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Physicist Unfolds Phenomena Of Rock With Super-Pressure

Testes Furnish New Information On Plasticity, Strength Of Formations


Pressures reaching up to 2,000,000 pounds per square inch are being applied to rocks and crystals by David T. Griggs, Junior Follow in Geophysics. in a study of geological forces deep underground. Tests used involve the highest one directional stress ever controlled and measured in laboratory research. High pressure equipment utilized was developed at Harvard by Professor Percy W. Bridgeman, '04.

Under high pressure the plasticity and strength of rocks undergo fundamental changes, yielding clues to the mechanism of earthquakes, mountain formation, and other phenomena.

Laboratory tests duplicate the pressure conditions in any part of the earth's outer crust--a granite layer extending down 30 to 50 miles. In some of the tests temperatures as high as 900 degrees Fahrenheit were employed.

A specimen of rock under test is placed in a thick steel cylinder. An hydraulic confining pressure is applied through a liquid, or at very high pressures through lead. The highest confining pressure used are about 300,000 pounds per square inch. In addition, a direct, differential pressure is exerted on the specimen by a steel piston. Different pressure used attain more than 1.500,000 pounds per square inch.

Under a confining pressure equivalent to a depth of 32 miles underground, a limestone cylinder was compressed 35 percent in length without shattering. Similar tests show that the flow of rocks under pressure is mathematically alike to the flow of metals.

Quarts Remains Brittle

Although some varieties of rock were observed to become plastic and flow under relatively low pressure, quartz remained brittle under the very highest confining and differential pressures. Studies of quartz, one of the most common minerals, are vital to investigation of conditions underground. Prior to these tests it was thought that quartz might become plastic udder the high pressures.

The fracture of quartz into needles under stress of natural rock deformation was duplicated in the laboratory tests and observations were made of the crystallographic details of these fractures.

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