BEST IELTS General Reading Test 415

BEST IELTS General Reading Test 415

IELTS General Reading Test

The Alvarez Hypothesis

In 1980, Luis and Walter Alvarez offered their meteorite impact theory after examining research sites around the world. Their hypothesis suggested that the mass extinction of the dinosaurs was caused by the impact of a large meteorite on Earth about sixty-five million years ago.

Meteorites are lumps of stony or metallic material that were parts of asteroids. Meteorites commonly collide with Earth, but the majority are destroyed in the impact with the Earth’s atmosphere. Asteroids are planetary bodies or objects that revolve around the Sun and most are found in the asteroid belt between Mars and Jupiter. Some meteorites survive after entering the Earth’s atmosphere and impact on the Earth’s surface.

IELTS General Reading Test

The Alvarez Hypothesis theorises that a meteorite the size of San Francisco, travelling faster than a bullet, slammed into Earth sixty-five million years ago. The impact delivered a destructive blast thousands of times more powerful than the combined yield of all the world’s nuclear weapons, setting off earthquakes greater than eleven in magnitude and widespread tsunamis, and shrouding the globe for years in a thick cocoon of sky-blackening dust and debris. This cataclysm effectively ended the reign of the dinosaurs and opened the door for the ascension of mammals.

The story began in 1977 in Gubbio, Italy, a tiny village halfway between Rome and Florence, where geologists Luis and Walter Alvarez were collecting samples of limestone rock for a paleomagnetism study. The limestone outside of Gubbio, which was once below the sea, provides total geological evidence of the end of the Cretaceous period and the beginning of the Tertiary period This time span is sometimes referred to as “the Great Dying,” because a massive extinction claimed nearly seventy-five per cent of all the species of life on our planet, including, in addition to the dinosaurs, most types of plants and many types of microscopic organisms.

IELTS General Reading Test

Luis and Walter Alvarez found that forming a distinct boundary between the limestone of the two periods was a thin layer of red clay. Immediately below this clay boundary, the Cretaceous limestone was heavily populated with a wide mix of the fossils of tiny marine creatures. Above the clay layer in the Tertiary limestone, however, only the fossils of a single marine species could be seen. The clay layer itself contained no marine fossils at al. The two geologists consulted two nuclear chemists in Berkley, who had developed a technique called neutron activation analysis. This enabled precise measurements of very low concentrations of elements.

Luis Alvarez believed that neutron activation analysis would help determine how long it took for the clay layer to form. To the amazement of everyone involved, the measurements showed that the clay layer was about six hundred times richer in iridium than the surrounding limestone. Iridium, a silvery-white metallic element related to platinum, is virtually absent from the Earth’s crust, but high concentrations are common in extra-terrestrial objects, such as asteroids.

IELTS General Reading Test

These same iridium results were subsequently also discovered in clay layers at locations in Denmark and New Zealand, and later dozens of other sites around the world where the geological record of the Cretaceous-Tertiary boundaries is also complete. These iridium-spiked layers of clay also contained an abundance of soot.

The four scientists published their paper: “Extra-terrestrial Cause for the Cretaceous – Tertiary Extinction”. This paper was immediately resisted by scientific critics, who argued that volcanic eruptions were behind the demise of the dinosaurs and cited as evidence the thousands of miles of volcanic rock in an area of India known as the Deccan Traps. However, that argument was weakened by two subsequent findings. First, there was the discovery of shocked quartz along with the iridium and soot in the clay layer samples from around the world, which could only have been produced in the heat and violence of a titanic smash.

IELTS General Reading Test

Second came the discovery in 1991 of the scene of the titanic smash – the Chicxulub crater. This meteorite impact was thought to have caused the broken up quartz. The Chicxulub crater is a 180-kilometerwide, 20-kilometer-deep impact crater off the northern coast of the Yucatan peninsula in the Gulf of Mexico that is buried a kilometre deep. The discovery of this impact site answered critics of the Alvarez Hypothesis who’d been demanding to know: if an asteroid impact killed the dinosaurs, where’s the crater?

The Chicxulub crater was explored by drilling over two months. The crater r nowadays is buried beneath a thousand meters of sediment. Under this is a layer of what is known as impact breccias and then a layer of the impact melt. Beneath that is the rock that was needed to be brought to surface for research. The rock was shown to be made of anhydrite, which is a mineral that, if it were vaporised in an impact event, would produce the particular gases that could alter the Earth’s climate.

IELTS General Reading Test

The question of why the dinosaurs died out is still not definitively answered. Sixty-five million years ago is a large age gap to find conclusive evidence. Today, most scientists agree with the ‘The Alvarez Hypothesis’ and that the Earth was indeed struck by a large meteorite at the end of the Cretaceous period. Whether this was the actual reason the dinosaurs died out or whether it was only a contributing factor will probably never be proved.

Choose the correct letter A, B, C or D.

28. Most meteorites colliding with Earth

A. usually have trajectories that head towards the sun.

B. are broken up and destroyed when they enter the Earth’s atmosphere.

C. are usually found beyond the farthest planet.

D. lost their power to threaten Earth millions of years ago.

IELTS General Reading Test

29. The Alvarez Hypothesis theorised that the impact of the meteorite

A. triggered a nuclear explosion.

B. was preceded by a series of earthquakes.

C. had no effect on sea life.

D. darkened the sky of the Earth.

30. Gubblo was a good place to research for Luis and Walter Alvarez, because

A. it was close to where they lived.

B. the area had a strong magnetic field.

C. rock there provides a complete geological record of the relevant time frame.

D. it is close to the sea.

IELTS General Reading Test

31. ‘The Great Dying” was a time when

A. all life on Earth was wiped out.

B. almost three-quarters of life on Earth was wiped out.

C. only plants were left alive on Earth.

D. only a few dinosaurs were left alive.

32. The thin layer of red clay found by Luis and Walter Alvarez

A. was coloured by the decay of the limestone on either side.

B. had no fossils from the sea.

C. contained only a few dinosaur fossils.

D. was a record of the first few million years of the Tertiary period.

IELTS General Reading Test

33, The surprise from the neutron activation analysis results was

A. how exact the measurements were.

B. how fast the clay layer took to form.

C. how slow the clay layer took to form.

D. how high the levels of a certain element were.

34. Samples around the world from the same time period also showed

A. uneven results of irridium.

B. the Cretaceous period ended at different times around the world

C. high levels of soot.

D. similar fossil records.

IELTS General Reading Test

Complete each sentence with the correct ending (A – F) below. Write the correct letter (A – F) in answer boxes 35-37 on your answer sheet.

35. The paper that was published on the Alvarez Hypothesis

36. The impact of the meteorite 65 million years ago

37. The site of the Chicxulub crater

A. was not published until some time later.

B. was discovered after the initial publication of the Alvarez Hypothesis.

C. was initially preferred to the theory of volcanic eruptions killing the dinosaurs.

D. was not accepted at once by fellow scientists.

E. was replaced by soot in some research sites around the world.

F. was later thought to be responsible for the broken up quartz in the rock samples

IELTS General Reading Test

Label the diagram below. Write NO MORE THAN THREE WORDS for each answer.

IELTS General Reading Test

IELTS General Reading Test

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BEST IELTS General Reading Test 415

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28. B

29. D

30. C

31. B

32. B

33. D

34. C

35. D

36. F

37. B

38. SEDIMENT

39. (THE) IMPACT MELT

40. GASES

IELTS General Reading Test

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