As recently as the mid 20th Century, most people believed that the world's continents were fixed in position, and that they had been located more or less where they are now since the earth was formed.
It was thought that the geographical distribution of different plants and animals throughout the world could be explained by three basic processes: evolution, climate change and land bridges (these are connections formed between land masses during geological periods when sea levels are low). These beliefs were based on studies of the earth's most recent, and therefore best known, past.
Everything made perfect sense, until scientists started studying the geographical distribution of fossils dating from around 300 million years ago. They found fossils of forests dominated by tree ferns called Glossopteris in South America, South and Central Africa, Australia, Antarctica and India. These plants could not be found in North America, Greenland, Europe and Asia. Geologists also found that just before Glossopteris evolved, all the southern continents, including India, had been affected by a huge glaciation from the south. In the case of India this meant that the glaciers came from the direction of today's equator. This definitely did not make sense.
Moving continents
Such observations could be explained if the continents had been joined together in a pattern different from today, also if they had all lain further south. Since 16th Century people have remarked on how the coastlines of West Africa and Eastern South America seemed to fit together, and scientists toyed with ideas that continents could have moved, for example, by expansion of the earth.
But it was only in 1915 that German astronomer Alfred Wegener tied all the evidence together and put forward his theory in a book called: "The Origin of Continents and Oceans". He suggested that at one time all the continents were bound together as a super-continent which he called "Pangaea" with a subgroup of continents to the south called Gondwana (the home of Glossopteris), and subgroup to the north called Laurasia. He included geological evidence to back up the fossil records and described characteristics of mountain ranges which started in Argentina, continued through South Africa and ended in Australia, and others that appear to start in North America and end in Europe.
Very few people believed him. Firstly, accepting this theory would mean radical changes in the ideas held by geologists. Secondly, Wegener was not a geologist himself, but a meteorologist and astronomer. There was therefore even more prejudice against ideas which had been thought up by an outsider. Scientists performed all sorts of mental gymnastics to find other solutions to their problems. For example, small fossil reptiles Lystrosaurus and Mesosaurus are found in South America and Africa, and so biologists proposed that a land bridge had once existed right across the Atlantic between the two continents. They said that Glossopteris must have spread to Australia by wind-blown seeds and ocean currents, and some people chose to disbelieve the evidence for glaciation entirely.
The main stumbling block to Wegener's ideas was, however, that no-one could think of a convincing theory to explain how continents could move about as he had described. Then, in the 1950's, deep sea exploration revealed mid-ocean ridges split by deep canyons. These canyons formed a neatly fitting patchwork of six large and several smaller plates over the earth's surface. They called the canyon network the "Great Global Rift", and the patchwork pieces "Tectonic plates" (from Greek: Tekton: carpenter).
The Atlantic grows
In 1960 American Harry Hess suggested that molten rock from beneath the earth's crust could ooze up between the plates in places like the mid-Atlantic. As this cooled in the ocean water, it would expand and push the plates away on either side of it. Thus North and South America would move westwards and Eurasia and Africa eastwards. In this way, the Atlantic Ocean would get wider but the coastlines of the landmasses would not change dramatically.
In the 1980's the rate of spreading was measured at 1.7cm per year. Today, the theory is almost universally accepted as being true. Like the theory of Evolution, it is accepted by all except those who hold conflicting religious beliefs.
We can now track how the continents have moved in the past through studying the direction of the magnetisation of rocks. Magnetic particles which occur in some types of rock align themselves with the earth's magnetic field as the rock is formed and act as tiny compass needles. These reveal in which direction and how far away the earth's magnetic poles were at the time the rock was laid down. Since the magnetic poles do not move very much, a diagram of the directions pointed to by these compass needles in rocks of different ages, creates a kind of route map showing where each continent has wandered over the surface of the globe.
Separate land masses
We find that during the Carboniferous period, some 350 mya (million years ago), the continents were rather spread out, sea levels were low because water was locked up in ice which glaciated much of the the southern land mass, Gondwana. The areas which currently have the best coal reserves (North America, Northern Europe and Australia) were positioned over the equator and surrounded by shallow seas.
Here, coal was formed in warm, acidic swamps through the accumulation of layer upon layer of plant remains and mud. Over a period of 60 million years, thousands of gigatons (109) of carbon were locked away. (Carbon which is now being released by Man over a period of only 200 years). Thus, coal-rich Germany had a tropical climate, not because the tropics had spread northwards, but because Germany was then positioned at 0º N and not 51ºN as it is today.
(Late Carboniferous 300 mya)
Pangea, the super-continent
Later, during the Permian period, the continents came together to form a single "C"shaped land mass (Pangaea) which extended North / South. The presence of a single continent undoubtedly changed the climate of the world. Vast areas of land were now far from cooling seas and scientists believe that the interior was very hot and dry, while the coasts were subject to seasonal monsoons.
The ocean currents, which strongly affect climate, must also have been very different. While this super-continent existed, the great Permian Triassic extinction took place and 95% of all species became extinct. Was this coincidence or something more?
The concentration of most living things on and around one land mass may have made them particularly vulnerable to the catastrophic events which initiated the extinction (see XAD nº62 August 2009). Firstly, there are fewer opportunities for different species to evolve when there is only one continent. So the variety of species (biodiversity) would have been low. Secondly, a single large continent has less shoreline than several smaller ones. This could have contributed to the extinction of marine species by reducing the extent of shallow coastal areas preferred by many marine organisms.
(Pangaea 240 mya)
During the age of the dinosaurs (Jurassic) Pangaea broke into two big lumps: northern Laurasia and southern Gondwana, though land connections persisted long enough for the dinosaurs to colonise the whole world. The break up continued as small mammals evolved. By the end of the Cretaceous (90 mya) sea levels were high and the continents looked more like chains of huge islands rather than the large connected land masses we are familiar with today. This isolation contributed to the independent evolution of many different mammals simultaneously in different areas.
(Late Cretaceous 90 mya)
The extinction of marsupials
The continental break up had a great effect on the survival of the marsupials. The earliest fossils of these unusual mammals (whose young leave the womb at a very early age and then complete their development in the mother's pouch) have been found in China, and have been dated as 123 million years old. Fossil marsupials are found throughout the world, but today, the more advanced placentals (where the young complete their entire development in the womb) dominate in every continent except Australia.
The shortest route from China to Australia would be to go south, but at that time they were separated by thousands of miles of empty ocean. Fossil evidence shows that the marsupials went the long way round, from Asia, across to North America, down to South America, then to Antarctica (which still had a warm climate) and across to Australia which was either connected, or at least had not drifted too far away.
Some researchers suggest that the marsupial colonisation of Australia was by only a few individuals of one species. These later evolved into the kangaroos and koala bears of today. Placental mammals also spread throughout the world, displacing the marsupials in almost every land. But by the time they reached Antarctica, the climate had turned cold and Australia had separated, safely isolating the marsupials in their refuge.
India's great voyage
So, how can Continental Drift explain the presence of Glossopteris in India and the curious glaciation of this continent from the south? Geologists describe a spectacular journey. India was originally part of southern Gondwana and located in the southern hemisphere. Thus it suffered Antarctic glaciation and Glossopteris grew there. India and Madagascar broke away about 130 mya. Then India left Madagascar behind, and went north. About 80 million years ago, India was located about 6,400 km south of the Asian continent and traveling at a rate of about 9m a century. It crossed the equator to collide with Asia 40 to 50 mya creating the Himalayas. It is interesting to note that neither marsupials nor placentals were able to reach India or Madagascar before they separated from Africa. All Indian mammals are recent colonisers.
300 million years in one minute
Thus if we could make a time lapse video of the earth's evolution over the past 300 million years, we would see not only the climate changing from sub-zero ice ages to stifling temperatures 10C higher than today, meteoric impacts and sea levels rising and falling, but also continents drifting about like globules of oil on the surface of a simmering soup, colliding with each other and pulling apart to cause huge volcanic eruptions and earthquakes, to create mountains and chasms and influence the climate and evolution of life.
Without plate tectonics, there may have been no life in the first place. As described in XAD August 2009, it is possible that life originated in hot water vents deep in the oceans where plates are moving apart.
This is far from the static view of the world we had less than 100 years ago. The very fabric of the world seems alive. It is no wonder that astronomers search the galaxy for signs of continental drift on other planets.
Christine Betterton Jones - BSc. (Zoology), PhD (Parasitology)
Boibliography:
1. "The origin and evolution of mammals" - Thomas Stainforth Kemp 2005
2. "A Science Odyssey: People and Discoveries: Hess proposes sea-floor spreading," http://www.pbs.org/wgbh/aso/databank/entries/do62se.html.
3. "The Himalayas [This Dynamic Earth, USGS]," http://pubs.usgs.gov/gip/dynamic/himalaya.html.
4. "Continental Drift - History Of Wegener's Theory," http://science.jrank.org/pages/1748/Continental-Drift-History-Wegener-s-theory.html.
5. "The Cretaceous Period," http://www.ucmp.berkeley.edu/mesozoic/cretaceous/cretaceous.html.
6. "Mollewide Plate Tectonic Maps of Phanerozoic," http://jan.ucc.nau.edu/~rcb7/mollglobe.html.
7. "Earth's biggest ' whodunnit': unravelling the clues in the case of the end-Permian mass extinction" - Rosalind V. White The Royal Society October 2002
8. "Biogeography - an ecological and Evolutionary approach" - C. Barry Cox and Peter D. Moore. Blackwell 7th Edn 2005.
