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Reading Passage: The Snowball Earth Hypothesis

A debate has been raging among scientists about the plausibility of the ‘snowball Earth’ hypothesis, which posits that the Earth was once completely covered in ice and snow. If this theory is true, it could explain many geological mysteries, but some claim the planet could not have recovered from such a deep freeze.

Imagine an Earth entirely covered by ice, from the poles to the equator, where little to no life survives and temperatures are perpetually far below zero. That is the proposition of the ‘snowball Earth’ hypothesis. This is a contentious theory that suggests that the Earth was entirely frozen for at least one period in its history, and scientists are divided on how to interpret the geological record and the debate over the hypothesis is ongoing.

The most recent period when this might have occurred was at the end of the Proterozoic eon, some 650 million years ago, in what is now known as the Karoo Ice Age. Scientists are however divided on both the geological evidence and the likelihood of Earth reviving, and fostering life, after such a period, as well as when exactly this could have taken place. Proponents nevertheless suggest that the theory explains some of the mysteries of the geological record.

The first geologist to put forward the idea of a snowball Earth was Douglas Mawson who discovered glacial sediments in southern Australia in the mid-20th century, which he took as evidence of global glaciation. However, this theory was superseded by the idea of continental drift, which more readily explained the existence of glaciers in Australia and other landmasses. The snowball Earth theory was revived in the 1960s by W. Brian Harland who suggested that glacial sediments in Greenland were actually deposited nearer to the tropics. According to Harland, the only thing that could account for these deposits was an extreme, worldwide ice age.

It was not until the 1990s that Joseph Kirschvink, a Professor of geobiology, coined the term ‘snowball Earth’ to describe this ice age. Kirschvink also proposed an ultra-greenhouse effect as a way in which the Earth may have escaped from this glacial condition. Kirschvink’s ideas were taken up by Franklyn who developed a model that suggested that the Earth had once indeed been ice-covered.

According to proponents of the snowball Earth hypothesis, there are several ways in which both the geological record and climate models support the theory. The most persuasive is perhaps the evidence of palaeomagnetism, a recent development in geology which can show when and where geological deposits were made, whilst taking into account continental drift. Through palaeomagnetism it has been possible to show that sediments of glacial origin were deposited near to the equator during the Neoproterozoic era. However scientists are sceptical of this conclusion due to other plausible explanations, such as the possibility that the Earth’s magnetic field has shifted over time and the existence of an earlier magnetic pole near the equator could account for these deposits.>> Form đăng kí giải đề thi thật IELTS 4 kĩ năng kèm bài giải bộ đề 100 đề PART 2 IELTS SPEAKING quý đang thi (update hàng tuần) từ IELTS TUTOR

As Eyles’ research suggests, one of the main points of contention of the snowball Earth debate is the extent to which this big freeze took hold across the planet. Many geologists and climate scientists claim that a ‘hard snowball’ effect, in which the Earth is completely enveloped in ice sheets, would have obliterated life on Earth forever. They thus say that the most likely scenario was a partial freeze, in which some parts of the ocean remained free of ice.

The lack of a mass extinction event in the geological record proves that the snowball Earth was more temperate than some contend. It may be that carbon dioxide built up during the cold period, when vegetation was reduced and could not remove as much of it from the atmosphere. In turn, this carbon dioxide warmed the planet, leading to widespread thawing and release of oxygen into the seas. Francis Macdonald, an Earth scientist at Harvard University, who has carried out research into volcanic rocks in Canada that suggested that the global glaciation did indeed occur, has suggested that in fact Earth wasn’t just a white ball, but more of a mud ball during this period.

This idea of a ‘mud ball’ rather than a snowball has gained many adherents among geologists. Richard Peltier of the University of Toronto says, “The suggestion is that the Earth was once entirely covered by ice – in combination with thick ice sheets and the oceans by thick sea ice – remains somewhat contentious. Peltier’s model demonstrates that despite deep glaciations across the planet, a large amount of water remained unfrozen in the tropical regions of the planet. This has been termed the ‘slushball Earth’ hypothesis, and it is supported by the survival of life during this period as evidence of a less extensive global freeze. If the snowball Earth hypothesis is true then a band of ice-free waters would have persisted around the equator; most experts now agree this was the most probable situation.”

The debate over the snowball Earth hypothesis seems to continue, although it is clear that some form of glaciation did occur. The idea of a completely frozen Earth nonetheless remains a thriving area of research, not only for geology but for climate science. A recent study at Columbia University revealed how climate models using the snowball Earth theory can shed insight into future climate change, and the potential repercussions of catastrophic global climate shifts. As Sohl states, “Studying snowball Earth glaciation can tell us just how bad it can get, in which case life as we know it would probably not survive.”

Questions

Questions 1 - 5
Do the following statements agree with the information given in the reading passage?
Write:
YES if the statement agrees with the claims of the writer
NO if the statement contradicts the claims of the writer
NOT GIVEN if it is impossible to say what the writer thinks about this

1. Scientists agree the snowball Earth occurred in the Palaeoproterozoic era.

2. Climate models developed in the 20th century led to theories about paleomagnetism.

3. Some experts believe that if Earth had been totally enclosed in ice, life would have been permanently erased.

4. The ‘slushball Earth’ hypothesis shows how life evolved on Earth.

5. Linda Sohl showed that the snowball Earth model can teach us about climate change.

Questions 6 - 9
Choose the correct letter, A, B, C or D.

1. In the first paragraph, the writer suggests that the ‘snowball Earth’ hypothesis
   A. has changed how geologists understand the formation of glaciers.
   B. is an experimental theory that has caused widespread confusion.
   C. has changed how scientists understand the origins of the planet.
   D. is a theory which has prompted extensive debate.

2. The writer says that scientists oppose the snowball Earth theory because
   A. it shows that climate change is not man-made.
   B. banded iron formations disprove it.
   C. there are other reasonable explanations.
   D. the theory of continental drift explains the evidence better.

3. As Eyles’ research suggests, one of the main points of contention of the snowball Earth debate is the
   A. speed at which the glaciers melted.
   B. extent to which the freeze covered the planet.
   C. effect the freeze had on volcanic activity.
   D. reason why the freeze began in the first place.

4. What confirmed that the snowball Earth was warmer than some maintain?
   A. A new climate model developed at a university
   B. An absence of evidence for a mass extinction
   C. Carbon dioxide samples taken from sea ice
   D. Vegetation patterns in the geological record

Questions 10 - 13
Complete the summary below.
Choose NO MORE THAN TWO WORDS from the passage for each answer.

The snowball Earth hypothesis was first put forward by Douglas Mawson, who found 10 _______________ in southern Australia. This prompted him to theorise that the entire planet had at one point been covered in ice. However, 11 _______________ was proposed to explain existence of glaciers in Australia and other landmasses and Mawson’s ideas were forgotten. W. Brian Harland brought the snowball Earth hypothesis back to prominence when he proposed that glaciers found in Greenland were originally deposited in the 12 _______________ due to a global freeze. This was reiterated by Joseph Kirschvink, who invented the term ‘snowball Earth’, and Franklyn Van Houten, whose discovery of phosphorus deposits and banded 13 _______________ formations provided evidence for the hypothesis.

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