Earth Sciences / 23separated from Antarctica. The Indian plate began subducting beneath the Eurasian plate and as
India moved north the collision, about 50 million years ago, raised the Himalayan mountain
range. India is still moving into Asia at about 5 cm a year and the mountains are still growing
higher (WINDLEY, 1984, pp. 161 and 310), although the situation is rather complicated. Rocks
exposed at the surface are eroded by ice, wind, and rain, so mountains are gradually flattened.
At the same time, the crumpling that produces mountains of this type increases the mass of
rock, causing it to sink into the underlying mantle. This also reduces the height of large mountain
ranges. It is possible, however, for the eroded material to lighten the mountains sufficiently to
reduce the depression of the mantle, causing them to rise, and there is reason to suppose this is
the case for the Himalayas (BURBANK, 1992). The Red Sea is opening and in time will become
a new ocean between Africa and Arabia.
The distribution of land has a strong influence on climates. If there is land at one or other pole, ice
sheets are more likely to form. The relative positions of continents modify ocean currents, which
convey heat away from the equator, and the size of continents affects the climates of their interiors,
because maritime air loses its moisture as it moves inland. The Asian monsoon is caused by pressure
differences to the north and south of the Himalayas. In winter, subsiding air produces high pressure
over the continent and offshore winds, with very dry conditions inland. The word ‘monsoon’ simply
means ‘season’ (from the Arabic word for ‘season’, mausim) and this is the winter, or dry, monsoon.
In summer, pressure falls as the land warms, the wind direction reverses, and warm, moist air flows
across the ocean toward the continent, bringing heavy rain. This is the summer, wet monsoon. Plate
tectonics exerts a very long-term influence, of course, and other factors modify climates in the
shorter term, but the distribution of land and sea determines the overall types of climate the world is
likely to have (HAMBREY AND HARLAND, 1981).
Plate tectonics affects the environment more immediately and more dramatically. The movement
of plates causes earthquakes, because it tends to happen jerkily as accumulated stress is
released, and is associated with volcanism due to weakening of the crust at plate margins.
Earthquakes cause damage to physical structures, which is the direct cause of most injuries,
and those which occur beneath the sea produce tsunami (www.geophys.washington.edu/tsunami/
general/physics/physics.html). These are shock waves affecting the whole water column. No
more than a metre high and with a wavelength of hundreds of kilometres, but travelling at
more than 700 km h-1, on reaching shallow water they rise to great height and destructive
power (ALLABY, 1998, pp. 54–60).
If volcanic ash reaches the stratosphere it can cause climatic cooling, but volcanic eruptions are
more usually associated with damage to human farms and dwellings. This arises partly because of
the beneficial effect volcanoes can have. Volcanic ash and dust are often rich in minerals and rejuvenate
depleted soils. Farmers can grow good crops on them, which is why there tend to be cultivated fields
at the foot and even on the lower slopes of active volcanoes.
7. The formation of rocks, minerals, and geologic structures
Volcanoes create environments. This was demonstrated very dramatically, and shown on televi-sion,
in 1963, when a new submarine volcano called Surtsey (volcano.und.nodak.edu/vwdocs/volc_images/
europe_west_asia/surtsey.html) erupted to the south of Iceland. The eruption was extremely violent,
because sea water entered the open volcanic vent, and steam, gas, pieces of rock, and ash were
hurled many kilometres into the air. Since then eruptions of this type have been called ‘Surtseyan’.
The lava cone was high enough to rise above the surface, where it formed what is now the island of