The extinction of the dinosaurs has fascinated the scientific world for decades. At first the scientists though that dinosaurs were slow-witted, lethargic, cold-blooded animals like their reptilian contemporaries. These scientists believed that smarter, more advanced mammals came along and drove the dinosaurs to extinction. They said mammals took the dinosaurs' eggs and out-competed them for food. But this scenario does not seem possible because mammals evolved around 230 million years ago, about the same time as the dinosaurs, and one would need to know the complete world ecological system (from 65 million years ago) from an incomplete fossil record (Dingus, Gaffney and Norell 63).
Other scientists believed theories that said dinosaurs could not digest flowers or that the flowers poisoned them; some even went so far as to speculate that dinosaurs might have developed deadly hayfever. There is no evidence to support these theories in the fossil record.
Another theory says that the dinosaurs got too big for their own good, but the fossil record shows that small dinosaurs thrived until the extinction of all dinosaurs. Also, the fossil record does not show the dinosaurs getting progressively larger (Dingus, Gaffney and Norell 63).
Because the evidence does not support the above theories, scientists searched for other explanations for the extinction of the dinosaurs. The sudden extinction theory, that speculated dinosaurs died out suddenly, not gradually as previously thought, opened the door for a whole new generation of theories.
The extinction has also been blamed on climatic alterations associated with plate tectonics (the moving of the continents). The late Cretaceous climate was mild with no extreme temperatures, because parts of the continents were underwater, which would have buffered the temperature. When the seas retreated at the end of the Cretaceous, the temperatures reached more extreme levels with hot days with cold nights and hot summers with cold winters (Dingus, Gaffney and Norell 63). Some scientists believed that the dinosaurs would not have been able to survive those temperatures. However, snakes, lizards, turtles, and crocodiles did survive.
A very recent theory is that dinosaurs were warm-blooded. Being warm-blooded would have implied that dinosaurs would not have needed the sun to stay warm, therefore, they could survive the cold temperatures. This would also explain why the extinction would was not caused by this climate fluctuation.
Some theories attempt to answer the question: Are mass extinctions periodic? Nemesis is one of these theories. When scientists counted the extinctions and graphed them, they discovered a pattern. The extinctions peaked about every 26-30 million years, but these peaks are very small. The theory states that there is a faint companion star to our sun with an orbit that comes close to the sun about every 26 million years. The theory goes on to say that this companion star pulls comets out of their orbit in our solar system causing some of them to collide with our planet. There is no star known that could be attributed to this. A star that would fit this theory would be faint and hard to find because it would not be near our solar system at this point in its orbit. The last extinctions blamed on this star were 10-15 million years ago, so it is not scheduled for a return for at least 10 million years (Hecht 124).
Another theory that attempts to answer the same question of periodic extinctions is the theory that hot molten rock from a layer close to the metallic core of the earth comes to the surface causing major eruptions every 26-30 million years. The Siberian traps and the Indian deposits are thought to have formed by this type of eruption. It is not known what effect these eruptions would have or if these types of eruptions would even be able to cause mass extinction (Hecht 125).
Towards the end of the Cretaceous there was an increase of volcanism. Lava flooded large areas of India and explosions in the South Atlantic and the Midwestern United States hurled ash over much of the globe. Large lava flows come from volcanoes over hot spots like the hot spot that formed the Hawaiian Islands. “Instead of exploding, these volcanoes pour hot molten lava onto the land. They put less of their material into the air -- but the eruptions are immense. The largest have ejected over 240,000 cubic miles (a million cubic kilometers) of lava in a series of eruptions spanning at least thousands of years. That’s enough to cover the entire state of California with 1.5 miles (2.4 kilometers) of rock” (Hecht 71). Other times in history, major eruptions have fallen close to mass extinctions -- 360,00 cubic miles (1.5 million cubic kilometers) of lava in southwest India around 65 million years ago at the end of the Cretaceous and 480,000 cubic miles of lava in Siberia around 245 million years ago (Hecht 71).
These massive eruptions would have had major consequences. By throwing gases and soot into the atmosphere, the eruptions would have blocked out the sunlight causing the climate to cool for a few years. About 73,400 years ago an eruption in Sumatra formed a crater, spread 670 cubic miles of molten rock and vapor (500 times larger than Pinatubo in 1991) and cooled the world 7 to 10 degrees Fahrenheit (4 to 5 degrees Celsius) for several years (Hecht 70). The debris in the atmosphere would have also caused acid rain. Over the long term the climate would warm through the greenhouse effect. However, this climate change does not explain the selectiveness of this mass extinction -- why some species survived and some did not.
The element iridium might hold a clue to the mystery. Iridium is uncommon on the earth’s surface but it is abundant in the clay layer, Cretaceous-Tertiary boundary layer, found at the time of the dinosaurs' extinction. This element is sometimes found in volcanic rock. Iridium has also been found in volcanic gas in a Hawaiian volcano. The amount of this element in the volcanic gas was found to be 10,000 times greater than in the lava. But the lava in Southwest India from about 65 million years ago thought to have caused the dinosaurs’ extinction has little Iridium in it (Dingus, Gaffney and Norell 66) .
The asteroid impact theory can account for the high amount of iridium. This element is common in asteroids and it is found in high concentrations. Microscopic meteors fall to Earth continually. “By measuring how many of the meteorites fall to Earth over a given period of time, scientists can estimate how long it might have taken to deposit the observed amount of iridium in the boundary clay. These calculations suggest that a period of about 1 million years would have been required. It has been estimated that dinosaurs went extinct in about half a million years” (Dingus, Gaffney and Norell 65). If this is true the natural meteor deposition would not have lasted 1 million years. This unusually high concentration of iridium seems to require a special explanation. “A few beds of the boundary clay contain microscopic grains if quartz that have small fractures oriented in several different directions. Proponents argue that the fractured texture could have been created only at the temperatures and pressures generated by an impact” (Dingus, Gaffney and Norell 67). Similar things are found in rare volcanic rock. Extremely small, glass spheres that have been found in Haiti are said to be cooled drops of molten rock thrown into the atmosphere by the impact.These spheres have been dated at 65.01 million years 80,000 years. Similar spheres have been found in other clay beds. The spheres are also thought to be fossilized algae, volcanic ejecta, microscopic meteorites or organic remains. Soot is often found in the clay layer, also. It may have come from massive global forest fires at impact or natural causes over a long period of time (Dingus, Gaffney and Norell 67).
Layers of rock are piled up like old newspapers -- the oldest ones on the bottom and the newer ones on top. There is a problem with the clay boundary layer it is about ten feet (3 meters) above the highest known dinosaur fossil, which is thought to mean that the dinosaurs were extinct before the impact, but since dinosaur fossils are rare it is highly improbable that one would be found directly under the boundary layer.
Scientists trying to understand the end of the Cretaceous have calculated the effects of the impact of a six mile (10 kilometer) asteroid, which is the size the authors of this theory, Luis and Walter Alvarez, have estimated the asteroid to be. Luis and Walter Alverez have also estimated the asteroid to have weighed 10 grams (4.54 x 10 pounds) and have traveled 31 miles per second (50 kilometers per second). “Even before the object hit the ground its energy would have split air molecules, converting nitrogen and oxygen into nitrogen oxides. That would have made acid rain as strong as the acid in an automobile battery near the impact. It would have burned the leaves off trees and the skins off the animals hiding beneath them. It would have made the upper layer of the oceans strongly acidic, killing the plants and animals that lived there” (Hecht 67). The six mile asteroid would have formed a crater about 100 miles (150-200 kilometers) wide.
Finally in the early 1990’s, the Chicxulub crater, 180 kilometers wide, was discovered on the Yucatan Peninsula in Mexico. This crater is partially submerged in the Gulf of Mexico now and it was submerged in shallow seas at the time of the extinction. The crater has been dated at 64.98 million years 50,000 years. “The most recent radio metric dating [uses atomic process of radioactive decay] from volcanic units near the Cretaceous-Tertiary boundary just above the highest dinosaur fossils in Montana place the age of the boundary at about 65.01 million years 30,000 years or 65.17 million years 40,000 years” (Dingus, Gaffney, and Norell 68). The impact of the asteroid not only would have been heard around the world, it would have shaken the whole planet. The material forced into the atmosphere would have blocked out the sun and caused freezing temperatures for a short period of time. Without the sun, plants died, then the plant eaters died and finally the scavengers would have died after the plant-eaters were eaten. For the long term it would have significantly raised temperatures through the green house effect. If the asteroid hit the ocean, there would have been a tsunami as high as the ocean is deep. There is tsunami evidence along the Brazos River in Texas, which has a thick layer of stones. There are large stones on the bottom and they get smaller as they go up, eventually ending with sand and then silt (Hecht 76). This was formed by a huge Caribbean tsunami when that part of Texas laid along the Caribbean Sea.
Another crater has been found near Manson, Iowa. It is 25 miles by 18 miles (40 kilometers by 29 kilometers) and has been dated at 65 million years old. This crater has raised the question: Were there two impacts? In 1989 and again in 1991, astronomers found that an asteroid passing near the earth was really two big rocks held together by gravity. Or there might have been a huge comet passing near the sun that broke into two or more parts on a path that crossed Earth’s orbit (Hecht 81). Or the earth may have gotten in the way of a shower of comets. If so, it may have been recovering when it was hit by another object or there may have been more than two comets. It is know that some craters are well hidden. The crater near Manson was not found until people who were drilling for water found some unusually shattered rocks. The crater was buried during the last Ice Age, and there may be more craters waiting to be discovered.
Realizing what an asteroid might have done to the dinosaurs makes one think about when or if it could happen again. In 1908, in the Tunguska region of Siberia, a 100 foot (30.5 meters) chunk of ice or rock exploded in the atmosphere leveling 800 square miles (2000 square kilometers) of forest. Luckily, no one lived there. It has been estimated by John Pike of the Federation of American Scientists that, if this had happened in a Midwest farming area, it would have killed 68,000 people and caused $4.5 billion in property damage. How many people would have been killed if this had happened in New York, Rome, Tokyo or another major city? In 1992, the comet Swift-Tuttle was thought that it would collide with the earth in 2126 but it was later estimated that it would miss the earth by 25 million miles. But what about other comets and asteroids? Many have been discovered, none as big as the asteroid that made the Chicxulub crater, and none are on a collision course with our planet. “However, a small impact could still cause big problems, and there probably are thousands of unknown objects larger than .3 mile (.5 kilometer) which could hit with little or no warning. An impact too small to cause mass extinction could devastate the planet. A species can still survive even if 90 percent of the individuals die. Human civilization might not” (Hecht 133).
The two remaining theories, the volcanism theory and the asteroid impact
theory, are the most studied and the most supported. While we may never know
what happened, this mass extinction needs to be studied to prepare for what
could happen in the future.
Dingus, Lowell, Eugene S. Gaffney, and Mark A Norell. Discovering Dinosaurs
in the American Museum of Natural History. New York: Alfred A. Knopf,
Inc., 1995. 62-67
Hecht, Jeff. Vanishing Life: The Mystery of Mass Extinctions. New York:
Charles Scribner’s Sons, 1993.
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