What evidence do scientists use to descriibe asteroid impacts as a case for mass extinctions on earth?

Answer 1

The end of the "Mesozoic Era" coincides with the abupt disappearance of almost all large animal life on the Earth, about 65 million years ago. Luis Alvarez determined that a number of interesting factors including a layer of "shocked quartz" and iridium dust that occurred at about the same time, and this led to his hypothesis that a large object had impacted the Earth causing the extinction.

Answer 2

It is not as simple to say that meteorite impact cause mass extinction. The best example of this is the KT Boundary Event that caused the mass extinction event 65 million years ago that marked the end of the Cretaceous Period.

For many years scientist noticed that rocks dating back to 66 million years ago where relatively rich in dinosaur fossils were as rocks from 64 million years ago showed no dinosaur fossils. It was widely excepted that an mass extinction event occurred 65 million years ago. Many theories were put forward. From a super virus, to climate change, to the rise in mammals were looked at.

The Deccan Trap Theory.

For a long time scientist were aware of the Deccan Traps on the Deccan Plateau in Western Central India. It was an unusually large deposit of volcanic rock. When studying it further it was discovered that it dated back to 68 - 60 million years ago. It is estimated that the majority of the rock comes from an eruption event that occurred 66 million years ago near modern day Mumbai. The scale of this is very hard to imagine. Scientist believe that before erosion and plate tectonic forces the eruption measured 1.5 million km2 about 1/2 the size of modern India. Carbon dioxide, acid rain causing sulphur dioxide and poisonous carbon monoxide would have been released in vast quantities.

This would have changed earths environment catastrophically. Global warming and acid rain would have pushed life to the brink of extinction. However this kind of event would cause a gradual mass extinction. The K-T extinction happened too quickly for this to be the only answer.

The Alvarez Impact Hypothesis.

Then in 1980 a group of scientists looking at the layer from 65 million years ago and started to notice an unusually high level of iridium, an element that is extremely rare on earth but much more common in asteroids. All over the world when the layer was analysed the same concentration of iridium was found. The idea that an impact event caused the mass extinction was gaining credibility. However to prove the theory scientist needed to find the "smoking gun", the crater itself. After only 65 million years some scientist argued that there should be some evidence of it still remaining.

Then in the late 1980's, while looking for oil of the coast of the Yucatan Peninsular in Mexico, scientist noticed that the the rocks underground looked strange, as if there was a hidden crater underneath. More research done by Glen Penfield showed in 1990 that it was indeed an impact crater near the town of Chicxulub and more importantly it dated back to 65 million years ago. This was the definitive proof that an asteroid measuring about 6km in diameter hit Earth creating a crater some 180km across.

Super Nova Theory

When scientist noticed the high levels of iridium, some put forward the theory that a nearby star could have exploded. This would certainly explain the iridium. However it was soon realised that there should also be detectable levels of Plutonium-244. Under further analysis the element was not found in any abundance disproving the theory.

Maastrichtian Sea Level Regression

When scientist looked at rocks from the Maastrichtian Age (70 - 65 Ma) they noticed that layers associated with the end of the age are terrestrial but in the same place layers associated with the early Maastrichtian are from a coastal region. Since the rocks show no sign of distortion due to mounting building the only explanation is that sea levels dropped dramatically.

This would have caused massive changes to marine environments. The Western Interior Sea that covered the center of North America would have disappeared completely. Also the continental shelf environments which are the richest in marine life so loss of this habitat would have been devastating.

There has been no real evidence to the cause for the changes in sea level. The usual suspects such as an ice age have been ruled out. It is generally accepted that the mid oceanic ridges became less active and sank under there own weight.

Multiple Impact Theory.

Along with the Chicxulub Crater there is evidence of several large impact events from around the same time. Some scientists suggest that the asteroid that hit earth broke up due to the gravity of earth in the same way the comet Shoemaker-Levy 9 broke up when it approached Jupiter soon before it impacted the giant planet. The biggest chunk hit the Yucatan and smaller fragments hit in the Ukraine and also the North Sea.

Conclusion

The truth is probably all of these contributed to the mass extinction. The planet was in a state of flux. Huge changes in volcanism and changes to the environment meant that the planet was already dieing before the asteroid hit. The impact was enough to tip the balance and cause the mass extinction.

Answer 3

The first evidence that scientists discovered suggesting that a meteor caused the extinction of the dinosaurs was the layer of iridium in the Cretaceous/Tertiary boundary a layer of sedimentary rock that formed at the same time as the dinosaur extinction. Iridium is very rare on Earth, but common in asteroids. This led scientists to hypothesize that a massive asteroid strike was the cause of the dinosaur extinction. This hypothesis was later confirmed when scientists discovered the remains of a massive crater on the Yucatan Peninsula. After studying the crater, they determined that it was formed 65 million years ago, the same time as the dinosaur extinction.