Earth Sciences / 21such as Ontario, Canada, and the Transvaal, South Africa, it is no more than 9 or 10°C per kilometre
(HOLMES, 1965, ch. XXVIII, p. 995). Because of the low thermal conductivity of rock, very little
of this heat reaches the surface and it has no effect on the present climate.
Where the gradient is anomalously high, however, it can be exploited as a source of geothermal
energy. In volcanic regions, such as New Zealand, Japan, Iceland, and Italy, water heated below
ground may erupt at the surface as geysers, hot springs, or boiling mud. More often it fails to reach
the surface and is trapped at depth, heated by the surrounding rock. A borehole drilled into such a
reservoir may bring hot water to the surface where it can be used. In some places a body of dry
subsurface rock is much hotter than its surroundings. In principle this can also be exploited, although
experimental drilling, for example some years ago in Cornwall, Britain, has found the resulting
energy rather costly. The technique is to drill two boreholes and detonate explosive charges at the
bottom, to fracture the rock between them and so open channels through it. Cold water is then
pumped at pressure down one borehole; it passes through the hot rock and returns to the surface
through the other borehole as hot water.
This exploitation of geothermal energy is not necessarily clean. Substances from the rock dissolve into
the water as it passes, so it returns to the surface enriched with compounds some of which are toxic.
The solution is often corrosive and must be kept isolated from the environment and its heat transferred
by heat exchangers. Nor is the energy renewable. Removal of heat from the rock cools it faster than it
is warmed by radioactive decay, so eventually its temperature is too low for it to be of further use.
Similarly, the abstraction of subsurface hot water depletes, and eventually empties, the reservoir.
Although subsurface heat has no direct climatic effect, there is a sense in which it does have an
indirect one. Material in the mantle is somewhat plastic. Slow-moving convection currents within
the mantle carry sections of the crustal rocks above them, so that over very long time-scales the
crustal material is constantly being rearranged.^2 On Earth, but possibly on no other solar-system
planet, the crust consists of blocks, called ‘plates’, which move in relation to one another. The
theory describing the process is known as ‘plate tectonics’ (GRAHAM, 1981). At present there
are seven large plates, a number of smaller ones, and a still larger number of ‘microplates’. The
boundaries (called ‘margins’) between plates can be constructive, destructive, or conservative. At
constructive margins two plates are moving apart and new material emerges from the mantle and
cools as crustal rock to fill the gap, marked by a ridge. There are ridges near the centres of all the
world’s oceans. Where plates move towards one another there is a destructive margin, marked by
a trench where one plate sinks (is subducted) beneath the other. At conservative margins two
plates move past one another in opposite directions (see Figure 2.2). There are also collision
zones, where continents or island arcs have collided. In these, all the oceanic crust is believed to
have been subducted into the mantle, leaving only continental crust. Such zones may be marked in
various ways, one of which is the presence of mountains made from folded crustal rocks. An
island arc is a series of volcanoes lying on the side of an ocean trench nearest to a continent. The
volcanoes are due to the subduction of material.
Slowly but constantly the movement of plates redistributes the continents carried on them. A glance at
a map shows the apparent fit between South America and Africa, but for 40 million years or more prior
to the end of the Triassic Period, about 213 million years ago, all the continents were joined in a
supercontinent, Pangaea, surrounded by a single world ocean, Panthalassa. Pangaea then broke into
two continents, Laurasia in the north and Gondwana in the south, separated by the Tethys Sea, of
which the present Mediterranean is the last remaining trace. The drift of continents in even earlier
times has now been reconstructed, with the proposing of a supercontinent called Rodinia that existed
about 750 million years ago (DALZIEL, 1995). The Atlantic Ocean opened about 200 million years
ago and it is still growing wider by about 3–5 cm a year. A little more than 100 million years ago India