Reading Comprehension

In 1979, a team of scientists from Berkeley working near Gubbio, Italy,
discovered a layer of clay that revolutionized theories concerning the
disappearance of the dinosaur, which had centred on the assumed
gradual climatic change. Beneath the two-centimetre-thick layer lay
limestone containing fossil organisms from the late Cretaceous, while
above it was limestone with early Cenozoic fossils.

Positionally, then, the Berkeley group could place the clay in a period
roughly contemporaneous with the disappearance of the dinosaur
approximately 63 million years ago. They found that the clay stratum
contained an iridium level thirty times greater than that of clays in
adjacent strata. As iridium is distributed fairly evenly over time through
micrometeoritic impact, the researchers knew that the anomalous matter
in the clay must have originated extra-terrestrially; the high iridium level,
moreover, indicated a sudden deposition in an exceptional, catastrophic
event.

Scientists are sharply divided on the possible causes of so cataclysmic
an event. The possibility that the deposition occurred as an aftereffect of
a supernova has been discounted: radioactive isotope Pu-244 was absent
from the clay, and neither Ir-191 nor Ir-193 were present in significant
proportions.

Those who maintain that the material came from within the solar
system contend that the earth must have collided during the late
Cretaceous with an astral body large enough to have distributed the
iridium-rich material over the globe. An asteroid of the required mass
would have been approximately ten kilometres in diameter; a comet
would have to have been twice as large, since comets are largely
composed of ice water.

Trying to fathom the scale of such an event as this is mind boggling.
It is true that from space, an object 10-20 miles across colliding with
earth would be akin to something smaller than a grain of sand landing on
a basketball, it is also the case that an object twenty miles across that
landed on earth would be nearly twice as tall as Mt. Everest (the tallest
mountain on Earth) and further across than the length of Manhattan.
Furthermore, when the body came crashing to Earth it would have been
ablaze in an inferno caused by the friction of entry into our atmosphere.
To the argument that there is no geological evidence of the impact of
such massive objects, Richard Grieve has replied that the clay layer could
have resettled after the impact in the form of fallout. Frank Kyte of UCLA
asserts that a comet, if disrupted by the earth's gravitational field, would
have exposed the surface to a deluge of debris that would not have
created major craters. Alternatively, the Berkeley group suggests that an
asteroid may have landed in the sea; such a collision would have
produced tidal waves eight kilometres high, swamping large areas of the
earth.

Whatever the type of body and mode of impact, Walter Alvarez of the
Berkeley team argues that the primary effect of the catastrophe was to
disrupt the planetary ecology through the suspension of vast clouds of
matter in the stratosphere. The effects of the initial impact would have
been greatly multiplied, Alvarez argues, as photosynthesis was impeded
by the blockage of sunlight; there would then have been a massive
disruption at the base of the dinosaur's food chain.


1. The passage discusses a new discovery that may change the way scientists
think about one aspect of dinosaurs. It can be inferred that the discovery
described in the passage may "•revolutionize"– (line 2) which aspect of current
theories about dinosaurs?
A. The geographical extent of the presumed habitation of the dinosaur
B. The approximate date at which dinosaurs are thought to have become
extinct
C. The assumption that dinosaurs became extinct because of a change in
their natural environment
D. The rate at which the extinction of the dinosaur is thought to have
occurred
E. The notion that dinosaurs became extinct because of the onset of an ice
age.


2. According to the passage, the Berkeley group used which of the following to
support their hypothesis on the disappearance of the dinosaur?
I. A comparison of the fossil records of various marine strata
II. A comparison of different clay strata near Gubbio, Italy
III. A comparison of marine strata in several locations
A. I only
B. III only
C. I and II
D. II and III
E. I, II and III


3. According to the information presented by the author throughout the
passage, scientists used the analysis of the isotopes present in the clay to:
A. estimate the age of the stratum more exactly.
B. determine the extent of meteoritic impact upon the earth.
C. derive a hypothesis concerning the effect of the impact of an
extraplanetary body on the earth's ecology.
D. eliminate a possible theory concerning the enriched clay's formation.
E. determine whether dinosaurs were allergic to these


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