Development of geology in the twentieth century

During the twentieth century the skills of the geologist have been in constant demand and the geological profession has in consequence expanded enormously. The technology basic to modern society lays strenuous claim upon all manner of materials locked into the Earth's crust, and the discovery of those materials has become one of the prime tasks of the Earth scientist. As a result of such explorations in all

Comers of the world, understanding of the Earth's structure has recently grown apace. The acquisition of this new knowledge of the Earth has been greatly assisted by the modern ability to view the Earth's surface from afar.

First, aerial photography gave geologists fresh perspectives on the terrestrial configuration, and more recently satellite imagery has revealed patterns in the Earth's structure that would never have been discovered by ground examination. Another important development is the realization that some of the Earth's rocks contain built-in natural chronometers .

It was Lord Rutherford (1871-1937) who in 1905 suggested that newly discovered radioactive properties of certain elements could employed as a key to the measurement of geological time in quantitative terms. The earliest efforts in this field of radiometric dating involved using the known decay rates of uranium and thorium into helium and lead, but today a variety of other methods is employed, allowing a remarkable precision in the dating of many of

the Earth's rocks.

The twentieth century has certainly seen many exciting developments within the Earth sciences, but there is one that transcends all others in its importance : the advent of the plate tectonic theory. The theory's origins go back to the early years of the century. In 1912 Alfred Lothar Wegener (1880-1930) suggested that during the Palaeozoic all the Earth's present continents had been grouped together to form one large supercontinent which he termed Pangaea. During Mesozoic times, he claimed. Pangaea began to break up and the fragments drifted apart to become the world's present continents (Figure 1.1 5 1).

1.15: Alfred Wegener's maps illustrating his conception of continental drift. The stippled areas are shallow seas. From Alfred Wegener's The Origin of Continents and Oceans (London. 1924)

In support of his theory of continental drift he offered a wide variety of evidence, but few geologists found his thesis convincing. The chief objection was that the geophysicists of the day regarded the Earth as a rigid body and there seemed to be no plausible mechanism that could result in the continents being dragged from their moorings and sent careering off across the face of the globe.

Arthur Holmes ( 1890-1965) did suggest that an appropriate mechanism might be found in convection currents operating within the Earth's mantle, but little attention was paid to his ideas. Attitudes towards continental movement nevertheless began to change in the 1950s when studies in the palaeomagnetism of rocks first suggested that the continents had indeed shifted their positions.

This discovery paved the way for the development in the 1960s of the modern concept of the Earth's surface as consisting of a series of mobile plates. This new and exciting conceptual model has reinvigorated the Earth sciences and encouraged fundamental re-appraisals of a type not witnessed in the Earth sciences since the early yean of the last century.

Strangely modern plate tectonic theory might well have appealed to the cosmogonists of the late seventeenth century. The conception of mobile plates colliding to form mountain ranges, grinding past each other to trigger earthquakes and being subducted to bring oceanic trenches into being seems strangely reminiscent of those catastrophic theories of the Earth of three hundred year ago. But there is a major differente.

Plate movement takes place with Infinite slowness against the vast time Beak of modern geology and not with a cataclysmic suddenness such as had to be invoked m those distant days when the whole of a complex Barth history had to be crammed into the few millennia permitted by

the Old Testament chronology.