One of the great mysteries in science is the extinction of the dinosaurs at the end of the Mesozoic era some 65 million years ago. Who (or more likely what) caused it is unknown and a subject of great debate.
Dinosaurs appeared at the beginning of the Mesozoic era and were the dominant form of life for the next 140 million years. They lived almost everywhere there was land including Antarctica. We can see their bones in the geological record laid down over time. The lower stratum of rock contains the earliest and most primitive species of dinosaur, and the upper stratum contains the newer species.
There have been many mass extinctions throughout the history of the Earth. Probably the most famous is the extinction that finally saw the end of the dinosaurs reign on the Earth, 65 million years ago. It wasn't just the dinosaurs that died out in this extinction. Whatever caused the death of the dinosaurs also caused the death of around 70% of all of the species on the Earth. Although the dinosaurs had been in a period of decline, it is thought that their recovery was prevented by some sort of catastrophic event. There are many theories about why the dinosaurs finally became extinct, some of which are more than a little outlandish.
Then, suddenly, at a geological strata line called the Cretaceous-Tertiary boundary (often referred to as the K-T Boundary), the dinosaurs disappear.
The K-T Boundary and Iridium
In the late 1970's Luis and Walter Alvarez (father and son) along with a team of scientists from the University of California were making a study of the rocks around the K-T boundary in Gubbio, Italy. In particular they were looking at an unusual layer of clay at the boundary point which contained an unusual spike in the amounts of the rare element iridium. This spike revealed that the levels of iridium contained in the clay were roughly 30 times the normal levels. In parts per million iridium is present in the following amounts,
Crustal Average - 0.001
Granite - 0.00001
Diabase - 0.003
(The term diabase is used to describe certain types of gabbro.) From these figures it can be seen that iridium is an extremely rare element, so it's discovery in 'large' amounts indicates that something serious happened. There are 2 sources of iridium, the main source comes from outer space in the form of cosmic dust which is constantly showering the planet. A second source is the Earth's core when there are eruptions of certain types of volcano. It is believed that the iridium, plus many other rare elements, were carried down and concentrated in to the Earth's core while the Earth was still largely molten. During this time certain types of primitive chondritic meteorites were formed where no concentration could have taken place due to rapid cooling. This means that it is possible that within the primitive chondritic meteorites there could be reasonable levels of iridium. From this information it can be seen that there are only two possible theories to explain the increased presence of iridium in the clay layer either an asteroid strike or a massive volcanic eruption.
Asteroid Signature
Tunguska Fireball
On June 30, 1908, the area near the Tunguska River in Siberia was the site of a remarkable explosion. The explosion, which took place at an altitude of roughly 8 km, had a force that was roughly equivalent to a 10-megaton atomic bomb. The explosion caused a shock-wave that flattened forests covering an area of more than 1000 square kilometers and killed herds of reindeer and other animals. No crater was formed, and aside from some microscopic nodules extracted from the soil, no recognizable fragments of the object remain. Scientists generally believe that the explosion was caused by an object with an approximate weight of 100,000 tons.
The Asteroid Theory
The Barringer Meteor Cater in Arizona though impressive, is only about a mile wide. Just a fraction of the 130 mile wide crater left by the asteroid that ended the Mesozoic Era. (Copyright Lee Krystek, 2008).
The first people to suggest the asteroid theory were the team lead by Luis and Walter Alvarez. It has been calculated that a chondritic asteroid approximately 10km in diameter would contain enough iridium to account for the iridium spike contained in the clay layer. Since the original discovery of the iridium spike other evidence has come to light to support the asteroid theory. Analysis of the clay layer has revealed the presence of soot within the layer. It is thought that the presence of the soot comes from the very large global fires that would have been the result of the large temperatures caused by an impact. Something else that was found within the clay were quartz crystals that had been physically altered. This alteration only occurs under conditions of extreme temperature and pressure and quartz of this type is known as shocked quartz. Despite all of this evidence many geologists did not believe in this theory and many were saying 'show us the crater'.
Blasted Rock
The parallel lines on this sample of quartz show what happens when this particular mineral is subjected to extremely high temperatures and pressures such as those obtained from meteorite impacts or nuclear explosions. (Courtesy of United States Geological Survey)
In 1990 a scientist called Alan Hildebrand was looking over some old geophysical data that had been recorded by a group of geophysicists searching for oil in the Yucatan region of Mexico. Within the data he found evidence of what could have been an impact site. What he 'found' was a ring structure 180km in diameter which was called Chicxulub. The location of this structure was just off the northwest tip of the Yucatan Peninsula. The crater has been dated (using the 40Ar/39Ar method) as being 65 million years old. The size of the crater is comparable to that which would have been caused by an impacting body with a diameter of roughly 10km.So we now have some of the proof of the asteroid theory. We know that a chondritic meteorite with a diameter of 10km contains enough iridium to cause a spike. We also know that about 65 million years ago there was an impact of a large object. The big question is what were the results, and how did they effect the dinosaurs.
The Yucatan Peninsuka
A map showing the probable location of the crater formed by the impact of the K-T meteorite.
Chicxulub, Yucatan Peninsula, Mexico
This three-dimensional map of local gravity and magnetic field variations shows a multi-ringed structure called Chicxulub named after a village located near its center. The impact basin is buried by several hundred meters of sediment, hiding it from view. This image shows the basin viewed obliquely from approximately 60° above the surface looking north, with artificial lighting from the south. (Courtesy of V. L. Sharpton, LPI)
If a 10km diameter object impacted at the point at which it struck it would have a velocity of roughly 100,000 km/h. At this velocity there would have been an initial blast (with an estimated force of many millions of tons of TNT) which would have destroyed everything within a radius of between 400 and 500km, including the object. At the same time large fires would have been started by the intense shock wave which would have traveled long distances. Trillions of tons of debris (dust, gases and water vapour) would have been thrown into the atmosphere when the object vaporized. Many enormous tidal waves would be started causing even more damage, the evidence of such waves has been found all the way round the Gulf of Mexico. Along with the tidal waves the blast would also start a chain reaction of earthquakes and volcanic activity there would have also been very high winds caused by the blast. In the days and weeks following the impact the cloud of debris would have been carried over large distances by the post blast high winds. This will have caused months of darkness and a decrease in global temperatures. After this there would have been an increase in temperatures caused by the large amounts of CO2 released by what would have been global fires. Eventually this would cause chemical reactions that would result in the formation of acid rains.
Dino Killer?
The Double Whammy Theory
It may be that the dinosaurs were not just the victim of a single event, but a string of bad luck. Professor Arens has carefully looked at this idea and has attempted to measure the extinction rate of species during periods when there were major asteroid impacts, periods of massive volcanic eruptions and periods when neither were happening. Surprisingly, the rate of extinction for each type of period is about the same. What she discovered, however, is that during periods when both massive volcanic eruptions and impacts were taking place at the same time extinction rate climbs. This suggests that no single disaster event might cause mass extinctions, but a combination of two or more are needed for it to happen, hitting life on earth with a "double whammy."
The fossil record shows that the mass extinction that killed the dinosaurs wasn't the only one that has occurred in our planet's history. The earth has suffered a number of such traumatic events over its life. In fact the K-T extinction wasn't even the largest. The Permian-Triassic (P-Tr) extinction event, sometimes referred to as the Great Dying, occurred about 250 million years ago and eliminated 90 percent of known species from the planet. As with the K-T extinction, nobody is quite sure what caused this disaster.
Are They All Dead?
Did any of the dinosaurs survive the extinction? Scientists have very rarely found bones of dinosaurs buried above the K-T Boundary. A single Hadrosaur leg bone found in the San Juan Basin, New Mexico, might suggest that a small population of these dinosaurs survived as long as a half a million years into the following Paleocene era. However, it is also possible that the fossils in question, which are very few in number, were unearthed by some geologic event, then reburied at a higher level.
Occasionally stories still appear about dinosaurs being found still alive today in some remote location of the world (for example, the legend of mok'ele-mbembe in Africa). While there are several famous fictional books like Arthur Conan-Doyle's The Lost World on this subject, there is no hard evidence that any dinosaur, other than the birds, their avian decedents, have survived into modern times.
It is likely scientists will continue to puzzle over the death of the dinosaurs for many years to come. Part of the mystery of K-T extinction is why certain species died out while others survived. Mososaurs went extinct while other marine reptiles, like crocodiles, are still around. If climate change is responsible why did the dinosaurs, hearty creatures that lived in all kinds of conditions all over the planet, die when frogs, who are much more sensitive to temperature change, still survive today?
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