Download Pangea torn: answer to the riddle Alfred Wegener`s theory is mainly

Pangea torn: answer to the riddle
Alfred Wegener's theory is mainly based on the mapping between the coasts both sides of the
Atlantic Ocean (cartographic arguments). Scientists objected that this could be because of a variable
level of the sea, however the topographic indicator is not the only argument presented by Wegener.
Numerous others indicators support the idea of continental drift, which would have formed some
200 million years ago a super-continent unique, Pangea, surrounded by a super-ocean Panthalassa:
• Geological arguments: similarity of geological units located on both sides of the Atlantic (age and
nature of the geology…)
• Paleontological arguments: fossils from American and African continents show that the flora and
fauna of the two continents were very similar to the beginning of the Mesozoic era (-200 million
years), time from which the fossils differ on each continent.
• Climate arguments: The presence of old glacial moraines dated Carboniferous in South Africa and
Australia demonstrates a glacial climate at this time, while fossil ferns found in Carboniferous
sediments in Europe and America North show a tropical climate at the same time.
What about our enigma?
Below are the documents collected in the manner of ... Wegener:
Document 1: Cartographic map
Conclusion: Complementarities of coasts are not very obvious ... we‘d choose the location 2, but that
is not enough ....
Document 2: paleontological evidence
Conclusion: The paleontological evidence (study of fossils) does not help us. There are 'Glossopteris,
everywhere. Impossible to decide for location!
Document 3: Climate evidence
Conclusion: Difficult to place Madagascar with traces of polar icecaps... Only position 1 seems to be
excluded!
Document 4: geological evidence
Conclusion: The geological evidence save us! Position 3 is only compatible with the geological data. It
is noted that Madagascar is characterized by major faults and old rocks whose extension is found in
India and in zone 3 of Africa.

Answer to the riddle:
It is clear that the map is not enough evidence to reconstruct Pangaea. The geologist must
accumulate evidence from different disciplines before concluding. Here the solution is not very
obvious! Position 3 was the solution.
To go further:
The arguments used by A. Wegener are not so obvious. If in the Atlantic, these arguments are
effective, it is not the same everywhere. But the main weakness of Wegener's theory is related to the
real reason for the drift of continents: Wegener explained the origin of these movements to the lunar
tides. This hypothesis allowed geophysicists to be against to this theory (including Harold Jeffreys)
because it is very easy to demonstrate that it is physically impossible. In addition, the seismic data
show, at this time, that the earth is solid. This strong opposition has resulted for a time to forgot the
idea of a continental drift.
Much later, however, Wegener found support in the scientific community that showed the reality of
horizontal movements of the Earth's surface.
The model of plate tectonics must be a gradual accumulation of the data mostly from observations of
Oceanography (beginning in the 1950s).
Among the major architects of the construction of the model of plate tectonics, we find Harry
Hammond Hess, a naval officer and American geologist (1906-1969). During his many dives, his
observations help him to assert that the ocean floor reflected the convection of Earth's mantle: the
ocean rifts highlighted the up convection currents in the mantle and oceanic trenches showed down
convection currents. Oceanic crust created at ridges and disappeared at ocean trenches was
continuously recycled while the continental crust, because of its lightness, was condemned to drift to
the surface of the Earth. Harry Hess proposed a model of the seafloor spreading. Hess's report was
highly controversial: it initiated a revolution in the earth sciences.
It remained to undertake a systematic exploration of the ocean floor, including an ocean deep drilling
program. Such a scientific program will begin in 1960! Harry Hess was a great fan for this program.
We are leaving soon to the Pacific over a rift which bears his name: Hess Deep Rift!