• What are mass extinctions and how do they occur?
• What evidence suggests an asteroid hit Earth 65 million years ago?
• Where is the crater and how can we confirm it?

Contents

Insights & Connections
Vocabulary
Resources
Main activity
Try this

INSIGHTS & CONNECTIONS

Trying to understand why the dinosaurs became extinct has become one of the great geological detective stories. Some recent findings from the small Mexican village of Chicxulub have given scientists new hope that the answer may soon be known.

The story starts a little over 15 years ago in the town of Gubbio, Italy, where geologist Walter Alvarez was collecting sediment from a layer of rock which marked the boundary between the Cretaceous and Tertiary time periods. Geologists had long known that this boundary was important because it marked a period in the Earth's history, some 65 million years ago, when almost half of all known species suddenly disappeared, including the dinosaurs.

Walter brought some of his sample back to the United States and his father, Nobel prize-winning physicist Luis Alvarez, analyzed it for any unusual chemicals. To their surprise, the sample showed a high concentration of the element iridium, a substance rare on Earth but common in meteorites. To make sure there was nothing unusual about the Gubbio sample, they analyzed other K-T boundary strata from around the world. They found extra iridium in these samples as well. Using the average thickness of the clay as a guide, they calculated a meteorite would require a diameter of about 10 kilometers (6 miles) to produce this much iridium.

If a meteorite that size had hit Earth, the results could explain the extinction of dinosaurs. The dust thrown up in the air would have caused major climatic changes to which many animals could not rapidly adapt. A major problem with this theory, however, was that a 10-kilometer meteorite would leave a very large crater, between 150 and 200 kilometers (93-124 miles) in diameter. While Earth has many impact craters on the surface, few are even close to this size.

Because 65 million years had passed since the hypothetical impact, scientists decided to shift the search underground. A crater that old would almost certainly have been filled in. Just by chance, a Mexican oil company drilling off the coast of Yucatan discovered what appeared to be a crater about one kilometer (0.6 miles) under the surface near the village of Chicxulub. When core samples were analyzed, they showed the crater to be about 180 kilometers (112 miles) in diameter and 65 million years old. Was this the cause of the dinosaurs' extinction? The jury is still out, but evidence strongly suggests that the case of the disappearing dinosaurs may finally be solved.

• Suppose a ten-kilometer meteorite hit Earth today. What effect would it have on humans?
• Massive meteorite impacts are only one possible cause of climatic change. Volcanoes, forest fires, and industrial pollutants can also affect weather. Many scientists feel that human activities may cause climatic change. What do you think?
• How have meteor impacts affected our moon, Mars, and other solar system members?
• What did astronomers learn when they watched fragments of the Shoemaker-Levy comet hit Jupiter in July 1994?

VOCABULARY

crater a large circular depression on the surface of a solid celestial object caused either by meteorite impact or volcanic eruptions

gravity anomaly the difference in the amount of gravity measured on Earth's surface compared with the theoretical value for that location

meteroite a small, rocky body from space that strikes Earth

strata distinct layers of sediment or rock

tekite a small, round- or oval-shaped, glassy object formed when ejecta is melted and rapidly cooled in the atmosphere as it lands back on Earth's surface

RESOURCES

• Asimov, I. (1988) Did comets kill the dinosaurs? Milwaukee: Gareth Stevens Publishing.
• Bakker, R. (1986) The dinosaur heresies. New York: William Morrow and Company.
• Bond, P. (1994, May) Assessing Earth impact hazards. Astronomy, pp. 18-20.
• Gore, R. (1989, June) Extinctions. National Geographic, pp. 662-699.
• Gore, R. (1993, Jan) Dinosaurs. National Geographic, pp. 2-53.
• Hecht, J. (1993) Vanishing life: The mystery of mass extinctions. New York: Charles Scribner's Sons.
• Kerr, R.A. (1993, Mar 12) Second crater points to killer comets. Science, p. 1543.
• Lemonick, M.D. (1993, Apr 26) Rewriting the book on dinosaurs. Time, pp. 42-49.
• Newton's Apple Show #1105--meteors segment. KTCA Video Services: (612) 229-1253. Or call your local PBS station to find out when it will be rerun.
• Seyfert, C. & and Sirkin, L. (1979) Earth history and plate tectonics. New York: Harper and Row.

Community resources

Contact a local astronomy club to go comet and asteroid hunting

The Geology department at a local college

MAIN ACTIVITY

Materials

• a cardboard shoe box
• 4 or 5 colors of plastic modeling clay, about 1 lb of each
• 25 wide plastic straws
• small, sharp scissors
• a metric ruler
• 2 blank pieces of paper and a pencil

1. Before starting this exercise, someone must create a geological structure in the shoe box. Begin by placing layers of clay, each a different color, one on top of each other in the box. Vary the thicknesses of the layers. In the middle of the box, either create a crater by cutting out a circular depression or make a dome by burying a big ball of one clay color in the different layers. Once the structure has been completed, give it to the students doing the drilling.
2. To "drill" the structure, slowly insert one plastic straw into the top and push it down all the way to the bottom of the box. Before removing the straw, measure the exact location of the core relative to two sides of the box and mark that point on the blank paper. Label it "core 1."
3. After marking the core location, slowly remove the straw. The clay should stay inside. With the scissors, carefully cut the straw lengthwise, starting at the bottom. Try not to disturb the clay inside. Using the ruler, start at the bottom of the straw and measure the thickness of each color of clay. Draw this on the second piece of paper and label it "core 1."
4. After marking the first core, take a second straw and repeat steps 2 and 3 in a second location. Continue until you have drilled, located, and measured 20 cores. Based on the location of the cores and the clay layers in each one, see if you can reconstruct a three-dimensional model of the geology in the shoe box. When you think you have done it, peel away the sides of the box and see how close you came.

Questions

1. Based on this exercise, how might increasing or decreasing the number of straws you use change the accuracy of your coring model?
2. When you are cutting open your straws, what error factors must you consider?
3. Besides coring, what other methods might geologists use to "look inside" Earth?
4. What hazards might a drill team encounter if they were drilling rock instead of clay?

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TRY THIS!

Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.