Paleontologists (geologists who study the fossils of ancient life forms) have long wondered what might have caused the phenomenonof mass extinction, wherebymore than 75 percent of all the life forms then on Earth vanished about 65 million years ago. The most dramaticloss involved the extinction of the dinosaurs, a group of animals that had roamed the planet for 150 million years. Numerous other life forms vanished as well—large and small, water and land dwelling, plant and animal. Many species, whether living in freshwater lakes, in rivers, in saltwater oceans, or on land, became extinct at roughly the same time.
Some early hypotheses focused on a soleorganism to explain these extinctions. Some hypotheses proposed that epidemic diseases triggeredthe elimination of dinosaur populations or that egg-stealing mammals destroyed dinosaur nests. But neither of these hypotheses conformedto the fact that two-thirds of all marine animal species were also lost, which led some scientists to propose that the oceans became lethally salty (though this idea did not explain why some marine creatures survived). To explain the extinction of enormousterrestrial reptiles, tiny marine organisms, and many life forms in between, a number of hypotheses invokedglobal
15 environmental change. Did Earth suffer from a period of drastic cooling 65 million years ago? Did a shift in the planets protective magnetic field allow harmful solar radiation to reach land and sea, eliminating a wide variety of life forms? Did a nearby star explode, bathing Earth in cosmic radiation? Surely, each of these events would have affected all life on Earth simultaneously. Why, then, did 25 percent of the planets species remain unaffected?
20 Several hypotheses agree that some catastrophic collapseof the global food chain preceded
widespreadextinction. One group of scientists has proposed that massive volcanic eruptions on India's Deccan plateau may have been such an event. The rocks of these eruptions have been dated to 65 million years ago, coincidingperfectly with the extinction of the dinosaurs. It is postulated that the eruptions sent a cloud of volcanic ash and gas around Earth, blocking out sunlight,
25 cooling the planet, and diminishingvegetation, including microscopic plants. Without the plants on which their diets were based, many plant-eating animals would have died, and their extinction would in turn have displacedthe meat-eaters, such as Tyrannosaurus rex, that ate them.
Essential Academic Vocabulary
Another group of scientists, led by geologist Walter Alvarez and his father, Nobel Prize-winning physicist Luis Alvarez, conceivedanother scenario: A meteorite at least 10
30 kilometers (6 miles) in diameter plowed into Earth, releasing a shower of pulverized rock into the atmosphere. The resulting dust veil would have blocked out sunlight (in much the same way volcanic ash would have), cooled the planet, and led to a so-called"impact winter" with a possible durationof decades—long enough to destroy the global food chain. The strongest empiricalevidence to support this impact hypothesis is a 2-centimeter- (1-inch) thick layerof
35 clay found around the world in rocks that date from approximately 65 million years ago. The clay contains iridium, an element that is extremely rare in rocks of terrestrial origin but is quite common in meteorites. Mineral grains shattered by very high pressures—as would occur if a meteorite had struck them—have also been found at the proposed impact sites. The Alvarezes and their associates make the logicalclaim that the iridium-rich layer resulted from the
40 widespread fallout of meteorite dust. Fossils of numerous species, including many now-extinct organisms, had accumulatedin the rocks that formed just before the iridium-rich layer was deposited, whereas only about one-fourth as many species are foundin the rocks formed just after this layer was deposited. This evidence suggests that numerous extinctions occurred during the time of deposition.
45 Just as hypotheses may be discarded, revised,or confirmedand elevated to theory status,
they also sometimes overlap.One group of scientists has recently proposed that Earth was indeed struck by a meteorite 65 million years ago somewhere in the Western Hemisphere, and that the impact initiated massive volcanism on the reverseside of the globe. The material thrown into the atmosphere by both events may have combined to inhibitthe growth of vegetation,
50 thus eventually bringing about the death of the dinosaurs.
As yet, no versionof the extinction hypothesis has attainedtheory status. Analysis of these 6 5-million-year-old deposits continues today, as scientists seek to clarifythe proportions of organisms that became extinct at that time and search, for additional evidence of a meteorite strike or of a catastrophic volcanic eruption that coincides with the time of the extinctions.
Adapted from Stanley Chernicoff, Haydn A. "Chip" Fox, and Lawrence H. Tanner, Earth: Geologic Principles and History (Boston: Houghton Mifflin, 2002), 4-5.
Chapter 17 • Geology
1. Vocabulary in Context
Determine how the boldface words are used in the reading. Then for each item, choose the word or phrase that is closest in meaning.
1.____ phenomenon(line 2) 5.
2.____ sole(line 8) 6.
3.____ preceded(line 20) 7.
b. predominated a. predated
4.____ diminishing(line 25) 8.
c. complemented empirical(line 34)
c. observed accumulated(line 41)
c. released confirmed(line 45)
9.____ inhibit(line 49)
10.____ attained(line 51)
2. Reading Comprehension
Getting the Facts
1. What do paleontologists study?
2. How long ago was the mass extinction of 75 percent of all life forms on Earth?
3. What kinds of animals became extinct at this time?
Essential Academic Vocabulary
4. Put a check mark («/) next to each hypothesis of mass extinction that is mentioned in the second paragraph of the reading.
a. A single organism was responsible.
b. Earth was flooded for a long period of time.
c. Epidemic diseases killed the dinosaurs.
d. Animals stole the eggs from dinosaur nests.
e. The largest dinosaurs ate all the smaller ones.
f. The oceans became too salty to support life.
g. Earth became much colder for a period of time.
h. Earth became a desert.
i. ____ Radiation from the sun killed many species.
j. ____ The climate became too hot to support life.
k. ____ Radiation from the explosion of a star killed many life forms.
5. "What is wrong with the hypotheses mentioned in the second paragraph?
6. One group of scientists has proposed that volcanic eruptions in India may have caused the extinctions. Put the following events from the third paragraph in the correct sequence, numbering them from 1 to 7, according to this hypothesis.
____ blanketing of Earth with volcanic ash and gas
____ cooling of the planet ____ blocking out of sunlight
____ massive volcanic eruptions ____ elimination of meat-eating animals
____ death of vegetarian animals great reduction of plant life
7. The Alvarezes proposed the hypothesis that a meteorite was responsible for the mass extinction. In your own words, summarize the empirical evidence they have discovered to support this hypothesis.
8. Explain the recent hypothesis proposed by a group of scientists that combines two previous hypotheses.
Chapter 17 • Geology 179
J Making Inferences
1. What is meant by an impact winter? (paragraph 4)
2. Why have there been so many conflicting hypotheses about the extinctions of dinosaurs and other life forms?
The effort involved in trying to remember a word helps you to learn it.
3. Dictionary Skills
Study the dictionary entries for confirm, confirmation, and confirmed. Complete each sentence with an appropriate ending from the following choices. Write the letter of the choice on the blank.
COn*firm (kan furm') tr.v. 1. To support or establish the truth or validity of (sthg.): The results of the experiment confirmed the theory. 2. To make (sthg.) firmer; strengthen: Reading about famous scientists confirmed her plan to study chemistry. 3. To make (sthg.) valid or binding by a formal or legal act: The judges appointment to the Supreme Court vjas confirmed by the Senate. 4. To admit (sbdy.) to full membership in a church by confirmation: Their 13-year-old was confirmed last weekend.
con^fir^ma^tion (ko n'far ma'shan) n. [C; U] 1. The act of confirming: The President needs the Senates confirmation to appoint an ambassador. 2. Something that confirms; proof: The driver's license was confirmation of the man's age. 3. A religious ceremony that allows sbdy. to be a full member of a church: We held a reception to celebrate our son's confirmation. COn»firmed (kan furmd') adj. 1. Firmly established; proved: a confirmed theory. 2. Settled in a habit or condition: a confirmed gossip.
a. of their identity
b. Dr. Jenkins had to leave the paleontology conference early
c. the hypothesis
d. as an extinction theory
e. as the chairperson of the Geology Department
f. so he always takes vacations in places where he can search for fossils
g. her intention to be a geologist
h. that dinosaurs existed in this area long ago
Essential Academic Vocabulary
1. The newly found fossils provided confirmation_J2
2.The analysis of the data confirmed____
3.Visiting the Grand Canyon as a child confirmed____
4.A family party celebrated Dr. Smithtons confirmation____
5.In order to attend her son's confirmation,_____
6.My father is a confirmedfossil hunter,____
7. Paleontologists have to show identification in some countries as confirmation