Worlds of David Darling > Children's
Encyclopedia of Science > Could You Ever Dig a Hole to China? > 4. The
Deepest Holes in the World
COULD YOU EVER DIG A HOLE TO CHINA?
a book in the Could You Ever? series by David Darling
4. The Deepest Holes in the World
Oil, like coal, is a fossil fuel. In other words, it formed from the squashed
remains of animals and plants that died hundreds of millions of years ago.
But instead of being mined, oil is brought to the surface through wells
that are drilled through the rock that lies above it. Today, scientists
employ the methods used to recover oil to drill deep holes. In this way,
they hope to learn more about the makeup of the Earth's crust.
|The Glomar Challenger helped scientists
learn more about the makeup of the Earth's crust beneath the oceans
A specially equipped American research ship, the Glomar Challenger,
carried out a worldwide survey of the rocks beneath the sea. In 1976, as
part of the United States Deep Sea Drilling Project, Glomar Challenger
drilled to a record depth of 5,709 feet below the ocean floor off the northwest
coast of Spain.
On land, where most permanent drilling rigs can be set up, much deeper bore-holes
have been sunk into the Earth. In Russia's Kola Peninsula, 155 miles north
of the Arctic Circle, scientists have been drilling what is now the world's
Hitting a New Low
The bottom of the Russian hole is now more than 7.6 miles beneath the Earth's
surface. Further progress, though, will only come very slowly. Every time
the drill bit that cuts into the rock wears out, the entire 39,000-foot-long
drill assembly has to be lifted out so that a new bit can be fitted.
|Some of the deepest holes,
natural and manmade, in the world
In an ordinary well, the whole drill stem is rotated in order to turn the
drill bit at the end. But as the drill gets deeper, more and more energy
is lost by the stem rubbing against the side of the hole. To solve this
problem, new drilling methods have been developed. In the Kola well, the
drill bit is mounted on a turbine, a device like a propeller. The turbine
is spun around by pressurized mud pumped down the hole. Since the only moving
part is the bit itself, friction is greatly reduced.
The mud that drives the turbo-powered drill also picks up fragments of rock
and brings them back to the surface where they can be examined. In this
way, Russian scientists have been able to learn a great deal abut the structure
of the crust below the Kola site. For the first half of the drilling, the
rocks were found to be about 1.1 billion years old. But in the lower half
of the well, the rocks have been dated at more than 2.7 billion years old.
Below a depth of 5.5 miles, nearly half the rock consists of ores rich in
metals such as iron and titanium. Most surprising of all, the Russian team
discovered large amounts of hot water deep within the hole that had cracked
the surrounding rock.
As the Kola well deepens, drilling becomes more and more difficult. Already,
the temperature of the rock at the bottom of the shaft is nearly 400°F,
and the pressure is hundreds of times that at the surface. Even so, Russian
scientists hope to extend the Kola well down to a depth of between 9 and
Sampling the Crust
The success of the new drilling techniques pioneered by the Russian scientists
has encouraged other groups to make plans for super-deep wells. In Russia,
a number of these will be drilled for studying oil- and gas-bearing rocks.
Others will be used to search for ores and for basic research. Improved
equipment such as high-strength alloy drilling pipes could lead to holes
being drilled even deeper than that on the Kola Peninsula.
Germany, too, has begun a huge drilling program close to the Czech border.
If all goes well, this project will sample the Earth's crust down to a depth
of 8½ miles. Similar projects are bring considered in France, Britain,
and the United States.
However great these accomplishments may be, they are like tiny pinpricks
compared with the size of our whole planet. The Kola hole, after more than
20 years of drilling, reached only a five-hundredth of the way to the center
of the Earth. What hope is there that people or machines might penetrate
much more deeply underground in the future? Could we eventually break through
the crust and travel down into the mantle or even to the Earth's core?