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Geologists drilling `ultradeep' holes hope for scientific pay dirt

Geologists are gearing up to turn Earth into a pincushion. Around the world, scientists are laying plans to bore a new series of ``ultradeep'' holes into the continental crust to help unravel some basic riddles of geology.

West German scientists are looking at two possible sites to spud a 46,000-foot (14,000-meter) hole beginning in 1988. A national continental drilling program is getting under way in the United States which includes plans for a possible 33,000-foot (10,000-meter) probe in the Appalachians -- the deepest ever drilled in North America.

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The Soviet Union, long the leader in the field, is expanding its ambitious deep-drilling program. Canada, too, is weighing plans to begin rummaging around in its continental basement.

All this is expected to lead to a great leap forward in geology over the next decade, aiding scientists in understanding such fundamental things as the origins of continents, how mountain chains form, and the forces that give birth to minerals.

``It will answer questions that have been raised and argued about literally for centuries,'' says G. Arthur Barber, president of a 24-university consortium overseeing a US drilling program, referring mainly to the American effort. ``Here we've been spending billions sending sensors out into space, yet we don't know what's eight miles beneath our feet.''

It is this knowledge gap that is driving the deep-drilling activity. The past few decades have seen a wealth of geologic information unearthed: The theory of plate tectonics -- that continents drift around the world atop a soupy layer of molten rock -- explains much of the earth's general geology today. So, too, have scientists gleaned much from a successful deep-sea drilling program. But the continental crust remains largely an unexplored frontier.

``We think this is one of the major areas that earth scientists have yet to come to grips with,'' says Benjamin Morgan of the US Geological Survey (USGS).

What is known has been inferred from surface rocks and studying eroded riverbeds or holes drilled by the oil and gas industry, which are usually in the wrong place and not deep enough for geologists. Lately, scientists have gained new insights by using seismic devices to create fuzzy ``snapshots'' of the earth's crust. Now, however, they need to drill into the basement rock itself to test these theories and to retrieve other information.

The nascent US continental drilling program is being funded by the National Science Foundation, the US Department of Energy, and the USGS. Some $7 million will be spent on boreholes this year.

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The most ambitious proposal being considered is the 6.5-mile-deep hole in the Appalachian Mountains, near the Georgia-South Carolina border. The area is believed to mark the leading edge of a sheet of crystalline rock that was pushed over sedimentary rock during a great continental collision between Africa and North America 270 million years ago. The deep hole would test this theory.

It may also, says University of South Carolina geologist Robert Hatcher, open a window on the evolution of continents and the formation of mountains here and around the world.

Funding for the $40 million-plus project has not been approved yet. But an advisory committee of the National Academy of Sciences has suggested it receive high billing in the national drilling program. At present, Dr. Hatcher and colleagues are boring shallow holes to test rock samples. Work on the deep hole itself could begin within three years, though garnering federal funding in these deficit-ridden times may be as tough as chipping granite with a wooden mallet.

For now, the deepest man-made hole in North America is the Bertha Rogers oil well in Oklahoma. Drilling stopped there at six miles (9.65 kilometers) when the bit hit molten sulfur. While the Appalachian hole would punch well beyond this -- and considerably boost the nation's scientific drilling program -- the US still trails the Soviets in the field.

After 15 years of boring and countless rubles, Soviet geologists have punctured the 7.4-mile (12-km) mark on the world's deepest hole in the Kola Peninsula, inside the Arctic Circle. Using an innovative drilling technique, they are boring through rock more than a billion years old.

The hole has already produced a harvest of data. Geologists have bumped into flows of hot, highly mineralized water where none were thought to exist. Such fluids, when able to filter up through other rock formations, can deposit dissolved minerals to form veins of ore. Soviet scientists have also found traces of copper, and other minerals far deeper than predicted, all of which could enhance their ability to search for new resources.

Nor is this the only super-deep hole they are working on. Last fall, Soviet bits began gnawing away at a crystalline shield in the Ukraine, where a 9.3-mile (15-km) hole is planned. At least four others are planned or under way that will pierce the 4.4-mile barrier.

In the US, drilling projects other than the Appalachian venture are being devised which may yield scientific pay dirt. Drilling will begin on two 2,000-foot holes in a silver-mining district of southwestern Colorado. Geologists will be plumbing volcanic rock 30 million years old to learn more about silver deposits. It will also be a test run for the type of technology that may be used in the Appalachian hole.

In California, Mark Zoback of Stanford University and others want to deepen a well drilled for oil near the San Andreas fault to study stresses in the rock. This could lead to more understanding of when and where faulting, and thus earthquakes, may occur.

In the geothermal fields near the California-Mexico border, a consortium has $7.4 million in US Department of Energy funding to bore to depths where temperatures reach 300 degrees C. Interest here is in the interaction of hot brines and rock and how geothermal reservoirs are fed. Several other holes have already been bored in this Salton Sea region.

From all this, geologists hope at least to begin to understand the continental basement. ``It will be 10 to 20 years before we understand the continental crust,'' says George Kolstad, DOE's manager of geosciences. CHART: Rummaging in Earth's `basement' Drilling `ultradeep' holes in Earth's crust sediments and volcanic rocks folded and metamorphosed `basement' `granitic' crust coarse-grained igneous rock very dense igneous rock, thought to be largely olivine upper crust Conrad discontinuity lower crust Mohorovici'c discontinuity (Moho) mantle (in kilometers) 10 20 30 40 Bertha Rogers well, USA Appalachian well, USA (proposed) Kola well, USSR Ukraine well, USSR (proposed) Hypothetical cross-section of continental crust, based on seismic data and outcrop information. Source: ``The Cambridge Encyclopedia of Earth Sciences'' 30{et