Cretaceous Extinctions

Even though the timing of plant and animal extinctions is usually impossible to determine precisely, it seems that an extraordinary number of organisms on land and in the oceans went extinct at the very end of the Cretaceous period (145 million to 66 million years ago).  Popular attention to this event long focused on the extinction of the dinosaurs, ignoring the simultaneous marine catastrophe.  Various hypotheses were offered as explanation, such as widespread disease, overpredation, drastic climate change, loss of habitat, or lethal doses of radiation from a supernova occurring nearby.  But most such explanations suffered from failing to explain the apparent suddenness of extinction of so many different groups of organisms in widely varying habitats or lacked support from the geological record.

In 1980, a team of scientists led by Nobel laureate Luis Alvarez and his son Walter published an article on a thin layer of clay found in marine sediments at the boundary between the layers formed during the Cretaceous and Tertiary (66 million to 2,588 million years ago) periods, commonly referred to as the K-T boundary, in several locations around the world. When analyzed, this clay layer was found to contain levels of the element indium, a heavy metallic element that is rare in Earth’s crust but more concentrated in metallic meteorites, at more than ten times the normal concentration. The scientists reasoned that the clay was deposited from a world-encircling layer of dust enriched in iridium created a result of the impact of a big piece of space debris. Their calculations suggested that a collision with an asteroid (a small rocky body orbiting the Sun) with a diameter of at least ten kilometers was responsible for this dust layer.

Paleontologists realized that such an impact would have had severe effects on the global environment. A sudden and disastrous period of global cooling could have resulted from the huge cloud of dust that would have been propelled into the atmosphere. A more gradual but equally devastating period of global warming might have been caused by greenhouse gases formed by the impact. If the asteroid struck marine sediments, vaporization of the rock would have formed sulfate aerosol droplets that might have created intense acid rain conditions. Widespread wildfires would have resulted from the blast itself. Any combination of these effects could easily have disrupted the food chain, with the most severe consequences for those at the top. The oceans also would have been devastated by the chemical effects of the disaster, with planktonic organisms, which live at the ocean surface, and other organisms directly dependent on them, most vulnerable.

The great appeal of the impact hypothesis is that many predictions that follow from this idea have been supported by observations. One is that the iridium-rich clay layer at the K-T boundary has indeed been found worldwide. Another is the presence in the dust layer of grains of quartz with intersecting sets of parallel fractures. These grains, called shocked quartz, are evidence that rocks were subjected to extremely high pressures in the vicinity of the impact. Small, spherical, glassy grains are also present in the layer; these formed by rapid cooling of droplets of melted rock thrown into the atmosphere by the blast.

Perhaps the most crucial prediction is that the impact of a body with a diameter of 10 kilometers would have created a crater about 100 kilometers wide. Where can such a crater be seen today? With no such structure readily visible at the surface of Earth, geologists were left with two possibilities. First, the impact may have occurred in the deep ocean; the crater therefore may have been buried by marine sediment. Second, the crater may have formed on land but have been buried by sediments in the 66 million years since the impact. Pure luck was on the side of geologists when exploration for oil on the Yucatan Peninsula of Mexico revealed a circular depression 180 kilometers wide buried beneath hundreds to over 1,000 meters of rock. Rocks just above the depression, called the Chicxulub structure, are Tertiary in age, whereas those just below are Late Cretaceous (100.5 million to 66 million years ago). Holes drilled into the structure found broken and melted rock right at the K-T boundary. Most geologists now believe that the Chicxulub structure is indeed the crater formed by the asteroid that caused the massive extinctions at the K-T boundary.

 

 

1

Even though the timing of plant and animal extinctions is usually impossible to determine precisely, it seems that an extraordinary number of organisms on land and in the oceans went extinct at the very end of the Cretaceous period (145 million to 66 million years ago).  Popular attention to this event long focused on the extinction of the dinosaurs, ignoring the simultaneous marine catastrophe.  Various hypotheses were offered as explanation, such as widespread disease, overpredation, drastic climate change, loss of habitat, or lethal doses of radiation from a supernova occurring nearby.  But most such explanations suffered from failing to explain the apparent suddenness of extinction of so many different groups of organisms in widely varying habitats or lacked support from the geological record.

The word “lethal” in the passage is closest in meaning to

Adeadly

Bhigh

Crepeated

Dsudden

 

2

Even though the timing of plant and animal extinctions is usually impossible to determine precisely, it seems that an extraordinary number of organisms on land and in the oceans went extinct at the very end of the Cretaceous period (145 million to 66 million years ago).  Popular attention to this event long focused on the extinction of the dinosaurs, ignoring the simultaneous marine catastrophe.  Various hypotheses were offered as explanation, such as widespread disease, overpredation, drastic climate change, loss of habitat, or lethal doses of radiation from a supernova occurring nearby.  But most such explanations suffered from failing to explain the apparent suddenness of extinction of so many different groups of organisms in widely varying habitats or lacked support from the geological record.

According to paragraph 1, what was one of the reasons why many of the hypotheses about the mass extinctions were considered unsatisfactory?

AThey could not determine the precise timing of the extinctions.

BThey exaggerated the suddenness of the extinctions.

CThey did not explain why only certain types of habitat were affected.

DThey were not supported by geological evidence.

 

3

In 1980, a team of scientists led by Nobel laureate Luis Alvarez and his son Walter published an article on a thin layer of clay found in marine sediments at the boundary between the layers formed during the Cretaceous and Tertiary (66 million to 2,588 million years ago) periods, commonly referred to as the K-T boundary, in several locations around the world. When analyzed, this clay layer was found to contain levels of the element indium, a heavy metallic element that is rare in Earth’s crust but more concentrated in metallic meteorites, at more than ten times the normal concentration. The scientists reasoned that the clay was deposited from a world-encircling layer of dust enriched in iridium created a result of the impact of a big piece of space debris. Their calculations suggested that a collision with an asteroid (a small rocky body orbiting the Sun) with a diameter of at least ten kilometers was responsible for this dust layer.

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.

AIridium is a heavy metallic element that is over ten times more concentrated in metallic meteorites than in Earth’s crust.

BWhen analyzed, this clay layer was found to have more than ten times the normal concentration of metallic elements like iridium.

CThis clay layer contained more than ten times Earth’s normal concentration of iridium, an element found more commonly in metallic meteorites.

DAnalysis found that most of the iridium in Earth’s crust is concentrated in this clay layer.

 

4

In 1980, a team of scientists led by Nobel laureate Luis Alvarez and his son Walter published an article on a thin layer of clay found in marine sediments at the boundary between the layers formed during the Cretaceous and Tertiary (66 million to 2,588 million years ago) periods, commonly referred to as the K-T boundary, in several locations around the world. When analyzed, this clay layer was found to contain levels of the element indium, a heavy metallic element that is rare in Earth’s crust but more concentrated in metallic meteorites, at more than ten times the normal concentration. The scientists reasoned that the clay was deposited from a world-encircling layer of dust enriched in iridium created a result of the impact of a big piece of space debris. Their calculations suggested that a collision with an asteroid (a small rocky body orbiting the Sun) with a diameter of at least ten kilometers was responsible for this dust layer.

According to paragraph 2, what did Luis and Walter Alvarez conclude in their 1980 article?

AMost of the extinctions were of organisms that resided in marine sediments.

BThe extinctions were caused by toxic levels of iridium.

CEarth was hit by a giant asteroid about 10 kilometers in diameter.

DMeteorites contain more iridium than scientists previously thought.

 

5

Paleontologists realized that such an impact would have had severe effects on the global environment. A sudden and disastrous period of global cooling could have resulted from the huge cloud of dust that would have been propelled into the atmosphere. A more gradual but equally devastating period of global warming might have been caused by greenhouse gases formed by the impact. If the asteroid struck marine sediments, vaporization of the rock would have formed sulfate aerosol droplets that might have created intense acid rain conditions. Widespread wildfires would have resulted from the blast itself. Any combination of these effects could easily have disrupted the food chain, with the most severe consequences for those at the top. The oceans also would have been devastated by the chemical effects of the disaster, with planktonic organisms, which live at the ocean surface, and other organisms directly dependent on them, most vulnerable.

The word “disrupted” in the passage is closest in meaning to

Amoved up

Bdestroyed

Cinterfered with

Dspread through

 

6

Paleontologists realized that such an impact would have had severe effects on the global environment. A sudden and disastrous period of global cooling could have resulted from the huge cloud of dust that would have been propelled into the atmosphere. A more gradual but equally devastating period of global warming might have been caused by greenhouse gases formed by the impact. If the asteroid struck marine sediments, vaporization of the rock would have formed sulfate aerosol droplets that might have created intense acid rain conditions. Widespread wildfires would have resulted from the blast itself. Any combination of these effects could easily have disrupted the food chain, with the most severe consequences for those at the top. The oceans also would have been devastated by the chemical effects of the disaster, with planktonic organisms, which live at the ocean surface, and other organisms directly dependent on them, most vulnerable.

According to paragraph 3, all of the following environmental effects might have occurred after the impact of a giant asteroid EXCEPT

Aextensive fires

Bsevere acid rain

Can immediate increase in global temperatures

Da gradual increase in the impact of greenhouse gases

 

7

Paleontologists realized that such an impact would have had severe effects on the global environment. A sudden and disastrous period of global cooling could have resulted from the huge cloud of dust that would have been propelled into the atmosphere. A more gradual but equally devastating period of global warming might have been caused by greenhouse gases formed by the impact. If the asteroid struck marine sediments, vaporization of the rock would have formed sulfate aerosol droplets that might have created intense acid rain conditions. Widespread wildfires would have resulted from the blast itself. Any combination of these effects could easily have disrupted the food chain, with the most severe consequences for those at the top. The oceans also would have been devastated by the chemical effects of the disaster, with planktonic organisms, which live at the ocean surface, and other organisms directly dependent on them, most vulnerable.

It can be inferred from paragraph 3 that which of the following marine organisms were LEAST likely to suffer from the negative effects of the impact?

AOrganisms that fed directly on plankton.

BOrganisms that lived near the ocean’s surface.

COrganisms that were at the top of the food chain.

DOrganisms that lived deep in the oceans.

 

8

The great appeal of the impact hypothesis is that many predictions that follow from this idea have been supported by observations. One is that the iridium-rich clay layer at the K-T boundary has indeed been found worldwide. Another is the presence in the dust layer of grains of quartz with intersecting sets of parallel fractures. These grains, called shocked quartz, are evidence that rocks were subjected to extremely high pressures in the vicinity of the impact. Small, spherical, glassy grains are also present in the layer; these formed by rapid cooling of droplets of melted rock thrown into the atmosphere by the blast.

The word “appeal” in the passage is closest in meaning to

Aattraction

Bsurprise

Coutcome

Dcharacteristic

 

9

The great appeal of the impact hypothesis is that many predictions that follow from this idea have been supported by observations. One is that the iridium-rich clay layer at the K-T boundary has indeed been found worldwide. Another is the presence in the dust layer of grains of quartz with intersecting sets of parallel fractures. These grains, called shocked quartz, are evidence that rocks were subjected to extremely high pressures in the vicinity of the impact. Small, spherical, glassy grains are also present in the layer; these formed by rapid cooling of droplets of melted rock thrown into the atmosphere by the blast.

The word “vicinity” in the passage is closest in meaning to

Ainstant

Barea

Cheat

Dremains

 

10

The great appeal of the impact hypothesis is that many predictions that follow from this idea have been supported by observations. One is that the iridium-rich clay layer at the K-T boundary has indeed been found worldwide. Another is the presence in the dust layer of grains of quartz with intersecting sets of parallel fractures. These grains, called shocked quartz, are evidence that rocks were subjected to extremely high pressures in the vicinity of the impact. Small, spherical, glassy grains are also present in the layer; these formed by rapid cooling of droplets of melted rock thrown into the atmosphere by the blast.

Why does the author point out that “Small, spherical, glassy grains are also present in the layer”?

ATo provide an example of a prediction from the impact hypothesis that has been confirmed by observation

BTo point out differences in the kinds of grains produced by the impact

CTo contrast the effects of high pressures and rapid cooling on rocks in the vicinity of the impact

DTo indicate that the iridium-rich layer contains different types of grains

 

 

11

The great appeal of the impact hypothesis is that many predictions that follow from this idea have been supported by observations. One is that the iridium-rich clay layer at the K-T boundary has indeed been found worldwide. Another is the presence in the dust layer of grains of quartz with intersecting sets of parallel fractures. These grains, called shocked quartz, are evidence that rocks were subjected to extremely high pressures in the vicinity of the impact. Small, spherical, glassy grains are also present in the layer; these formed by rapid cooling of droplets of melted rock thrown into the atmosphere by the blast.

According to paragraph 4, the presence of fractured grains of quartz is a sign that rocks were

Ahit by pieces of extremely fast-moving dust and clay

Bexposed to a huge amount of pressure

Cexposed to extremely high temperatures

Dformed through melting followed by rapid cooling

 

12

Perhaps the most crucial prediction is that the impact of a body with a diameter of 10 kilometers would have created a crater about 100 kilometers wide. Where can such a crater be seen today? With no such structure readily visible at the surface of Earth, geologists were left with two possibilities. First, the impact may have occurred in the deep ocean; the crater therefore may have been buried by marine sediment. Second, the crater may have formed on land but have been buried by sediments in the 66 million years since the impact. Pure luck was on the side of geologists when exploration for oil on the Yucatan Peninsula of Mexico revealed a circular depression 180 kilometers wide buried beneath hundreds to over 1,000 meters of rock. Rocks just above the depression, called the Chicxulub structure, are Tertiary in age, whereas those just below are Late Cretaceous (100.5 million to 66 million years ago). Holes drilled into the structure found broken and melted rock right at the K-T boundary. Most geologists now believe that the Chicxulub structure is indeed the crater formed by the asteroid that caused the massive extinctions at the K-T boundary.

According to paragraph 5, which of the following is NOT true of the Chicxulub structure?

AIt was circular in shape.

BIt was buried under 180 meters of rock.

CIt was located right at the K-T boundary.

DIt was probably formed by the impact that caused the massive extinctions.

 

13

Look at the four squaresthat indicate where the following sentence could be added to the passage

Eventually, however, a theory emerged that has won wide acceptance.

Where would the sentence best fit？Click on a square  sentence to the passage.

14

Scientists have long sought to explain why an extraordinary number of organisms went extinct at the end of the Cretaceous period.

AOne theory is that high levels of iridium in the atmosphere may have caused widespread disease, loss of vegetation, and weakness that increased animals’ vulnerability to predation.

BA thin layer of iridium-rich clay found in marine sediments in several locations around the world suggested that an asteroid with a diameter of at least 10 kilometers had hit Earth.

CThe lack of an obvious crater and the prevalence of iridium in marine sediments as opposed to terrestrial rocks suggest that the impact occurred in the deep ocean.

DEffects of such a collision would have included changes in global climate, fires, acid rain, and loss of plankton and other ocean life.

EAmong the evidence found in support of the impact hypothesis was the presence of the iridium-rich clay worldwide, droplets of melted rock, and the discovery of a huge crater in the Yucatan Peninsula.

FThe impact hypothesis is now favored over the iridium hypothesis because it better explains the suddenness of the extinctions and their occurrence on both land and sea.

 

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