24 / Basics of Environmental Science
Surtsey. As it cooled, sea birds began to settle on it.^3 They carried plant seeds and slowly plants and
animals began to colonize the new land.
Even the damage caused by destructive eruptions is repaired, although this can take a long time. The
1883 eruption of Krakatau, in the Sunda Strait between Java and Sumatra, Indonesia, destroyed
almost every living thing on Krakatau itself and on two adjacent islands. Three years later the lava
was covered in places by a thin layer of cyanobacteria, and a few mosses, ferns, and about 15 species
of flowering plants, including four grasses, had established themselves. By 1906 there was some
woodland, which is now thick forest. The only animal found in 1884 was a spider, but by 1889 there
were many arthropods and some lizards. In 1908, 202 species of animals were living on Krakatau
and 29 on one of the islands nearby, although bats were the only mammals. Rats were apparently
introduced in 1918. Species continued to arrive and 1100 were recorded in 1933 (KENDEIGH,
1974, pp. 24–25).
Rock that forms from the cooling and crystallization of molten magma is called ‘igneous’, from the
Latin igneus, ‘of fire’, and all rock is either igneous or derived from igneous rock. This must be so,
since the molten material in the mantle is the only source for entirely new surface rock. If the magma
reached the surface before cooling the rock is known as ‘extrusive’; if it cooled beneath the surface
surrounded by older rock into which it had been forced, it is said to be ‘intrusive’. Intrusive rock may
be exposed later as a result of weathering. It is not only igneous rocks that can form intrusions. Rock
salt (NaCl) can accumulate in large amounts beneath much denser rocks and rise through them very
slowly to form a salt dome. Salt domes are deliberately sought by geologists prospecting for oil but
occasionally they can break through the surface. When this happens the salt may flow downhill like
a glacier.
The character of the rock depends first on its chemical composition. If it is rich in compounds of iron
and magnesium it will be dark (melanocratic); if it is rich in silica, as quartz and feldspars, it will be
light in colour (leucocratic). Rock between the two extremes is called ‘mesocratic’. The rock comprises
minerals, each with a particular chemical composition, and minerals crys-tallize as they cool. Whole
rock is quarried for building and other uses; many minerals are mined for the chemical substances
they contain, especially metals, and some are valued as gemstones. Crystallization proceeds as atoms
bond to particular sites on the surface of a seed crystal, forming a three-dimensional lattice. It can
occur only where atoms have freedom to move and so the more slowly a molten rock cools the larger
the crystals it is likely to contain. The crystal size gives the rock a grain structure, which also contributes
to its overall character. The type of rock is also determined by the circumstances of its formation.
Lava that flows as sheets across the land surface or sea bed often forms basalt, a dark, fine-grained,
hard rock. Basalt covers about 70 per cent of the Earth’s upper crust, making it the commonest of all
rocks; most of the ocean floor is of basalt overlain by sediments and on land it produces vast plateaux,
such as the Deccan Traps in India. Intrusive igneous rocks are usually of the light-coloured granite
type. Beyond this, however, the identification and classification of igneous rocks are rather
complicated.^4
Rocks formed on the ocean floor may be thrust upward to become dry land or exposed when the
sea level falls. Tectonic plate movements are now believed to be the principal mechanism by
which this occurs. Where two plates collide the crumpling of rocks can raise a mountain chain,
as is happening now between the Indian and Eurasian plates, raising the Himalayan chain. The
Himalayas, which began to form some 52–49 million years ago following the closure of the
Tethys Sea, are linked to the Alps, which began forming about 200 million years ago owing to
very complex movements of a number of plates (WINDLEY, 1984, pp. 202–308). The formation
of a mountain chain by the compression of crustal rocks is known as an ‘orogeny’ (or
‘orogenesis’).