May 9, 1993
Why the Dinosaurs Became Extinct
........Sixty-five million years ago the biggest creatures that ever walked the earth, the dinosaurs, became extinct. But why? This is a question that people have been trying to answer for a long time, and scientists are still unsure of what the true answer is.
........There are many different theories, and some of the theories, such as the asteroid impact theory and the volcanic eruption theory, have a lot of evidence. This paper will discuss the evidence for these two theories, the effects of the mass extinction 65 million years ago, and the opinions of 8th graders and one 4th grader about why the dinosaurs became extinct.
........The mass extinction of many species of plants and animals, including the dinosaurs, occurred at the end of the Cretaceous period, 65 million years ago. This mass extinction created the geological time boundary between the Cretaceous and the Tertiary period, which is called the K-T boundary (Milne, 1991).
........According to some evidence, the dinosaurs didn't all die at the same time (Milne, 1991). Some species survived right up to the K-T boundary, while others may have died a little before then. One of the biggest problems with dating the dinosaurs' extinction is that our dating techniques just aren't good enough to get exact numbers. With things that are so old, we can only estimate their ages to within approximately a million years.
........At the Hell Creek formation in the upper Great Plains of the United States, the fossil record was thought to show a gradual extinction of the dinosaurs. However, new data from Hell Creek suggests that the dinosaurs probably became extinct suddenly (Sheehan, Fastovsky, Hoffman, Berghaus, & Gabriel, 1991).
........Besides the dinosaurs, the mass extinction brought an end to the flying and swimming reptiles, many microscopic organisms of the ocean, and many other animals and plants (Whitfield, 1991). One idea for what caused this mass extinction is that the sun gave off abnormally large amounts of solar radiation, slowly killing off the dinosaurs, or that a supernova of a nearby star sent gamma radiation into the Earth, causing many plants and animals to die out (Milne, 1991). There are a lot of other ideas and theories for how the dinosaurs became extinct. The main theories have to do with climate changes due to volcanic eruptions, an asteroid hitting the earth, or gradual natural changes causing the dinosaurs to dwindle in number and become extinct (Sheehan et al., 1991).
........People are curious about dinosaurs. Half of the 8th graders at my school wanted to know why the dinosaurs died, mostly because they thought dinosaurs were interesting or because they thought it could happen to us. Most of the 8th graders thought that the dinosaurs died out from slow changes in their environment, but many thought that an asteroid hit the earth, sending up clouds of dust and blotting out the sun, or that a combination of volcanic eruptions and an asteroid caused the mass extinction.
........The one 4th grader I interviewed wanted to know why the dinosaurs died because he thought that dinosaurs were interesting, and because he thought that knowing about creatures of the past would help us know more about creatures of the present. He believed that a meteor hit the earth, throwing up huge clouds of dust and debris that blocked out the sun. He thought the plant-eating dinosaurs then died from lack of food, followed by the meat-eating dinosaurs.
........In summary, the main theories for how the dinosaurs became extinct are related to the effects of climate changes. The 8th graders and the one 4th grader that were surveyed seemed to be pretty well-informed regarding dinosaur extinction.
The Asteroid Impact Theory
........If an asteroid had hit the earth, it would have shot thousands of cubic miles of debris skyward, like a nuclear bomb except thousands of times more powerful (Dane, 1991). Three mile high tsunami waves would have devastated the coasts, and the entire planet would have been covered in darkness and enveloped in cold for months or possibly years. A 65-million-year-old buried crater has been found in Mexico's Yucatán Peninsula (Gore, 1993). The crater is about the right size to have been made by the possible extinction causing asteroid. Other possibilities are that more than one impact was responsible or that the responsible asteroid left a crater in an area that has since been melted and consumed by the subduction zones at the edges of oceanic plates (Alvarez & Asaro, 1990).
........The Hell Creek formation, which recorded the dinosaurs' last two million years, has a thin layer of the metal iridium (Gore, 1993). Iridium is very rare on Earth but is common in certain asteroids. An extraterrestrial impact is probable because the ratio of iridium to similar elements such as platinum and gold is the same in the K-T boundary layer as it is in meteorites (Alvarez & Asaro, 1990). Iridium bands have been found in 65-million-year-old deposits all around the world (Gore, 1993).
........The sudden heating of the atmosphere during an asteroid impact would have raised temperatures high enough to combine atmospheric nitrogen and oxygen into nitrous oxide, which would have fallen from the sky as nitric acid (Alvarez & Asaro, 1990). This acid rain would have been much more acidic than the acid rain of today and would have caused the mass extinction of many species of tiny marine plants and animals whose shells would have dissolved in the acidified water.
........Shocked quartz crystals, which were formed under very high pressure, have been found in the rock layers that date back to the mass extinction (Alvarez & Asaro, 1990). This is strong circumstantial evidence for an asteroid impact or some similar high-pressure event because shocked quartz grains have been found only in impact craters and at nuclear test sites.
........Basaltic mineral grains have been found in the same rock layers as the shocked quartz crystals (Alvarez & Asaro, 1990). Rock on the earth's surface may have been shock-melted and blasted into space by an asteroid impact. Before falling back to Earth, the melted particles may have cooled rapidly and formed tiny basaltic mineral grains. The impact probably took place in the ocean because the ocean floor is made of basaltic rock and this is where the basaltic mineral grains most likely came from.
........In summary, according to the asteroid theory, the impact of an asteroid created enormous clouds of dust which covered the entire planet in darkness and resulted in a sudden climate change. There is geologic evidence of a major extraterrestrial impact which occurred 65 million years ago, the time of the mass extinction.
The Volcanic Eruptions Theory
........Nearly all the results of an asteroid impact (lack of sunlight, sudden cold, mass death of many species, etc.) could instead have been caused by the tremendous volcanic eruptions that took place in what is now India around the time the dinosaurs and many other species perished (Courtillot, 1990; Whitfield, 1991). These huge eruptions left the lava flows now called the Deccan Traps.
........The unusual iridium and clay band at the K-T boundary could have come from a hot spot in Earth's mantle (where iridium is abundant) just as easily as from an asteroid (Courtillot, 1990). Therefore, the geological evidence supports both the asteroid and the volcanic theories.
........In spite of the new evidence that the mass extinction was a sudden phenomenon, there is other new evidence that suggests the extinction occurred over a period of tens or even hundreds of thousands of years (Courtillot, 1990). The violent eruptions in India lasted for around 500 thousand years--about the same length of time. Also, the abnormal iridium concentrations in rocks in Europe at the K-T boundary are spread out for as much as 500 thousand geological years. The Deccan Traps eruptions lasted for just the right amount of time to account for the time distribution of the higher than normal iridium levels. Since an asteroid impact would have created a sudden, well-defined rise in iridium, the longer geological time distribution of iridium supports the volcanic theory rather than the asteroid theory.
........The shocked quartz crystals mentioned before seemed to be proof that the mass extinction was caused by a meteorite. However, shocked quartz grains can be formed at much lower pressures if the rock is heated before the shock, like in a volcanic eruption (Courtillot, 1990). Thus the iridium supports a theory involving gradual extinction rather than the more sudden extinction expected from an asteroid. The quartz crystal evidence does not point only to the asteroid theory; the quartz phenomenon could have possibly been caused by intense volcanic eruptions.
........There is a lot of evidence for both the asteroid impact and volcanic eruption theories. It's hard to decide which one best explains the evidence because the immediate and long-term effects of either scenario would be very similar (Courtillot, 1990). Even so, some scientists believe they almost have the answer to the riddle of what caused the mass extinction. Of course, these scientists have opposite opinions, so it may be some time, if ever, before we know the true reason for why half of all life on Earth (Alvarez & Asaro, 1990) was wiped out 65 million years ago.
........Many people think that it was bad that the dinosaurs became extinct, but was it really? By the end of the Cretaceous Period, most of the life on Earth had become very adapted to its surroundings, and evolution was slowing down (Alvarez & Asaro, 1990). But when the mass extinction occurred, the surviving plants and animals, particularly the mammals, had a lot of space in which to develop and evolve (like the excellent career opportunities law students would have if half of all lawyers were suddenly fired). When the dinosaurs existed, they ruled the earth as the dominant life forms (Alvarez & Asaro, 1990; Courtillot, 1990). After the dinosaurs became extinct, the mammals took over and became even more advanced than the dinosaurs. Mass extinctions may be necessary for the evolution of complex life. After all, we humans might never have existed if the dinosaurs hadn't become extinct.
Alvarez, W., & Asaro, F. (1990). An extraterrestrial impact. Scientific American, 263(4), 76-84.
Courtillot, V. E. (1990). A volcanic eruption. Scientific American, 263(4), 85-92.
Dane, A. (1991). Why the dinosaurs died. Popular Mechanics, August, 96-97.
Gore, R. (1993). Dinosaurs. National Geographic, 183(1), 2-53.
Milne, A. (1991). The fate of the dinosaurs: New perspectives in evolution and extinction. Garden City Park, New York: Avery Publishing Group.
Sheehan, P. M., Fastovsky, D. E., Hoffman, R. G., Berghaus, C. B., & Gabriel, D. L. (1991). Science, 254, 835-838.
Whitfield, P. (1991). Why did the dinosaurs disappear? New York: Viking Penguin.