Lecture 4 – The puzzle of the continents

From the 1995 lecture programme:

Curiously, although plate tectonics is very good at describing what happens to the sea floor, and also describes the changing distribution of the continents, it is little help in describing what happens when the continents themselves deform. They behave in a completely different way from the oceanic plates.

In the oceans, plate boundaries are very narrow and localised, but on the continents they are spread out and diffuse; it is hard to identify a 'boundary' at all.

When continents collide they crumple up over vast areas — such as from India to Mongolia. When they stretch, they neck and thin over wide areas to make basins that fill up with sediment and contain much of the world' s oil and gas. Moreover, the continents seem to have a memory, as they break again and again in the same place, even though the deformation episodes may be many millions of years apart. Why should continents be so different?

The continental crust is much, much older than the oldest surviving oceanic crust. It is also made of different stuff; in particular it is much lighter and weaker than the rocks of the oceans. It is virtually indestructible, too buoyant to be pushed into the Earth's interior in significant quantity, and if it is eroded, the sediment that is dumped on the seafloor is mostly scraped off again at the trenches and returned to the land. When dealing with deforming continents we need a different language from that of plate tectonics. We shall look at what is appropriate, and also address the major question of where the continents came from. 




James Jackson



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