Dinosaur Extinction

By: A. Buckley

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.

The two main 'serious' theories are the asteroid and volcano theories, both of which make some use of the analysis of the rocks in and around the K-T boundary (the Cretaceous - Tertiary boundary). The use of K comes from the Greek word for chalk (Kreta) which is found in great quantities in the rocks of the Cretaceous.

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, 

(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

The thin clay layer that marks the boundary between the Cretaceous and Tertiary rocks. This layer has been found at many localities around the Earth. (Courtesy of Canadian Museum of Nature, Ottawa)

Of the two more serious theories perhaps the most well supported theory is concerned with the impact of a large asteroid type body. It is a well-known fact that throughout the history of the planet there have been many thousands of impacts some large and some small. The results of these impacts can have a wide variation. Two recent (in geological time) events show these differences in results very well. The results are known as the Tunguska Fireball and the Barringer Meteor Crater. 

  

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. 

  

 Barringer Meteor Crater 

Barringer Meteor Crater in northern Arizona, is a bowl-shaped depression 180m deep and 1.2km in diameter. It was named after American scientist Daniel M. Barringer who in 1905 theorized that the crater was meteoric in origin. The crater was the first impact site of an object to be identified on the Earth. Scientists think that the crater was formed sometime between 25,000 and 50,000 years ago by an iron meteorite, somewhere between 30 and 100m in diameter, weighing roughly 60,000 tons. The energy released by the impact was roughly equivalent to 3.5 million tons of TNT. Most of the object vaporized, but about 30 tons of fragments have been collected. There are other craters on the planet but due to erosion and plate movement it takes a trained eye to find them. From these two examples it can be seen that when large objects enter the atmosphere a great deal of energycan be released causing large amounts of damage. This evidence helps to support the Asteroid theory.

Barringer Meteor Crater, Arizona, USA

A photograph of the 50,000 year old crater in Arizona. (Courtesy of United States Geological Survey) 

The Asteroid Theory 

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?

An artists impression of the meteorite that was responsible for the death of the dinosaurs. 
Artist: Don Davis (No Copyright-see NASA "USE" policy)

On the land the effects of the impact on the flora and fauna would have been devastating, especially on the large animals which would need large food supplies and on the dinosaurs which would need sun light to keep warm. The global fires would have destroyed considerable amounts of vegetation (by the analysis of the soot in the K-T boundary it is estimated that 25% of the vegetation cover was destroyed), the immediate effect of this would have resulted in the death of the large herbivores. A knock on effect of this would have killed off the large carnivores. Only the small active scavengers, like birds and mammals with the ability to find food from a wide range of sources would have survived. Analysis of the K-T boundary fossils shows that there was a short term takeover of the land by the hardy ferns, which moved into the areas were there had been fires.

In the sea the effects would have been just as dramatic. There would have been a decrease in the oxygen levels in the seawater as low oxygen deep seawater would have been brought up by massive under water currents. This would have resulted in a massive disturbance of the marine food chain through the death of much of the plankton. This would have resulted in the eventual death of the marine reptiles which would have relied on the food chain. There would also have been a massive death rate amongst the shelled sea animals like the ammonites. There could also have been a serious increase in the acidity of the seas caused by the acid rains. This may have also killed off some of the sea species.

The period of recovery would have seen the surviving species moving into the ecological niches left vacant by the dead species. After a short period of time some of the plants that had been burnt down would have regrown from buried seeds or rootstock. As is common with all mass extinctions there would have a sudden evolutionary burst as new species developed. The age of the mammals was beginning.

except taken from http://web.ukonline.co.uk/a.buckley/dino.htm