Exploring the Earth from above and below Recent years have seen basic geology merge with other sciences to produce the all-embracing discipline of earth science, and the advent of sophisticated data-gathering systems. There has been much debate amongst the scientists involved. Traditionalists maintain that the only way to study the Earth is go out and sample it, record it, and interpret it directly. Progressives insist that the only way to a full picture of the Earth is through remote sensing and computerized assessment of the data. No matter how the science changes, there will always be room for both approaches.
Measuring the Earth The greatest advances in remote sensing have been in distance measurement. The surface of the Earth can be measured extremely accurately, using global positioning geodesy (detecting signals from satellites by Earth-based receivers), satellite laser ranging (in which satellites reflect signals from ground transmitters back to ground receivers), and very-long-baseline interferometry, comparing signals received at ground-based receivers from distant extraterrestrial bodies. These techniques can measure distances of thousands of kilometres to accuracies of less than a centimetre. Movements of faults can be measured, as can the growth of the tectonic plates. Previously such speeds were calculated by averaging displacements measured over decades or centuries. The results show that in the oceanic crust plate growth is steady: from 12 mm/0.5 in per year across the Mid-Atlantic Ridge to 160 mm/6.5 in per year across the East Pacific Rise. The major continental faults seem to be very irregular in their movement; the Great Rift Valley of East Africa has remained stationary for 20 years, when long-term averages suggest that it would have opened up by about 100 mm/4 in in that time.
Offshore petroleum exploration Petroleum is society's most important raw fuel. The traditional oilfields were on land, but in 1896 wells were drilled into the Summerland oilfield from piers extending from the California coast. Oilwells were sunk in the Caspian Sea, the Gulf of Mexico, and Venezuela's Lake Maracaibo in the 1930s and 1940s, but only in the 1970s with the development of the North Sea and Canadian Arctic oilfields did offshore exploration really start. Even under such hostile sea conditions, wells could be drilled into the continental shelf 125 m/400 ft below sea level.
As the more accessible deposits become depleted, petroleum exploration is going into ever-deeper waters of the world's continental shelves. In 1991 Brazil, currently leading the way in offshore oil exploration, sank a production well in water 752 m/2,467 ft deep off the northeast coast. It is estimated that by the year 2000 more than 30% of the world's petroleum production will be from offshore wells.
Alternative sources of petroleum However, the main oil reserves may lie somewhere else altogether. Accepted theory says that petroleum was formed from the remains of plant and animal matter, in the sedimentary rocks in which they were entombed. An alternative theory proposes inorganic origins for oil, from methane and hydrogen trapped when the Earth was formed in granites or metamorphic rocks, from which the oil seeps into the known traps in surrounding sedimentary basins. One team has been drilling into granite rocks in Sweden since 1987, with apparent success. Another will drill into granite in Canada in 1993, seeking vast new oil deposits in totally different geological sites from those traditionally investigated.
Climatic change The burning of oil and other carbon-based fuels, pumping vast quantities of carbon dioxide into the atmosphere, raises the possibility of climatic change. The prospect is alarming, especially if such change is brought about by human interference with nature. However, the Earth's climate has never been stable, and human effects may be masked by natural variations. Beds of coal in Spitzbergen and glacial debris in the middle of the Australian desert attest to large-scale climate variations over hundreds of millions of years. Other studies show shorter-term variations: temperate tree fossils in Antarctica, 58 from the South Pole, and in the far north of Canada, show that climates were much warmer than now right up to the beginning of the Ice Age.
The international Pliocene Research Interpretation and Synoptic Mapping programme, begun in 1990, should produce fine details of climatic changes before the Ice Age. The Greenland Icecore project has shown that, during the Ice Age, temperatures over the ice caps varied between cold – with temperatures about 12°C lower than at present – and mild, with temperatures about 78°C lower than at present. Each cold period lasted between 500 and 2,000 years, starting abruptly and ending gradually.
Several sudden, catastrophic climate changes may have been caused in the past by the impact of giant meteorites or comets. The crater of one, 214 million years old, has been found in Canada; this seems to relate to a mass extinction in the Triassic period. Scientists are still seeking remains of the meteorite believed to have wiped out the dinosaurs 65 million years ago.
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