Sunday, 28 September 2014

How Do We Know About Pangaea and plate tectonics?

Pangaea is the name given to the super continent that existed up until about 100 million years ago, when it started to break up into the continents we know today. This is all very exciting, but what is more exciting to me is how we even came to know about this in the first place. I found most of this information in a fantastic book by Bill Bryson called "A Short History Of Nearly Everything" which I thoroughly enjoyed reading, in fact probably so much that I have no doubts that I will try to emulate his style of writing in this post and it will most likely end up as a concise summary of one of the chapters from this book. 

Pangaea with  the current boarders of today

In 1908, when geology was still in it's infancy, a wealthy American geologist named Frank Taylor was among the first people to notice that the facing coastlines of Africa and South America appeared to be extremely similar, in fact they were identical. He went on to develop the idea that the continents had moved around, at least at one point in time. He went on to hypothesise that the smashing together of continents could have created the great mountain ranges found all across the globe, although he had very little in the way of evidence to substantiate this idea of his. Naturally, as with most ground breaking ideas in science, he was largely ignored and even actively ridiculed by many of the top American geologists of the time, such as Charles Hapgood who felt the need to write a whole book that steadily demolished the idea and labelled Taylor a crackpot.

Fortunately, a German meteorologist named Alfred Wegener continued his work. Wegener put his mind to investigating strange anomalies in the fossil record that could not be explained by conventional knowledge. In fact, very little of the fossil record could be explained by conventional means of the time. How did land animal fossils keep showing up on opposite sides of oceans? How did semi-tropical remnants appear in the Arctic circle?

Wegener was the one who proposed that all of the world's landmass had once existed as a super-continent that he called Pangaea, where all of the world's plants and animals had intermingled, before being separated by it's break up and moving to their current positions over millions of years. His book was published in 1912 but it wasn't until 1920 (after the first world war) that it gained any attention. Unfortunately, this was mostly negative attention as many of the mainstream geologists were not happy that a meteorologist was questioning the entire foundation of geological science, despite it in fact explaining quite nicely many questions that were as yet unanswered. 

They went to terrible trouble to try and dismiss his ideas, suggesting that ancient "land bridges" had once existed, providing a highly convenient highway with which animals could have crossed the oceans at their leisure, before obligingly disappearing without a trace at just the right time. These land bridges became more and more numerous and more and more ridiculous. The best examples of which come from trying to explain the appearances of certain species of Trilobites. One of which was found throughout Europe, but also in one side of Newfoundland. It was proposed that these trilobites had crossed 3,000 Km of ocean, but then failed to cross to the other side of an island only 300 Km across. Another species of trilobite was found only in Europe and the Pacific north-West of America and nowhere in between. quoting Bryson, "This would have required not so much a land bridge as a flyover", yet even in the mid '60's people were still positing Wegener's theory as the one with "numerous grave theoretical difficulties". It later turned out that Newfoundland had once been a part of Europe, as did a portion of Staten island. 

Trilobites, proof of an ancient super-continent or masters of teleportation?

Although Wegener was correct, he had no way of actually showing how this all worked. It was not until after world war II and the discovery of the mid-Atlantic ridge that things started to fall into place. This mid-Atlantic ridge was rather accidentally discovered by Harry Hess, a mineralogist who was put in charge of a ship in the war with a high-tech depth sounder, designed for inshore manoeuvres, which he kept on at all times in order to create a rudimentary map of the sea floor. Given the rate of deposition of minerals from rivers into oceans, it was calculated that there should be at least 20 Km of sediments on the sea floor, but instead found nothing but smooth ancient sea bed (the reasons for which will be explained shortly). The mid-Atlantic ridge is part of the world's most extensive network of mountains, mostly buried under water, but occasionally protruding to form islands such as the Bahamas and Hawaii. The full network extends to around 75,000 Km!

The largest mountain network in the world stretches for 75,000 Km and is almost entirely underwater!

It was also found that through the middle of the mid-Atlantic there was a 20 Km wide canyon, stretching for 19,000 km, as if the earth was splitting apart at the seam! Core samples also showed that the ocean floor at the ridge was relatively young, and got progressively older as you moved East or West. This could mean only one thing, new crust must be being created and pushed out of the mid-Atlantic ridge and on towards both North America and Europe, known as sea floor spreading. When the ocean floor met it's boundary with the continents, it subducted back into the Earth's mantle, taking all of the missing sediment with it. This explained why no-one had ever found ocean floor older than 175 million years, whereas it was well known by then that the Earth was billions of years old. Hess put all of this together in a very important paper, which was of course almost universally ignored. 

Shortly after, two scientists found that the Earth's magnetic poles have switched several times, and that certain rocks (such as tiny grains of iron ore) could "remember" their position at their time of birth. It was noted that the magnetic history of North America and Europe fitted together almost perfectly, and that at some point the UK must have spun through 180 degrees and migrated north. naturally, this was also ignored. Eventually two scientists from Cambridge university finally managed to put all of this together and suddenly in 1964, even some of the most belligerent deniers of sea floor spreading were immediate converts. 

This phenomenon became known as continental drift, and quickly became the new science of plate tectonics. We now know that there are about 30 continental plates. These plates range greatly in size, from the truly huge North American plate (which also protrudes well into the Atlantic and parts of Asia) to the comparatively tiny Caribbean and Scotia plates (amongst many others). These plates move around at various speeds and in many different directions. not only did this create the many mountain ranges we have today and explain the causes of Earthquakes (the smashing together of continental plates with tremendous force) it also contributes to some of the natural causes of climate change and also can result in abrupt changes to the chemical composition of the oceans and atmosphere, though opening and shutting of huge ocean floor rifts.  These challenges may also have contributed to the necessity and development of intelligent species we have today, including us.  

Major tectonic plate boundaries with mid-ocean ridge

While plate tectonics explains many things about the history of Earth, it cannot explain everything. Firstly, there is much debate as to just how all of our landmasses were once joined together, and even more debate as to how they split up, with numerous other super-continents such as Gondwana  and Laurasia. There are also many interesting locations and phenomena that cannot yet be explained fully by plate tectonics (although recent research is getting closer), such as how Denver and a 1600 Km portion of Southern Africa have risen massively (1.5 Km), with no signs of tectonic activity. There is also the fact that Australia has been steadily tilting and sinking as it moves towards Asia.

When Bill Bryson's book was written in 2003, there were no other planets known to have plate tectonics, despite there being many with other similar charatceristics. In September this year, evidence was found that one of Jupiter's moons, Europa, exhibited plate tectonics in its vast ice sheets much like those seen in rocks on Earth, this holds promise that there may be other planets out there that also exhibit plate tectonics and also bolsters Europa as one of the most likely candidates for harbouring extra-terrestrial life. 


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