Table of Contents
Passage
A June 30,1908, 7:14 a.m., central Siberia—Semen Semenov, a local farmer, saw “the sky split in two. Fire appeared high and wide over the forest… From… where the fire was, came strong heat….Then the sky shut closed, and a strong thump sounded, and I was thrown a few yards…. After that such noise came, as if … cannons were firing, the earth shook …” Such is the harrowing testimony of one of the closest eyewitnesses to what scientists call the Tunguska event, the largest impact of a cosmic body to occur on the earth during modern human history. Semenov experienced a raging conflagration some 65 kilometers (40 miles) from ground zero, but the effects of the blast rippled out far into northern Europe and Central Asia as well. Some people saw massive, silvery clouds and brilliant, colored sunsets on the horizon, whereas others witnessed luminescent skies at night—Londoners, for instance, could plainly read newsprint at midnight without artificial lights. Geophysical observatories placed the source of the anomalous seismic and pressure waves they had recorded in a remote section of Siberia. The epicenter lay close to the river Podkamennaya Tunguska, an uninhabited area of swampy taiga forest that stays frozen for eight or nine months of the year.
B Ever since the Tunguska event, scientists and lay enthusiasts alike have wondered what caused it. Although most observers generally accept that some kind of cosmic body, either an asteroid or a comet, exploded in the sky above Siberia, no one has yet found fragments of the object or any impact craters in the affected region. The mystery remains unsolved, but our research team, only the latest of a steady stream of investigators who have scoured the area, may be closing in on a discovery that will change our understanding of what happened that fateful morning.
C Part of the enduring mystery of the Tunguska event harks back to the stark physical isolation of central Siberia and the political turmoil that raged in Russia during the early 20th century, a time when the czarist empire fell and the Soviet Union emerged. These two factors delayed scientific field studies for nearly 20 years. Only in 1927 did an expedition led by Leonid Kulik, a meteorite specialist from the Russian was confronted with some almost unbelievable scenery. Amazingly, the blast had flattened millions of trees in a broad, butterfly-shaped swath covering more than 2,000 square kilometers (775 square miles). Furthermore, the tree trunks had fallen in a radial pattern extending out for kilometers from a central area where “telegraph poles,” alone stand of partially burned tree stumps, still remained. Kulik interpreted this ravaged landscape as the aftermath of an impact of an iron meteorite. He then began to search for the resulting crater or meteorite fragments.
D Kulik led three additional expeditions to the Tunguska region in the late 1920s and 1930s, and several others followed, but no one found clear-cut impact craters or pieces of whatever had hit the area. The dearth of evidence on-site gave rise to various explanatory hypotheses. In 1946, for instance, science-fiction writer Alexander Kazantsev explained the puzzling scene by positing a scenario in which an alien spacecraft had exploded in the atmosphere. Within a few years, the airburst theory gained scientific support and thereafter limited further speculation. Disintegration of a cosmic object in the atmosphere, between five and 10 kilometers above the surface, would explain most of the features investigators observed on the ground. Seismic observatory records, together with the dimensions of the devastation, allowed researchers to estimate the energy and altitude of the blast.
E The lack of an impact crater also suggested that the object could not have been a sturdy iron meteorite but a more fragile object, such as a relatively rare, stony asteroid or a small comet. Russian scientists favored the latter hypothesis because a comet is composed of dust particles and ice, which would fail to produce an impact crater. Another explanation for the tumult in the Tunguska region claimed that the destruction resulted from the rapid combustion of methane gas released from the swampy ground into the air.
F In 1975 , Ari Ben-Menahem, a seismologist at the Weizmann Institute of Science in Rehovot,Israel, analyzed the seismic waves triggered by the Tunguska event and estimated that the energy released by the explosion was between 10 and 15 megatons in magnitude, the equivalent of 1,000 Hiroshima atomic bombs. Astrophysicists have since created numerical simulations of the Tunguska event to- try to decide among the competing hypotheses. The airburst of a stony asteroid is the leading interpretation.
G Models by Christopher F. Chyba, then at the NASA Ames Research Center, and his colleagues proposed in 1993 that the asteroid was a few tens of meters in diameter and that it exploded several kilometers above the ground. Comparison of the effects of nuclear test airbursts with the flattened pattern of the Tunguska forest seems to confirm this suggestion.
H More recent simulations by N. A. Artemieva and V. V. Shuvalov, both at the Institute for Dynamics of Geospheres in Moscow, have envisioned an asteroid of similar size vaporizing five to 10 kilometers ab ove Tunguska. In their model, the resulting fine debris and a downward propagating gaseous jet then dispersed over wide areas in the atmosphere. These simulations do not, however, exclude the possibility that metersize fragments may have survived the explosion and could have struck the ground not far from the blast.
I Late last year Mark Boslough and his team at Sandia National Laboratories concluded that the Tunguska event may have been precipitate by a much smaller object than earlier estimates had suggested. Their supercomputer simulation showed that the mass of the falling cosmic body turned into an expanding jet of high-temperature gas traveling at supersonic speeds. The model also indicated that the impactor was first compressed by the increasing resistance of the earth’s atmosphere. As the descending body penetrated1 deeper, air resistance probably caused it to explode in an airburst with strong flow of heated gas that was carried downward by its tremendous omentum. Because the fireball would have transported additional nergytoward the surface, what scientists had thought to be an explosion between 10 and 20 megatons was more likely only three to five megatons, according to Boslough. All this simulation work only strengthened (and continues to strengthen) our desire to conduct fieldwork at the Tunguska site.
Questions
Questions 1-6 The reading passage has seven paragraphs, A-I
Choose the correct heading for paragraphs A-F from the list below.
Write the correct number, i-ix, in boxes 1-6 on your answer sheet.
List of Headings
i. Specialists investigate the bizarre scene
ii. Model proved by atomic experiment
iii. Mystery remains for further explanation
iv. Terrifying description from witness
v. Rare clue arouses fantasy assumptions
vi. Some scientists assume the explosion never happened
vii. Meteorite theory is still a dominant explanation
viii. The Crater is hided in messy scene
ix. Scientific explanation for an untraceable crater
1 Paragraph A
2 Paragraph B
3 Paragraph C
4 Paragraph D
5 Paragraph E
6 Paragraph F
Questions 7-11 Use the information in the passage to match the people (listed A-I) with opinions or deeds below. Write the appropriate letters A-E in boxes 7-11 on your answer sheet.
NB you may use any letter more than once
A Alexander Kazantsev
B Christopher F. Chyba
C Ari Ben-Menahem
D Mark Boslough
E Leonid Kulik
7 The traceless crater is due to explosion above ground.
8 Fictional depiction for Tunguska explosion.
9 Proposed a hypothesis of atomic quantity estimate.
10 Regarded messy scene caused by a meteorite.
11 The magnitude of the explosion is different from previous one.
Questions 12-13 Choose the TWO correct letters from following options. Write your answers in boxes 12-13 on your answer sheet.
Which TWO of the following statements are correct according to this passage?
A More recent simulations prove the previous model has flaws.
B Craters of explosion has not been found yet
C Semen Semenov is the only witness of Tunguska Event
D Hard evidence of simulation indicates unnecessary field research.
E Friction of atmosphere may lead to meteorite explosion
Answers
Below are the answers, key words, synonyms and full explanations for the IELTS Reading passage The Tunguska Mystery (Passage 1, Questions 1–13).
Questions 1–6: Matching Headings (i–ix) for paragraphs A–F
1. Paragraph A
Answer: iv — Terrifying description from a witness
Key words: Semen Semenov, sky split in two, harrowing testimony
Synonyms: terrifying description = “harrowing testimony of one of the closest eyewitnesses”
Explanation: Section A is the farmer’s frightening first-hand account of the event.
2. Paragraph B
Answer: iii — Mystery remains for further explanation
Key words: wondered what caused it, mystery remains unsolved
Synonyms: remains for further explanation = “The mystery remains unsolved”
Explanation: Section B states the cause is still unknown and being investigated.
3. Paragraph C
Answer: i — Specialists investigate the bizarre scene
Key words: expedition, Leonid Kulik, flattened millions of trees
Synonyms: specialists investigate = “an expedition led by Leonid Kulik, a meteorite specialist”; bizarre scene = “almost unbelievable scenery”
Explanation: Section C describes Kulik’s team studying the strange flattened forest.
4. Paragraph D
Answer: v — Rare clue arouses fantasy assumptions
Key words: dearth of evidence, Kazantsev, alien spacecraft
Synonyms: fantasy assumptions = “an alien spacecraft had exploded”; rare clue = “The dearth of evidence on-site gave rise to various … hypotheses”
Explanation: The lack of evidence gave rise to imaginative theories such as an alien spacecraft.
5. Paragraph E
Answer: ix — Scientific explanation for an untraceable crater
Key words: lack of an impact crater, comet, dust particles and ice
Synonyms: untraceable crater = “The lack of an impact crater”; scientific explanation = “a comet … would fail to produce an impact crater”
Explanation: Section E explains why no crater exists — a fragile comet/asteroid leaves none.
6. Paragraph F
Answer: vii — The asteroid (impact) theory is the dominant explanation
Key words: Ben-Menahem, megatons, leading interpretation
Synonyms: dominant explanation = “The airburst of a stony asteroid is the leading interpretation”
Explanation: Section F establishes the stony-asteroid airburst as the leading (dominant) interpretation.
Questions 7–11: Matching People (A–E)
A Alexander Kazantsev · B Christopher F. Chyba · C Ari Ben-Menahem · D Mark Boslough · E Leonid Kulik
7. The traceless crater is due to an explosion above the ground.
Answer: B
Key words: exploded several kilometers above the ground
Synonyms: explosion above ground = “it exploded several kilometers above the ground”
Explanation: Chyba’s 1993 model proposed the asteroid exploded above the ground, so no crater formed.
8. A fictional depiction of the Tunguska explosion.
Answer: A
Key words: science-fiction writer, alien spacecraft
Synonyms: fictional depiction = “science-fiction writer Alexander Kazantsev … an alien spacecraft had exploded”
Explanation: Kazantsev, a science-fiction writer, imagined an exploding alien spacecraft.
9. Proposed an estimate of the explosion’s atomic-scale magnitude.
Answer: C
Key words: 10–15 megatons, 1,000 Hiroshima atomic bombs
Synonyms: atomic quantity estimate = “the equivalent of 1,000 Hiroshima atomic bombs”
Explanation: Ben-Menahem estimated the energy at 10–15 megatons — about 1,000 Hiroshima bombs.
10. Regarded the ravaged scene as caused by a meteorite.
Answer: E
Key words: Kulik, aftermath of an impact of an iron meteorite
Synonyms: messy scene caused by a meteorite = “Kulik interpreted this ravaged landscape as the aftermath of an impact of an iron meteorite”
Explanation: Kulik believed the destruction was caused by an iron meteorite impact.
11. The magnitude of the explosion differs from previous estimates.
Answer: D
Key words: Boslough, smaller object, three to five megatons
Synonyms: different from previous = “more likely only three to five megatons” (vs the earlier 10–20)
Explanation: Boslough concluded the blast was smaller (3–5 megatons) than earlier estimates.
Questions 12–13: Choose TWO correct statements
Answer: B and E (either order)
B — Craters of the explosion have not been found yet
Synonyms: “no one has yet found fragments of the object or any impact craters”
Explanation: No crater or fragments have ever been located.
E — Friction with the atmosphere may cause a meteorite to explode
Synonyms: friction of atmosphere = “air resistance probably caused it to explode in an airburst”
Explanation: Boslough’s model attributes the explosion to increasing air resistance.
Why not the others: A (“proves previous model has flaws”) overstates — the estimate was revised, not disproved; C is wrong (Semenov was “one of the closest eyewitnesses”, not the only one); D is contradicted — the simulations “strengthened … our desire to conduct fieldwork”.
Answer Key (Quick Reference)
| Q | Answer | Q | Answer |
|---|---|---|---|
| 1 | iv | 8 | A |
| 2 | iii | 9 | C |
| 3 | i | 10 | E |
| 4 | v | 11 | D |
| 5 | ix | 12 | B |
| 6 | vii | 13 | E |
| 7 | B |
