# Francis Birch (geophysicist)

Albert Francis Birch
Born August 22, 1903
Died 30 January 1992 (aged 88)
United States
Occupation Geophysicist

Albert Francis Birch (August 22, 1903-January 30, 1992) was an American geophysicist.[1]

## Biography

He was born in Washington DC, United States to a Virginia born father involved in business and real estate and a church choir singer and soloist mother at St. Matthew's Cathedral in Washington DC. He graduated from Harvard University and became a full time professor at Harvard in 1946.

Albert Francis Birch is 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,[2] 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

## Nuclear bombing

During World War II he took a leave of absence from Harvard and was assigned to the Manhattan Project, first in New York City, then in New Mexico. Because of his knowledge of metals and metallurgy he was in charge of designing and engineering the triggering mechanism for the Hiroshima atomic bomb code-named Little Boy; and went to Tinian to supervise loading it onto Enola Gay (the Boeing B-29 Superfortress tasked with dropping the bomb) and assembling the bomb before takeoff. He devised the 'double plug' system that allowed for actually arming the bomb after Enola Gay took off (so that if it crashed, there would not be a nuclear explosion on Tinian).

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.[3]

In 1961, Birch published two papers [4] [5] 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 $\rho$ as:

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

## Post-war

Birch was the Sturgis Hooper Professor of Geology at Harvard University from 1949 to 1974. He served as the president of the Geological Society of America in 1963-1964 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 Gold Medal of the Royal Astronomical Society (1973), and the 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.