In the late 1970s, several research groups were trying to determine the cause of the boundary between the geological layers corresponding to the Cretaceous and Tertiary periods, about 65 million years ago. At this boundary, called the K/T boundary (where “K” represents the German spelling of “Cretaceous” and “T” represents “Tertiary”), a sudden change in the fossils that are found in the rocks means that a substantial fraction of all living species on Earth died out. Included in this extinction event were the dinosaurs (the most famous of the extinctions), most land-dwelling animals, and a significant fraction of the marine invertebrate species. In fact, this sudden extinction event defines the boundary between the two geologic periods. In most places where the K/T boundary is seen in the geologic record (that is, where erosion has not destroyed it), it is marked by the presence of a layer of clay that can be up to several meters thick.
The scientists measured the abundance of what are called siderophile elements within the clay. Siderophile elements are of interest because they are not very abundant within Earth’s crust-they tended to remain with the iron and nickel when iron and nickel concentrated into the planet’s core early in Earth’s history. However, they are abundant in meteorites, which, unlike Earth, never differentiated into a core, mantle, and crust. One of the siderophiles is the element iridium. Within the clay layer, the amount of iridium was found to be ten times that of adjacent layers of rock. This was such an enormous increase that the scientists postulated that a large influx of extraterrestrial material must have occurred at that time. In addition to the iridium and other siderophile elements, the scientists noted that the boundary-layer clay contained grains of the mineral quartz that had been shocked by high pressure and temperature into a denser form. The dense forms of quartz, called coesite and stishovite, occur almost exclusively as a result of the tremendous forces that result from the impact of an asteroid onto Earth’s surface; they are found in abundance at Meteor Crater in Arizona in the United States, for example.
The iridium excess and the presence of shocked quartz suggested that an impact of a single, large asteroid onto the surface of Earth had occurred at the time of the K/T boundary. Based on the worldwide distribution of the clay layer, and on the abundance of iridium within it, it was suggested that the impact of a single, 10-kilometer- sized object could have been responsible. A 10-kilometer object impacting onto the surface of Earth would produce a crater that was almost 100 kilometers across. Unfortunately, there was no known 100-kilometer crater that had formed 65 million years ago. The only known crater of that age was the Manson impact crater in lowa in the United States; this crater is only 10 kilometers across, however, and is much too small to account for the observed amount of iridium.
There were no guarantees that the search for a crater would be successful. The crater, if it existed, might have been obliterated by erosion; and there was about a 40 percent chance that it had formed on the ocean floor and subsequently had been submerged beneath the continents during the 65 million years since its formation. However, in a remarkable confirmation of the hypothesis, the crater was found, located just off the Yucatan peninsula in Mexico. Known as Chicxulub, it is about 200 kilometers in diameter and lies half on land and half on the ocean floor. Lying half on the ocean floor, and having been weathered substantially where it lies above the surface, it had not been identified prior to the search for the K/T impact crater. There is a suggestion in the gravity and topography data that the crater might actually have been about 3,000 kilometers across. This large a crater would have been formed by the impact of an asteroid 10 to 20 kilometers across.
1
In the late 1970s, several research groups were trying to determine the cause of the boundary between the geological layers corresponding to the Cretaceous and Tertiary periods, about 65 million years ago. At this boundary, called the K/T boundary (where “K” represents the German spelling of “Cretaceous” and “T” represents “Tertiary”), a sudden change in the fossils that are found in the rocks means that a substantial fraction of all living species on Earth died out. Included in this extinction event were the dinosaurs (the most famous of the extinctions), most land-dwelling animals, and a significant fraction of the marine invertebrate species. In fact, this sudden extinction event defines the boundary between the two geologic periods. In most places where the K/T boundary is seen in the geologic record (that is, where erosion has not destroyed it), it is marked by the presence of a layer of clay that can be up to several meters thick.
Paragraph 1 supports all of the following statements about the K/T boundary layer EXCEPT:
AIt was formed about 65 million years ago.
BIt marks a place where the fossil record changes suddenly.
CIt is responsible for destroying the surrounding geologic record in some areas.
DIt contains clay that in some places is several meters thick
2
The scientists measured the abundance of what are called siderophile elements within the clay. Siderophile elements are of interest because they are not very abundant within Earth’s crust-they tended to remain with the iron and nickel when iron and nickel concentrated into the planet’s core early in Earth’s history. However, they are abundant in meteorites, which, unlike Earth, never differentiated into a core, mantle, and crust. One of the siderophiles is the element iridium. Within the clay layer, the amount of iridium was found to be ten times that of adjacent layers of rock. This was such an enormous increase that the scientists postulated that a large influx of extraterrestrial material must have occurred at that time. In addition to the iridium and other siderophile elements, the scientists noted that the boundary-layer clay contained grains of the mineral quartz that had been shocked by high pressure and temperature into a denser form. The dense forms of quartz, called coesite and stishovite, occur almost exclusively as a result of the tremendous forces that result from the impact of an asteroid onto Earth’s surface; they are found in abundance at Meteor Crater in Arizona in the United States, for example.
The word “exclusively” in the passage is closest in meaning to
Aautomatically
Binstantly
Ccertainly
Donly
3
The scientists measured the abundance of what are called siderophile elements within the clay. Siderophile elements are of interest because they are not very abundant within Earth’s crust-they tended to remain with the iron and nickel when iron and nickel concentrated into the planet’s core early in Earth’s history. However, they are abundant in meteorites, which, unlike Earth, never differentiated into a core, mantle, and crust. One of the siderophiles is the element iridium. Within the clay layer, the amount of iridium was found to be ten times that of adjacent layers of rock. This was such an enormous increase that the scientists postulated that a large influx of extraterrestrial material must have occurred at that time. In addition to the iridium and other siderophile elements, the scientists noted that the boundary-layer clay contained grains of the mineral quartz that had been shocked by high pressure and temperature into a denser form. The dense forms of quartz, called coesite and stishovite, occur almost exclusively as a result of the tremendous forces that result from the impact of an asteroid onto Earth’s surface; they are found in abundance at Meteor Crater in Arizona in the United States, for example.
What can be inferred from paragraph 2 about Earth’s iridium content?
AIt is more abundant in Earth’s core than in its crust
BIt increased as a result of rising temperature and pressure during Earth’s formation
CIt circulates regularly between Earth’s core, mantle, and crust.
DIt requires millions of years to separate from iron and nickel
4
The scientists measured the abundance of what are called siderophile elements within the clay. Siderophile elements are of interest because they are not very abundant within Earth’s crust-they tended to remain with the iron and nickel when iron and nickel concentrated into the planet’s core early in Earth’s history. However, they are abundant in meteorites, which, unlike Earth, never differentiated into a core, mantle, and crust. One of the siderophiles is the element iridium. Within the clay layer, the amount of iridium was found to be ten times that of adjacent layers of rock. This was such an enormous increase that the scientists postulated that a large influx of extraterrestrial material must have occurred at that time. In addition to the iridium and other siderophile elements, the scientists noted that the boundary-layer clay contained grains of the mineral quartz that had been shocked by high pressure and temperature into a denser form. The dense forms of quartz, called coesite and stishovite, occur almost exclusively as a result of the tremendous forces that result from the impact of an asteroid onto Earth’s surface; they are found in abundance at Meteor Crater in Arizona in the United States, for example.
Why does the author include the information that dense forms of quartz “are found in abundance at Meteor Crater in Arizona”?
ATo explain the difference between the structure of Earth and that of meteorites
BTo contrast the formation of dense forms of quartz to that of siderophile elements
CTo help explain how the iridium in the clay boundary layer was formed
DTo support the idea that dense forms of quartz are usually produced on Earth as a result of extraterrestrial impacts
5
The scientists measured the abundance of what are called siderophile elements within the clay. Siderophile elements are of interest because they are not very abundant within Earth’s crust-they tended to remain with the iron and nickel when iron and nickel concentrated into the planet’s core early in Earth’s history. However, they are abundant in meteorites, which, unlike Earth, never differentiated into a core, mantle, and crust. One of the siderophiles is the element iridium. Within the clay layer, the amount of iridium was found to be ten times that of adjacent layers of rock. This was such an enormous increase that the scientists postulated that a large influx of extraterrestrial material must have occurred at that time. In addition to the iridium and other siderophile elements, the scientists noted that the boundary-layer clay contained grains of the mineral quartz that had been shocked by high pressure and temperature into a denser form. The dense forms of quartz, called coesite and stishovite, occur almost exclusively as a result of the tremendous forces that result from the impact of an asteroid onto Earth’s surface; they are found in abundance at Meteor Crater in Arizona in the United States, for example.
According to paragraph 2, which of the following led to the idea that a large influx of extraterrestrial material occurred at the time that the K/T boundary was formed?
ALarge chunks of meteorites in the boundary-layer clay
BThe lack of iron and nickel in the boundary-layer clay
CMuch higher levels of iridium in the boundary-layer clay than in the rock layers immediately above and below that layer
DHigh levels of coesite and stishovite in the rock layers immediately above and below the boundary layer
6
The iridium excess and the presence of shocked quartz suggested that an impact of a single, large asteroid onto the surface of Earth had occurred at the time of the K/T boundary. Based on the worldwide distribution of the clay layer, and on the abundance of iridium within it, it was suggested that the impact of a single, 10-kilometer- sized object could have been responsible. A 10-kilometer object impacting onto the surface of Earth would produce a crater that was almost 100 kilometers across. Unfortunately, there was no known 100-kilometer crater that had formed 65 million years ago. The only known crater of that age was the Manson impact crater in lowa in the United States; this crater is only 10 kilometers across, however, and is much too small to account for the observed amount of iridium.
According to paragraph 3, scientists were able to infer all of the following from the composition and distribution of the clay layer EXCEPT
Athe location of the crater produced by the impact
Bthe number of extraterrestrial bodies responsible for the impact
Cthe approximate size of the crater that would have been produced by the impact
Dthe approximate size of the extraterrestrial body responsible for the impact
7
The iridium excess and the presence of shocked quartz suggested that an impact of a single, large asteroid onto the surface of Earth had occurred at the time of the K/T boundary. Based on the worldwide distribution of the clay layer, and on the abundance of iridium within it, it was suggested that the impact of a single, 10-kilometer- sized object could have been responsible. A 10-kilometer object impacting onto the surface of Earth would produce a crater that was almost 100 kilometers across. Unfortunately, there was no known 100-kilometer crater that had formed 65 million years ago. The only known crater of that age was the Manson impact crater in lowa in the United States; this crater is only 10 kilometers across, however, and is much too small to account for the observed amount of iridium.
According to paragraph 3, which of the following is true of the Manson impact crater in lowa?
AIts small size cannot explain why it contains such high levels of iridium.
BIt was formed by the impact of an extraterrestrial body that was about 10 kilometers across.
CIt is not large enough to explain the amount of iridium at the K/T boundary layer.
DIt was formed by an impact that contributed no iridium to the K/T boundary layer.
8
There were no guarantees that the search for a crater would be successful. The crater, if it existed, might have been obliterated by erosion; and there was about a 40 percent chance that it had formed on the ocean floor and subsequently had been submerged beneath the continents during the 65 million years since its formation. However, in a remarkable confirmation of the hypothesis, the crater was found, located just off the Yucatan peninsula in Mexico. Known as Chicxulub, it is about 200 kilometers in diameter and lies half on land and half on the ocean floor. Lying half on the ocean floor, and having been weathered substantially where it lies above the surface, it had not been identified prior to the search for the K/T impact crater. There is a suggestion in the gravity and topography data that the crater might actually have been about 3,000 kilometers across. This large a crater would have been formed by the impact of an asteroid 10 to 20 kilometers across.
According to paragraph 4, which of the following helps to explain why scientists did not know about the Chicxulub crater for a long time?
AThe part of the crater lying above the ocean surface has been damaged by erosion.
BThe crater lies in a deep part of the ocean that had not been fully explored
CThe crater has been submerged beneath the continents over the past 65 million years.
DThe crater is located in a different area of the world than scientists initially expected
9
The scientists measured the abundance of what are called siderophile elements within the clay. Siderophile elements are of interest because they are not very abundant within Earth’s crust-they tended to remain with the iron and nickel when iron and nickel concentrated into the planet’s core early in Earth’s history. However, they are abundant in meteorites, which, unlike Earth, never differentiated into a core, mantle, and crust. [■] One of the siderophiles is the element iridium. Within the clay layer, the amount of iridium was found to be ten times that of adjacent layers of rock. [■] This was such an enormous increase that the scientists postulated that a large influx of extraterrestrial material must have occurred at that time.[■] In addition to the iridium and other siderophile elements, the scientists noted that the boundary-layer clay contained grains of the mineral quartz that had been shocked by high pressure and temperature into a denser form. [■] The dense forms of quartz, called coesite and stishovite, occur almost exclusively as a result of the tremendous forces that result from the impact of an asteroid onto Earth’s surface; they are found in abundance at Meteor Crater in Arizona in the United States, for example.
Look at the four squaresthat indicate where the following sentence could be added to the passage
Other evidence from the boundary supported this idea
Where would the sentence best fit？Click on a square sentence to the passage.
10
Sudden and massive changes in the fossil record at the K/T boundary indicate the occurrence of a worldwide extinction at that time.
AUnusual amounts of iridium as well as dense forms of quartz in the clay layer of the boundary suggested that an extraterrestrial impact had occurred at the time of its formation
BIridium and other siderophiles were concentrated into Earth’s core early in the planet’s history, and thus any traces of siderophiles found on Earth’s surface have extraterrestrial origins
CScientists had a great deal of difficulty locating the impact crater responsible for the K/T boundary because they believed it to be much smaller in size than it actually turned out to be.
DThe presence of coesite and stishovite in boundary-layer clay, in particular, indicated that an asteroid had hit Earth’s surface because those dense forms of quartz are found only on asteroids.
EBased on the composition and distribution of the clay layer, scientist estimated that a 10-kilometer-sized body had hit Earth’s surface, but no known crater had the right age and size.
FThe impact crater was discovered, although its location had made it difficult to identify and it was much larger than expected.
