Francis Birch (1903–1992)

Francis Birch was born in Washington, D. C., on August 22, 1903. He entered Harvard in 1920 and graduated magna cum laude in 1924 with a bachelor’s degree in electrical engineering. After working 2 years for the New York Telephone Company, he decided to shift to the study of physics. He obtained a fellowship that led to 2 years of study in France at the University of Strasbourg in the laboratory of Pierre Weiss, one of the founders of modem magnetism. Encouraged by this experience, he returned in 1928 to Harvard as a graduate student in physics, working chiefly in the high-pressure laboratory of Percy W. Bridgman, who was to receive the Nobel Prize for Physics in 1946. Birch was an instructor and tutor in physics from 1930 to 1932, received his master’s degree in 1929, and was awarded his Ph.D. in 1932. During this period, an almost daily visitor and consultant at Bridgman’s laboratory was Reginald A. Daly, the renowned Harvard professor of geology and textbook author. Long interested in the origins of igneous rocks and the structure and properties of the Earth’s deep interior, Daly clearly recognized the need to extend the very limited data on high-pressure physical properties of rocks and minerals in order to better interpret measurements made by seismological and gravity techniques. Daly frequently discussed such problems with his friend Bridgman, and together they set up the continuing interdepartmental Committee on Experimental Geology and Geophysics. In 1932 the Committee initiated two new research ventures at Harvard: a revitalized program in seismology and a program for comprehensive high-pressure studies devoted to geophysical problems. Birch was invited to lead the new high-pressure research program as Harvard’s first research associate in geophysics. Research on the physical and chemical properties of the Earth’s interior became his lifelong task.

Except for a 1942–1945 wartime leave of absence, Francis Birch spent his entire career at Harvard, advancing from research associate through the academic ranks to become the prestigious Sturgis-Hooper Professor of Geology and chairman of the Division of Geological Sciences. Retiring as professor emeritus in 1974, he continued active research and writing until his death, at age 88, on January 30, 1992.

Birch combined impressive theoretical as well as experimental competence in physics, electrical engineering, and geology. By bringing the full power of these disciplines to bear in his researches, he successfully treated many difficult and long-standing fundamental problems in a rigorous and decisive manner. His career was characterized throughout by clear insight into the physical basis of geological problems, practical ideas on how to go about solving them, and lucid presentations of the results in lectures and publications. His experiments, as well as most of his resulting general conclusions and interpretations pertaining to the Earth, have stood the test of time extremely well. Birch’s work was concerned chiefly with elasticity, phase relations, thermal properties and heat flow, and the composition of the Earth’s interior. Extensive laboratory studies of elastic velocities in rocks and their variation with pressure and temperature by himself and others provided the first realistic estimates of density-pressure relationships (equations of state) at high compressions, which Birch used to interpret global seismic data in terms of the composition and structure of the interior.

Probably Birch’s most significant contribution appeared in one of classic papers of geophysics in 1952. Here, he conclusively demonstrated that (1) the mantle is predominately composed of silicate minerals; (2) the upper mantle and lower mantle regions, each essentially homogeneous but of somewhat differing compositions, are separated by a thin transition zone associated with silicate phase transitions; and (3) the inner and outer core are alloys of crystalline and molten iron, respectively, rather than alternative interpretations proposed at the time. The essential details of this model are still valid; only a few refinements have been necessary in the light of subsequent research.

Birch and his students also made major contributions to our knowledge of terrestrial heat flow. Again combining experimental data on thermal conductivities of rocks with temperature gradient measurements from boreholes and tunnels, they helped establish heat flow as one of the key boundary conditions of terrestrial geophysics.

Another of Birch’s important contributions to science was his ever helpful role as mentor, teacher, advisor, and friend to numerous colleagues, postdoctoral fellows, and students who worked with him over more than 40 years; many became outstanding leaders in geophysics in subsequent years. Birch was a gracious person with a wry sense of humor, and associates held him in great respect.

During his lifetime, Birch received many honors, including election to the National Academy of Sciences (1950), the National Medal of Science (1968), and the Vetlesen Medal for 1960. A Fellow of the American Physical Society and of the Royal Astronomical Society of Great Britain, he received the Royal Astronomical Society’s Gold Medal in 1973. Particularly active in the Geological Society of America, he was awarded both its Arthur L. Day Medal (1950) and the Penrose Medal (1969) and served as president during 1964. For his distinguished contributions to geophysics, Birch received the William Bowie Medal, the highest award of the American Geophysical Union, in 1960. AGU has instituted a series of named lectureships in honor of former Bowie Medalists; these invited lectures for the Tectonophysics Section are known as Francis Birch Lectureships. A Francis Birch Lecture has been delivered annually since 1992 by a noted researcher in this field.

—Kenneth H. Olsen

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