It turned out that a few meters away, there was the most unusual fossil bed ever found in North America: a dry water cave, which became the collective grave of dozens of animals-including rhinoceros, zebra-like wild horses, deer with saber teeth, camels and turtles. They all died in a mysterious catastrophe less than120 thousand years ago. That era is called Miocene in geology. At that time, Nebraska was located on a vast and hot plain, just like the Serengeti Plain in Africa today. Animals were found buried under volcanic ash as deep as 3 meters. Strangely, Nebraska has never had a volcano.
Today, Voorhees was discovered in ashford Fossil Bed Park. A beautiful visitor center and museum has been built here, which creatively displays the geological discoveries and fossil bed history of Nebraska. There is a laboratory in the center, and visitors can see paleontologists cleaning up bones through the glass wall. One morning, I passed by and saw a man in blue overalls and gray hair busy alone in the laboratory. I recognized him as Mike Walhees, the host of the BBC Horizon documentary. Ashford Fossil Bed Park is located in an insignificant place, so there are not many tourists. Walhus seems happy to show me around. He also took me to the top of the 6-meter-deep mountain pass and looked at his discovery.
"It is foolish to come to a place like this to look for bones," he said happily. "But I'm not looking for bones. I was thinking about drawing a geological map of eastern Nebraska, but I was just walking around here. If I hadn't climbed this pass, if the heavy rain outside hadn't washed away the skull, I would have walked straight over and never found this thing again. " He pointed to a covered place nearby, which has become the main excavation site. They found about 200 animals sprawled together.
I asked him why he said it was stupid to find bones in such a place. "Oh, if you want to find bones, you must have exposed rocks. Therefore, most paleontological work is carried out in hot and dry places. Not because there are many bones there, but because there is a great possibility of finding them. In this place, "he waved at the vast grassland," you can't start at all. There may be amazing things there, but there are no clues on the ground to tell you where to start looking. "
At first, they thought that these animals were buried alive, as Voorhees said in an article in 198 1 National Geographic magazine. "The article called this place' Pompeii of Prehistoric Animals'," he told me. "It's unfortunate, because it won't be long before we find that animals don't die suddenly at all. They all suffer from a disease called "pulmonary osteodystrophy". This is the problem when people inhale a lot of corrosive ash-they must inhale a lot of this ash, because the ash several meters thick stretches for several kilometers. " He picked up a mass of gray clay-like soil, crushed it and put it in my hand. The soil is powdery, but a bit like sand. "Inhaling this thing is very uncomfortable," he continued. "It's thin and sharp. Anyway, they came to this puddle, probably because they wanted to have a rest. It was killing them. This kind of ash can destroy everything. It will drown weeds and stick firmly to leaves, turning water into gray mud that is not suitable for drinking. It will definitely be very uncomfortable to drink. "
The Horizon documentary pointed out that there were so many ashes in Nebraska that it was unexpected. In fact, people have long known that Nebraska has a lot of volcanic ash deposits. For almost a century, ashes powder has been mined as raw materials and made into household decontamination powders such as Comet brand and Ajax brand. Interestingly, however, no one thought about where so much ash came from.
"It's a little embarrassing," Voorhees said with a smile, "but an editor of National Geographic magazine asked me where so much volcanic ash came from. I have to admit, I don't know. Nobody knows. At this time, I thought of this problem. "
Voorhees sent samples to colleagues all over the western United States and asked them if they could deny what it was. A few months later, a geologist named Bill Bonnici from the Geological Survey of Idaho contacted him and told him that the ashes were completely consistent with a volcanic deposit and came from a place named Bruno Jabich in southwest Idaho. The killing of animals on the plains of Nebraska was a volcanic eruption on a scale never imagined before-but it was enough to leave a layer of volcanic ash 3 meters thick in eastern Nebraska 1600 kilometers away. It turns out that there is a large area of magma and huge volcanic hot spots under the western United States. It explodes catastrophically about every 600,000 years. The last such eruption was 600,000 years ago. That hot spot is still there. Today, we call it Yellowstone National Park.
We know very little about what is happening under our feet. It is really interesting to think that Ford began to produce cars, and the baseball world series was held longer than we knew that the earth had a core. Of course, people all know that the mainland moves around the earth's surface like water lily leaves floating on the water, which is far less than a generation. "Unbelievable as it is," richard feynman wrote, "we know more about the distribution of matter in the sun than on earth."
The distance from the ground to the center of the earth is 6370 kilometers. Not too far. Some people calculate that if you drill a well towards the center of the earth and drop a brick, it will fall to the bottom in 45 minutes (although it has no weight there, because all the gravity of the earth is above and around, not below). In fact, few people try to go deep into the center of the earth. One or two gold mines in South Africa have reached a depth of more than 3 kilometers, but most of the mines on the earth are below 400 meters. Suppose the earth is an apple, and we haven't poked its skin yet. Actually, it's still a long way from puncturing the skin.
Until less than a century ago, the most knowledgeable scientists didn't know much more about the interior of the earth than miners-that is, you can dig down a distance in the soil and hit a rock, that's all. Then in 1906, an Irish geologist named R.D. Alderman, while looking at the seismograph reading of an earthquake in Guatemala, noticed that some shock waves penetrated deep into the earth and then bounced back at a certain angle, as if encountering some obstacles. He deduced from this that the earth has a core. Three years later, Croatian seismologist Andrei Mohorovic, while studying the chart of the Zagreb earthquake, suddenly noticed a similar turning point, only in the shallow layer. He discovered the boundary between the crust and the lower layer, that is, the mantle. Since then, this area has been called the Moho-Holovics Discontinuity, or Moho Surface for short.
We began to have a vague concept of the level inside the earth-although it was only vague. During the period of 1936, Danish scientist Ying Lehmann found two cores-an inner core and an outer core-while studying the seismograph readings of the New Zealand earthquake. We now think that the kernel is hard; The outer core is considered as a liquid, where magnetic force is generated.
Just as Lyman improved our basic understanding of the earth's interior by studying seismic waves, two geologists at California Institute of Technology invented a method to compare the previous earthquake with the next one. They are Charles richter and Beno Gutenberg. For reasons unrelated to fairness, the name of magnitude is called Richter scale almost immediately. (These reasons have nothing to do with richter himself. Richter is a very humble person. He never put his name before quantity, but always just called it "quantity". )
Many non-scientists always misunderstand the Richter scale, although the situation may be improved now. In the early years, visitors to richter's office often asked to see his masterpiece and thought it was a machine. Of course, the Richter scale is a concept, not a thing, but an arbitrary number of earthquakes based on ground measurements. It is expressed by the increase of index, so the earthquake of magnitude 7.3 is 50 times stronger than the earthquake of magnitude 6.3 and 2500 times stronger than the earthquake of magnitude 5.3.
Theoretically, there is no upper limit for earthquakes, so there is no lower limit. Magnitude is a simple method to measure intensity, but it does not explain the degree of damage. An earthquake of magnitude 7 occurring deep in the mantle-for example, below 650 kilometers-may have no destructive impact on the ground, while a much smaller earthquake occurring six or seven kilometers below the ground may cause extensive damage. To a great extent, it also depends on the nature of the subsoil, the duration of the earthquake, the frequency and intensity of aftershocks and the specific situation of the disaster area. All this means that the most terrible earthquake is not necessarily the strongest earthquake, although the intensity is obviously valuable.
Since the invention of magnitude, the biggest earthquake (depending on which data you use) is either1March 1964, which was centered in prince william sound, Alaska, or1June 1960, which was in the Pacific Ocean near Chile. The former is 9.2 on the Richter scale; The latter was initially recorded as 8.6, but was later upgraded to 9.5 by some authoritative organizations (including the US Geological Survey). From this, you can know that measuring earthquakes is not always a very accurate science, especially when it involves reading from far away. Anyway, these two earthquakes are very big. The 1960 earthquake not only caused extensive damage to the western coastal areas of South America, but also triggered a huge tsunami. The tsunami spread almost 654.38 million kilometers in the Pacific Ocean, washed into many places in downtown Shiloh, Hawaii Island, destroyed 500 buildings and killed 60 people. Similar storms reached as far away as Japan and the Philippines, killing more people.
However, in terms of concentrated destruction, the strongest earthquake recorded in history is probably the one that happened in Lisbon, Portugal on Halloween (1 1 October) in 775. The earthquake actually turned Lisbon into ruins. Just before 10 in the morning, the city suddenly swung from side to side. The violent shaking lasted seven minutes. It is now estimated that the magnitude of that earthquake is 9. The force of the vibration is so great that the seawater in the port of this city surges out and then returns with a huge wave exceeding 15 meters, causing more damage. When the vibration finally stopped, the survivors only enjoyed three minutes of peace, and then a second earthquake happened, only slightly weaker than the first one. The third and last earthquake happened two hours later. By the time it was all over, 60,000 people had died, and almost all the buildings within a few kilometers of Fiona Fang were razed to the ground. In contrast, the San Francisco earthquake of 1906 was only 7.8 on the Richter scale and lasted less than 30 seconds.
Earthquakes are very common. On average, there are two earthquakes of magnitude 2.0 or above every day in the world-strong enough to make people nearby feel scared. Earthquakes are often concentrated in some areas, especially along the Pacific coast, but earthquakes can happen almost anywhere. In the United States-so far-only Florida, eastern Texas and the northern Midwest seem to have almost completely survived. In the past 200 years, there have been two earthquakes of magnitude 6 or above in New England. In April 2002, an earthquake with magnitude of 5. 1 occurred near lake champlain, which is located at the junction of new york and Vermont, causing great damage to the local area. Even in New Hampshire (I can testify), the photos on the walls were knocked down and the children in the beds were overturned.
The most common earthquakes occur where two plates meet, such as California along the San Andreas fault. When the two plates push and shove each other, the pressure increases, and finally either one or the other makes concessions. Generally speaking, the longer the interval between two earthquakes, the greater the accumulated pressure and the wider the scope of major earthquakes. Tokyo is particularly worried about this situation. Bill McGuire, an expert on dangerous events at University College London, described Tokyo as a "city waiting to die" (you will find that many tourist leaflets don't have such famous words printed on them). Japan is already a country with frequent earthquakes, and Tokyo happens to be at the intersection of three tectonic plates. You will remember 1995, a 7.2-magnitude earthquake struck Kobe, nearly 500 kilometers west, killing 6,394 people. It is estimated that the loss is as high as $99 billion. But this is nothing-alas, relatively small-if compared with the losses that Tokyo may suffer in the future.
Tokyo suffered a devastating earthquake in modern times. 1, 65438, 0923 Shortly before noon in September, the famous Great Kanto Earthquake occurred in this city-an earthquake more than 10 times stronger than the Kobe earthquake. 200,000 people died. Since then, Tokyo has been mysteriously silent, and underground tension has accumulated for 80 years. It is bound to break out in the end. 1923, the population of Tokyo is only about 3 million. Today, the population is nearly 30 million. No one wants to guess how many people will die next time, but it is estimated that the potential economic loss may be as high as $7 trillion.
What is even more worrying is a relatively rare earthquake, called a cross-plate earthquake. People don't know much about that kind of earthquake, which can happen anytime and anywhere. It happens far from where the plates meet, so it is completely unpredictable. Because the epicenter is deep, it often spreads to a wider area. Among such earthquakes experienced by the United States, the most famous one is a series of three earthquakes that occurred in New Madrid, Missouri in the winter of181-18/2. February16,65438 started just after midnight. People are first awakened by the panic of livestock (livestock will be restless before the earthquake, which is not nonsense, but actually recognized by everyone, although the reason is still unclear), and then they hear a loud noise coming from the depths of the earth. The local people quickly ran outdoors, only to see the earth turn up a wave one meter high, and the open gap was several meters deep. There is a strong smell of sulfur in the air. The earthquake lasted for four minutes, and as always, it caused huge property losses. Among the witnesses was the painter john james audubon, who happened to be in this area. The earthquake radiated outward with powerful force and collapsed the chimney in Cincinnati, more than 600 kilometers away. According to at least one report, it "destroyed the ships in the east coast port" ... and even knocked down the scaffolding around the Capitol in Washington. 65438+1On October 23rd and February 4th, two earthquakes with similar magnitude occurred continuously. Since then, New Madrid has been safe-not surprisingly, it is reported that such an earthquake never happens twice in the same place. As far as we know, they are as irregular as lightning. The next such earthquake may happen in Chicago, Paris or Kinshasa. People are too lazy to guess. How did this cross-plate earthquake happen? The reason is deep in the earth. We don't know much about it.
By the 1960s, scientists were very sad because they knew too little about things inside the earth, so they decided to take some measures. Specifically, they want to drill a hole in the seabed (the continental crust is too thick) all the way to the Moho surface, and take out a mantle sample to study it slowly. They think that as long as we can understand the nature of rocks on the earth, we may begin to understand their interaction, so that we can predict earthquakes and other unwelcome events.
This project will almost certainly be named "Moho Drilling", which is simply disastrous. They hope to put the drill bit into the Pacific Ocean more than 4000 meters deep off the coast of Mexico, and then drill down more than 5000 meters to penetrate thinner crustal rocks. In the words of an oceanographer, drilling from a boat on the coast is "like trying to drill a hole in the sidewalk of new york with spaghetti on the top of the Empire State Building". All efforts ended in failure. They only reached 180 meters at most. Moho drilling was finally called "impossible drilling". 1966 because of the rising cost, there was no result, and Congress was angry and annoyed and cancelled the project.
Four years later, Soviet scientists decided to try their luck on land. They set out and chose a place in kola peninsula, Russia, not far from the Finnish border, hoping to drill to a depth of 15km. The work is more difficult than expected, but the Soviets have commendable tenacity. By the time they finally gave up after 19, they had drilled to the depth of 12262 meters. However, we have not forgotten that the earth's crust only represents about 0.3% of the earth's volume, and Cora drilling has not penetrated into one third of the earth's crust, so it is difficult for us to claim that we have conquered the earth's interior.
Despite the limited depth of drilling, almost all the findings surprised the researchers. The study of seismic waves has always made scientists predict that they will encounter sedimentary rocks at a depth of 4700 meters, followed by granite and basalt with a thickness of 2300 meters. It was found that the sedimentary rock layer was 50% thicker than expected, but the basalt layer was not found at all. Moreover, the underground world is much warmer than expected, and the temperature at the depth of 1000 meters is as high as 180 degrees Celsius, almost twice as high as expected. The most surprising thing is that the deep rocks are full of water-this has always been considered impossible.
We can't see the depth of the earth, so we have to use other methods, mainly including observing the propagation form of waves inside the earth, so as to infer the situation there. We know something about the mantle from the so-called kimberlite cone (where diamonds are formed). The situation there is that an explosion occurs deep in the earth, which can send magma shells to the ground at supersonic speed. This is a completely irregular phenomenon. When you read this book, a kimberlite cone may explode in your back garden. Because it goes deep underground-as deep as 200 kilometers-the kimberlite cone brings all kinds of things that are usually not found on or near the ground: peridotite, olivine crystals and diamonds. It is very accidental to bring diamonds. 100 rock cones, only about 1 will do such a good thing. The material ejected from the kimberlite cone brings a lot of carbon, but most of it is Huasong steam or turned into graphite. Only occasionally, a lump of carbon is ejected at a proper speed, cooled at a necessary speed, and finally turned into a diamond. It is because of this cone that South Africa has become the country that produces the most diamonds in the world, but it is likely that other countries have even richer reserves, which we still don't know. Geologists know that there is a place near the northeast of Indiana, and there are signs that there are huge cones or cones. Diamonds of 20 carats or more were found in different parts of the whole area. However, no one has found the source. John mcphee pointed out that it may be buried under glacial sediments, like Manson Pit in Iowa, or under the Great Lakes.
So, how much do we know about the interior of the earth? Very few. Scientists generally believe that the world under our feet is divided into four layers-a rock crust, a mantle composed of hot and sticky rocks, a liquid outer core and a solid inner core. We know that the main composition of the ground is silicate. Silicate is relatively light, and its weight is not enough to explain the overall density of the planet. So, there must be something heavier inside. We know that in order to generate a magnetic field, there must be a concentrated area of liquid metal elements somewhere inside. These are all recognized by everyone. In addition, almost everything-how these structural layers interact, why they are like this, and what they will do at some point in the future, is still a very uncertain problem in general.
Even what we can see-the earth's crust-is a controversial issue. Almost all geological documents will tell you that the crust thickness is 5- 10 km under the ocean, about 40 km under the mainland and 65-95 km under the mountain, but there are still many puzzling variations in these general laws. For example, the crust under the Sierra Nevada is only about 30-40 kilometers thick, and no one knows why. According to all geophysical principles, the Sierra Nevada should sink like quicksand. Some people think that this mountain range may be sinking. )
How and when did the earth have a crust? These two problems divide geologists into two camps-one thinks it happened suddenly at the beginning of the earth's history; The other school thinks it happened gradually later. Everyone is very emotional on these issues. Richard Armstrong of Yale University put forward the theory of early explosion in 1960s, and then spent the rest of his life fighting people with different views. He died of cancer on 199 1. However, according to 1998 Earth magazine, shortly before his death, he "severely criticized his critics in a debate in an Australian geoscience magazine, accusing them of continuing the myth". "He died unsatisfied." A colleague said.
The crust and part of the outer mantle are collectively called lithosphere (from the Greek lithos, meaning rock). On the other hand, the land boundary floats on a soft rock, which is called asthenosphere (from Greek meaning "no power"), but these names have never been satisfactory. It is not entirely correct to say that the lithosphere floats on the asthenosphere, that is to say, it has certain buoyancy. Similarly, it is misleading to think that rocks flow like flat objects. Rock is sticky, but it is like glass. This seems unlikely, but under the constant pull of gravity, all the glass on the earth is flowing downwards. Take a very old piece of glass from the window of a European church, and you will find that its bottom is obviously thicker than the top. This "flow" is what we call. The movement speed of the hour hand on the clock face is about 654.38+00,000 times faster than the "flow" speed of mantle rocks.
Motion not only really happens, just like the plates of the earth move in a plane, but also moves up and down, just like rocks rise and fall under the so-called convection agitation. Convection as a process was first inferred by Count Lunford at the end of 18. Sixty years later, an English priest named osmond Fisher presciently suggested that the earth's interior may be liquid, and things can move freely on it, but it will take a long time to get the support of others.
About 1970, the news was really shocking when geologists realized that the underground was a mess. Shawna vogel said in her book Naked Earth: New Geophysics: "It's like it took scientists decades to discover the layers of the earth's atmosphere-troposphere, stratosphere, etc.-and then suddenly discovered the wind."
Since then, how deep the convection process is has always been a controversial issue. Some people say it started below 650 kilometers, while others say it started above 3000 kilometers. James Trevor believes that the problem is that "two sets of data from two different disciplines are irreconcilable". Geochemists say that some elements on the earth's surface cannot come from the upper mantle, but must come from deeper inside the earth. Therefore, the materials in the upper mantle and the lower mantle are at least occasionally mixed. Seismologists believe that there is no evidence to support this argument.
Therefore, it can only be said that in the process of moving towards the center of the earth, we will leave the asthenosphere and enter the pure mantle in an uncertain place. The mantle accounts for 82% of the earth's volume and 65% of its mass, but it has not attracted enough attention, largely because scientists and ordinary readers on earth are interested in either the deep underground (such as magnetism) or the proximity to the ground (such as earthquakes). We know that at a depth of about 150 km, the mantle is mainly composed of a kind of rock called peridotite, but what is below 2650 km is not clear. According to the report of Nature, it doesn't seem to be peridotite. We don't know much about it.
Below the mantle are two inner cores, a hard core and a liquid outer core. Needless to say, our understanding of the two core properties is indirect, but scientists can make some reasonable assumptions. They know that the pressure in the center of the earth-about 3 million times the maximum pressure on the ground-is enough to harden the rocks there. They also know from the history of the earth (among many other clues) that the core is good at conserving its own heat. Although it is only a guess, it is believed that the temperature of the earth's core has dropped by less than 1 10 degrees Celsius in the past four billion years. No one knows how high the temperature of the earth's core is, but it is estimated to be 4000-7000 degrees Celsius-roughly equivalent to the temperature of the sun's surface.
The outer core is less understood in many aspects, although everyone thinks that it is a liquid and a place where magnetic force is generated. 1949, E.C. Brad of Cambridge University put forward a theory that the liquid part of the earth's core is rotating in some way and actually becomes a motor, creating the earth's magnetic field. He believes that the liquid convection in the earth, in a sense, plays the role of current in the wire. It is not clear what is going on now, but it is certain that the formation of magnetic field is related to the rotation of the core and the fact that the core is liquid. Objects without liquid nuclei, such as the moon and Mars, have no magnetism.
As we know, the intensity of the earth's magnetic field is constantly changing: in the age of dinosaurs, the magnetic field intensity was three times that of today. We also know that it reverses every 500,000 years on average, although this average contains a great degree of unpredictability. The last reversal took place about 750 thousand years ago. Sometimes, it hasn't changed for millions of years-the longest time seems to be 37 million years, and sometimes it happens less than 200 thousand years. In the past 65438+ billion years, the earth's magnetic field has reversed about 200 times, and the reason is still unclear. This has always been called "the biggest unsolved problem in geological science".
We may be going through a reversal now. In the past century alone, the earth's magnetic field has weakened by about 6%. The weakening of magnetic force may be bad news, because in addition to ensuring the normal operation of refrigerator and the compass pointing in the right direction, magnetic field plays an important role in maintaining our lives. Space is full of dangerous cosmic rays. Without the protection of magnetic field, cosmic rays will penetrate our bodies and tear a lot of our DNA into useless pieces. If the magnetic field works normally, these rays will be safely blocked from the earth's surface and driven to two areas in the air called "Van Allen radiation belt". It also interacts with particles in the upper atmosphere to produce a beautiful light curtain called aurora.
The reason why we are ignorant is largely because we have not made great efforts to coordinate the situation on the earth with the situation inside the earth. Shawna vogel said: "Geologists and geophysicists rarely attend the same meeting or cooperate on the same issue."
Perhaps, what best illustrates our lack of understanding of the dynamics inside the earth is the serious mistakes we made when that dynamics caused trouble. It is hard for us to think of an example that better illustrates our limited knowledge than the eruption of Mount St. hellens in Washington in 1980.
At that time, 48 States in the United States had not experienced volcanic eruptions in the past 65 years. Therefore, most government volcanologists called in to monitor and predict the activity of San hellens volcano have only seen the volcanic eruption in Hawaii. It turns out that the two are not the same thing at all.
On March 20th, Mount St. hellens began to rumble ominously. Within a week, it has been spewing out magma, although the amount is not large, but as many as 100 times a day, often accompanied by earthquakes. People are evacuated to places considered safe 13 kilometers away. With the increasing rumble of mountains, Mount St. hellens has become a tourist attraction in the world. The newspaper reports the best places to see every day. TV reporters kept flying to the top of the mountain by helicopter, and even saw someone climb over the mountain. One day, more than 70 helicopters and light planes circled the top of the mountain. However, with the passage of time, the rumbling sound did not produce dramatic results, and people became more and more impatient. It is generally believed that this volcano will not erupt at all.
On April 19, the north side of the volcano began to bulge obviously. Incredibly, no one in charge understands that this obviously indicates that one side of the volcano is about to erupt. Volcanologists have concluded that according to the way of volcanic activity in Hawaii, volcanoes will not erupt sideways. Almost only one person thinks that something is really going to go wrong. His name is Jack Hyde, and he is a professor of geology at a community college in Tacoma. He pointed out that the San hellens volcano does not erupt as openly as the Hawaiian volcano, so the pressure accumulated inside it is bound to be released sharply, and it may be disastrous. However, Hyde is not a member of the formal team. His observation didn't attract much attention.
We all want to know what happened next. May 18 is a Sunday. At 8: 32 am, the north side of the volcano collapsed, and avalanche-like dust and rocks washed down the hillside at a speed of nearly 250 kilometers per hour. This is the biggest landslide in human history.