HISTORY OF GEOLOGY FROM 1900 TO 1962 195 (refracted) around the core (and thus delayed) It was not until 1913 that he accepted the idea of a liquid core which caused the ‘shadow’ for S waves if the angular distance from the focus of the earthquake were more than 120 Oldham and most other researchers were unaware that the Russian Leonid Leybenzon had suggested the same idea in 1911 There then followed the recognition of the ‘Moho discontinuity’, discovered by Andriya Mohorovicic in 1909, separating, he thought, the crust from the mantle However, what was the nature of this discontinuity? Was it the result of chemical or merely physical differences? Were the concepts of two separate crustal layers, the heavier ‘sima’ (silica and magnesium) and the lighter ‘sial’ (silica and aluminium), introduced by Suess, still valid? Ideas and terms remained in turmoil, as some, such as Bailey Willis, equated ‘crust’ with sial Did such crustal divisions explain the major differences in magmas and the resultant intrusions and extrusions that could be observed at the surface, or were deeper levels involved in the evolution of surface geology? Other workers suggested different models of the Earth’s interior, and in about 1914 Joseph Barrell introduced the concept of the ‘lithosphere’ as a thin crust, underlain by the ‘asthenosphere’, a relatively weak 100 km or so, with an even weaker layer of unknown thickness beneath, in which adjustments to isostasy took place Between 1940 and 1942 Keith Bullen divided both the mantle and the core into three separate zones, but there were still unresolved questions In 1954, a conference ‘The Crust of the Earth’ was convened at Columbia University, New York, and many prominent geologists and geophysicists contributed, reaching some agreement on terms and directions for future research into the crust and mantle However, the question of the Earth’s inner composition remained largely unsolved The increasing sophistication of geophysical equipment, particularly seismic, showed that the ‘Moho’ was not a continuous feature, but was ‘replaced’ in places, such as below mountain belts, by a disturbed ‘mixed’ zone In the early 1950s, evidence from marine surveys showed that the ocean floors were covered in many areas by only a thin layer of sediments With the increasing ability to drill deep within the Earth it was suggested in the late 1950s that some ‘real evidence’ of the nature of the Moho discontinuity could be obtained by drilling a hole or km through the crust on the seafloor The ‘Mohole project’ was taken up with enthusiasm, but eventually failed because the ideas of the geologists involved differed from the approach advocated by the preferred drilling company, and the project was terminated in 1966 While the records of earthquakes were used to study the Earth’s interior, seismologists also devoted considerable efforts to predicting and classifying earthquakes In the 1930s a number of seismologists began work at the California Institute of Technology They included Harry Wood, Beno Gutenberg, Hugo Benioff, Frank Press, and Charles Richter, who were essentially the founders of modern seismology, although there were important researchers such as Fiusakichi Omori in Japan Nevertheless the accurate prediction of earthquakes has not yet been achieved (see Seismic Surveys) Impact Craters While actualism (uniformitarianism) ruled during the early part of the twentieth century, there was still a suspicion that ‘catastrophes’ might have played a part in shaping the Earth Late in the nineteenth century Grove Karl Gilbert argued for impacts as the origin of most of the Moon’s craters However, it was a long time before such features on the Earth’s surface were accepted as being due to meteorites Specimens from small craters at Henbury in Central Australia were described in 1932 and accepted as meteorites, but it was not until the 1950s that the larger famous Barringer (Meteor) Crater in Arizona was accepted as an impact crater, despite information obtained from drilling in the early 1900s As early as 1942, Harvey Nininger argued that large impacts might have influenced the evolution of life This idea was not really taken up until the 1980s, although when the Yucata´ n Puerto Chicxulub crater was investigated for oil in the 1950s the drill cores showed impact melt material, and, in 1970, Digby McLaren also talked about the idea In the mid-1950s research on craters began in earnest, initially in two independent fields: photographic and field studies investigated the origin of craters and the possible significance of accretion of impact bodies in the formation of heavenly bodies; and the mode of formation of craters was investigated by nuclear and chemical explosions and detailed studies of shock-wave propagation Important researchers in the former (geological) field included Ralph Baldwin, Robert Dietz, and Eugene Shoemaker; Donald Gault’s experimental work on hypervelocity impacts was typical of the latter By the late 1950s there was considerable interaction between these two groups, and the first international impact symposium was held in 1961 Pioneering work in the 1940s and 1950s by George Baker and others on the glassy meteorites (tektites), notably their shape, proved important in the study of the problem of re-entry by spaceships into the Earth’s atmosphere