Show number #1207
Peggy travels to Mexico to look for evidence that an asteroid may have caused the
Segment length: 9:27
- 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?
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
- 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?
core sample a section of a cylindrical soil or rock
sample taken from below the ground by using a hollow drill
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
- 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
- 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.
Contact a local astronomy club to go comet and asteroid
Geology department at local
Because many geological features like craters are buried, geologists must find ways of
probing beneath Earth's surface to understand what's there. Since it would be too
expensive to simply dig one big hole, geologists do the next best thing--they drill
long, slim holes and correlate the sections. In this activity, you can try your hand
at unmasking a hidden geological structure.
- 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
- 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.
- 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."
- 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."
- 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
- Based on this exercise, how might increasing or decreasing the number of straws
you use change the accuracy of your coring model?
- When you are cutting open your straws, what error factors must you consider?
- Besides coring, what other methods might geologists use to "look inside" Earth?
- What hazards might a drill team encounter if they were drilling rock instead of
Go to a local museum or planetarium and check out some meteorite exhibits. How do the
meteorites differ? What's the largest one researchers have found and where is it
Meteorite impact is only one theory used to explain the extinction of the dinosaurs.
Do a little research and see what other extinction theories are being offered. Are
any of them valid? Hold a debate with your friends and discuss these
While the Chicxulub crater is a large one, it's not the only impact site on Earth.
Research other impact sites in the United States and the world. Are any of these
possible candidates for a dino crater as well? Do any of them support the idea of a
multiple meteorite impact?
Try making a model meteorite bombing range by using marbles of different sizes and a
large baking dish filled with about an inch of flour. Try dropping the marbles from
different angles and at different speeds and see what types of patterns develop. How
might this activity be used to tell the age of craters on the moon?
We encourage duplication for educational use!
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