WHAT KILLED THE DINOSAURS?

Key Concepts

  • Dinosaurs died out during the Cretaceous mass extinction, which occurred 65 million years ago.
  • A large asteroid or comet struck the Earth 65 million years ago.
  • The asteroid/comet impact may have triggered the mass extinction.

(1) Dinosaurs died out during the Cretaceous mass extinction, which occurred 65 million years ago.

Dinosaurs represented the most complex lifeforms on Earth during the Jurassic and Cretaceous periods (from 210 million to 65 million years ago). Although the 145 million years of their flourishing represents a long run, the curtain finally went down on the dinosaurs during the Cretaceous mass extinction. During this mass extinction, which happened at the end of the Cretaceous period, 65 million years ago, about 70% of all species of living things became extinct, including ALL surviving dinosaur species. (Some species, such as Apatosaurus and Stegosaurus, went extinct long before the end of the Cretaceous.) In addition to the dinosaurs, the mass extinction also killed off all the ammonites (a marine invertebrate which was previously extremely abundant.) On land, many plant species died; in the sea, many species of plankton were wiped out.

Whatever hypothesis we select to explain the mass extinction must not be dinosaur-specific. The mass extinction killed a wide variety of species from Tyrannosaurus Rex to microscopic plankton.

The Cretaceous mass extinction was abrupt, by geological standards. From fossil records, we can only estimate that it took less than a million years to be completed, but it may have taken a considerably shorter time than that. Sedimentary rocks laid down during the Cretaceous period contain dinosaur fossils; rocks laid down during the Tertiary period (the period immediately after the Cretaceous) do NOT contain dinosaur fossils.

Mammals, although they coexisted with dinosaurs during the entire Jurassic and Cretaceous periods, were exclusively small, mouse-like creatures prior to the mass extinction. Afterwards, they evolved to fill the ecological niches left vacant by the defunct dinosaurs. Today, of course, there's a wide variety of mammals, from shrews to blue whales. If the Cretaceous mass extinction had not occurred, then mammals might still be small and mousey today, and the universities would be full of intelligent dinosaurs.

(2) A large asteroid or comet struck the Earth 65 million years ago.

A few decades ago, there were literally dozens of hypotheses about what may have caused the extinction of the dinosaurs. The current favorite, however, is the collision hypothesis, which is supported by an increasing mass of evidence.

The collision hypothesis states that the Cretaceous mass extinction was triggered when an asteroid (made of rock and/or metal) or a comet (made of dusty ice) struck the Earth. A large asteroid or comet, no matter what it's made of, packs quite a wallop.

Consider an asteroid with a diameter of 10 kilometers. Its mass will be roughly a trillion tons. If it is traveling at a speed of 10 kilometers/second relative to the Earth (about 20,000 mph), its collision with the Earth will release an energy of 100 million megatons. (The ``megaton'' is the energy content of a million tons of TNT. A hydrogen fusion bomb releases about 10 megatons of energy.) Such an enormous release of energy will blast a crater into the Earth's crust about 100 kilometers across, and will (as you might imagine) have a severely detrimental impact on the environment.

Postponing the question of whether the impact will have enough of an effect to kill off ALL the dinosaurs, let's first address the question of whether a large asteroid/comet hit the Earth at all 65 million years ago. (If there was no collision, then the collision hypothesis goes out the window.)

Look for the ``smoking gun'' - the hundred kilometer wide crater left by the asteroid or comet. We know that impact craters exist on the Earth today - for instance, Barringer Meteorite Crater in northern Arizona (shown below). However, this crater is only 1 kilometer across, and 50,000 years old -- far too tiny and far too young for our purposes.


Hiding a 100+ kilometer crater is surprisingly easy if it is 65 million years old. Erosion is very effective over the course of millions of years. (In addition, if the crater happened to fall on a section of the ocean floor that has since been subducted, all evidence of it would now be destroyed.) The crater corresponding to the Cretaceous impact wasn't discovered until recently. Geologists working for the Mexican national oil company discovered rocks that had been shocked and melted, hidden away beneath the sediments of the Yucatan peninsula. Further investigation showed that the disturbed rocks formed a circle 180 kilometers in diameter. The circular structure was actually a buried impact crater. (It has been given the name of the ``Chicxulub crater'', after a community close to the crater's center.) Dating of the molten rocks gave an age for the Chicxulub crater of 64,980,000 years (plus or minus 50,000 years). The age is right, the size is right; the Chicxulub crater is just what was predicted by the collision hypothesis.

Location of the Chicxulub crater:

Additional evidence, which emphasizes the widespread effects of the asteroid/comet collision, is provided by the iridium layer which separates Cretaceous rocks from younger rocks in many regions of the world. Iridium is a dense metal, like platinum and gold, which is very scarce in terrestrial rocks. When the Earth was molten, dense metals like iridium had a tendency to sink to the Earth's core. However, asteroids and comet dust are relatively rich in iridium. Thus, if an asteroid or comet struck the Earth, we would expect iridium-rich eject to be thrown high in the atmosphere and settle down gradually all over the Earth.

A photograph of the iridium layer in Italy (thousands of miles from the Yucatan) is shown below:


The rocks at the bottom right date from the Cretaceous period. The rocks at the upper left date from the Tertiary (post-Cretaceous) period. The dark band between them is a layer of clay in which the iridium density is hundreds of times the normal level. The total amount of iridium in the worldwide iridium layer is about what you would expect to find in a 10 kilometer wide asteroid. (And somewhat more than you'd expect to find in a comet, which points to an asteroid rather than a comet being the culprit.)

(3) The asteroid/comet impact may have triggered the mass extinction.

Okay, so the dinosaurs (and many other species) died out 65 million years ago, and an asteroid (or maybe a comet) hit the Earth 65 million years. Did the asteroid impact cause the mass extinction, or was it merely coincidental? Relatively few dinosaurs were killed by being hit directly by the asteroid. Slightly more were killed by the blast which formed the Chicxulub crater. However, the crater (180 kilometers in diameter) only covers a small fraction of the Earth's surface. To kill EVERY dinosaur, an indirect method is required -- nothing so crude as hitting them on the head.

The killing method worked like this:

  • The impact of the asteroid (or comet) throws a layer of dust high into the atmosphere (the mother of all mushroom clouds).
  • Fine dust stays in the upper atmosphere for months prior to settling down to earth and forming the iridium layer.
  • Dust in the atmosphere blocks out sunlight, making the Earth much darker and colder than normal.
  • Plants die from lack of sunlight.
  • Herbivores die (particularly those who browse on leaves).
  • Carnivores die.
The death certificate for the dinosaurs would read ``died of starvation, with hypothermia as a contributing factor''. The omnivorous, versatile, mousey mammals manage to squeak through (if you'll excuse the pun) because they can eat seeds and roots which lie dormant during the time of darkness (not to mention nibbling on dead dinosaurs).
Can a mass extinction, triggered by the impact of a large asteroid, happen again?
Yes.
Given the number of asteroids whose orbits cross that of Earth, it is estimated that every century, there's a one-in-a-million chance of a killer asteroid striking the Earth. This estimate is supported by the fossil record, which shows roughly one mass extinction per 100 million years.

Worried about the possibility of an asteroid striking Earth? More information is available from the Asteroid and Comet Impact Hazard page, provided by the NASA Ames Space Science Division.

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