Geoscience Reference
In-Depth Information
Granite may be the inevitable result of a tectonically active planet of silicate rocks and
plenty of water. But there can never be a Waterworld, a planet with no continents but global
oceans. Once there is water, it finds its way into the chemistry of the rocks, lubricating
them as they melt so that they can rise as great masses of granite to form the peaks of con-
tinents above the oceans. Without water, you get the situation on Venus: tectonics without
the plates. Without the inner fire of molten magma, you get the situation on Mars, an old,
cold surface where life, if it exists, is deep in hiding. On Earth, you get oceans and contin-
ents in dynamic and sometimes lethal interaction.
Riches in the earth
One of the first incentives to geological exploration was the search for mineral wealth.
Rare and valuable substances can be formed or concentrated by a number of geological
processes. Organic remains in sedimentary basins can be gently cooked by the Earth's heat
to produce coal, oil, and natural gas. We've already seen how sulphides of valuable metals
can be concentrated around deep-sea hydrothermal vents and how manganese nodules can
form on the deep ocean floor. Minerals can be concentrated in continental rocks in a num-
ber of ways. In molten rock, crystals will begin to form and the densest ones will sink to
the bottom of the melt chamber. As well as concentrating minerals within it, a mass of mol-
ten rock rising through other rocks will drive super-heated water and steam ahead of it.
Under pressure, that can dissolve many minerals, especially those rich in metals, forcing
them through cracks and fissures where they are deposited as veins of mineral. Other min-
erals can become concentrated near the surface when water evaporates or when other com-
ponents in a rock are eroded away. If we have the technology to recover them, the Earth's
riches are there for the taking.
The search for lost continents
If continental scum has been accumulating on the surface of the planet for most of its his-
tory, when did it begin? Where is the first continent? It is not easy to say. The most ancient
continental rocks have been so reworked, folded, fractured, buried, partially melted, folded
and fractured again, and shot through by younger intrusions, that it is hard to make sense
of them. It's a bit like trying to identify the remains of an individual car within the com-
pacted scrap from a junk yard. But the search for the oldest rocks on Earth may be near-
ing its end. Some of the first contenders were from the Barberton greenstone belt in South
Africa. These are more than 3.5 billion years old, but they are the remains of pillow lavas
and ocean islands, not continents. Similar rocks have now been discovered in the Pilbara
region of western Australia, and there are rocks in southwest Greenland that yield dates of
3.75 billion years, but these again are ocean volcanic rocks. The best candidate for the first
