8. Extinction Theories - Background

Teacher Background
There have been many mass extinctions throughout the history of the Earth. A mass extinction may be defined as an episode in geologic history where over half of the species in existence become extinct in a relatively short amount of time (just a few million years). The worst mass extinction came at the end of the Paleozoic Era 245 million years ago when nearly 95% of plant and animal life in the seas disappeared. Another mass extinction may be happening today. Evidence from the fossil record shows that, on average, only 10-100 species become extinct per year. Some estimates show that current rates of extinction are as high as 27,000 species per year.

Probably the most famous mass extinction happened 65 million years ago when the dinosaurs disappeared. This is generally called the K-T extinction since it occurred at the boundary between the Cretaceous (K) and Tertiary (T) periods. Whatever triggered the extinction of the dinosaurs also caused the death of nearly 60-70% of all the other species on Earth. Interestingly, not all groups of organisms were affected equally. Ocean species were hit harder than land-based species, with 90% of them becoming extinct. Birds were the only survivors of the dinosaur lineage. Interestingly, mammals, lizards, snakes, and other smaller terrestrial creatures were hardly affected. For some reason, ferns actually expanded and thrived during this time.

So what caused the dinosaur extinction? The clues can be found in the rocks that date from 65 million years ago. Some pieces of evidence are agreed upon by nearly all scientists:

  1. Around the time of the K-T extinctions, there was global climate change. What was once a warm, mild climate changed to one that was more varied (sometimes very hot, sometimes very cold).
  2. In India, there was extensive volcanic activity, known as the Deccan Traps. Rocks that date from 65-68 million years ago in southern India are almost entirely igneous (volcanic). Almost 200,000 square miles was covered in lava over a period of 3 million years (an area equivalent to the entire mid-western United States). In some places, the lava beds are a mile deep.
  3. A scientist called Alan Hildebrand found evidence of what is most likely an enormous asteroid impact site in the Yucatan region of Mexico. He called the 110 mile wide crater Chicxulub. The crater has been dated as 65.3 million years old (300,000 years before the mass extinctions). The size of the crater is comparable to that which would have been caused by an asteroid 6 miles wide.
  4. Rocks that date from 65 million years ago contain high amounts of soot
  5. Rocks that date from 65 million years ago contain unusual amounts of the rare element iridium. The levels of iridium contained in the rocks from this time period are roughly 30 times the normal levels. The most likely sources of high levels of iridium are:
    • from outer space in the form of cosmic dust from a nearby exploding supernova
    • from outer space carried to Earth by an asteroid or meteor
    • from eruptions of massive volcanoes.
  6. Major changes in the organization of the continents were occurring around 65 million years ago due to plate tectonics. There is evidence that the oceans were receding. For example, a shallow sea once covered what is now the mid-western United States. This sea drained away over several million years around the time of the dinosaur extinctions as the Colorado Plateau rose.

Yet, even with an abundance of evidence, there is no consensus among scientists as to what happened at this time. Generally speaking, scientists are divided between two camps:

Gradualists – These scientists believe that the fossil record indicates a gradual decline over 5-10 million years. This time frame is more consistent with long term events such as plate tectonic forces and massive volcanic activity. These scientists believe that plate tectonic forces caused extensive volcanic activity in India and perhaps elsewhere that resulted in dense clouds of soot being released into the air. The soot darkened the skies resulting in global climate change and the reduced survival of plants, algae and plankton. In addition, the volcanoes likely released large quantities of carbon dioxide, further aggravating the climate through global warming and acid rain.

Catastrophists – These scientists believe that the fossil record indicates a sudden decline that is more consistent with a catastrophic event such as a massive asteroid impact. This theory was first proposed in 1980 by Walter Alvarez and is often referred to as the Alvarez Theory. He based his claims on the high iridium levels in rocks of that time period – suggesting that the isotopic profile of the iridium is more consistent with an extraterrestrial origin (a meteor or comet impact) than a volcanic origin. This is consistent with the presence of shocked quartz (metamorphically transformed quartz resulting from intense shock waves) in the rocks of that age. The resulting blast would have destroyed everything within 250-300 miles, including the object. Trillions of tons of debris (like dust, smoke, and steam) would have been thrown into the atmosphere when the object vaporized, darkening the sky around the globe in just a few weeks. Earthquakes, tsunamis, and wildfires would almost certainly have been triggered. The darkness may have only persisted for a few years but the effects on plant life would have been devastating.

The biggest problem with the catastrophic theory is that no conclusive crater has been identified. The most promising is evidence of an ancient crater called Chicxulub that was discovered in the Yucatan peninsula of Mexico. It is widely believed that Chicxulub is indeed the result of a massive asteroid nearly 6 miles across. Unfortunately, the crater itself is dated to 300,000 years before the K-T extinctions themselves. Could there have been another meteor impact or even a series of impacts? An even larger crater, the Shiva crater, was reported by Sankar Chatterjee under the Arabian Sea, off the coast of India. It is called the Shiva cater and dates from 65.0 million years ago. The Shiva crater is about 370 miles across and 7.5 miles deep. However, what created the crater is unknown. If it were made by an asteroid or meteoroid, the object must have been at least 25 miles wide. Other geologists claim the Shiva crater is the result of a sinkhole in the Earth’s surface, not an asteroid.

This ongoing debate offers an exciting opportunity for students to sort through the clues and propose a theory to explain the extinction of the dinosaurs. The key to this activity is for students to begin by organizing the evidence into sets of related information and then use the evidence to support a logical theory. Since there is no right answer students have an opportunity to engage in a true scientific debate over the same set of data that paleontologists, geologists, and astronomers argue over. Furthermore, there are endless directions in which the debate may travel, opening endless opportunities for further exploration.

Student Prerequisites
Students should have an understanding of how fossils form and have experience with the geologic time scale. It is helpful to have a good foundation in the rock cycle and stratigraphy so as to better understand how the evidence provided may have been gathered.