Introduction - The Solid Earth - page 3

like a thin crust over a fluid.' This was the great Concepcion earthquake. Several
days later, near Concepcion, Darwin reported that 'Captain Fitz Roy found beds
of putrid mussel shells still adhering to the rocks, ten feet above high water level:
the inhabitants had formerly dived at low-water spring-tides for these shells.' The
volcanoes erupted. The solid Earth was active.
By the early twentieth century scientific opinion was that the Earth had cooled
from its presumed original molten state and the contraction which resulted from
this cooling caused surface topography: the mountain ranges and the ocean basins.
The well-established fact that many fossils, animals and plants found on separated
continents must have had a common source was explained by either the sinking
of huge continental areas to form the oceans (which is, and was then recognized
to be, impossible) or the sinking beneath the oceans of land bridges that would
have enabled the animals and plants to move from continent to continent.
In 1915 the German meteorologist Alfred Wegener published a proposal that
the continents had slowly moved about. This theory of continental drift ,which
accounted for the complementarity of the shapes of coastlines on opposite sides
of oceans and for the palaeontological, zoological and botanical evidence, was
accepted by some geologists, particularly those from the southern hemisphere
such as Alex Du Toit (1937), but was generally not well received. Geophysicists
quite correctly pointed out that it was physically impossible to move the con-
tinents through the solid rock which comprised the ocean floor. By the 1950s,
however, work on the magnetism of continental rocks indicated that in the past
the continents must have moved relative to each other; the mid-ocean ridges ,
the Earth's longest system of mountains, had been discovered, and continental
drift was again under discussion. In 1962 the American geologist Harry H. Hess
published an important paper on the workings of the Earth. He proposed that
continental drift had occurred by the process of seafloor spreading . The mid-
ocean ridges marked the limbs of rising convection cells in the mantle. Thus, as
the continents moved apart, new seafloor material rose from the mantle along the
mid-ocean ridges to fill the vacant space. In the following decade the theory of
plate tectonics, which was able to account successfully for the physical, geological
and biological observations, was developed. This theory has become the unifying
factor in the study of geology and geophysics. The main difference between plate
tectonics and the early proposals of continental drift is that the continents are no
longer thought of as ploughing through the oceanic rocks; instead, the oceanic
rocks and the continents are together moving over the interior of the Earth.
References and bibliography
Brush, S. J. 1980. Discovery of the earth's core. Am. J. Phys ., 48 , 705-24.
Darwin, C. R. 1845. Journal of Researches into the Natural History and Geology of the
Countries Visited during the Voyage of H.M.S. Beagle round the World, under the
Command of Capt. Fitz Roy R.N ., 2nd edn. London: John Murray.