Twelve Key 20th-century Discoveries in the Geosciences

Abstract

This paper reviews 12 major discoveries in geology and geophysics during the 20th century, reasonably mature and discrete with respect to subject, time, or discoverer. The Textbook of Geology (Geikie, 1903) and Understanding the Earth (Brown and others, 1992) are used as benchmarks, supplemented by Yoder's 0007-1992 “Timetable of Petrology” (1993). The discovery of the radioactive decay law by Rutherford and Soddy was fundamental, bearing on the age of the Earth, internal energy, nature of the crust, and a quantitative geologic time scale. Discoveries of the internal structure of the Earth, investigated seismically, included definition of the core boundary (Gutenberg and Weichert), the inner core (Lehmann), the mantle/crust boundary (Mohorovic), the lithosphere, and the asthenosphere. Deep structure of the continental crust, revealed since 1975 by reflection profiling and study of exposed sections, has been found to be drastically different from previous concepts. The magnetic reversal time scale was developed by correlating reversed magnetization in terrestrial and marine rocks with radiometric and stratigraphic data, interpreted in light of sea-floor spreading. Sea-floor spreading, the key element of plate-tectonic theory, discovered by several independent lines of inquiry, has been directly confirmed by space-geodesy measurements of plate motion and intraplate rigidity in the Pacific Basin. Elastic rebound, the cause of shallowfocus earthquakes, was proposed after the 1906 California earthquake by Reid, using geodetic evidence; deep-focus events are still not understood. The mechanism of overthrust faulting, a major problem as late as the 1950s, was discovered by Hubbert and Rubey, who demonstrated that high fluid pressure could reduce normal stress to the point that gravity sliding would be effective. X-ray diffraction by the crystal lattice was discovered in 1912, and its applications revolutionized mineralogy, confirming solid solution and clarifying many crystallographic phenomena. The origins of basaltic and granitic magmas, completely unknown in 1900, were definitively explained for basalts by Bowen as partial melting of a peridotitic mantle, and for most granitic magmas as partial melting of crustal rocks by Tuttle and Bowen. Space exploration has revealed common patterns of planetary differentiation, including a first (or felsic) differentiation forming global primitive crusts, followed by a second (or basaltic) differentiation, including continued basaltic magmatism. Another major result of space exploration and related research has been discovery of the importance of impact cratering, proposed as being responsible for formation of continental nuclei, the first ocean basins, and many mass extinctions in the geologic record. The most important scientific achievement of the 20th century is suggested to be the discovery of the DNA structure, revealing not only the molecular basis for all life on Earth but a critical line of investigation, formation of RNA, for study of the origin of life.