Tuesday, March 07, 2006
The Expanding Earth: Part 1
Many people have heard of continental drift. It was one of the big scientific ideas of the last century proposed by the German geologist Alfred Wegener in 1912. Wegener proposed that America and Africa were once joined together as a supercontinent called Pangaea. He provided evidence to show that the two continents had once been joined together by the shape of their coastlines and by the occurrence of the same fossil species on both continents.
Wegener's ideas were strongly rejected in America until the 1950s when new evidence from oceanographical studies showed that there were stripes of magnetic polarity reversal running either side of the mid-Atlantic ridge and that the two sides of the ridge formed a mirror image of each other. Knowledge of magnetic polarity reversals had existed for much of the 20th Century and the reversals had been dated using lava flows on land. It only remained for Vine and Matthews in the 1960s to match the land reversals to the ones observed in the oceans to show the timing of the ocean spreading. The two continents had moved apart with lava emerging at the mid-Atlantic ridge between them.
The concept of plate tectonics evolved from that of continental drift in the late 1960s. The model proposes that the surface of the earth is covered by plates of solid rock some 100 km thick that float on a weaker rock layer called the asthenosphere. At their boundaries, the plates may move apart to form mid-ocean ridges and basaltic oceanic crust, or thy may converge together, one plate diving down underneath another, a phenomenon which is thought to occur on the western seaboard of America where the Pacific ocean plate is subducted underneath the American plate. A consequence of this subduction is the uplift of the Rockies and the Andes and the related volcanic activity.
With the plate tectonic model in place, many aspects of the structural evolution of the earth were finally understood. The previous model of the earth which dated all the way back to the French philosopher Rene Descartes in 1664 came to seem completely inadequate. This model proposed that the earth had original been hot and molten and that during its cooling a crust had formed which wrinkled and deformed to create mountains. The picture at the top of the page shows Descartes original illustration of this deformation under cooling and contraction.
The plate tectonic model suggested that the earth was not cooling and contracting but simply staying the same size. While some parts of it were moving apart, other parts were moving together. But some geologists have questioned whether the expansion of the Atlantic ocean, today more than 4000 km from one side to the other, is really matched by an equal and opposite subduction effect. Might it not be possible that the earth was in fact expanding?