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.
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.
29. Scientists disliked Wegener’s idea because he
A was German.
B made simple observations.
C was a meteorologist.
D made too many suggestions.
30. Both Wegener and Semmelweis
A died prematurely.
B lacked crucial evidence.
C were never given recognition.
D were German.
31. The motion of tectonic plates
A is faster than hair growth.
B does not change.
C is well understood.
D can start cyclic swirls.
32. Volcanos are formed away from plate boundaries due to
A buckling and melting.
B oceanic effects.
C geological events.
D heated regions.
Questions 33-35
Complete the diagram. Choose NO MORE THAN TWO WORDS from the passage for each answer.
