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The Grand Canyon of the United States is a world famous natural wonder composed of towering peaks and deep leaf canyons, it is majestic and ravine. Because the Colorado River flows through it, also known as the Colorado Grand Canyon. It is selected by UNESCO as protected natural heritage. The Grand Canyon is majestic and ravines, forming a huge natural labyrinth. For more than a century, scientists still unable to explain why the river can cut such a deep canyon on the earth surface. The Grand Canyon, as a result of geological activities are invisible to the naked eye. Scientists are about to reveal what kind of geological forces created this fascinating natural wonder. The Grand Canyon is huge, so it can be clearly seen from the universe. It is 277 miles long, 18 miles wide, and one mile deep. The abyss of the Grand Canyon in the United States is also one of the deepest on earth, it is composed of rocks and sand. There are many stripes in the Grand Canyon, and they are all neat and regular. There are fossils of the ancient ocean at the top, and the remains of the ancient mountains at the bottom.....
世闻名科罗拉多大峡谷:--
美国大峡谷是由高耸孤峰和深叶峡谷组成一个举世闻名的自然奇观, 威风兀立, 沟壑纵横, 由于科罗拉多河穿流其中, 故又名科罗拉多大峡谷, 它是联合国教科文组织选为受保护的天然遗产之一. 大峡谷威风兀立, 沟壑纵横, 组成了巨大的天然迷宫. 一个多世纪以来科学家仍无法解释, 为什么一条小河能在地表上切割出如此幽深的峡谷. 大峡谷是地球深处地质活动的结果, 这些地质活动肉眼根本看不见. 如今, 新的发现让大峡谷的形成初现端倪. 科学家即将揭晓, 是何种地质力量创造出了这迷离奇幻的自然奇观. 大峡谷的面积很大, 所以从宇宙上都能清楚地看到, 一共长277哩, 宽18哩, 深一哩, 美国大峡谷的深渊也是地球上最深的深渊之一. 由岩石,沙子组成的大峡谷有很多条纹, 而且都很整齐, 也有规律, 顶端有古老海洋的化石, 底端确有古老山脉的遗迹.
世闻名科罗拉多大峡谷:--
美国大峡谷是由高耸孤峰和深叶峡谷组成一个举世闻名的自然奇观, 威风兀立, 沟壑纵横, 由于科罗拉多河穿流其中, 故又名科罗拉多大峡谷, 它是联合国教科文组织选为受保护的天然遗产之一. 大峡谷威风兀立, 沟壑纵横, 组成了巨大的天然迷宫. 一个多世纪以来科学家仍无法解释, 为什么一条小河能在地表上切割出如此幽深的峡谷. 大峡谷是地球深处地质活动的结果, 这些地质活动肉眼根本看不见. 如今, 新的发现让大峡谷的形成初现端倪. 科学家即将揭晓, 是何种地质力量创造出了这迷离奇幻的自然奇观. 大峡谷的面积很大, 所以从宇宙上都能清楚地看到, 一共长277哩, 宽18哩, 深一哩, 美国大峡谷的深渊也是地球上最深的深渊之一. 由岩石,沙子组成的大峡谷有很多条纹, 而且都很整齐, 也有规律, 顶端有古老海洋的化石, 底端确有古老山脉的遗迹.
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00:03The Grand Canyon, a colossal labyrinth of towering buttes and deep-sided canyons.
00:10For over a century, scientists have puzzled over how one small river could cut such a deep slice through the
00:19Earth.
00:20The Grand Canyon formed through processes deep within the Earth, invisible to the eye.
00:27Today, new discoveries point towards new answers.
00:31Scientists are on the brink of understanding the geological forces that created one of the natural wonders of the world,
00:39the Grand Canyon.
00:59It's a landmark so immense that you can see it from space.
01:05446 kilometers long, up to 29 kilometers wide, and one and a half kilometers deep, the Grand Canyon is one
01:14of the deepest chasms on Earth.
01:16Its walls plunge sheer into the Earth's crust.
01:21The Colorado River sliced through 2,000 million years of our planet's history in less than 5 million years, exposing
01:30layer after layer of geological time.
01:35The first humans walked here thousands of years ago.
01:39Since then, the canyon has inspired awe.
01:43How did it come to be here, and what secrets lie hidden in its walls?
01:49To many Native Americans, this land is sacred.
01:53They have revered the canyon for centuries.
01:56The canyon is a very spiritual and powerful place.
02:01Our legends say that we actually came from the bottom of the canyon.
02:05Scientists like senior geologist John Spencer consider the canyon a treasure and a mystery.
02:12It's a bit startling because all around it is very flat.
02:16You know, you can be 100 feet from the edge of the canyon and think this is a rather boring
02:20landscape,
02:21and then you walk over and look into the canyon and all of a sudden there's this monster canyon in
02:26front of you.
02:28The layers of rock in the canyon walls formed over thousands of millions of years.
02:37The deeper you go, the further back in time you'd travel.
02:43At the top, the newest rocks. At the bottom, the oldest.
02:49The Grand Canyon appears to be upside down.
02:54On top, fossils from an ancient sea.
02:57At the bottom, remains of an old mountain range.
03:03Karl Karlström, geology professor at the University of New Mexico, is an expert on the oldest rocks of the canyon.
03:10One and a half kilometers down.
03:12They reveal what the southwestern U.S. looked like two billion years ago.
03:18We were interested in what's the oldest chapter of Grand Canyon history,
03:23and these rocks are so spectacularly exposed that we have the opportunity to study the earliest history in Grand Canyon.
03:34What's strange about these rocks at the bottom is that they're part of an ancient mountain range,
03:40now buried under many meters of rock.
03:44They're the remains of a range called the Vishnu Schist, a layer of metamorphosed rock that dates from around 1
03:52.7 thousand million years ago.
03:55The human eye sees a line of rock.
03:59Karlström's instruments see evidence of a mountain range that once towered nine and a half kilometers above.
04:05His techniques bring the past back to life.
04:09His clue lies with the minerals in the rock that change chemical composition under pressure.
04:15More rock on top means more pressure on the rock below.
04:19That forces a mineral in the rock, garnet, to take up more calcium.
04:23The higher the calcium concentration within the garnet, the brighter it looks under a microscope.
04:30That tells scientists how deep a rock was buried.
04:34We've learned that these rocks were buried to great depth,
04:38and the amount of pressure that they experienced was equivalent to about six miles of rock above them.
04:49Karlström's findings have helped other geologists visualize how this area may have looked 1.7 thousand million years ago.
04:58Ron Blakey is a professor of geology at Northern Arizona University in Flagstaff.
05:04He studies ancient continents, and he discovered that California wasn't always America's west coast.
05:11It's very likely that the coastline of North America at this period of time was just south of Wyoming,
05:19perhaps along the Wyoming-Colorado border, and it extended up into the Lake Superior region and then up into Canada.
05:28So I would say that perhaps only half to two-thirds of North America had been built at that period
05:34of time.
05:42The Vishnu Mountains stretched across a very different landscape.
05:47The south of North America did not exist as we know it today.
05:53The mountains towered up out of an ocean along the ancient coast.
05:58They eventually became part of the North American continent and formed the basement rocks of the Grand Canyon.
06:05I imagine it looked like a bit of a train wreck where each of these island chains was perched offshore.
06:14A series of island chains that had substantial mountains but fairly high peaks like in the Aleutians.
06:24Nine and a half kilometres beneath the summits of these mountainous islands,
06:28the rocks were developing that distinctive calcium signature.
06:33The mountains above didn't last long.
06:36In around 500 million years, erosion ground them down until all that was left was this at the bottom,
06:43the Grand Canyon's most ancient lair.
06:48As we rise up the side of the canyon, we travel forward in time.
06:53Every metre takes us closer to the present day, and each period has its own history.
07:00Over hundreds of millions of years, the land masses continually change shape.
07:09Forces deep inside the earth cause continents to break apart, drift away, and form new ones.
07:17Deep trenches open up beneath the ocean.
07:20The sea level changes, and low-lying areas flood.
07:27At least eight separate seas have flooded the area since erosion cut down the Vishnu Mountains.
07:34They covered the land, stayed for a few million years, and retreated again.
07:41The evidence is here, around 750 metres above the modern Colorado River, and 1,600 metres above present-day sea
07:50level.
07:52These are the remains of oyster beds that flourished here nearly 100 million years ago.
08:00The Cretaceous Sea stretched from Kansas in the east, to Nevada in the west.
08:05From Mexico in the south, to the Arctic in the north.
08:08It was a shallow seaway that covered much of the continent.
08:13Sea creatures sank to the bottom of the ocean, and fossilised in a thick grey layer of mud.
08:20Scientists call it the Tropic Shale.
08:25Wayne Ranney is fascinated by this region, rich in fossils.
08:29It gives him a clear idea of the area's history.
08:33This oyster bed, which contains millions of oysters, was from a sea that was a shallow sea
08:38that covered the interior of North America between about 80 and 90 million years ago.
08:44Oysters weren't the only creatures living in this ancient sea.
08:48Life was abundant.
08:49Sharks, turtles and ammonites also inhabited the waters.
08:55Today, as erosion slowly wears away the layers of rock, new fossils appear.
09:01And in 2005, just over 100 kilometres away from the edge of the canyon,
09:09paleontologists found the well-preserved skeleton of a plesiosaur,
09:13a dinosaur-like fish-eating reptile.
09:16For David Gillette, it was a dream come true.
09:19He is a curator of paleontology at the Museum of Northern Arizona in Flagstaff.
09:25For a vertebrate paleontologist like myself, finding plesiosaur skeletons like we're finding in the Tropic Shale
09:32is almost a once-in-a-lifetime experience.
09:35The one in front of me was a plesiosaur that was about half complete.
09:40We found the front half of the body.
09:42And that's more exciting almost than anything I can imagine.
09:47Plesiosaur were seagoing reptiles.
09:49They were last seen about 65 million years ago,
09:53when a catastrophic mass extinction hit the Earth.
09:58Gillette's plesiosaur dates back much earlier.
10:02It lived around 90 million years ago, clear proof that this area was under the sea at that time.
10:11The seas left deposits of mud, sand, and the remains of living creatures.
10:17Sea currents moved this material, which then settled.
10:22And over time, the deposits turned into layers of sedimentary rock.
10:27Ron Blakey explains.
10:29What I'm going to show you now is a form of sedimentation.
10:33I have here a glass cylinder that's filled with water and sediment, mostly sand.
10:38And what I'm going to do is shake this up.
10:40And the shaking up provides energy that allows the sand grains to move around this cylinder of water.
10:47And then when I set it down, that energy is removed and the grains will immediately settle out.
10:53Let's see what happens.
10:59As you can see, the coarsest grains have already settled out.
11:03And finer grains and finer grains are settling.
11:06And now most of the sand is out.
11:08The water is still cloudy because there's some fine material still in suspension within this cylinder.
11:13But all the energy of the system has been removed.
11:16And so now even the finest grains over time will settle out to form a layer of sediment.
11:24This process demonstrates how sedimentary layers built the Grand Canyon.
11:29Over the course of time, the glass cylinder turned many times.
11:33And when the seas come and go, this is all that remains.
11:37Millions upon millions of years worth of sediment compressed over time to form layers of rock.
11:43The building blocks of the canyon.
11:45But powerful forces were building deep underground.
11:49Forces that would thrust these rocks one and a half kilometers into the...
11:56We are on a journey to discover the geological processes that created the Grand Canyon.
12:02We're in the Cretaceous period, where plesiosaurs hunted in the shallow seas that covered the area,
12:09near to where the Grand Canyon is now.
12:11Seas flooded and retreated over this area for millions of years,
12:15overlaying the ancient Vishnu mountains with three kilometers of sedimentary rock.
12:23Half of that has since eroded away to leave today's canyon.
12:28One and a half kilometers deep.
12:31The next major transformation will push the land hundreds of meters into the air.
12:37Deep beneath the Earth, a monumental force builds up.
12:40It's one of the greatest forces on Earth.
12:44The gigantic power of colliding tectonic plates.
12:54Over millions of years, the planet's surface changes.
13:00Scientists explain the process using the theory of plate tectonics.
13:05The surface of the Earth is made up of a series of large plates.
13:10They float about on the soft plastic part of the Earth's mantle.
13:15The inner core of the Earth may reach over 6,650 degrees Celsius.
13:22Heat escaping from the core creates convection currents in the next layer, the lower mantle,
13:28pushing the plate slowly across the surface.
13:32Scientists call it continental drift.
13:34Over time, whole plates move apart and crash back into each other, shaping and reshaping our world.
13:44Scientists believe that these colliding tectonic plates were instrumental in creating the Grand Canyon.
13:52Around 130 million years ago, the oceanic plate began to collide into the western edge of the North American plate.
14:00Oceanic plates are 50 to 100 kilometers thick and lie beneath the sea.
14:06They consist of dense basalt and get pushed down.
14:13Continental plates are mostly granite and lighter.
14:16Oceanic plates subduct underneath them when they collide.
14:21So when the oceanic plate hit the lighter North American plate, the oceanic plate moved under the west coast of
14:27America.
14:31Over tens of millions of years, the continental crust compressed and thickened.
14:38As the oceanic plate thrust downwards, the whole of the west of America lifted slowly upward.
14:46As the land rose, the sea retreated, creating the landmass we see today.
14:54The collision dragged down portions of the leading edge of the continental crust.
14:58Beneath the surface, it melted and liquid rock pushed up through the crust.
15:05The rocky mountains formed in the north.
15:08In the south, a range of ancient mountains, the Mogollon Highlands grew higher.
15:13The area that today is the Colorado Plateau rose about one and a half kilometers, lifting the ancient sea floor
15:21to where it is today.
15:222,100 meters above the present sea level.
15:29So what do we see when we look in the Grand Canyon?
15:33When we have marine rocks now exposed at 7,000 feet above sea level, there's a big question here.
15:40Did the sea fall that far or was the land uplifted that far?
15:44Well, clearly, after the marine rocks were deposited in the sea that formed the rim of the Grand Canyon, the
15:52area had to be uplifted.
15:56The region of the Grand Canyon was now high above sea level.
16:00But at this point, it was an unbroken plain.
16:04There was no canyon.
16:08Making the canyon took another force of nature, erosion.
16:14Even as the land rose up, a river flowed across the plain and began to cut down through the rocks.
16:21Here we have a colorful layer cake, which represents the rock formations on the Colorado Plateau in the Grand Canyon
16:27region.
16:28And the river itself is represented by this knife.
16:31Now, as the Colorado Plateau is uplifted, the river chisels its way down through and actually makes a cut.
16:39Which is, whoa, I made a canyon.
16:4370 million years ago, water and snow melt flowed from the Mogollon Highlands onto the surface of the future Colorado
16:50Plateau.
16:52They began slowly cutting into the plateau area.
16:57Ron Blakey demonstrates how flowing water can cut through layers of sedimentary rock to make a canyon.
17:03When I cut a hole in the side of the tub, we're going to see the sediment is going to
17:09be eroded.
17:10Let's see if this works.
17:13There you can see the erosion taking place.
17:16As I have lowered the base level by cutting apart the plastic tub,
17:22you can see the water cutting down through the sediment and exposing it.
17:27So if erosion cut through the rock, the next part of the puzzle is to find the ancient river that
17:33created the canyon.
17:38Some of the old riverbeds are still visible, but they pose more questions than they answer.
17:46Richard Young, professor of geology at the State University of New York,
17:50has been trying to solve the riddle of the old riverbeds since he was a student.
17:56Young pieces together the history of rivers by looking at the shape and composition of rocks found on riverbeds.
18:03The modern Colorado River flows from the Rocky Mountains in the east to the Gulf of California in the southwest.
18:11But as Young investigated the ancient canyons, he found that something just didn't add up.
18:17It was a shock for a student who just is getting started and you want to believe what your professors
18:21are telling you,
18:22but everything you see out in the field says that something very different is happening.
18:31To us, these look like large pebbles. To geologists, they're a missing link.
18:37We see two things. We see how round they are, which indicates how far they've come.
18:41And secondly, the rock types are completely far into this area and they come from at least 100 miles away
18:46from here.
18:48The old stream pebbles are not native to the region where Young found them.
18:53They came from around 100 miles southwest of the plateau.
18:58The old river must have transported the pebbles from central Arizona north and west to the plateau region.
19:05The orientation of the pebbles in the riverbed shows the direction of the water flow.
19:12Rivers cause pebbles to tumble in the river channels.
19:15They move until they reach this position with the current flowing this way.
19:19And that's a stable position just like the shingles on a roof.
19:22And then that tells us that the river was flowing in that direction.
19:26As the ancient pebbles settled on top of each other, they left a lasting record of how the river flowed.
19:33This was strong evidence that the rivers had to have flowed northeast, the opposite direction to today.
19:41But not only did the water flow in the opposite direction, it flowed in a different canyon six kilometers south
19:48of western Grand Canyon.
19:51The old channel is now empty.
19:54What happened?
19:58Deep within the Earth's core, powerful forces were about to shatter the surface of what would become the Grand Canyon.
20:09About 20 million years ago, the oceanic plate began moving north relative to the North American plate.
20:22Heat built up under the Mogollon Highlands southwest of the canyon.
20:33The Earth's crust began stretching.
20:39Over millions of years, vast areas of land deformed and cracked, sheared upward or snapped down.
20:47The Mogollon Highlands fell hundreds of meters. Huge basins appeared on low-lying land.
20:55Today, geologists call it the Basin and Range province.
20:59Wayne Ranney uses dominoes to explain.
21:02Once faulting begins, we tilt all the fault blocks down on individual faults,
21:07and you can see the surface tilting back into the Colorado Plateau edge.
21:11Rock layers that were once horizontal are now almost vertical.
21:16The region tilted and then collapsed.
21:20Ancient history is written in the orientation of every outcrop.
21:24The collapse redrew the topography of the region.
21:28Before, the highlands to the south towered many hundreds of meters over the Colorado Plateau.
21:34After the collapse, the basin and range area was almost 8,000 meters lower.
21:41It was a major piece in the scientific puzzle.
21:46The rivers may have changed direction because the whole landscape changed.
21:51When the basin and range faulting occurred, it dropped that area.
21:55It made it actually lower than the Colorado Plateau.
21:58And that's the reason that the rivers were able to change direction and flow in the opposite direction toward California.
22:05Scientists think they have solved the mystery of how the rivers reversed.
22:09But they were six kilometers away from where the canyon is today.
22:13These ancient rivers did carve canyons, but not where today's canyon is,
22:18and on nothing like the same scale.
22:23Sometime after the collapse, a new river formed.
22:26The Colorado River.
22:28And it took a new course.
22:30But when, and why?
22:32The challenge was to find the earliest sign of this river.
22:38Surprisingly, a clue came from outside the Grand Canyon.
22:41In the lower Colorado River, about 160 kilometers before the river enters the Gulf of California,
22:48scientists discovered a tiny fossil.
22:52Kristin McDougall, research geologist with the U.S. Geological Survey, has studied these fossils.
22:59They're called foraminifera.
23:01Foraminifera are like specks of dust.
23:05It's a microscopic seashell.
23:07The Cretaceous, um, planktic foraminifera that we find look like three golf balls hooked together.
23:17Foraminifera are tiny.
23:19A hundred of them could fit on a grain of salt.
23:22But they could show when the river began cutting the canyon.
23:27The foraminifera were not native to the lower Colorado River.
23:30Their home was much further north.
23:33The only way their fossil remains could have been deposited in sediments in the lower Colorado River
23:38is when the river transported them down.
23:42They were originally deposited in Cretaceous shales on the Colorado Plateau north of the Grand Canyon.
23:51And they were eroded and brought down along the modern course of the Colorado River and redeposited.
24:04Dating the sediments where the fossils were redeposited helps to solve the mystery.
24:10The sediments seemed to have formed when the river took its present course.
24:14And geologically speaking, it happened just a short time ago.
24:18These Cretaceous fossils indicate that the Colorado River became a through-flowing river by at least four and a half
24:27million years ago.
24:29That's a very exciting find.
24:32Evidence indicates that four and a half million years ago, the river flowed along its present course from the northeast
24:38to the southwest.
24:40Scientists thought they had solved the mystery of the age of the river.
24:44But the solution raised a new question.
24:47They knew when the river had formed, but they were still not certain how.
24:55Most rivers begin energetically at their source.
24:58They rip through the terrain downward, eroding anything in their path.
25:04Downstream, things seem calmer.
25:06The water moves slower and the channel is wider.
25:12But back near the source, the river lengthens its channel upstream as the rushing waters erode more rock.
25:19They cut their channel headward.
25:23One theory is that the Colorado River got started this way.
25:28This headward erosion usually happens where the steepness of a riverbed rises.
25:32It gives the river more energy to cut deeper into the rocks.
25:37The steeper the gradient, the more rainwater channels into the river and the faster it pulls down the slope.
25:44The river eats into soil and rock at the top and may also lengthen its channel headwards.
25:51Could headward erosion have been the mechanism that made the Colorado River head out across the plateau?
25:58We know that the Colorado Plateau rose about a kilometer and a half above sea level 70 million years ago,
26:04creating a steep slope towards the present-day Gulf of California.
26:09It's a clever explanation, but it's controversial.
26:14The modern Colorado River is only around four and a half million years old, and the plateau rose 65 and
26:21a half million years earlier.
26:23Perhaps there was another, more recent uplift that would explain how the river crossed the plateau.
26:30Searching for clues, scientists discovered a layer of rock downstream in the lower part of the region called the Bouse
26:37Formation.
26:40These rocks are around six million years old, and some parts are nearly 610 meters above sea level.
26:48When scientists took a closer look and discovered marine fossils in the rock, they knew they were onto something extraordinary.
26:56There are three key fossils that we find in the Bouse Formation.
27:01The first is a planktic forminifer, which is one of the ones that floats on the surface of the water.
27:06And then there's a specimen called pneumonia, and it's a very shallow water form.
27:12And then there's a third kind that lives much deeper in the ocean.
27:17The fossils meant the rock layer is likely to have formed in seawater.
27:22For Dr. Ivo Lukita, the fossils led to a new theory of what happened here.
27:28He believes that when the Gulf of California first opened up, it extended about 160 kilometers further north,
27:36than it does today.
27:38Dead marine organisms sank to the seabed, and became entombed in the white sedimentary deposit, the Bouse Formation.
27:46Yet today, the fossils are many meters above sea level.
27:50That was the clue that he had been searching for.
27:52One day, by looking at the present distribution and elevation of remnants of the Bouse,
27:58it occurred to me that the Bouse and the land with it had been uplifted as much as perhaps 1
28:02,700 feet in the last 5 million years.
28:08He believes that this area was uplifted to its current elevation after the fossils were deposited around 6 million years
28:16ago.
28:17And this second uplift created an ideal scenario for headwood erosion.
28:23By uplifting the plateau, you basically steepen the gradient of the rivers that emptied into the Gulf,
28:30and therefore allow them to erode more rapidly.
28:34The trouble is, not everyone agrees.
28:38The apparent existence of marine fossils in the Bouse Formation 300 meters above sea level
28:43is not irrefutable evidence that the area was uplifted.
28:47Senior geologist John Spencer from the Arizona Geological Survey is not convinced about the second uplift theory.
28:55I just was uneasy with the idea that the Earth would just go up and down like that.
28:59I called it trampoline tectonics.
29:01I didn't believe in trampoline tectonics.
29:05Spencer and his team examine the Bouse rocks in the lab.
29:09They're especially interested in the levels of the strontium isotopes.
29:15Strontium isotopes found in these rocks are irregular atoms undetectable to conventional microscopes.
29:21They come in heavier and lighter forms.
29:25The relative concentration of heavy and light forms varies.
29:29It depends on the origin of the water.
29:32River water usually has a different strontium isotope concentration than ocean water.
29:40Depending on the concentration, scientists can often determine if sediment was formed in freshwater or in the sea.
29:50Spencer's graph of the isotope concentration of the Bouse Formation
29:54confirms his suspicions.
29:57The green dots represent isotopic concentrations of seawater.
30:01The blue dots represent ratios typical for river water.
30:06The red dots are his readings from the Bouse Formation.
30:09They indicate fresh water.
30:12When we got our first results, it was immediately clear that we were seeing no marine signature in any of
30:20the Bouse samples.
30:21We were a little surprised.
30:23We didn't expect that every number would come out looking like river water.
30:29These results question the validity of the second more recent uplift theory.
30:37If the Bouse Formation was never under the sea, the uplift theory couldn't apply, and headward erosion along the lower
30:43Colorado could not have happened.
30:48Scientists had to look for another model explaining how the river cut across the plateau.
30:54About 400 kilometers west of Grand Canyon Village, there's a canyon system called the Lake Mannix Basin.
31:02Norman Meek, Professor of Geography at California State San Bernardino, believes this area may offer a clue of how its
31:10bigger cousin got started.
31:12The Lake Mannix Basin has sometimes been called the Grand Canyon of the Mojave River because of the spectacular landscape
31:18that's portrayed here, a miniature version of the Grand Canyon.
31:23The side of Lake Mannix Basin is connected to Silver Soda Lake via a canyon.
31:29Meek wondered if the formation of this canyon could throw light on the formation of the Grand Canyon.
31:34First, he examined the site of Lake Mannix.
31:38Lake Mannix is an extinct desert lake.
31:41It used to stretch over an area of more than 230 square kilometers.
31:45Yet, many thousands of years ago, this lake disappeared.
31:50Meek wanted to find out when and why the lake drained.
31:54At the edge of Lake Mannix, he found sediments of an ancient shoreline from the old lake.
31:59In the deposits, he found freshwater shells.
32:04He took them to a radiocarbon dating lab to find out their age.
32:09Radiocarbon is an isotope that occurs in all living creatures.
32:13Its half-life is almost 6,000 years, which means after around 6,000 years, half of the original amount
32:20is gone.
32:21By measuring how much radiocarbon is left in the shells, scientists can determine their age.
32:30Meek found that the shellfish in the Mannix Basin were alive 19,000 years ago, but after that, they died.
32:38At this point, Lake Mannix emptied.
32:41When he looked at Silver Soda Lake, he found that it existed from around the time when Lake Mannix emptied.
32:49Could these two lakes be connected?
32:54Meek suggested the canyon here was cut by a mechanism called spillover.
32:59He theorized that 19,000 years ago, heavy rainfall or snow melts filled up the lake until finally the water
33:07spilled over the rim.
33:09The lake drained dramatically.
33:12Over a few thousand years, it cut down through the rock, carved a canyon, and then created Silver Soda Lake.
33:18It's just like a reservoir breaking. There's a lot of energy. That energy rips away the sediments and the bedrock
33:27that make up the canyon, and you get a through-flowing river.
33:32Scientists speculated that if spillover explained how a canyon formed in the Mojave Desert, it could also explain how the
33:39Grand Canyon formed just 400 kilometers to the east.
33:43Now, they just had to find the lake.
33:48Today, there is no lake, but there are 60 meters of green-layered deposits, remnants of an ancient lake called
33:56Lake Bidahochi.
33:57And so this green layer right here reflects the idea that Lake Bidahochi was in existence east of the Grand
34:03Canyon between 16 and 6 million years ago.
34:06Over 10 million years, a lake deposited fine-grained sediments like silt, clay and sand mixed with gray volcanic ash.
34:15The deposits indicate the existence of a lake, but they don't show how the lake spilled over and carved the
34:22canyon.
34:23John Douglas, geography professor at Paradise Valley Community College in Arizona, wanted an effective way to test the theory.
34:31He built a small-scale model of the Grand Canyon to demonstrate how the lake could have overflowed.
34:38Six million years ago, the canyon wasn't yet carved.
34:42But Lake Bidahochi was full to the point of overflow.
34:46And then as the lake came in, the lake filled.
34:49It filled and filled higher and higher and higher until it eventually spilled across the lowest point, across the Kaibai
34:55Plateau.
34:57And then water poured down the other side.
34:59Those waterfalls continued to retreat back, and the whole time you're cutting Grand Canyon.
35:05Douglas suggests that Lake Bidahochi drained and formed a stream.
35:11The steep gradient of the riverbed caused the stream to cut deep into the rock.
35:17Over time, the size of the stream increased, and more and more water poured out of the lake, which in
35:23turn cut deeper and deeper into the rock.
35:28But not all geologists agree.
35:31They say there is no evidence of there ever being a lake large enough to carve the Grand Canyon.
35:37So at present, there's still no definitive answer as to how the river came into existence.
35:45What we do know is that when it began flowing around four and a half million years ago, it cut
35:50an 800 meter deep scar in the surface.
35:55They cut that whole canyon in five million years is a considerable feat.
36:00How did the Colorado River manage to cut a canyon?
36:04There are bigger rivers in the United States which haven't cut so deep.
36:10The biggest and longest river in North America is not the Colorado River, but the Mississippi.
36:16It flows nearly 4,000 kilometers from its source in Minnesota to its mouth at the Gulf of Mexico.
36:25It carries ten times more water than the Colorado.
36:29Yet the Mississippi did not carve a Grand Canyon.
36:35Evidently, volume of water is not the only factor.
36:38Something else gave the Colorado River its enormous cutting power.
36:46The Colorado River begins in the Rocky Mountains 2,700 meters above sea level.
36:53On its course, it drops three meters for every 800 meters it flows.
36:59The Mississippi, on the other hand, begins its journey in Minnesota, almost 450 meters above sea level, dropping less than
37:0830 centimeters every 800 meters.
37:11Although the Mississippi has ten times more water, it is a less erosive river because it has a lower gradient.
37:19All rivers want to run downhill. It's the golden rule of gravity.
37:25Two rivers can have the same amount of water, but for every one degree of extra gradient, the steeper river
37:32gets a 1% increase in power.
37:36My observations have been that this river, and even some of its little tributary streams, have enough power that they
37:44cut through any kind of rock like butter.
37:46The speed and depth of cutting also depend on the rock it's carving into.
37:53Hard rock like granite is much tougher, so the river cuts a narrow gorge.
38:00Soft rock like shale is easy for water to cut through, so the river carves a broad canyon.
38:09With soft rock, as the sides erode away, the canyon widens.
38:15Joel Pedersen, geomorphologist at Utah State University, is on a quest to discover how the river cut away the canyon
38:23walls.
38:26The information that we collect from this, what we're doing is relating it to the history of the river.
38:30And so what the river is doing is, in the past, as it's eroding, it is also stopping sometimes and
38:36depositing sediment.
38:37Pedersen is interested in finding out how fast the canyon erodes.
38:43He uses a technique called luminescence dating.
38:48This uses light to measure the age of a rock.
38:51It works on sediments containing minerals such as quartz or feldspar that have been buried in the dark.
38:58As sediment builds up, it's gradually sealed away from the sun.
39:04Pedersen measures the amount of luminescence a sample gives off when exposed to light.
39:10This method can date sediments between a few hundred to several hundred thousand years old.
39:15We're collecting sediment that hasn't yet seen light, and the metal tube prevents it from seeing light.
39:21And we pull the tube out and take it back to the laboratory.
39:26In the laboratory, scientists expose the samples of ancient river sediments to specific wavelengths of light.
39:33This energy excites the electrons in the sediment.
39:36Some respond by releasing their own energy in the form of light, and this minute signal is measured.
39:43The greater the response from the particles, the longer they spent buried.
39:50So it's telling us that moment in time, how many years ago it was, that the river actually deposited that
39:56sediment,
39:57and then buried it under more sediment so it couldn't see light anymore.
40:05When Pedersen measured the incision rates along the whole length of the river, he found something unexpected.
40:12Two hundred and ninety kilometers downstream, the canyon appeared less deep.
40:19Something changed and lessened the river's cutting power.
40:23Could it be that the river lost its power due to changes beneath the land itself?
40:29It was an intriguing puzzle scientists were desperate to solve.
40:353.5 million years ago, the Colorado Plateau ruptured and created a deep chasm across the canyon.
40:43It's called the Toruweep Fault.
40:46Tectonic activity along the fault slowly lowered the land of the plateau to the west,
40:51and as the land fell away, it weakened the cutting power of the river flowing over it.
40:57Karl Karlström explains with a loaf of bread.
41:01We'll imagine this loaf of bread as a stack of rocks that is going to be incised by the river
41:08to be the Grand Canyon,
41:10and we'll imagine this knife as the Colorado River itself,
41:15which over time can erode through even the hardest rocks to form a canyon.
41:20But many times there's a break in the crust called a fault,
41:24and so if I incise the river at the same time that I'm moving one block down,
41:31and I think you can see this if I turn it up like this,
41:34and you can see that on one side the canyon is quite deep, and on the other much less deep.
41:42Scientists had their answer.
41:45The river stayed the same, but the land west of the Toruweep Fault sank lower,
41:50and so escaped the full cutting force of the water.
41:54The Colorado River has cut a masterpiece of a canyon,
41:57but water doesn't only destroy rock.
42:00Amazingly, it can also create it.
42:05Scientists are intrigued by new rocks within the canyon that are still growing.
42:14The Colorado River has been a great destroyer.
42:18In the last four and a half million years, it has excavated over 3,335 cubic kilometers of rock.
42:27It created a gorge so big that all of the river water in the world would fill the canyon to
42:33just over a third.
42:36But the river can also create rocks.
42:39At certain points in the canyon, scientists have discovered new rocks forming today.
42:48Laura Crossy, professor at the University of New Mexico, is an expert on these rocks, the youngest in the canyon.
42:55The big story of Grand Canyon is the carving of this beautiful place.
42:59There are other processes such as travertine that cause changes to occur,
43:03and younger rocks to form inside that very canyon.
43:09These travertines are special because they're created from water saturated with carbon dioxide,
43:15and they actually form at the bottom of the canyon itself.
43:20The carbon dioxide comes from deep inside the earth.
43:24It leaks out to the surface.
43:29Here, it dissolves in water, acidifying it and giving it the power to dissolve limestone.
43:38The carbon dioxide escapes from the water forming bubbles like fizzy soda.
43:44As the gas escapes, it leaves behind calcium carbonate,
43:49which coats objects in its path.
43:52In some places, it forms stone waterfalls.
43:56In others, it covers sticks and twigs.
44:00These are just small examples of the coatings of travertine that form on every stick and twig and cobble that
44:07comes down the stream bed.
44:08It accumulates into vast piles of sediment which are instantly formed into rock, the youngest rocks in Grand Canyon.
44:18Every drop in the river has in some ways shaped the landscape here.
44:23But water does far more than destroy rocks and build them.
44:28Without the river, the current plant and animal life would struggle to survive.
44:32And it was the river which attracted early settlers here.
44:37When the ancestors of the Hualapai arrived in the Grand Canyon about 700 years ago,
44:42they celebrated the Colorado River as an important source for life.
44:47And it remains that today.
44:50The river's all source of life up here.
44:53The canyon, to me personally, is something that our ancestors, they fought hard to retain because they knew the importance
45:01of it.
45:03Our wish is to maintain it in its natural state or its natural beauty.
45:10Modern visitors to the canyon are as mesmerized as the first Native Americans would have been several hundred years ago.
45:18It is one of the natural wonders of the world.
45:23A monument to the age-old colossal forces that created it.
45:341.7 thousand million years ago, it began.
45:40Layers of flat-lying stratified sediments build up high canyon walls.
45:46Uplift elevates the plateau, and rivers cut deep into the rocks.
45:52And the climate today is arid. Nothing is lost. Everything is visible.
45:59It's difficult to find a canyon as impressive as this one.
46:04Possibly in south-eastern Tibet, home of the biggest, longest, and deepest canyon.
46:10The Yalung-Sangpo Great Canyon.
46:15The climate is different, and the area is covered with dense vegetation.
46:20The Euphrates is a very large river running through arid Iraq, a climate not dissimilar to Arizona.
46:27But this area never uplifted and formed a canyon.
46:32The Grand Canyon might not be the biggest and longest canyon on Earth.
46:38But the powerful forces that created it and made it unique are still hard at work.
46:45Nobody can predict how much uplift there will be.
46:48But if the river stays on course, then the future of the Grand Canyon looks good.
46:55In another million years or two, the canyon might be even deeper, with walls just as sheer as they are
47:02today.
47:04The Grand Canyon is getting grander.
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