Lecture 11: Plate Tectonics and the Earth’s Geography
continents of South America, Africa, Antarctica, Australia, and India had once
been part of a single supercontinent that he named Gondwanaland. Wegener
expanded this idea to suggest that all the world’s continents had once been
joined together. Wegener showed that fossil species, such as the ancient plant
Glossopteris, could be found on most of the continents of Gondwanaland.He also found evidence of ancient glaciers in tropical regions of Africa, and
of ancient tropical forests in Antarctica, which suggested that both continents
had traveled huge distances. But how could whole continents possibly
move? Wegener could not explain this. As a result, most geologists rejected
his ideas from the 1920s to the 1960s. However, they had to get very creative
to ignore the powerful evidence he had presented. For example, it became
fashionable to argue that there had once been thin land bridges linking
the continents.New research after World War II suggested a solution to Wegener’s dilemma
and revived interest in his ideas. In the 1960s, the new theory of plate
tectonics emerged as the core idea of modern geology. American geologist
Harry Hess (1906–1969), who was a naval commander during World War
II, used the technology of sonar (developed to detect submarines) to survey
the Paci¿ c seaÀ oor. Like most geologists, he assumed the ocean À oors would
consist of a huge, À at ooze, from sediments washed into the seas. Instead,
he was astonished to ¿ nd underwater chains of volcanic mountains. These
turned out to be one section of a chain linking all the major oceans.In 1962, studies of the deep seaÀ oor showed that while regions close to the
mid-oceanic ridges had normal magnetism, further out, the magnetism was
reversed, and further out it was reversed again. In 1966, it was shown that the
Earth’s magnetic ¿ eld periodically reverses, so that each band represented
seaÀ oor laid down in a different period. Improved dating techniques
eventually con¿ rmed that the bands closest to the ridges consisted of new
oceanic crust and those further away were much older.This evidence con¿ rmed an idea Hess had proposed in 1962: that molten
magma rose from the Earth’s interior at the oceanic ridges, creating new
seaÀ oor and pushing existing seaÀ oor aside. Where did the old seaÀ oor
end up? Hess suggested that millions of years later it plunged back into the