Continental Drift
The process by which the continents drift about the world is called plate tectonics. The movement of the Earth's plates, on which the continents ride, is very slow, being only a few centimetres each year. However, over tens or hundreds of millions of years, both the size and position of land areas can change appreciably.
At times in Earth history, there have been super-continents in which all the continental plates were locked together in one area of the globe. The last of these occurred about 250 million years ago, and is named Pangea. Since that time, the continents have gradually moved apart, the most recent separation occurring between Europe and North America, during the last 60 to 70 million years, to form what is now the North Atlantic Ocean. What is now the Pacific Ocean used once to be the vast expanse of water called the Panthalassa Ocean that surrounded Pangea.
Changes in the distribution of landmasses are believed to explain climate changes that occur over tens or hundreds of millions of years. Of course, we have no direct way of knowing what the Earth's climate was like hundreds of millions of years ago, but we can use geological records of sea floor sediments to reconstruct what the climate may have been like. We can also use computer models to estimate how different arrangements of continents may influence the global climate. Currently, it is believed that the arrangement of continental landmasses significantly affects the ocean currents. Since ocean circulation is involved in the transfer of heat around the Earth, so the wandering of landmasses over tens and hundreds of millions of years may influence climate changes over similar time scales.
Such long-term changes to ocean circulation as a result of continental drift may explain the gradual return to an ice-covered world during the last 40 million years. Prior to that period, there was little ice covering the polar regions. As the supercontinent of Pangea continued to break up, so the continent of Antarctica became isolated at the Earth's southern pole. With ocean all around it, a new circumpolar ocean current formed, and heat from lower latitudes was prevented from reaching the continent. The subsequent expansion of the (white) ice sheet on Antarctica about 35 million years ago increased the amount of sunlight the Earth as a whole reflected, and led to a drop in average global surface temperature. Today, although we live in a period of relative warmth since the end of the last Ice Age 14,000 years ago, the Earth as a whole is still gripped by a much longer period of global frigidity. Temperatures today are still perhaps 10°C cooler on average than they were during the age of the dinosaurs.
As continents break apart, new oceans form between them, through a process known as sea-floor spreading. A major zone of sea-floor spreading is today located along the length on the Atlantic Ocean, and is called the Mid-Atlantic Ridge. At these zones large amounts carbon dioxide are released. During time of enhanced tectonic activity and sea floor spreading, elevated levels of carbon dioxide emissions may increase the strength of the Earth's natural greenhouse effect.
Different rates of sea-floor spreading can also affect the shape of the seafloor. When tectonic activity is greater, the sea floor is pushed up, leaving a smaller volume to hold the water of the oceans. Consequently, sea levels can rise by several hundred metres, covering large areas of the continents with warm shallow seas. Indeed, during the age of the dinosaurs about 100 million years ago, the sea level was much higher than it is today and the climate was much warmer.
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| Last 50 million years |
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