> 00:14:41,007 He's studying the Palisades, 206 00:14:41,007 --> 00:14:44,607 a dramatic geologic feature that hugs the Hudson River, 207 00:14:44,607 --> 00:14:50,887 beginning across mid Manhattan and running into northeast New Jersey. 208 00:14:50,887 --> 00:14:53,727 They too are made of basaltic rock, 209 00:14:53,727 --> 00:14:58,447 the same rock implicated in the mass extinction of land animals. 210 00:14:58,447 --> 00:15:03,167 But the Palisades are on an altogether different scale. 211 00:15:03,167 --> 00:15:10,527 The Palisades are a sheet of basaltic magma, about 1,000 feet thick, 212 00:15:10,527 --> 00:15:12,567 it's about 40 miles long, 213 00:15:12,567 --> 00:15:14,247 so it's a very prominent set of cliffs 214 00:15:14,247 --> 00:15:17,603 that run all the way up the west side of the Hudson River. 215 00:15:19,367 --> 00:15:23,246 Here is proof of massive volcanic activity. 216 00:15:24,687 --> 00:15:28,127 Hot lava flooded out of ruptures in the Earth's crust 217 00:15:28,127 --> 00:15:32,359 and covered ancient North America in a mile-deep sheet. 218 00:15:33,847 --> 00:15:36,281 The lava cracked as it cooled. 219 00:15:37,407 --> 00:15:41,764 The vertical ruptures formed regular pencil-shaped columns. 220 00:15:43,287 --> 00:15:46,687 These distinctive rock formations have been known to geologists 221 00:15:46,687 --> 00:15:48,837 since the 19th century. 222 00:15:51,607 --> 00:15:55,327 Intriguingly, they appear on both sides of the Atlantic, 223 00:15:55,327 --> 00:16:00,847 in North and South America, Europe and Africa. 224 00:16:00,847 --> 00:16:04,727 Geologists suspected that their presence pointed to the spot 225 00:16:04,727 --> 00:16:09,687 where Africa and Europe separated from America. 226 00:16:09,687 --> 00:16:14,807 But that was just an unproven theory, until the 1950s, 227 00:16:14,807 --> 00:16:18,207 when scientists developed a revolutionary technique 228 00:16:18,207 --> 00:16:20,447 called paleomagnetism. 229 00:16:20,447 --> 00:16:25,043 Now, they could study the magnetic properties of rocks. 230 00:16:26,727 --> 00:16:31,607 Many rocks, including basalt, have a distinctive magnetic signature, 231 00:16:31,607 --> 00:16:34,527 formed as the rock is born. 232 00:16:34,527 --> 00:16:38,127 Tiny crystals inside the rock act like compass needles. 233 00:16:38,127 --> 00:16:41,447 When the magma that forms the rock is fluid, 234 00:16:41,447 --> 00:16:45,759 the crystals align to the Earth's magnetic field, pointing north. 235 00:16:48,327 --> 00:16:51,607 As the rock solidifies, the crystals freeze, 236 00:16:51,607 --> 00:16:55,647 forever locked in that fixed magnetic alignment. 237 00:16:55,647 --> 00:16:58,927 As continents move and the rocks travel, 238 00:16:58,927 --> 00:17:03,607 the crystals end up pointing in a different direction than north. 239 00:17:03,607 --> 00:17:09,159 Gorring and his team investigate the magnetic signature of the Palisades. 240 00:17:12,607 --> 00:17:17,237 To get a sample, they bore into the rock with a water-cooled drill. 241 00:17:21,727 --> 00:17:27,127 They measure the exact orientation of the crystals today with a compass. 242 00:17:27,127 --> 00:17:30,007 When they offset this reading with magnetic north, 243 00:17:30,007 --> 00:17:33,767 they can calculate the original location of the rock. 244 00:17:33,767 --> 00:17:36,767 Uh... 14. 245 00:17:36,767 --> 00:17:39,567 One of the useful things that you can do with this rock 246 00:17:39,567 --> 00:17:43,007 is you can take it back in the lab and measure its magnetic orientation, 247 00:17:43,007 --> 00:17:46,807 and that magnetic orientation will be when this rock crystallised 248 00:17:46,807 --> 00:17:48,767 or solidified 200 million years ago. 249 00:17:48,767 --> 00:17:51,687 So this rock would have minerals that would be pointing 250 00:17:51,687 --> 00:17:54,838 in some other direction other than north today. 251 00:17:57,647 --> 00:18:01,407 When scientists compared the magnetic orientation of the Palisades 252 00:18:01,407 --> 00:18:04,367 with the other basalt outcrops around the Atlantic, 253 00:18:04,367 --> 00:18:08,407 they discovered they formed at approximately the same latitude. 254 00:18:08,407 --> 00:18:11,647 Not only did the rocks have the same age, 255 00:18:11,647 --> 00:18:15,207 they were also born at the same location. 256 00:18:15,207 --> 00:18:21,043 For example, 200 million years ago, New York and Morocco were neighbours. 257 00:18:26,047 --> 00:18:29,447 The geologists had all the proof they needed. 258 00:18:29,447 --> 00:18:33,156 They could now confidently piece together what happened. 259 00:18:35,487 --> 00:18:40,127 It began with a global volcanic disaster. 260 00:18:40,127 --> 00:18:42,167 About 200 million years ago, 261 00:18:42,167 --> 00:18:45,239 North America and Africa began to pull apart from each other. 262 00:18:46,727 --> 00:18:48,927 There were gigantic lava outpourings. 263 00:18:48,927 --> 00:18:54,327 These lava flows erupted along very long cracks in the Earth's crust, 264 00:18:54,327 --> 00:18:56,287 that would have produced fountains of lava 265 00:18:56,287 --> 00:19:00,644 extending thousands and thousands of feet into the atmosphere. 266 00:19:02,567 --> 00:19:05,527 They covered an almost inconceivably large area, 267 00:19:05,527 --> 00:19:08,967 roughly four million square miles, 268 00:19:08,967 --> 00:19:13,767 from southwestern France to southwestern Brazil, 269 00:19:13,767 --> 00:19:16,607 from New York to central Mali in Africa. 270 00:19:16,607 --> 00:19:20,247 This area was covered in ponded lava flows 271 00:19:20,247 --> 00:19:24,286 that in some places ended up being nearly a mile thick. 272 00:19:27,847 --> 00:19:31,527 Volcanic eruptions led to soaring temperatures. 273 00:19:31,527 --> 00:19:37,204 Half of the plants and animals died. Postosuchus didn't stand a chance. 274 00:19:40,647 --> 00:19:43,847 As enormous forces tore Pangaea apart, 275 00:19:43,847 --> 00:19:47,727 a giant sea formed between the separating land masses - 276 00:19:47,727 --> 00:19:50,447 the Atlantic Ocean. 277 00:19:50,447 --> 00:19:54,520 The city of New York was now at the coast. 278 00:19:59,407 --> 00:20:03,367 200-million-year-old footprints beneath a layer of basalt 279 00:20:03,367 --> 00:20:07,047 and the Palisades towering above the Hudson 280 00:20:07,047 --> 00:20:09,367 provide evidence that Pangaea split apart 281 00:20:09,367 --> 00:20:12,518 to create the east coast of North America. 282 00:20:13,847 --> 00:20:17,527 But the story of New York City was far from over. 283 00:20:17,527 --> 00:20:21,967 After being built by fire, the region was about to be overcome 284 00:20:21,967 --> 00:20:25,277 by another destructive force. 285 00:20:28,927 --> 00:20:34,399 Scientists are piecing together the story of New York's violent geological past. 286 00:20:36,287 --> 00:20:40,367 250 million years ago, the Atlantic Ocean opened up, 287 00:20:40,367 --> 00:20:44,327 leaving the area of New York on the coast. 288 00:20:44,327 --> 00:20:48,559 But the maritime city still had a long way to go. 289 00:20:53,647 --> 00:20:56,527 There was no deep Hudson River channel. 290 00:20:56,527 --> 00:21:00,361 It was nothing but a small stream. 291 00:21:02,007 --> 00:21:05,887 What forces transformed it into the wide river 292 00:21:05,887 --> 00:21:09,197 capable of carrying heavy freighters far inland? 293 00:21:12,807 --> 00:21:17,687 A clue to how the Hudson Valley was created is the strange boulders 294 00:21:17,687 --> 00:21:21,367 that are scattered throughout Manhattan's Central Park. 295 00:21:21,367 --> 00:21:24,487 Some of them weigh several tons. 296 00:21:24,487 --> 00:21:30,960 But they're strangers to these parts, totally unlike the surrounding rocks. 297 00:21:33,087 --> 00:21:37,046 Geologist Charles Merguerian investigates where they came from. 298 00:21:38,887 --> 00:21:41,767 This boulder is a boulder from the Palisade sheet 299 00:21:41,767 --> 00:21:44,087 on the other side of the Hudson in New Jersey. 300 00:21:44,087 --> 00:21:46,447 You can see it's very nicely polished. 301 00:21:46,447 --> 00:21:49,607 Compositionally, it's totally different than the surrounding bedrock, 302 00:21:49,607 --> 00:21:51,167 which is Manhattan schist, 303 00:21:51,167 --> 00:21:53,767 and the Manhattan schist here is very rich in mica. 304 00:21:53,767 --> 00:21:57,047 This rock has no light-coloured mica in it whatsoever. 305 00:21:57,047 --> 00:22:01,563 The Palisade sheet is located to the west and north of us. 306 00:22:05,367 --> 00:22:09,887 The Palisades run for 40 miles along the Hudson River. 307 00:22:09,887 --> 00:22:14,127 Something immensely powerful must have moved the boulders 308 00:22:14,127 --> 00:22:16,118 such a great distance. 309 00:22:18,087 --> 00:22:22,444 Merguerian knows the answer is ice. 310 00:22:25,207 --> 00:22:29,767 Scientists have noticed a similar phenomenon 4,000 miles away 311 00:22:29,767 --> 00:22:31,723 in the Swiss Alps. 312 00:22:33,287 --> 00:22:36,967 As huge glaciers grind their way across the landscape, 313 00:22:36,967 --> 00:22:41,757 they gouge out lumps of rock and carry them along in the base of the ice. 314 00:22:42,807 --> 00:22:48,807 These rocks act like sandpaper and carve out deep scratches. 315 00:22:48,807 --> 00:22:52,807 When the ice melts, it leaves the boulders behind. 316 00:22:52,807 --> 00:22:59,007 Merguerian is convinced the same thing happened in Central Park. 317 00:22:59,007 --> 00:23:01,487 Ice moved the Palisade boulders 318 00:23:01,487 --> 00:23:05,367 and carved out grooves in the bedrock underneath. 319 00:23:05,367 --> 00:23:10,167 This bedrock exposure in Central Park shows the profound effects of glaciation 320 00:23:10,167 --> 00:23:13,207 in the form of these spectacular glacial grooves 321 00:23:13,207 --> 00:23:17,927 that move up the outcrop and show this pattern 322 00:23:17,927 --> 00:23:20,327 where glaciers grabbed huge boulders 323 00:23:20,327 --> 00:23:24,843 and those huge boulders acted like tools to produce these scratches. 324 00:23:27,767 --> 00:23:31,847 To Merguerian, the rocks in Central Park are compelling evidence 325 00:23:31,847 --> 00:23:34,998 that New York once was covered in ice. 326 00:23:36,367 --> 00:23:39,887 Over millions of years, growing and receding ice 327 00:23:39,887 --> 00:23:43,960 has repeatedly turned North America into a frozen wilderness. 328 00:23:48,087 --> 00:23:52,887 But the grooves on the rocks in New York don't tell the full story. 329 00:23:52,887 --> 00:23:58,607 The destruction caused by the ice points to a gigantic glacial event 330 00:23:58,607 --> 00:24:01,883 that would dwarf the future metropolis. 331 00:24:04,327 --> 00:24:07,047 To find out the extent of the ice, 332 00:24:07,047 --> 00:24:08,767 Merguerian travelled to Bear Mountain, 333 00:24:08,767 --> 00:24:12,123 some 50 miles north of New York City. 334 00:24:15,207 --> 00:24:18,085 Once again, the clue was in the rocks. 335 00:24:19,087 --> 00:24:23,444 He found glacial marks similar to those in Central Park. 336 00:24:25,007 --> 00:24:27,607 What we're looking at here are chatter marks. 337 00:24:27,607 --> 00:24:31,847 Chatter marks are very diagnostic features of glacial erosion, 338 00:24:31,847 --> 00:24:36,807 they're produced by boulders embedded in the base of a thick sheet of glacial ice. 339 00:24:36,807 --> 00:24:40,087 Those boulders impinged on this bedrock surface, 340 00:24:40,087 --> 00:24:44,807 polishing it, smoothing it off and then plucking pieces of rock off 341 00:24:44,807 --> 00:24:49,756 as the glacial ice moved over with the boulders embedded in the base. 342 00:24:51,407 --> 00:24:55,207 The gouges in the rocks could mean just one thing. 343 00:24:55,207 --> 00:24:59,087 The glacier must have been thousands of feet thick. 344 00:24:59,087 --> 00:25:01,527 In this case, although we're standing at an elevation 345 00:25:01,527 --> 00:25:04,287 of about 1,280 feet above sea level, 346 00:25:04,287 --> 00:25:08,724 the glacial ice sheet covered Bear Mountain as if it weren't even there. 347 00:25:11,127 --> 00:25:14,887 Scientists have found identical chatter marks on nearby peaks 348 00:25:14,887 --> 00:25:17,447 up to a mile above sea level. 349 00:25:17,447 --> 00:25:20,127 It was unmistakable proof. 350 00:25:20,127 --> 00:25:25,997 A glacier at least one mile thick ground its way across these mountains. 351 00:25:28,087 --> 00:25:32,647 Did this same ice sheet also plough through Central Park? 352 00:25:32,647 --> 00:25:36,447 Another nearby rock face provided the answer. 353 00:25:36,447 --> 00:25:38,567 The feature that we're looking at here 354 00:25:38,567 --> 00:25:41,247 are a series of sub-parallel glacial scratches and grooves 355 00:25:41,247 --> 00:25:45,647 and these are, again, are produced by the glacial ice sheet 356 00:25:45,647 --> 00:25:49,207 dragging boulders across this very durable granite surface, 357 00:25:49,207 --> 00:25:51,327 it's kind of polished the surface. 358 00:25:51,327 --> 00:25:55,407 And, in addition, it's produced these rather subtle but... but... but obvious, 359 00:25:55,407 --> 00:25:59,127 when the lighting is right, striae or grooves in the bedrock. 360 00:25:59,127 --> 00:26:01,800 Now if we measure the orientation of these... 361 00:26:06,927 --> 00:26:13,327 ...these... these come out about north 20 degrees west, 362 00:26:13,327 --> 00:26:18,037 just about identical in orientation to the striae that we measured at Central Park. 363 00:26:20,447 --> 00:26:24,607 It's significant evidence that the ice sheet that covered Bear Mountain 364 00:26:24,607 --> 00:26:28,287 also flowed over the surface of Central Park. 365 00:26:28,287 --> 00:26:33,047 Proof that New York City was covered by a glacier four times higher 366 00:26:33,047 --> 00:26:35,767 than the Empire State Building. 367 00:26:35,767 --> 00:26:37,607 MERGUERIAN: Just imagine glacial ice, 368 00:26:37,607 --> 00:26:40,727 a huge thick ice sheet over a mile thick, 369 00:26:40,727 --> 00:26:44,047 exerting tremendous pressure on the surface 370 00:26:44,047 --> 00:26:47,881 and sculpting the surface into the landscape that we see today. 371 00:26:50,247 --> 00:26:54,087 The ice sheet's crushing weight bulldozed everything in its path 372 00:26:54,087 --> 00:26:58,127 and cut through the remains of the ancient mountains. 373 00:26:58,127 --> 00:27:00,487 Before the ice arrived, 374 00:27:00,487 --> 00:27:04,087 the waters of the Hudson River had gently cut down through the landscape 375 00:27:04,087 --> 00:27:06,043 to form a V-shaped valley. 376 00:27:07,487 --> 00:27:11,167 But a mile-thick glacier takes no prisoners. 377 00:27:11,167 --> 00:27:15,407 It gouged out the sides and the bottom of the river valley 378 00:27:15,407 --> 00:27:17,921 and turned it into a U-shaped riverbed. 379 00:27:20,087 --> 00:27:24,046 The Hudson River was now navigable for big ships. 380 00:27:26,327 --> 00:27:30,445 The picture of modern-day New York was almost complete. 381 00:27:32,527 --> 00:27:36,287 Long grooves in the rocks in Central Park showed scientists 382 00:27:36,287 --> 00:27:41,047 that a vast ice sheet flowed over the eroded remains of these mountains. 383 00:27:41,047 --> 00:27:44,967 And marks and glacial striations on Bear Mountain 384 00:27:44,967 --> 00:27:48,167 proved this ice sheet was at least four times higher 385 00:27:48,167 --> 00:27:49,805 than the Empire State Building. 386 00:27:52,967 --> 00:27:54,127 When the ice melted, 387 00:27:54,127 --> 00:27:58,882 it left a vast ridge of debris blocking the Hudson River from the Atlantic. 388 00:28:00,087 --> 00:28:05,447 The final challenge for geologists was to find out how the ridge was destroyed 389 00:28:05,447 --> 00:28:09,565 and how New York's harbour opened to the oceans. 390 00:28:17,767 --> 00:28:22,687 New York today boasts one of the largest natural harbours in the world. 391 00:28:22,687 --> 00:28:26,127 But it wasn't always that way. 392 00:28:26,127 --> 00:28:32,487 Towards the end of the last Ice Age, the port's wide entrance was blocked. 393 00:28:32,487 --> 00:28:39,727 16,000 years ago, the melting glaciers left behind a 220-foot-high wall of debris. 394 00:28:39,727 --> 00:28:42,567 The ridge stretched from Long Island to Staten Island 395 00:28:42,567 --> 00:28:45,167 and forced the Hudson River through a narrow, 396 00:28:45,167 --> 00:28:47,317 more westerly course to the ocean. 397 00:28:49,567 --> 00:28:53,247 What powerful forces destroyed this rock jam? 398 00:28:53,247 --> 00:28:57,525 The prime suspect was a flash flood. 399 00:29:00,807 --> 00:29:05,756 But scientists needed evidence to prove that such a flood had happened. 400 00:29:06,767 --> 00:29:10,487 In the 1960s, fishermen made an unexpected find 401 00:29:10,487 --> 00:29:12,847 at the mouth of the Hudson River. 402 00:29:12,847 --> 00:29:19,007 They dredged up a giant mammoth tusk from the depths of the sea floor. 403 00:29:19,007 --> 00:29:21,680 (TRUMPETS) 404 00:29:24,647 --> 00:29:28,767 Herds of these giant beasts roamed the plains of North America 405 00:29:28,767 --> 00:29:33,602 before they became extinct at the end of the last Ice Age. 406 00:29:38,127 --> 00:29:42,087 Finding the odd mammoth tusk here and there is not so surprising. 407 00:29:42,087 --> 00:29:44,647 But since the initial discovery, 408 00:29:44,647 --> 00:29:47,887 scientists have found hundreds more tusks and bones 409 00:29:47,887 --> 00:29:51,007 in the mouth of the Hudson River. 410 00:29:51,007 --> 00:29:55,167 It was as though a violent torrent swept the mammoths away 411 00:29:55,167 --> 00:29:57,567 and dumped their remains off the coast. 412 00:29:57,567 --> 00:30:01,446 And there were more clues nearby. 413 00:30:04,087 --> 00:30:09,047 Huge boulders resting on the sandy sea floor, 414 00:30:09,047 --> 00:30:12,687 some of them as big as cars. 415 00:30:12,687 --> 00:30:16,167 The boulders must have been part of the ancient moraine 416 00:30:16,167 --> 00:30:19,796 that once ran between Long Island and Staten Island. 417 00:30:21,527 --> 00:30:27,238 Geologist David Franzi knows that only a raging torrent could have shifted them. 418 00:30:28,567 --> 00:30:31,047 Based on the size of the boulders that we see here, 419 00:30:31,047 --> 00:30:33,847 we know that that flood must have discharged 420 00:30:33,847 --> 00:30:37,920 on the order of 1.5 million cubic feet per second. 421 00:30:39,247 --> 00:30:43,160 That's three times larger than the largest Mississippi River flood ever recorded. 422 00:30:44,407 --> 00:30:48,007 And all that water had to come from somewhere. 423 00:30:48,007 --> 00:30:51,087 Scientists began looking for the source of this flood, 424 00:30:51,087 --> 00:30:57,447 a flood powerful enough to transport huge boulders all the way to the sea. 425 00:30:57,447 --> 00:31:00,727 Rocks to a geologist are like pages in a history book. 426 00:31:00,727 --> 00:31:05,327 For us, erosion oftentimes rips some of the pages out of our history book, 427 00:31:05,327 --> 00:31:09,127 so it's the job of the geologist to put together a fragmentary record 428 00:31:09,127 --> 00:31:12,517 into a coherent history of the events that happened in the past. 429 00:31:14,287 --> 00:31:18,207 300 miles north of the city, in upstate New York, 430 00:31:18,207 --> 00:31:20,527 Franzi tracked what might have been the flood's path 431 00:31:20,527 --> 00:31:22,047 to an unusual grove of trees 432 00:31:22,047 --> 00:31:23,767 on Covey Hill 433 00:31:23,767 --> 00:31:25,598 in the Adirondack Mountains. 434 00:31:28,327 --> 00:31:31,607 The trees are jack pines. 435 00:31:31,607 --> 00:31:35,885 They are rare in this area, where the soil is usually fertile and deep. 436 00:31:37,927 --> 00:31:40,327 But on Covey Hill, their presence shows 437 00:31:40,327 --> 00:31:44,559 that there is no more than a few inches of soil on top of the bedrock. 438 00:31:45,847 --> 00:31:47,687 The jack pine is essentially rooted 439 00:31:47,687 --> 00:31:49,887 right on the top of a rock's surface here. 440 00:31:49,887 --> 00:31:53,567 This is a bare sandstone surface, very little mineral soil, 441 00:31:53,567 --> 00:31:57,887 and it's subject to prolonged periods of dryness during the summertime. 442 00:31:57,887 --> 00:32:00,847 Jack pine's adaptations make it able to survive here 443 00:32:00,847 --> 00:32:02,803 where no other tree species can. 444 00:32:04,767 --> 00:32:07,327 What happened to the soil? 445 00:32:09,767 --> 00:32:11,727 The jack pines continue to grow 446 00:32:11,727 --> 00:32:15,686 at the entrance of a long gorge over 300 feet wide. 447 00:32:17,127 --> 00:32:21,247 Usually, gorges like this are cut down over thousands of years, 448 00:32:21,247 --> 00:32:25,647 but in this case, the missing topsoil points to a sudden flood event. 449 00:32:25,647 --> 00:32:31,085 A raging torrent must have ripped away the soil and cut deep into the rock. 450 00:32:36,367 --> 00:32:39,882 In a helicopter, Franzi follows the gorge west. 451 00:32:42,487 --> 00:32:45,287 Eventually, it opens into a vast, empty basin 452 00:32:45,287 --> 00:32:50,567 located next to one of the Great Lakes, Lake Ontario. 453 00:32:50,567 --> 00:32:54,047 It doesn't take much of a stretch of the imagination here 454 00:32:54,047 --> 00:32:56,847 to imagine this valley filled with water 455 00:32:56,847 --> 00:33:00,635 and then with these hills poking up through as islands. 456 00:33:02,287 --> 00:33:09,247 16,000 years ago, this basin was filled with two billion cubic miles of water - 457 00:33:09,247 --> 00:33:12,487 a huge lake geologists call Lake Iroquois. 458 00:33:12,487 --> 00:33:16,196 It formed at the end of the last Ice Age. 459 00:33:19,367 --> 00:33:24,447 As glaciers receded, the melt waters slowly filled up the lake. 460 00:33:24,447 --> 00:33:27,647 The ice dams holding the waters weakened. 461 00:33:27,647 --> 00:33:34,447 Eventually, the dams collapsed, causing sudden and devastating flash floods. 462 00:33:34,447 --> 00:33:38,007 Lake level dropped on the order of 70 feet 463 00:33:38,007 --> 00:33:41,087 and about 160 cubic miles of water 464 00:33:41,087 --> 00:33:44,047 were released into the Champlain Valley, catastrophically. 465 00:33:44,047 --> 00:33:47,967 That floodwater would have coursed down the Champlain Valley, 466 00:33:47,967 --> 00:33:51,403 through the Hudson Valley and ultimately out into the Atlantic Ocean. 467 00:33:52,447 --> 00:33:57,247 The torrent raced towards New York City, 300 miles to the south, 468 00:33:57,247 --> 00:34:00,247 then took the straightest course to the sea. 469 00:34:00,247 --> 00:34:03,807 The floodwaters smashed into the ancient moraine, 470 00:34:03,807 --> 00:34:08,676 the huge pile of debris blocking the direct exit of the Hudson River. 471 00:34:13,367 --> 00:34:16,887 The bridge we see behind me spans the channel that was cut 472 00:34:16,887 --> 00:34:18,447 by the flood event. 473 00:34:18,447 --> 00:34:21,407 When the flood wave came through, it was of sufficient intensity 474 00:34:21,407 --> 00:34:25,639 to over-top the dam and very rapidly cut the channel. 475 00:34:28,207 --> 00:34:31,447 The gap that was created by the flood still exists. 476 00:34:31,447 --> 00:34:35,486 It's now a tidal strait called the Narrows. 477 00:34:37,007 --> 00:34:41,567 Today, the gap is spanned by the Verrazano Bridge. 478 00:34:41,567 --> 00:34:45,447 The channel is deep enough for even the biggest ocean-going ships. 479 00:34:45,447 --> 00:34:50,316 It's the most important entrance to New York City's harbour. 480 00:34:54,087 --> 00:34:57,567 Mammoth tusks and huge boulders at the mouth of the river 481 00:34:57,567 --> 00:35:01,207 showed scientists that there was a torrent big enough to blast a hole 482 00:35:01,207 --> 00:35:03,407 through the ancient moraine. 483 00:35:03,407 --> 00:35:08,927 And a channel leading towards the Great Lakes revealed the source of the flood. 484 00:35:08,927 --> 00:35:11,647 It was this flood that created the Narrows 485 00:35:11,647 --> 00:35:16,167 and gave New York a wide entrance to its port. 486 00:35:16,167 --> 00:35:20,957 A unique geology laid down the foundations for New York City. 487 00:35:24,727 --> 00:35:28,447 But the same forces that constructed it may also have sown the seeds 488 00:35:28,447 --> 00:35:30,802 for New York's destruction. 489 00:35:36,647 --> 00:35:43,327 Scientists have pieced together the half-billion-year history of New York City. 490 00:35:43,327 --> 00:35:49,327 Huge mountains, volcanic eruptions and glacial ice shaped the area. 491 00:35:49,327 --> 00:35:52,797 But New York's story doesn't end here. 492 00:35:55,087 --> 00:35:59,367 The geology that created one of the greatest cities on Earth 493 00:35:59,367 --> 00:36:02,723 also has the potential to destroy it. 494 00:36:04,647 --> 00:36:08,127 Experts have been studying the potential threat to the city. 495 00:36:08,127 --> 00:36:10,727 We're standing here in Lower Manhattan 496 00:36:10,727 --> 00:36:12,047 on one of our major thoroughfares, 497 00:36:12,047 --> 00:36:13,087 Canal Street. 498 00:36:13,087 --> 00:36:15,567 And it's important, because in 1821, 499 00:36:15,567 --> 00:36:21,887 a Category 2 hurricane raised the water level at the Battery 13 feet in one hour 500 00:36:21,887 --> 00:36:25,407 and, literally, the Hudson River met the East River 501 00:36:25,407 --> 00:36:28,727 and Canal Street was covered by water 502 00:36:28,727 --> 00:36:33,926 and Manhattan was actually two islands for three hours, until the water receded. 503 00:36:36,687 --> 00:36:41,124 New York City is vulnerable because of its position on the coast. 504 00:36:43,767 --> 00:36:49,647 Long Island stretches northeast at a right angle from the New Jersey shore. 505 00:36:49,647 --> 00:36:54,047 New York City is nestled behind the western end of Long Island. 506 00:36:54,047 --> 00:36:55,887 Normally, the island protects the city 507 00:36:55,887 --> 00:36:57,407 from the sea, 508 00:36:57,407 --> 00:37:01,167 but when hurricanes threaten, the opposite is true. 509 00:37:01,167 --> 00:37:05,080 Long Island becomes a dangerous liability. 510 00:37:07,287 --> 00:37:11,407 Hurricanes racing north along the beaches of the Atlantic coast 511 00:37:11,407 --> 00:37:14,567 pile up huge bulges of water in front of them. 512 00:37:14,567 --> 00:37:17,035 They're called storm surges. 513 00:37:18,087 --> 00:37:20,647 Hitting the right-angled junction at Long Island, 514 00:37:20,647 --> 00:37:23,927 the winds funnel the storm surge in through the Narrows, 515 00:37:23,927 --> 00:37:27,407 the gap between Long Island and New Jersey. 516 00:37:27,407 --> 00:37:31,607 This is the place, at the actual apex of the right angle in New York 517 00:37:31,607 --> 00:37:36,407 where all the water being pushed by a hurricane would be concentrated. 518 00:37:36,407 --> 00:37:38,967 And in the distance is the Verrazano Bridge, 519 00:37:38,967 --> 00:37:42,927 and all that water is gonna go through the passage we call the Narrows 520 00:37:42,927 --> 00:37:45,647 and it's gonna be accelerated towards New York City, 521 00:37:45,647 --> 00:37:49,276 where it will rise to abnormal heights. 522 00:37:54,647 --> 00:37:57,527 Experts believe that in the United States, 523 00:37:57,527 --> 00:38:02,287 New York is the third most vulnerable city after Miami and New Orleans 524 00:38:02,287 --> 00:38:04,323 to a hurricane disaster. 525 00:38:05,367 --> 00:38:09,607 If it was hit today, the consequences would be serious. 526 00:38:09,607 --> 00:38:14,887 New York City is hit by hurricanes only infrequently. 527 00:38:14,887 --> 00:38:20,487 Like, in 1821 and in 1893 and in 1938. 528 00:38:20,487 --> 00:38:23,287 However, the point is that the hurricane 529 00:38:23,287 --> 00:38:27,847 that will eventually hit New York City again will be catastrophic, 530 00:38:27,847 --> 00:38:30,927 and what is going to happen when the utilities are knocked out? 531 00:38:30,927 --> 00:38:34,807 What is going to happen when salt water reaches into the subways 532 00:38:34,807 --> 00:38:37,607 and ruins the electrical system? 533 00:38:37,607 --> 00:38:39,887 We're talking about unbelievable amounts of money 534 00:38:39,887 --> 00:38:42,087 to restore the infrastructure. 535 00:38:42,087 --> 00:38:45,447 We're talking about setbacks and delays in commerce 536 00:38:45,447 --> 00:38:50,840 and banking and transportation, a catastrophe that's never been seen. 537 00:38:53,487 --> 00:38:57,685 Storm surges are not the only threat to New York's future. 538 00:38:59,767 --> 00:39:05,727 Earthquakes are also part of the vast geological forces that shape this area. 539 00:39:05,727 --> 00:39:07,877 They are still at work today. 540 00:39:10,207 --> 00:39:14,041 Some could change the city in an instant. 541 00:39:16,367 --> 00:39:19,727 November 4th 1884. 542 00:39:19,727 --> 00:39:24,447 New York City was shaken by an earthquake that lasted ten seconds. 543 00:39:24,447 --> 00:39:28,407 The Brooklyn Bridge swayed and people panicked. 544 00:39:28,407 --> 00:39:33,765 The earthquake showed 5.5 on the Richter scale. 545 00:39:34,767 --> 00:39:40,080 January 17th 2001, New York City was struck again. 546 00:39:41,647 --> 00:39:46,087 This time the quake was relatively small, only 2.4, 547 00:39:46,087 --> 00:39:50,807 but it struck right under 125th Street. 548 00:39:50,807 --> 00:39:54,727 The earthquake in 2001 is the first earthquake 549 00:39:54,727 --> 00:39:58,687 that we could confidently locate in Manhattan, 550 00:39:58,687 --> 00:40:03,397 that's its claim to fame, it was felt widely. 551 00:40:13,247 --> 00:40:17,798 It's impossible to study the cause of the quakes at the surface. 552 00:40:19,447 --> 00:40:22,757 The evidence is buried beneath the city. 553 00:40:25,527 --> 00:40:27,967 Deep within New York's bedrock, 554 00:40:27,967 --> 00:40:32,961 seismologist Leonardo Seeber studies the cause of these quakes. 555 00:40:33,967 --> 00:40:35,480 (HORN BLARES) 556 00:40:37,847 --> 00:40:42,447 In a subway tunnel 100 feet beneath the bedrock under the East River, 557 00:40:42,447 --> 00:40:44,687 there is a ready-made laboratory. 558 00:40:44,687 --> 00:40:49,124 Here, Seeber can study the rocks up close and personal. 559 00:40:51,207 --> 00:40:54,967 It is the same bedrock Manhattan is built on. 560 00:40:54,967 --> 00:40:59,085 But Seeber fears it isn't as solid as once was thought. 561 00:41:00,247 --> 00:41:06,127 New York area is considered a seismic zone, 562 00:41:06,127 --> 00:41:10,047 meaning there is a cluster of known earthquakes 563 00:41:10,047 --> 00:41:11,807 that have occurred in this area. 564 00:41:11,807 --> 00:41:18,047 So we are, as geologists, very eager to discover which faults are responsible 565 00:41:18,047 --> 00:41:19,560 for these earthquakes. 566 00:41:20,967 --> 00:41:23,687 The majority of earthquakes occur at the boundaries 567 00:41:23,687 --> 00:41:26,807 between separate sections of the Earth's crust, 568 00:41:26,807 --> 00:41:30,647 the tectonic plates on which the continents sit. 569 00:41:30,647 --> 00:41:34,127 But New York's earthquakes are different. 570 00:41:34,127 --> 00:41:37,367 The city is firmly in the middle of a tectonic plate, 571 00:41:37,367 --> 00:41:40,447 halfway between the mid-Atlantic ridge to the east 572 00:41:40,447 --> 00:41:43,325 and the San Andreas Fault to the west. 573 00:41:46,087 --> 00:41:49,523 Seeber is anxious to discover what's going on. 574 00:41:50,767 --> 00:41:55,397 As he examines the walls of the tunnel, he comes across a possible clue. 575 00:41:56,767 --> 00:42:00,527 The long fractures in the rock are fault lines that formed 576 00:42:00,527 --> 00:42:02,887 when pressure built up. 577 00:42:02,887 --> 00:42:07,047 As the tension was released, the rock cracked and shifted. 578 00:42:07,047 --> 00:42:10,847 This is felt on the surface as an earthquake. 579 00:42:10,847 --> 00:42:13,407 SEEBER: This is an example of a very small fault, 580 00:42:13,407 --> 00:42:19,727 but it's a fault that probably did generate some small earthquakes. 581 00:42:19,727 --> 00:42:21,847 When one of these faults generates an earthquake, 582 00:42:21,847 --> 00:42:28,605 we think that perhaps other faults of the same family can generate earthquakes. 583 00:42:30,527 --> 00:42:33,087 These faults in New York's bedrock are evidence 584 00:42:33,087 --> 00:42:37,527 that the area was hit by earthquakes in the past. 585 00:42:37,527 --> 00:42:43,716 But Seeber has no way of knowing if the faults are still active and dangerous. 586 00:42:47,367 --> 00:42:50,167 If a large earthquake hit New York today, 587 00:42:50,167 --> 00:42:53,523 the consequences would be catastrophic. 588 00:42:54,647 --> 00:42:56,687 A large proportion of New York City buildings 589 00:42:56,687 --> 00:42:59,647 are simply not built to withstand earthquake shaking. 590 00:42:59,647 --> 00:43:02,727 We worry about transportation tunnels, 591 00:43:02,727 --> 00:43:05,527 in particular, tunnels that traverse rivers 592 00:43:05,527 --> 00:43:08,847 where parts of the tunnels are rooted in solid rock 593 00:43:08,847 --> 00:43:11,167 and other parts are resting on soft sediment. 594 00:43:11,167 --> 00:43:13,407 The oscillation of these two different materials 595 00:43:13,407 --> 00:43:16,167 could cause severe cracking and fracturing. 596 00:43:16,167 --> 00:43:19,487 The infrastructure would be severely damaged, 597 00:43:19,487 --> 00:43:24,322 it would take tens of years to repair the damage caused by such a large event. 598 00:43:25,927 --> 00:43:28,847 With the evidence geologists have collected, 599 00:43:28,847 --> 00:43:33,204 the story of the creation of New York City can now be told. 600 00:43:34,287 --> 00:43:39,607 The city's bedrock was formed under a chain of mountains over a mile high. 601 00:43:39,607 --> 00:43:43,887 Volcanoes and lava fields over millions of square miles 602 00:43:43,887 --> 00:43:49,727 split up the ancient supercontinent and created the east coast of North America. 603 00:43:49,727 --> 00:43:53,487 Glacial ice, four times as high as the Empire State building, 604 00:43:53,487 --> 00:43:56,127 carved out the deep Hudson River. 605 00:43:56,127 --> 00:43:59,447 A catastrophic flash flood broke through the moraine 606 00:43:59,447 --> 00:44:04,646 to form the Narrows and opened up New York City's harbour to the oceans. 607 00:44:09,007 --> 00:44:11,527 Looking ahead to the distant future, 608 00:44:11,527 --> 00:44:15,122 geologists see more challenging times for the city. 609 00:44:16,167 --> 00:44:18,047 In 40,000 years, 610 00:44:18,047 --> 00:44:22,040 they predict this region will be engulfed by another ice sheet. 611 00:44:23,047 --> 00:44:28,727 And in 250 million years, the Atlantic will start to shrink again. 612 00:44:28,727 --> 00:44:33,807 Europe and Africa will eventually crash back into the American coast. 613 00:44:33,807 --> 00:44:37,207 The fossilised remains of the once great city of New York 614 00:44:37,207 --> 00:44:42,367 will become just another layer of rock in a vast new mountain range. 615 00:44:42,367 --> 00:44:47,680 A footnote in the immense, ever-changing story of planet Earth.