Final Years

Having accomplished his two career goals of dating the Earth and constructing a geological timescale that could be applied to common rocks, Holmes concentrated on his professorial duties. In 1943 the university of Edinburgh appointed him regius professor of geology, a position subsidized by the king of England. The outbreak of World War II forced Holmes to reduce his geology course from one year to six months. Though it would have saved lecture time to assign students reading material before coming to class, no geology textbook contained information about the recent developments in the field, such as radiometric dating and continental drift. Holmes took it upon himself to compose a book based on his lecture notes. When he published Principles of Physical Geology in 1944, it became an immediate best seller and was reprinted more than 18 times during the following 20 years.

Holmes became ill in 1948 and lost all interest in his work. The doctor ordered complete rest, and he and his wife spent the summer in Ireland. After recovering, Holmes focused on Precambrian geology and revised Africa's geological map based on radiometric dates. His heart began to deteriorate, and in 1956 he retired from the University of Edinburgh.

The distinguished geologist belonged to numerous scientific organizations and received many honors and awards during his career. The Geological Society of London gave Holmes their Murchison Medal in 1940 and their highest award, the Wol-laston Medal, in 1956. The Geological Society of America awarded Holmes their Penrose Medal in 1956 for his outstanding contributions in the science of geology. In 1964 Holmes received the Vetle-sen Prize, the greatest honor for a geologist, for his "uniquely distinguished achievement in the sciences resulting in a clearer understanding of the Earth, its history, and its relation to the universe." His health was too frail to travel to Columbia University for the award ceremony, but he did find the strength to tackle one more major project, revising Principles of Physical Geology. He finished just a few months before he died of bronchial pneumonia on September 20, 1965, in London.

Arthur Holmes was a quiet man but did not avoid the controversial topics of geology in his day, in particular, the antiquity of the Earth and continental drift. His background in physics convinced him radiometric dating was the most accurate means for determining the age of rocks and the Earth. This made him the right man for the job of providing actual ages for Earth's geological episodes. The implications of Holmes's estimate for an ancient Earth were widespread; they forced astronomers to reexamine the age of the universe and gave biologists reasonable time to allow for the occurrence of evolutionary processes. Though his contributions toward advancing the idea of drifting continents are often overlooked, Holmes was the first to propose convection currents as a plausible moving force. Today scientists believe the Earth formed 4.55 billion years ago because the father of geological time had a passion for seeking the truth and dedicated himself to laying the groundwork for using the natural geological clocks within the rocks.

See also geochronology; Hess, Harry; Precambrian.

FURTHER READING

Dunham, K. C. "Arthur Holmes (1890-1965)." In Biographical Memoirs of Fellows of the Royal Society. Vol. 12 (November 1966), 291-310. London: Royal Society, 1966.

Lewis, Cherry. The Dating Game: One Man's Search for the Age of the Earth. New York: Cambridge University Press, 2000. U.S. Geological Survey. Geologic Time: Online Edition. Available online. URL: http://pubs.usgs.gov/gip/geo-time/. Accessed January 30, 2009.

hot spot A hot spot is a center of volcanic and plutonic activity not associated with an arc and generally not associated with an extensional boundary. Most hot spots are 60-125 miles (100-200 km) across and are located in plate interiors. A few, such as Iceland, are found on oceanic ridges and are identified on the basis of unusually large amounts of volcanism on the ridge. Approximately 200 hot spots are known, and many others have been proposed but their origin is uncertain.

Hot spots are thought to be the surface expression of mantle plumes that rise from deep in the Earth's mantle, perhaps as deep as the core/mantle boundary. As the plumes rise to the base of the lithosphere, they expand into huge, up to 600-mile (1,000-km) wide plume heads, parts of which partially melt the base of the lithosphere and rise as magmas in hot spots of plate interiors.

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