separated 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.
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’).
The British landscape was formed by a series of orogenies. The first, at a time when Scotland was still joined to North America, began about 500 million years ago and produced the Caledonian-Appalachian mountain chain (WINDLEY, 1984, pp. 181–208) as well as the mountains of northern Norway. The Appalachians were later affected by the Acadian orogeny, about 360 million years ago, and the Alleghanian orogeny, about 290 million years ago. Europe was affected by the Hercynian and Uralian orogenies, both of which occurred at about the same time as the Alleghanian. Figure 2.3 shows the area of Europe affected by several orogenies.5
Igneous intrusions can be exposed through the weathering away of softer rocks surrounding them. Such an exposed intrusion, roughly circular in shape and with approximately vertical sides, is called a ‘boss’ if its surface area is less than 25 km2 and a ‘batholith’ if it is larger (and they are often much larger).
Dartmoor and Bodmin Moor, in Devon and Cornwall, Britain, lie on the surface of granite batholiths.
Mountains are not always formed from igneous rocks, however. There are fossil shells of marine organisms at high altitudes in the Alps and Himalayas, showing that these mountains were formed by the crumpling of rocks which had formed from sea-bed sediments.
Many sedimentary rocks are composed of mineral grains eroded from igneous or other rocks and transported by wind or more commonly water to a place where they settle. Others, said to be of
‘biogenic’ origin, are derived from the insoluble remains of once-living organisms. Limestones, for example, are widely distributed. Most sediments settle in layers on the sea bed, to which rivers have carried them. Periodic changes in the environmental conditions in which they are deposited may cause sedimentation to cease and then resume later, and chemical changes in the water or the sediment itself will be recorded in the sediments themselves and in the rocks into which they may be converted.
Figure 2.3 The mountain-forming events in Europe Note: The thick lines (- • - • -) mark the Alpine orogeny
Sandstones are perhaps the most familiar sedimentary rocks, consisting mainly of sand grains, made from quartz (silica, SiO
2
) which crystallized originally into igneous rock. Clay particles, much smaller than sand grains, can pack together to make mudstones. Sediments rich in calcium carbonate, often consisting mainly of the remains of shells and containing many fossils, form limestone and dolomite (sometimes known as ‘dolostone’ to distinguish it from the mineral called dolomite) (HOLMES, 1965, ch. VI, pp. 118–141). Particles deposited as sediments are changed into rock by the pressure of later deposits lying above them and the action of cementing compounds subsequently introduced into them. The process, occurring at low temperature, is called ‘diagenesis’. Some sedimentary rocks are very hard and many, especially sandstones and limestones, make excellent and durable building stone. Once formed, a sedimentary rock is subject to renewed weathering, especially if it is exposed at the surface, so sedimentary rocks continually form and re-form.
Sediments are deposited in horizontal layers, called ‘beds’, but subsequent movements of the crust often fold or fracture them. It is not unusual for beds to be folded until they are upside down, and the reconstruction of the environmental conditions under which sediments were deposited from the study of rock strata often begins by seeking to determine which way up they were when they formed.
All in all, the interpretation of sedimentary structures can be difficult.6 Figure 2.4 shows the sequence of events by which sedimentary structures may be folded, sculptured, and then subside to be buried beneath later beds producing an unconformity.
Figure 2.4 Stages in the development of an unconformity
The extreme conditions produced by the folding and shearing of rock can alter its basic structure by causing some of its minerals to recrystallize, sometimes in new ways. This process, called
‘metamorphism’, also happens when rock of any type comes into contact with molten rock, during the intrusion of magmatic material for example. Marble is limestone or dolomite (dolostone) that has been subjected to metamorphism at high temperature. Such shells as it contained are completely destroyed as the calcium carbonate recrystallizes as the mineral calcite. If quartz or clay particles are present, new minerals may form, such as garnet and serpentine. Hard limestone containing fossils is often called marble, but there are no fossils in true marble.
Slate is also a metamorphic rock, derived from mudstone or shale, in which the parallel align-ment of the grains, due to the way the rock formed, allows the rock to cleave along flat planes (HOLMES, 1965, pp. 168–170). It may contain fossils, although they are uncommon and usually greatly deformed, because slate forms when the parent sedimentary rock is squeezed tightly between two bodies of harder rock that are moving in parallel but opposite directions, so its particles, and fossils, are dragged out. It is this that gives slate its property of ‘slaty cleavage’ which, with the impermeable surface imparted at the same time, makes it an ideal roofing and weatherproofing material. Metamorphic rocks are widely distributed and with practice you can learn to recognize at least some of them.7 All the landscapes we see about us and the mineral grains that are the starting material for the soils which form over their surfaces are produced by these processes. The intrusion or extrusion of igneous rock supplies raw material. This weathers to provide the mineral grains which become soil when they are mixed with organic matter, or is transported to a place where it is deposited as sediment.
Pressure converts sediments into sedimentary rocks, which may then be exposed by crustal movements, so that erosion can recommence. Metamorphic rocks, produced when other rocks are subjected to high pressures and/or temperatures, are similarly subject to weathering. It is the cycling of rocks, from the mantle and eventually back to it through subduction, that produces the physical and chemical substrate from which living organisms can find subsistence.