Meteor Impact: the Day the Dinosaurs Died
By Charles Shaw
Chicken Little was right, the sky can fall. At least rocks do fall out of the sky from time to time.
Take one hugely significant example. Sixty-five million years ago an asteroid about ten kilometres (six miles) in diameter and moving at a velocity of 20 kilometers per second slammed into the Earth at a point near the modern city of Merida on the Yucatan Peninsula. Figure 1 shows an artist´s conception of the moment of impact into the ocean.
The center of the impact is found on the coast of the Gulf of Mexico, just north of Merida, at a Maya fishing village called Chicxulub (chick-shoo-lube). A huge circular crater surrounds the spot. It measures 180 kilometers (108 miles) from one side to the other. Impressive? Of course! But you can´t see it on the ground because it is buried under a kilometer (3000 feet) of limestone that was deposited after the impact.
At the time this rock fell out of the sky, theYucatán Peninsula did not exist in its present form. The area then was a shallow rise on an ancient ocean floor which has been uplifted from the sea some time after the meteor hit the Earth. On Figure 2, a radar map of the modern Yucatán Peninsula made from space by NASA, the crater can be seen in the northwest corner of the peninsula as a faint half-circle onshore with the other half under the Gulf of Mexico. The darkest green color near the coast marks the lowest altitudes just above modern sea level and the light blue-gray color at the south edge of the map indicates altitudes of over 200 meters (600 feet). The crater outline is reflected through the kilometer of younger rock strata much like a dinner plate hidden under a rug.
The consequences of such a large impact were immediate and severe. A large plume of melted and vaporized limestone rock was blasted into the air along with vast quantities of pulverized rock material and enormous boulders. The hot gases formed by vaporization of the limestone rock consisted largely of carbon dioxide, which we know is a potent greenhouse gas. The vapor, saturated with calcium and magnesium released from the rock, rolled outward from the impact site as a plume hugging the land surface with a temperature measured in thousands of degrees and with its front moving at 60 kilometers per second. Limestone and dolomite (a magnesium-rich limestone) from the plume began to condense as very fine dust as the gas expanded and cooled. The dust collected like a blanket of hot snow over the land surface, reaching a thickness of 40 feet near Chetumal and nearby parts of Belize. Figure 3 shows a quarry cut into the deposits, which are called ejecta. The front of the plume would have covered the 200 kilometers between Chicxulub and Chetumal in about 90 seconds! The devastation caused by the plume was immediate and total.
At greater distances other phenomena came into play. Large quantities of finely pulverized material along with larger blocks, some big as a house, were lofted high into the stratosphere and some into space beyond the atmosphere. The finest material spread out around the Earth along the top of the stratosphere where they blocked the sunlight, causing a dense twilight over the Earth. The larger blocks that fell back to Earth from space were heated to incandescence by atmospheric friction. Where these fell into the global forest, they started wildfires. Soot from the fires flooded the atmosphere and completely shut off sunlight from reaching the surface for months, or even years. As these particles slowly fell to Earth, they accumulated as a distinct layer a few inches or less thick. The layer is found all over the deep ocean basins and, wherever rocks of the correct age are found on the land.
The Earth was not a lifeless place at the time of impact. It was home to a highly developed ecosystem of microbes, plants, insects, and animals. The largest and most famous of the animals were the dinosaurs. Figure 4 shows Tyrannosaurus Rex getting ready for dinner. The dinosaurs and the ecosystem of which they were a part had developed over 190 million years, since the end of the Paleozoic Era, 255 million years ago. The impact sixty-five million years ago destroyed the ecosystem of that time completely. This is not the same as saying there were no survivors, there were, or the modern world around us, including ourselves, would not exist. But, sixty-five percent of all living species went to extinction, a disaster by any reckoning. The modern world evolved from the survivors.
The survivors included numerous species of small mammals, creatures with warm blood, which lived in burrows underground, perhaps also creatures that hibernated. Without sunlight there was no photosynthesis, so no plants could grow. Creatures that could eat dead plants, seeds, and carrion had the best chance of survival among the vertebrates. Insects that laid their eggs in protected places also could survive. But upon the return of sunlight, the survivors saw a bleak, destroyed world that lacked a functioning ecosystem to sustain them. A new ecosystem had to evolve from the pieces of the old.
One inheritance from the impact was an atmosphere heavily laden with carbon dioxide and sulfur dioxide. These produced two things; a very warm greenhouse world and a long period of acid rain. Water vapor reacted with carbon dioxide to make carbonic acid and with sulfur dioxide to make sulfuric acid. These caused acidified lakes, streams and ocean and slowed regeneration of plants. Estimates vary, but it probably took several thousand years for a functioning ecosystem to evolve.
If you would like to learn more about the death of the dinosaurs and the birth of the modern world, the story is available on CD with numerous illustrations and includes a sample of the ejecta material from near Chetumal. Copies of the CD are available at Cyberakumal, the internet café just inside the Akumal arch.
Charles Shaw holds a doctorate in geology and is a former Director of Centro Ecológico Akumal. www.envirocurricula.com
Charles Shaw's presentation takes place every Thursday at 6:30 at the CEA Center in Akumal.
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