In this video we will learn how Alfred Wegener proposed a theory in 1912 that the great continents of the Earth had drifted over geological time and were once all joined together in a giant land mass we now call Pangaea.

His idea was based on the way present day continents fit neatly together, and by the way bands of fossil-bearing rocks join up across continents. Unfortunately Wegener was unable to provide a mechanism for this movement, and his theory was ridiculed by the scientific community until 1965 when the theory of plate tectonics was published. This proposed that the continents were moved around on great plates driven by convection currents in the hot mantle of the earth. Perhaps the most dramatic evidence came from the magnetism of rocks each side of the mid ocean ridges. When rocks solidify they become weakly magnetic in the direction of the earth’s magnetic field.

However every few million years the earth’s magnetic field ‘flips’ so the North magnetic pole becomes South pole, and this change is signalled in the rocks. Scientists found that the rocks each side of the mid ocean ridge were magnetised first in one direction and then in another. Magma solidifies, locking in the magnetism; new magma forms as the plates move apart, the magnetic field reverses, more magma solidifies locking in the reversed polarity, and so on, creating a pattern that repeats each side of the ridge with the rocks getting older the further you are from the ridge – clear evidence of the ocean moving apart over millions of years. Further evidence came from plotting where the earth’s volcanoes and earthquakes were situated. From this map you can see that they form lines around the globe, marking out the edges of the great tectonic plates. You get almost all earthquakes and volcanoes at plate boundaries. And finally, we find the great mountain ranges right alongside where two plates are pushing into each other.

The Himalaya are being thrust upwards as India moved, and is still moving, slowly into Asia. As the Pacific plate is subducted under the Asian plate, the islands of Japan with many earthquakes and volcanoes and the deep ocean trench to the east were formed and are still being formed. To summarise, the theory of continental drift was published by Wegener in 1912 with evidence that the continents were once all joined together. In 1965 it began to be accepted when plate tectonics provided a mechanism for moving the continents. This was supported by evidence of spreading at mid ocean ridges and from the pattern of earthquakes and volcanoes which outlined the earth’s tectonic plates. Drifting Continents and Shifting Theories This innovative book uses the story of how a modern science achieved its present shape and focus to introduce the question of the nature of scientific change and its philosophical analysis.

The "modern revolution in geology" of the 1960s and 1970s saw the triumph of the global theory of plate tectonics, and decisive turning point in fifty years' controversy and competition first sparked in 1912 by Wegener's proposal of continental drift.

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