Shulou(Shulou.com)11/24 Report--

The author's words:

I believe that many people are no stranger to the film "Journey to Dreams", and even they will be deeply moved every time they watch the movie (figure 1). In this film, the protagonist's great grandmother Coco is the most important connection between the past and the future, between disappearance and existence. And when the background music "Please remember me" sounded, I thought of my great grandmother who passed away a few years ago.

My great grandmother is an amiable person. When I was a child, I always liked to sleep in her arms, because every time I woke up, she would secretly snack me. But at the moment of her death, I was the only one with those memories. As a result, a question comes to mind: how to prove that something really exists?

For people, it is a memory stored in the brain. So what is the memory of the earth? Without neurons as a storage intermediary, is the earth's memory a geological age finally measured after several twists and turns, or is it a record of rocks transformed by countless geological movements?

Figure 1 "Journey to Dream" is from Pixar Animation Studio. Although the film is a comedy, the story's profound discussion of life and death is still tear-jerking.

First, the tortuous history of geochronology in the 17th century, under the influence of the modern scientific movement, the physical theological movement tried to explain the history of the creation of the earth through science. Under the influence of this trend of thought, James Osher, Archbishop of Ireland, deduced from the Bible in 1650 (James Ussher) that the world was created on October 23, 4004 BC. As a result, most European Christians at that time believed that the earth existed for less than 6000 years.

Of course, science will not stop here. Geologists all believe that the age of the earth is much more than 6000 years old, but how to date it scientifically is a problem. Isaac Newton first challenged the problem, assuming that the earth was a hot iron ball. After experiments and conversion, Newton thought that it would take 50, 000 years for the surface temperature to drop to its current level. After Newton, Buffon experimented with increasing this time to 75,000 years (or even privately thought to be 3 million years). But Newton was not familiar with thermodynamics after all, and finally Kelvin made improvements in 1862 (Lord Kelvin), and his result was that he was between 0.2 million and 400 million years old.

In addition to thermodynamic methods, British geologist John Phillips (John Phillips) calculated the age of the earth to be about 9600 million years by simulating the rate of deposition in 1841. But contemporary geologists such as Charles Lyle (Charles Lyell, author of principles of Geology) say it is difficult to accept such a short geological age. Darwin believed that weathering would change the rock to its present state for at least 300 million years in order to meet the long process of genetic variation accumulation and natural selection. In addition, scientists have calculated the Earth time by measuring the salt content of sea water and the rate of the moon. However, limited by the level of science and technology at that time, the inferred figures are far lower than the real age of the earth.

Humans will not be able to determine the age of the real earth until the early 20th century, which has something to do with a familiar figure, Ernest Rutherford. In 1905, Rutherford made a scientific report on radioactive dating at Yale University. Rutherford realized that uranium fission would produce helium, and the age could be estimated by measuring the mass of helium in rocks. After preliminary measurements of some rocks, the age of the earth was pushed to 500 million years, and Rutherford thought the actual age would be longer. because some of the helium could leak.

Fig. 2 Ernest Rutherford, who is recognized as the "father of nuclear physics" and the theoretical founder of radioactive dating, but radiological dating is very difficult, this method puts forward high requirements for the sensitivity and accuracy of the instrument, it is not easy to go further. Harrison Brown (Harrison Brown), a professor of nuclear physics at the University of Chicago, and his student Claire Clair Patterson (figure 3) were the first to make breakthroughs in this field. The establishment of U-Pb dating method is very tortuous. Patterson's measurement results always have a lot of errors. after excluding various influences, he finally realized that it was the cause of lead pollution everywhere. In order to eliminate the effect of lead pollution on the experimental results, he even built the world's first super-clean laboratory. After a tortuous exploration, Professor Brown's team finally got the true age of the earth for the first time-4.55 billion years old.

Figure 3 Harrison Brown, a professor of nuclear physics at the University of Chicago (left) and his student Claire Patterson (right). Claire Patterson was the first person in the world to know the exact age of the earth. Today, although various radioisotope dating methods have been gradually improved, the history of the earth is still difficult to sort out, especially those ancient events. Because the larger the time span, the longer the half-life of the isotopes used, and the lower the resolution, the greater the error. Therefore, it is difficult for people to know the details of ancient history.

Second, incomplete rock records

On May 24, 1869, self-taught one-armed geologist John Wesley Powell (John Wesley Powell) led a scientific expedition to explore the canyons of the Green and Colorado River basins in the western United States (figure 4). However, when they entered the canyon, there was no news for several weeks. Soon after, the Chicago Tribune made a headline saying that the Powell team had been caught in a deadly torrent and disappeared in the canyon forever. Just when everyone was at a loss, the Powell team miraculously appeared in front of the public. However, Powell, who is still alive, did find a disappearance in the Grand Canyon, which has plagued geologists for more than 150 years.

Fig. 4 present-day Colorado Riverside Canyon, photographed in Horseshoe Bay of the Colorado River. It turned out that after Powell entered the Colorado River, he saw a large area of hard crystalline rock, mostly granite and schist, which, unlike ordinary rocks stacked horizontally, arranged vertically. Above these crystalline rocks is a layer of 305 meters thick horizontal red sandstone zone.

However, after Powell inspected the vertically crystalline rock, he estimated that this part of the rock should have reached 3050 meters, but after the actual measurement, it was found that the rock more than 2000 meters thick had completely disappeared. He called this strange thing "stratigraphic large-angle unconformity" (The Great Unconformity). Powell's observation is indeed correct. More than 100 years later, the results of isotope dating show that the youngest part of these crystalline rocks is about 1.7 billion years old, while the oldest red sandstone in the upper layer is only 550 million years old. In other words, there is a gap of more than a billion years, and the rock formations that record a billion years of history have disappeared.

In fact, as early as the 18 th century, James, the father of modern geology. Hutton found a similar phenomenon on the cliffs off the coast of England, so he proposed Hutton unconformity, which is now the stratigraphic unconformity in textbooks (figure 5). Stratigraphic unconformities are common around the world, but coincidentally, two puzzling events followed the missing 1 billion-year history of the Earth: the Cambrian explosion of life (Cambrian explosion) 540 million years ago, and the Snowball Earth Ice Age (Snow ball earth) between 800 million and 550 million years ago. Therefore, one hypothesis is that the wet glaciers of the Snowball Earth during the Great Ice Age were able to erode and transport rock layers 3, 000 to 5, 000 meters thick, which may be the reason for the loss of strata for 1 billion years. At the same time, some people speculate that the lack of strata led to the outbreak of Cambrian life. However, different hypotheses have their own flaws, and how to explain such deficiencies has also become a part of people's tectonic history of the earth.

Fig. 5 stratigraphic unconformities are revealed on the dyke: Cambrian sandstone is directly pressed on the bedrock of the older Weinue schist. The photo was taken in Blacktail Canyon, Arizona.

[2.2 lost continents] at the beginning of the Earth's formation (about 4.55 billion years), the earth's interior released 2-4 times as much heat as it is today, and there was a red "ocean" everywhere. Today, we can only imagine the environment through the lava lake when the volcano erupted (figure 6). During this period, the surface did not crystallize rocks on a large scale to form continents (left of figure 7), and even the formed rocks were quickly melted at high temperatures, making it difficult to preserve.

Fig. 6 aerial photo of the world's largest lava lake, located at Nyiragongo Volcano in Africa. Photographer: the earliest land of Bradley Ambrose was born about 43.5-4.4 billion years ago. In the history of more than 4 billion years after its formation, with the movement of plates and volcanic eruptions and the subduction between plates, the material on the earth's surface was taken to the depths of the earth, melted and disappeared. Only those rocks that can withstand high temperatures will be retained. Even if a small amount of rocks are preserved, they have been fully metamorphic, such as the Dabie Mountain-Sulu ultrahigh pressure metamorphic belt. There have been several supercontinents in the history of the earth, such as Pangea continent, Rodinia continent and so on.

Fig. 7 simulated map of the early stage of earth formation (left) and satellite photos (right). At present, the oldest known rock on Earth is the Acasta gneiss in the Slave Craton, northwest of the Canadian Shield, with an age of 4.025 billion years. But we can hardly find earlier rocks because it is very difficult for rock mass to withstand early high temperature conditions. It is also difficult for us to restore the lost continent, because all the evidence comes from rock records, but the older the rock, the stronger it is, and the harder it is to recover. How to restore the memory of the earth is an important driving force to promote the progress of geoscience.

Third, disappearance itself is also a memory. Lyle pointed out in the principles of Geology that "the present is the key to understanding the past". The realistic method of "discussing the past with the present" makes geologists search tirelessly for clues in geological time. However, in the long geological history, the face of the earth has changed greatly, and not every change will leave a mark. These vanishing traces hinder human exploration of unknown events that occurred on the earth in ancient times, and become the memory of the disappearance of the earth.

But history also tells us not to be pessimistic. More than 200 years ago, James James Hutton, the father of modern geology, said that "real geological time may be far beyond human imagination." But he did not expect that just over a century later, mankind would get the true age of the earth. Charles. Charles Lyell noticed the "fault" of biodiversity 6500 million years ago, but he can only attribute it to the loss of fossils or the absence of fossils themselves. A hundred years later, Walter Alvarez found extremely high levels of iridium in fossil-free clay layers, which are rare on Earth but common in meteors. Therefore, Walter finally put forward the catastrophe theory of the mass extinction of dinosaurs. Thus it can be seen that the emergence of new technologies will continue to reveal the unsolved mysteries of the Earth in the future, while comparative planetary studies may be able to fill the missing history of the Earth.

Explorer Magellan once said: "from Bitch to Mengzhong, from Mengzhong to Cangyue Island, and then back to Beech, so the earth is round!" But the reality is that whether human beings travel around the world or not, the earth is always round. The earth has evolved a habitable environment for human beings, which means that the disappearance of rock records is also part of the earth's evolutionary history.

With the change of things and the vicissitudes of life, the geological traces left to mankind are always limited, but perhaps disappearance itself is an important information for understanding the earth.

This article comes from the official account of Wechat: stone Science Popularization Studio (ID:Dr__Stone), by Ye Xiangying

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