Passage 2 | Paleontology
Two Extinction Theories
[A] What caused the mass extinctions about 65 million years ago at the end of the Cretaceous Period has long been an enticing scientific riddle, and the reasons for the resulting extermination of the dinosaurs have been the topic of fierce dispute for some time. [B] By studying the layers of the Earth’s crust that correspond to the time period of the mass extinctions, scientists have uncovered clues that, over the past few decades of debate, have evolved into two major theories about the cause of the mysterious mass extinctions. [C] Whatever the cause, 65 million years ago, as much as 70 percent of the species on the Earth vanished. [D]
The main support for both arguments comes from the evidence contained in the 65-million-year-old geological boundary in the planet’s crust, where rock layers from the Cretaceous Period meet rock layers from the Tertiary Period. At this Cretaceous-Tertiary (K-T) boundary, inexplicably large quantities of iridium, an element that is very rare in the Earth’s crust, have been found. Speculation about the origin of the unusual amount of iridium has led to the development of the impact theory and the volcano-greenhouse theory.
The widely accepted impact theory, proposed by Luis Alvarez in 1980, explains that the high levels of iridium at the K-T boundary may have been caused by a huge asteroid—probably miles in diameter—that slammed into the Earth 65 million years ago, and the result of that impact was a colossal explosion that created sky-darkening dust clouds around the entire planet. Iridium is common in some extraterrestrial objects, and this fact quickly earned support for the impact theory. An impact of this magnitude would have had a catastrophic effect on the planet’s ecosystem, as dust from the blast would have clouded the atmosphere and initiated an “impact winter.” The absence of sunlight and the sudden temperature drop would have quickly destroyed plant species, gradually starved larger herbivores that fed on those plants, and eventually killed the carnivores that relied on those herbivores for food. The impact theory was quite provocative when it was initially presented, and some scientists were skeptical about the idea of an enormous asteroid. Because objects of that size make impressive craters, they argued that the impact mark from the Alvarez asteroid should still be observable. However, for years no one was able to identify a crater of the appropriate size and age. Ten years after the proposal of Alvarez’s theory, researchers stumbled upon a probable impact site—a huge crater partially submerged in the ocean off the coast of Mexico’s Yucatan Peninsula. The Chicxulub Crater, which is the result of a collision that occurred approximately 65 million years ago, is believed es by many to be the impact site where Alvarez’s ten-kilometer-wide asteroid struck, ushering in a period of mass extinctions for species across the face of the Earth.
Although it is sometimes overlooked, there is a second viable theory that simultaneously accounts for the abundance
of iridium and proposes a reasonable cause of the K-T extinctions. This theory, known as the volcano-greenhouse theory, suggests that the iridium in the K-T boundary originated in the center of the Earth, released by long-term volcanic activity at the end of the Cretaceous Period. At that time, the Deccan Traps—a volcanic region on the Indian subcontinent exhibited some of the most severe volcanism in the entire history of the planet. The Deccan Traps would have expelled enough lava to cover more than 2.5 million square kilometers of the Earth’s surface and would have thrown massive amounts of carbon dioxide into the atmosphere, creating a greenhouse effect that would have drastically affected the planet’s climate. The volcano-greenhouse theory asserts that the planet’s temperature would so have risen and fatally disrupted the biosphere. Mass migrations would have occurred, but, eventually, populations of mammals, reptiles, and birds would have become extinct as the planet’s temperature exceeded each species upper limit for successful reproduction.
Currently, there is far too little evidence to exclude either theory about the mass extinctions at the end of the Cretaceous Period. Both theories appear to be equally supported by the limited evidence scientists have been able to unearth so far.
15. The word enticing in the passage is closest in meaning to
16. In paragraph 2, what can be inferred about the iridium in the Earth’s crust?
(A) It is an element that is often found during paleontological excavations.
(B) It is the result of a major event between the Cretaceous and Tertiary periods.
(C) It has been contained in a variety of fossils from different periods in the Earth’s history.
(D) It has not helped scientists hypothesize about the extinction of the dinosaurs.
17. The word Speculation in the passage is closest in meaning to
18. According to paragraph 3r some scientists initially doubted the validity of the impact theory because
(A) it seemed unlikely that an asteroid could cause so much damage
(B) it did not explain the iridium levels at the K-T boundary
(C) evidence of a massive crater could not be found
(D) asteroids rarely hit the Earth’s surface
19. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
(A) In 1980, Luis Alvarez proved that, 65 million years ago, an asteroid impact caused global extinctions.
(B) When an asteroid hits the surface of a planet, it causes a major explosion that releases high levels of iridium.
(C) Luis Alvarez proposed that an asteroid crash deposited iridium on the planet and caused a global disaster.
(D) When the K-T boundary was hit by a large iridium asteroid 65 million years ago, it released a dust cloud that surrounded the planet.
20. The word they in the passage refers to
21. The phrase ushering in in the passage is closest in meaning to
(C) specializing in
(D) converting to
22. According to the passage, what does the author imply about the volcano-greenhouse theory?
(A) It is not as popular as the impact theory.
(B) It is more probable than the impact theory.
(C) It will be easier to prove than the impact theory.
(D) It does not have as much supporting evidence as the impact theory.
23. The word fatally in the passage is closest in meaning to
24. According to paragraph 4, the volcano- greenhouse theory claims that species became extinct because
(A) they were forced to migrate long distances
(B) the Earth became too warm
(C) they did not have enough oxygen
(D) the planet’s surface was covered with lava
25. Look at the four squares m that indicate where the following sentence could be added to the passage.
Since the 1970s, scientists have extensively researched the matter, attempting to ascertain why dinosaurs, which had been the dominant land animals for approximately 160 million years, suddenly vanished at the end of the Cretaceous Period.
Where would the sentence best fit?
26 Directions: Complete the table by matching the phrases below.
Select the appropriate phrases from the answer choices and match them to the theory to which they relate. TWO of the answer choices will NOT be used. This question is worth 4 points.
Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it To review the passage, click View Text
(A) Involves a destructive effect caused by dust from a major explosion
(B) Theorizes that the iridium at the K-T boundary is native to the Earth
(C) Describes the catastrophes caused by blockage of sunlight
(D) Proposes that the mass extinctions occurred far earlier than 65 million years ago
(E) Suggests that there were disastrous results of a change in the gas composition of the Earth’s atmosphere
(F) Hypothesizes that the Earth’s temperature rose during the extinction period
(G) Refers to geologic phenomena located on the Indian subcontinent
(H) Has a major source of evidence just off the Yucatan Peninsula in Mexico
(I) Explains that the continents were separated by a catastrophic event
Impact Theory***Volcano Greenhouse Theory****