🔴Il Grande Pianeta - L'era Glaciale La Scienza prima della #SCIENZAH

Good evening everyone. Who can tell us, if Aldous Huxley once asked, if the Earth was or isn't the hell of some other planet? Well, I don't want to trivialize the joke, but a fact is certain, that from a physical point of view, in the long history of the Earth, made of millions of years, the Earth has really been a great hell. A hell of heat, of great energies, of fire and flames at its origin. And then, in the last two million years, at least seven times, scientists tell us, a hell of ice, of cold. By the way, have you noticed how, for a tradition that dates back to the Bible, it's much easier to talk about a hellish heat than a hellish cold? However, great artists, great poets, those who sometimes manage to break the habits, the common places of people's imagination, have managed to imagine a real hell of ice. Yes, both Dante and Milton, the Englishman Milton, when they wanted to represent the deepest and most terrible part of the infernal afterlife, the one where the most cruel, most serious, most tremendous sinners are punished, like the traitors of friends, the traitors of relatives, for example in Dante, well, they imagined a hell of ice. Cooked just for Dante, it's a big frozen lake covered by a very thick layer. So thick, Dante says, that even if you fell on top of the mountains, you would scream, that is, you would scream. Well, in 1837, a Swiss scientist, Louis Agassiz, has managed to imagine, scientifically, a real hell of ice on Earth. Taking inspiration from the glaciers that are still seen in Europe today, he imagined that they were the residue of large glacial eras due to the sudden cooling of the Earth. Then, it happened for still mysterious reasons. A hypothesis that was much contested, aroused much controversy at the beginning, and only after long research, work on the field of geologists, who showed how these great ice languages once had flattened, filed the entire walls of the mountains, well, this hypothesis managed to convince. Today's film of the series The Great Planet, makes us go through many of those interrogations of Agassiz, and also his first observations and his answers to the problem of glacial eras. Still 10,000 years ago, a third of the Earth's surface was covered by ice, but the last glacial period was about to end. In North America there are still traces evident of the last glacial era, which testify to the strength of nature. Here, in this immense field of grain, in Alberta, in Canada, this lonely rock is erected, absolutely out of place in a similar landscape. This 10-meter-high megalith, which weighs 20,000 tons, is the same type of rock found in the rocky mountains, more than 500 kilometers away. The people of the place called this rock erratic mass. Here we are in Central Park, in New York, a park of entertainment for the inhabitants, and the gathering place of many of these erratic masses, here among the skyscrapers of the Great Apple. The composition of these stones is completely different from that of the rocks found in the subsurface of New York. How did they get here? What force can have transported them so far? Around the middle of the 19th century, when the park was under construction, and these stones were found, it was thought that they must have been transported by a flood, like the Great Universal Flood described in the Bible. After the end of the century, however, scientists began to think that the rocks did not come from a flood of water, but from a flood of ice, glaciation. 20,000 years ago, a handful of seconds compared to the long geological history of our planet, the city of New York was buried under a glacier, often 100 meters thick. What caused the glaciation? Could such an event repeat itself? These are two questions to which we will try to give an answer in this episode. This is how the North American continent must have appeared during the Ice Age. The central part of the ice cove reached a thickness of 4,000 meters. It is believed that the thickness of the glaciers gradually decreased towards the edges of the continent. The glaciers could easily cover up to 40 degrees of latitude in the north, that is, those areas where New York and Chicago now arise. Almost at the same time, a large-scale glaciation advanced in the Euro-Asian continent, finally covering a third of the land that emerged. Large glaciations of these proportions did not occur only once. It has been ascertained that there were no less than seven glaciations in the last million years. The one that transported the huge rocks in the Central Park of New York, probably, began about 115 million years ago and ended about 10,000 years ago. What was the world like when its surface was covered by glaciers for more than 100,000 years? To give a partial answer to this question, we went where these glaciations are still active. We are in Groenlandia, in the north of the Atlantic. Groenlandia extends for 1,000 kilometers from east to west and 2,500 kilometers from north to south. Most of its surface is covered by a thick layer of ice. The few people who live here survive thanks to the fishing practiced during the short summer, when the sea is not frozen. The waters are very fishy. There are a large number of aragosts and seagulls. The thick glaciers that cover Groenlandia, when they reach the sea, fragment and produce numerous icebergs. We have explored the innermost area of this glacier, which gives rise to the icebergs. It has been calculated that the amount of ice that covers Groenlandia reaches the appalling weight of about 2.5 million billion tons. If it were to melt, the level of the oceans around the world would rise by 5 meters, flooding many coastal areas. These spectacular waterfalls, fed by the melting snow, have created a basin in the glacier. This beautiful lake has also been created by the melting snow. The light reflected by the white frozen bottom gives the lake a very particular color, a bright blue. The land of Groenlandia is located more than 1,500 meters below the thick layer of ice. 20,000 years ago, the entire northern part of the land had to look like this. How does the ice form on the surface of Groenlandia? Let's take a look at this computerized projection. The ice completely covers the island, with the exception of areas limited along the coastline. The highest point of the island is 3,300 meters above sea level. This projection, based on the information collected by the Institute for Geological Research of Groenlandia, shows that the surface of the island, the land below the glaciers, has surprising characteristics. A third of the island's surface is below sea level. The lowest point is even 300 meters below the level of the Atlantic Ocean. The land has undergone a downward pressure and has re-entered until it takes on this squiggly shape caused by the weight of this huge amount of ice that covers it. We were curious to know how the accumulation of such a large amount of ice was possible. In search of an answer, we arrived here, in Juneau, the capital of Alaska, in its narrow southern peninsula. A glacier reaches up to the most peripheral areas of the city, so frequent research on glaciers is carried out here, with the feeling of having to do it quickly. To find out how an ice mantle is formed, we visited an observation station where the conditions of the ice mantle are studied. The journey to the Mendenhall glacier led us to a vast snowfield. Juneau's ice mantle is at an altitude of 1,600 meters. A huge amount of snow has accumulated on this snowfield, 50 kilometers wide and 100 kilometers long. In this frozen landscape, you can see sharp peaks of mountains everywhere. They are a particular topographic feature of the glacial regions that scientists call Nunn attack. At the foot of these peaks is the camp where we were directed. In this prohibitive but splendid environment, scientists from various parts of the world, Americans, Canadians, Germans, are trying to unravel the mysteries of glaciers. Deep holes are dug into the ice to gather information on the amount of snow that accumulates annually and what changes it undergoes in the process of ice transformation. Let's listen to Professor Maynard Miller from the University of Idaho. We are looking at the wall of a test tube dug into the upper part of Juneau's ice bank, where the glacier is formed. Here, the annual accumulation exceeds 6 meters and includes not only the snow, but also the layers of ice that can be seen here between a wide section of old snow and another material called granulated. Granulated is the transition material from snow to ice resulting from the compression of the layer of snow produced by its own weight and the cooling caused by the dissolved water that filters to great depths. This metamorphosis is a very important aspect for the growth of a glacier. I am standing on the surface of last summer. Below me, there is 294 meters of ice. How does the gradual transformation of snow into ice occur? This phenomenon is connected to the depth, or rather, to the amount of snow that has accumulated above. On the walls of a crevice in the granulated, horizontal stripes are visible. They graphically mark the borders of the annual layer of snow. Between each strip, there is the snow that fell that year, now compressed in the granulated. Going down further, we can see how the snow changes in relation to the depth. This crevice is 25 meters deep. Part of the granulated on the bottom has already been transformed into ice. Now we are observing the snow that fell 12 years ago. The snow turns into ice due to the compression, but even the ice undergoes transformations due to the same compression. To examine what these transformations of ice are, we use polarizing plates. The red, blue and green particles are ice crystals. The diameter of an ice crystal at a depth of 15 meters is 2 millimeters. The black particles are air sacs that form large interstices between the ice crystals. At a depth of 30 meters, the ice undergoes a pressure of 30 tons per square meter. Because of this pressure, each ice crystal enlarges to a diameter of about 5 millimeters, while the air sacs become smaller. At a depth of 50 meters, the ice is subjected to a greater pressure that enlarges the crystals even more. When these become larger, the ice tends to slide in a certain direction because its molecular structure is modified. It is for this characteristic that the ice flows forward, giving rise to the movement of the glacier. The accumulation of snow is well observed in these images taken from the Landsat satellite. When summer arrives, the snow begins to melt in the flat areas and then gradually disappears. At an altitude of 1,000 meters, the snow does not melt and it is this snow that turns into ice. About 30 glaciers have originated here. The blue mass, which is seen in the center of the photo, is the Mendenhall Glacier. It descends along the slope, which has a slope of 1,000 meters. During its descent, a certain amount of ice melts and forms an ice lake at its feet. This strange hill on the edge of the ice lake was formed by the glacier. It is called Morena and was produced by the glacier that deposited its load of earth, sand and rocks on its front edge. How was all this material transported by the glacier? We have climbed the Morena, proceeding through the crevasses, some of which are more than 30 meters deep. The entire side of a mountain must have been swept away by the glacier. These stones were slowly brought to the front edge of the moving glacier and gave rise to a Morena. We found this extraordinarily large mass 10 kilometers from the top of the glacier. The Mendenhall Glacier flows at a speed of one meter per day. This means that this gigantic mass will reach the edge of the glacier in about 30 years. Those strange masses found in the Central Park of New York or those found in the Grano field in Canada must have been transported far from their original site in the same way. Towards the end of the 1960s, scientists from the University of Washington in the United States filmed the movements of the glaciers for five months. With these photographs taken over long periods of time, it is clearly seen how the huge masses of ice are continuously in motion, proceeding like a river, sometimes quickly, sometimes more slowly. But the most interesting thing was that the group of the University of Washington managed to film the movements of the bottom of a glacier. To make this video, they dug a tunnel two meters in diameter on one side of a glacier often 100 meters thick. From there they were able to examine the movement of the ice inside the glacier. On the left, below the frame, you can see the black rock at the base. Here it flows slowly, sliding along the rocky base. The glacier, which contains many rocks, behaves like a gigantic crack, levitating the rocky base as it crosses it. Non attac. These sharp pinnacles of rock that rise on Junot's glacier were created by the erosion produced by the rocks and the ice. Like the smaller glaciers, even the gigantic mantle that covered a third of the Earth during the last glaciation was produced by the accumulation of snow transformed into ice. This gave rise to the glaciers that flowed on the ground, slowing down the rock and collecting rocks and other debris. The glaciers then left behind traces and evidence of their passage. This remarkable and precious video shows us the movement of the surface of a glacier during a period of five years. Produced in some distant snowy area, the ice flows continuously as if it were mounted on a conveyor belt. The mantle that covered half of the North American continent must have also been continuously moving like what we see in this video. Many rocks of various sizes were brought by slow movement to a point where, by loosening the ground, an unmistakable slope was left behind. These signs can help us discover the origins of the huge ice mantle that covered North America. Now, New York is covered with skyscrapers. But only 20,000 years ago, this territory was covered by a thick layer of ice. In addition to the erratic masses in Central Park, you can find other signs left by the glacier. The signs left on the rocky base were produced by the slow movement of the glacier that scratched the ground. The slopes provide an important clue to discover the direction of the glacier. Professor Robert Ridkey of the University of Maryland tells us. In this part of the park we have very evident glacial slopes. Masses eroded by erosion, pebbles, sand, mud and clay working along the base of the glacier have shaped these recesses. All the slopes are parallel to this line that I'm drawing in one of the slopes. With a compass, we can see that the North is in that direction. It is deduced that the thick layer of ice that proceeded to this part of the park flowed from northwest to southeast, moving away to the lower end of Manhattan. This glacier came from an area located northwest of the city. We have climbed the path of the glacier about 300 km back in this direction until we reached an area south of the big lakes. An area dotted with long lakes similar to rivers, known as Finger Lake. A small lake of this type can reach 5 km in width, while a larger one can extend up to 65 km. Another photograph of the Tellandsat satellite shows us about 10 lakes of this region, both large and small, but all oriented from north to south. The glaciers lightened the bottom of soft rocks called clay, and created narrow and long valleys. The water flooded the depressions of the ground left by the glacier in motion, forming these lakes. Proceeding north, we arrive at Kelly Island, which is located in the western part of Lake Eyre, one of the five big lakes. Even in the region of the big lakes, the glaciers left many traces of their passage. On the rocky bottom of this island there is a large slope produced by the action of scraping the glacier. The slope is 10 meters wide and 5 meters deep, a clear demonstration of the surprising strength of the glacier that at that time had crossed the area. Already at the beginning of this century, the island was a well-known center for the extraction of limestone. When the workers removed sand and earth from the limestone, they always found these strange grooves in the rock. It is said that the people of the place have been interrogated for a long time about the origin of these grooves that go in the direction northeast. Each of the big lakes in turn controlled the flow of the glacier in this region. And here we have a splendid example of the control of the glacier by Lake Eire. In the spectacular ice sheets of the island of Kili, we see a marked orientation from northeast to southwest of the ice flow. Overflowing from the basin of Lake Eire, it crossed a soft territory and upon this more resistant limestone it underwent a compression and a significant crushing. There was also a considerable lateral thickness of the ice, which is very plastic and behaves almost like a fluid. The thickness of the glacier that covered the Finger Lakes was about 1,000 meters. But the glacier that passed in this area was only the final part of the immense ice sheet. Continuing our journey, we follow the tracks of the glaciers. 800 kilometers north of New York, we cross the Canadian border and enter the region of Quebec. The southern part of Canada is covered by a coniferous forest called Taiga. Here, too, there are traces of the glaciers. Let's listen to Professor Pierre Lassalle of the Institute of Geological Research of the State of Quebec. Usually, we wet the surface of the glaciation so that in the light of the sun you can easily see the marks. On the surface of the base rock concave depressions become visible that have a shape similar to the imprint that leaves the base of a horse in the dust. These marks were made by the glacier that passed in this direction. We call it plucking because there is something at the base of the glacier that removes everything that it finds on the path. These characteristic marks are usually made on particular types of rocks. Moving north, we find numerous lakes, all of glacial origin. The amount of conifers decreases as we continue in this direction. In this area, the thickness of the glacier has been calculated around 2,000 meters. The huge layer of ice carved the rocky base on its way south. The erratic masses transported by the glacier also appear in this area. Since the height of the trees is about 5 meters, it is easy to see that this mass is of considerable proportions. There is a place called the field of erratic masses, an area where conifer forests become rare and the ground becomes a desolate Nordic tundra. Large and small masses abound and there are many puddles between the large masses and the rocky base. It may seem a game of prestige, but this huge mass transported by the glacier was left unbalanced on the puddles after the ice melted. These puddles show that the force of the glacier in moving is incredible. In this area, the thickness of the ice was about 2,500 meters. At 52 degrees north latitude and at a distance of 1,200 km from the traces and erratic masses left in the Central Park of New York, we notice that the puddles left by the glaciers still tend to the northeast. Among the different tracks there are those in the shape of an iron horse and the puddles that we have already seen in the field of erratic masses. The tracks left by the glaciers and followed by scientists are all oriented in one direction, Canada. The place of origin of the glaciers is a vast plateau called Labrador. An immense and desolate Nordic plain like this hosts, however, a numerous wild fauna. Our journey to discover the path of glaciers started in Central Park in the middle of the bustle of a metropolis ends here, in this desolate Nordic plain, a territory covered mainly by the tundra under the eye of a weak sun. The huge spread of ice that once extended for more than half of North America was formed here. This valley is 300 meters deep. The glacier flowed from here completely submerging the valley and scratching both walls. When the ice cap reached the maximum extension, the thickness of the glaciers in the plateau of Labrador reached the incredible size of 3,000 meters. Professor John Banden-Lieden of the Institute of Geological Research of the State of Quebec tells us. These rocks formed 15,000 years ago. There was a continental glacier in this area at an altitude of about 1,000 meters. It was higher than the same canyon and covered the plateau of Labrador. When the glaciers melted, this area retained a reduced glacial mantle. The plateau of Labrador is buried under the snow for most of the year and its terrain is visible only for a short period of time during the short summer. The temperature rarely exceeds 10 degrees and although it increases a little during the summer, this plain still maintains its ability to produce glaciers. What gives rise to a glacial era? We can explain this by studying the movement of the Earth around the Sun. Being constantly under the influence of the Moon, of other planets and in particular of the Sun, the motion of the Earth undergoes slight changes due to the balance variation between the different influences. One of these variations is the one that the Earth undergoes during its revolution around the Sun. The orbit of the Earth varies regularly from a circular orbit to an elliptical orbit and then again circular and so on for a period of 100,000 years. When the Earth during the summer months of the northern hemisphere is in a point far from the Sun, in the plateau of Labrador the summer is colder. In addition to the orbital variations, there are also variations of the inclination of the Earth's axis. The current inclination is 23.4 degrees, but this angle can become greater or smaller in a period of 40,000 years. The plateau of Labrador is particularly sensitive to changes in the inclination of the Earth's axis because it is located far north. In this region, when the angle of inclination becomes greater, the summer is warmer because it receives more directly the solar energy. On the contrary, when the angle of inclination decreases, the plateau has a colder summer because the solar energy is reduced. About 115,000 years ago, the Earth was faced with both of these phenomena at the same time. The Earth's orbit became elliptical and the angle of inclination of the axis reached its lesser measure. These factors, therefore, contributed together to produce cold summers in the plateau of Labrador. 115,000 years ago, the plateau of Labrador was about to experience a cold summer. What is curious is that the arrival of the Ice Age was not triggered by a cold winter, but by a cold summer. When the plateau of Labrador is affected by a cold summer, the accumulated snow does not melt, but remains relatively warmer during the months. The mantle of snow reflects a lot of that solar energy that it would normally absorb, determining, in fact, a decrease in the absorption of solar energy by the Earth. This produces a further lowering of the temperature. This chain reaction is called the Albedo Effect. The Albedo Effect becomes even stronger when the vast plateau of Labrador is completely covered by snow, transforming the entire region into a freezing environment. The sea, however, does not cool down as quickly as the land, even in those periods in which the conditions of a cold summer prevail. At the time of the last Ice Age, the Gulf Stream continued to reach the coast near the plateau of Labrador, bringing from the tropics the warmest sea water. Therefore, despite the frozen land, the sea on the coast of Labrador was heated by the Gulf Stream. When the temperature difference became relevant, an unusual climate change occurred. Hot air, which contained a large amount of steam and water, moved northward from the southern regions along with the Gulf Stream. This mass of air came into contact with the cold water of the plateau of Labrador and the warmer air cooled down, forming clouds that produced abundant snow on the plateau. The water vapor rising from the Gulf Stream caused the fall of fresh snow in summer on the plateau of Labrador. Once started, this vicious circle continues for thousands of years. The water vapor in contact with the mass of cold air descends again in the form of snow. The snow cools the land reflecting solar energy and causes a further increase in the difference between the temperatures of the cold air and the hot current coming from the sea. This causes a greater fall of snow on the plateau of Labrador. This accumulation creates the conditions necessary for the formation of ice. The snow turns into ice almost immediately for its same weight and the ice that derives from it, pushed out of its original location because of the continuous snowfall, begins to slide from the plateau. Similar phenomena occurred in many places of the northern hemisphere and the consequent effect of the snow caused a clear cooling of the soil of the entire area. In the American continent glaciers developed in the rocky mountains and in the areas west of Hudson Bay. These glaciers united forming a huge ice cap 4,000 meters thick. A similar icy mantle was also formed in the Eurasian continent so that more than a third of the planet was under a thick layer of ice. The widespread diffusion of the icy areas increased the effect of snow which caused further cooling of the temperatures and a long ice age throughout the Earth. Although 10,000 years have passed since the dissolution of the glaciers of the last ice age, there are still plenty of testimonies of that time. The cliff that extends along the Hudson Bay in northern Canada was caused by the elevation of the ground. Like the ground under the glaciers in Greenland, when half of North America was covered by a huge ice mantle, the ground underwent a downward pressure caused by the enormous weight of the ice. When this withdrew and dissolved, the ground was freed from the pressure and began to rise slowly. In this area the highest elevation of the North American continent In fact, the coast here has risen by about 200 meters. Inside, we can find the very clear traces of a significant rise. Professor LaSalle of the Institute of Geological Research of the State of Quebec explains. This material is very slippery and extends like a kind of water sphere along the valley. The gravel was imprisoned by the waves and was produced during the storms. This is what we call a storm beach. The rise of the coast after the pressure produced by the glacier for 10,000 years still continues at a rate of about 2 centimeters per year. These lines, old traces of the previous coasts, are a bit like the rings of the trees and show us how much the ground has risen compared to the level of the sea. The earth has constantly oscillated between glacial eras and eras with a rainy climate. Some scientists predict that the next glacial era could start in about 2,000 years. New York, then, could be covered by ice in 20,000 years. If the earth were to be completely covered by ice and snow, the temperature would drop up to 50 degrees below zero due to the effect of the Bedo. Our hospital and planet would be transformed into a very hostile cold that could never return to its current state. However, since its birth, 4.6 billion years ago, the earth has never been completely covered by ice. The glaciers have appeared and disappeared for relatively short periods.