In my travels I have visited a number of locations where I was very aware of the Earth’s crust being so thin that the heat of the molten rock the crust floats on, causes water to boil, steam to rise and geysers to spout. The first and most spectacular site was Yellowstone National park, with its variety of thermal features, then came Iceland, full of steaming landscapes and geothermal plants. These experiences and a vague awareness of tectonic plates has caused me to do some reading this week [thank goodness for those who enrich cyberspace with so many well informed online articles].
I had always thought that when our planet was being formed some 4.6 billion years ago it was a huge ball of molten rock that cooled down in a few hundred million years or so, a crust formed, and life evolved and things were pretty stable after that.
Nothing could be further from the truth. Scientist now theorize that in the cooling process large slabs of solid rock very likely did form and float on Earth’s boiling molten rock mantle in its early years but that they drifted into each other, partly melted again, broke apart and reformed many times over the first billions of years.
Alfred Wegner, a brilliant multidiscipline scientist, was the first to propose the idea of continental drift as early as 1911, though he wasn’t sure why. What prompted his proposal was that he had happened on a some research that gave a list of fossils of identical creatures and plants that existed on land that was on opposite sides of the Atlantic Ocean. When you look at a map of the Earth, especially if you take continental shelves into consideration, you can easily see that the continents seem to fit together like the pieces of a big puzzle. The the east coastline of Brazil fits into the west coast of Africa almost perfectly and the same is true of other continents.
As more fossil records have been found, along with evidence from glacial scrapings over land surfaces, all the indications point back to the existence of a supercontinent called Pangea some 300 million years ago [see picture upper left]. As scientists from many disciplines continue to find new evidence, they can go further and further back in time and now postulate other similar continental reconstructions with names like Ur, Atlantica, Nina, Rodinia, Gondwana & Laurasia.
By recognizing many clues about the way land masses are displaced, it is now recognize that the Earth’s surface is composed of some 14 large rigid slabs or plates that move relative to one another – hence the name plate tectonics, or ‘surface built of plates’ - see diagram on the right.
It has taken decades of controversy and accumulating evidence to finally gain acceptance of the far-reaching theory of plate tectonics and I found an animated video very helpful in visualizing the process of why continental plates move.
The global positioning satellites [GPS] tracking system can now accurately measure plate motions and they are in the order of only a few centimeters a year. In the last 100 years we’ve learned a lot about our planet – lets hope it won't take the next 100 to learn better how to predict and cope with volcanic and earthquake disasters caused by still boiling molten rock not too far under our feet! Rie