Page 18 - Global Tectonics

HISTORICAL PERSPECTIVE 5

Toit’s idea of it being made up of two supercontinents ably from the present concepts of convection and ocean

(du Toit, 1937) (Fig. 11.27). The more northerly of these floor creation, Holmes laid the foundation from which
is termed Laurasia (from a combination of Laurentia, a modern ideas developed.
region of Canada, and Asia), and consisted of North Between the World Wars two schools of thought
America, Greenland, Europe, and Asia. The southerly developed – the drifters and the nondrifters, the latter
supercontinent is termed Gondwana (literally “land of far outnumbering the former. Each ridiculed the other’s
the Gonds” after an ancient tribe of northern India), ideas. The nondrifters emphasized the lack of a plausi-
and consisted of South America, Antarctica, Africa, ble mechanism, as we have already noted, both convec-
Madagascar, India, and Australasia. Separating the two tion and Earth expansion being considered unlikely. The

supercontinents to the east was a former “Mediterra- nondrifters had difficulty in explaining the present sepa-
nean” sea termed the paleo-Tethys Ocean (after the ration of faunal provinces, for example, which could be
Greek goddess of the sea), while surrounding Pangea much more readily explained if the continents were

was the proto-Pacific Ocean or Panthalassa (literally formerly together, and their attempts to explain these
“all-ocean”). apparent faunal links or migrations also came in for
Wegener propounded his new thesis in a book Die some ridicule. They had to invoke various improbable
Entstehung der Kontinente and Ozeane (The Origin of Con- means such as island stepping-stones, isthmian links, or
tinents and Oceans), of which four editions appeared in rafting. It is interesting to note that at this time many
the period 1915–29. Much of the ensuing academic dis- southern hemisphere geologists, such as du Toit, Lester
cussion was based on the English translation of the 1922 King, and S.W. Carey, were advocates of drift, perhaps
edition which appeared in 1924, consideration of the because the geologic record from the southern conti-
earlier work having been delayed by World War I. Many nents and India favors their assembly into a single super-
Earth scientists of this time found his new ideas diffi cult continent (Gondwana) prior to 200 Ma ago.
to encompass, as acceptance of his work necessitated a Very little was written about continental drift

rejection of the existing scientific orthodoxy, which was between the initial criticisms of Wegener’s book and
based on a static Earth model. Wegener based his theory about 1960. In the 1950s, employing methodology sug-
on data drawn from several different disciplines, in gested by P.M.S. Blackett, the paleomagnetic method
many of which he was not an expert. The majority of was developed (Section 3.6), and S.K. Runcorn and his
Earth scientists found fault in detail and so tended to co-workers demonstrated that relative movements had
reject his work in toto. Perhaps Wegener did himself a occurred between North America and Europe. The
disservice in the eclecticism of his approach. Several of work was extended by K.M. Creer into South America
his arguments were incorrect: for example, his estimate and by E. Irving into Australia. Paleomagnetic results
of the rate of drift between Europe and Greenland became more widely accepted when the technique of
using geodetic techniques was in error by an order of magnetic cleaning was developed in which primary
magnitude. Most important, from the point of view of magnetization could be isolated. Coupled with dating
his critics, was the lack of a reasonable mechanism for by faunal or newly developed radiometric methods, the
continental movements. Wegener had suggested that paleomagnetic data for Mesozoic to Recent times
continental drift occurred in response to the centripetal showed that there had been signifi cant differences,
force experienced by the high-standing continents beyond the scope of error, in the motions between
because of the Earth’s rotation. Simple calculations various continents.
showed the forces exerted by this mechanism to be An important consideration in the development of
much too small. Although in the later editions of his ideas relating to continental drift was that prior to
book this approach was dropped, the objections of the World War II geologists had, necessarily, only studied
majority of the scientific community had become estab- the land areas. Their findings had revealed that the con-

lished. Du Toit, however, recognized the good geologic tinental crust preserves a whole spectrum of Earth
arguments for the joining of the southern continents history, ranging back to nearly 4000 Ma before the
and A. Holmes, in the period 1927–29, developed a new present, and probably to within a few hundred million
theory of the mechanism of continental movement years of the age of the Earth and the solar system itself.
(Holmes, 1928). He proposed that continents were Their studies also revealed the importance of vertical
moved by convection currents powered by the heat of movements of the continental crust in that the record
radioactive decay (Fig. 1.4). Although differing consider- was one of repeated uplift and erosion, subsidence, and