雅思阅读第093套P3-Continents Collide!

Continents Collide!

The idea that the continents are moving was first proposed by a German meteorologist,.Alfred Wegener, in a book published in 1915. He had gathered a great deal of careful and tantalising evidence, the most obvious being the simple observation that the great landmasses of the world seem to fit together, jigsaw-like, a striking example being the coastlines of either side of the Atlantic ocean. Wegener was even able to theorise, correctly, that all the continents were once assembled into a supercontinent (now called Pangaea). Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).

Unfortunately, Wegener could propose no propulsive force for this movement, apart from the vague and erroneous suggestion that it might be centrifugal forces. He also severely overestimated the speed of this motion. These problems, and the fact that he was a meteorologist (rather than a geologist), meant that, upon publishing his ideas, the scientific community was resolutely and implacably hostile. It is an interesting example of that not uncommon instance in which a scientist who was fundamentally correct was denied any recognition in his lifetime. Semmelweis, who advocated the washing of hands before surgery as a way to reduce hospital fatalities, is another example. Wegener was to unexpectedly die on an expedition in Greenland, probably of a heart attack – in his death, as in his life, left out in the cold.

The first hints of the existence of Gondwana came from the similarity of fossil plants and animals distributed in the same geological period over South America, Africa, Antarctica, India, and Australia. Similarly, the composition and nature of the rocks along relevant coastlines spoke the same story, yet to become scientifically credible, the theory needed evidence of a propulsive force to move such huge continents (in the same way that Semmelweis’s ideas needed the germ theory of disease). It was only in the 1960s, decades after Wegener’s death, that hard evidence for his theory began amassing to eventually become overwhelming.

The theory is now called ‘plate tectonics’, since it was proven that the Earth’s surface is fractured into ‘plates’. These bump and grind as they steadily move at infinitesimally slow rates in given directions, driven by ‘convention forces’. These are formed by the vast circular rising of superheated rock from the planet’s molten interior. This material cools as it nears the surface, eventually sinking once again towards the centre. Add to this the rotation of the Earth itself, and there is a complicated and barely understood set of cyclic swirls of molten rock, producing drags and pulls on each tectonic plate, the sum of which results in a steady migration.

Of course, this motion is slow, typically at the speed at which fingernails grow, and at its fastest, the rate at which hair does. But by being consistent and essentially unstoppable, the results can be spectacular, particularly when plates meet. Here, the release of heat, as well as the buckling and melting which results, gives rise to geological events such as earthquakes, and geological features such as mountains, volcanoes, and oceanic ridges and trenches. Plate boundaries see most of the world’s active volcanoes, with the Pacific Plate’s ‘Ring of Fire’ being a good example. Volcanism may sometimes occur in the middle of plates, but this has been theorised to be a result of ‘hotspots’: anomalously hot areas of interior rock which melt through the plate, forcing an escape to the surface.

Plate boundaries come in three types. First, Transform boundaries, where the plates grind past each other. It was once thought that the well-known Aegir Ridge was an example, until studies showed that it had never been active, whereas the periodic earthquakes along California’s San Andreas Fault show the very opposite case. The second type is Divergent boundaries, where the two plates slide apart from each other. Mid-oceanic ridges, such as in the Atlantic, and active rift zones, such as in East Africa, are examples. Finally, there are Convergent boundaries, where the two plates slide towards each other. This can form either a subduction zone (if one plate moves underneath the other) or a continental collision. Deep marine trenches are formed in the former case, and with the descending plate releasing its trapped water on being heated in the Earth’s interior, huge amounts of heat and pressure rise to the surface, causing mountains and volcanoes to form, such as in the Andes mountain range.

The best example of a continental collision is the Indian plate, which is steadily and implacably migrating straight into central Asia. The Himalayas of Nepal and Northern India, the Karakoram Ranges of Northern Pakistan, and the highlands of Afghanistan, are all part of the complex fold system that resulted, producing some of the highest peaks in the world. There are also some deep valleys receiving the run-off melt-water from the far side of these mountains, creating some mighty rivers, such as the Indus, the Irrawaddy, and the Mekong. Interestingly, the Himalayas are still growing, meaning that the summit of Mount Everest is perhaps a couple of metres higher now than when people first stood there in 1953, presumably making it just that little bit harder to reach.

SECTION 3: QUESTIONS 27-40

Questions 27-28

Complete the sentences.

Choose ONE WORD from the passage for each answer.

The combination of North America and Eurasia had the name 27 _________________

The combination of Laurasia and Gondwana had the name 28 _________________

Questions 29-32

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

29Scientists disliked Wegener’s idea because he

Awas German.

Bmade simple observations.

Cwas a meteorologist.

Dmade too many suggestions.

30 Both Wegener and Semmelweis

Adied prematurely.

Blacked crucial evidence.

Cwere never given recognition.

Dwere German.

31The motion of tectonic plates

Ais faster than hair growth.

Bdoes not change.

Cis well understood.

Dcan start cyclic swirls.

32Volcanos are formed away from plate boundaries due to

Abuckling and melting.

Boceanic effects.

Cgeological events.

Dheated regions.

Questions 33-35

Complete the diagram.

Choose NO MORE THAN TWO WORDS from the passage for each answer.

Tectonic Forces at Work

33 _________________

34 _________________

35 _________________

Questions 36-40