# Francis Birch (geophysicist): Wikis

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# Encyclopedia

Albert Francis Birch (August 22, 1903-January 30, 1992) was the Sturgis Hooper Professor of Geology at Harvard University from 1949 to 1974. A geophysicist best known for his experimental work on the properties of Earth-forming minerals at high pressure and temperature, in 1952 he published a well-known paper in the Journal of Geophysical Research [1], where he demonstrated that the mantle is chiefly composed of silicate minerals, the upper and lower mantle are separated by a thin transition zone associated with silicate phase transitions, and the inner and outer core are alloys of crystalline and molten iron. The most famous portion of the paper, however, is a humorous footnote he included in the introduction:

"Unwary readers should take warning that ordinary language undergoes modification to a high-pressure form when applied to the interior of the Earth. A few examples of equivalents follow:"

High Pressure Form Ordinary Meaning
Certain Dubious
Undoubtedly Perhaps
Positive proof Vague suggestion
Pure iron Uncertain mixture of all the elements

In 1947, he adapted the isothermal Murnaghan equation of state, which had been developed for infinitesimal strain, for Eulerian finite strain, developing what is now known as the Birch-Murnaghan equation of state[2].

In 1961, Birch published two papers [3] [4] on compressional wave velocities establishing a linear relation (now called Birch's law) of the compressional wave velocity Vp of rocks and minerals of a constant average atomic weight $\bar{ M}$ with density ρ as:

$V_p = a (\bar{ M}) + b \rho$.

Birch served as the president of the Geological Society of America from 1963-64 and he received numerous honors in his career, including the GSA's Arthur L. Day Medal (1950) and Penrose Medal (1969), the American Geophysical Union's William Bowie Medal (1960), the National Medal of Science (1967), the Vetlesen Prize (1968) (shared with Sir Edward Bullard), the Royal Astronomical Society's Gold Medal (1973), and International Association for the Advancement of High Pressure Research's Bridgman Medal (1983). Since 1992, the AGU's Tectonophysics section has sponsored a Francis Birch Lecture, given at its annual meeting by a noted researcher in this field.

## References

1. ^ Birch, F. (1952), Elasticity and constitution of the Earth's interior, J. Geophys. Res., 57, 227-286.
2. ^ Birch, F. (1947), Finite elastic strain of cubic crystals, Phys. Rev., 71, 809-824.
3. ^ Birch, F. (1961), The velocity of compressional waves in rocks to 10 kilobars. Part 2. J. Geophys. Res., 66,2199-2224.
4. ^ Birch, F. (1961), Composition of the Earth's mantle, Geophys. J. R. astron. Soc., 4, 295-311.