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Pressure at the Center of the Earth

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Bibliographic Entry Result
(w/surrounding text)
"Density and Pressure." The New Book of Popular Science. Philippines: Grolier, 1194. "It [pressure] is about 3900 metric tons/cm" 380 GPa
"Petrology and Elements of Geochemistry." Constitution of the Earth's Interior. Warszawa: Polish Scientific Publishers, 1984. "The pressure at the centre is about 0.36 TPa" 360 GPa
"Density, gravity, pressure, and ellipticity in the interior of the earth." Internal Constitution of the Earth. New York: Dover. 1951 "Depth, 6371 km, pressure, 3,900,000 bars" 390 GPa
Handbook of Chemistry and Physics, 77th ed. 1996. "Depth 6371 km, pressure 3617 kbars" 362 GPa

A number of advances in physical sciences took place before any assessment of the pressure distribution within the earth were possible. Newton's gravitational law provided the stepping stone for this venture. The attractive force between two objects (according to Newton) was directly proportional to the product of the two masses and inversely proportional to the square of the distance between their centers, or

F ~ m1m2/r2

Written with a constant, this expression becomes

F = Gm1m2/r2

The constant G had to be determined experimentally; it could not be calculated. Henry Cavendish arrived at the value of G through experimental means

G = 6.7 × 10−11 Nm2/kg2

Evaluation of G has prompted the measurement of the earth's mass and consequently its density through the expression d = me/ve. The value of the Earth's density is approximately 5497 kg/m3.

Like many of the concepts in geophysics, density distribution within the earth is being studied through indirect means. Of particular value is the study of seismic wave propagation. Transverse and longitudinal seismic waves (shear and compressional waves respectively) behave differently depending on the medium in which they travel. A distinguishing property of shear waves is that they cannot pass through liquids and gases, while compressional waves can. Experimental evidence shows that, up to a depth of some 2900 km, shear wave movements are observed; material in this region s apparently rigid enough to allow such movement. Beyond 2900 km depth, no shear wave movements are observed, leading to a hypothesis that space beyond the 2900 km depth is filled with liquid. There is a fair amount of certainty that this liquid region is composed primarily iron, which is by far the most common of the dense materials on earth. Because of the immense pressures created by gravitational forces, the earth's core could have been squeezed to a solid, but high temperatures (roughly 2000 °C, according to one estimate) probably melted the iron.

Results of the study of seismic wave propagation in the form of density distribution yielded a hypothetical pressure distribution model of the earth's interior. At the centre, the pressure is about 380 GPa (380,000,000,000 pascal).

Pavel Khazron -- 1999

Bibliographic Entry Result
(w/surrounding text)
Bundy, F. P. The Diamond Makers. Physics Today. (November 2000): 59. "The last part of the book deals with efforts of scientists to harness the remarkable physical properties of diamond in miniature ultrahigh-pressure apparatuses (diamond anvil cells), using opposing, single-crystal diamond anvils to generate extremely high pressures in very small regions--pressures nearly as great as that at the center of planet Earth (about 3.5 megabar)." 350 GPa

Editor's Supplement -- 2001