Spinning plates
How huge geological changes hit the fast-forward button on evolution.
About 200 million years ago, there was only one continent – the supercontinent Pangaea, but immense forces generated deep below the surface of our planet were about to change all that. The continent was torn asunder, splitting first into two megacontinents, and then again and again, until the seven continents, perched on their respective tectonic plates, drifted across the planet and reached the locations in which we find them today. With the break-up came new lands, new seas and new habitats. Ocean currents, atmospheric circulation and the seasons changed dramatically. Many plants and animals became adapted to their new homes, some separating from their original populations and evolving into new species. The whole process supercharged evolution and that, of course, has not stopped.
Forces of nature
One of the most striking results of movement of the tectonic plates is mountain building, which profoundly affects the distribution of wildlife. When India crashed into southern Asia, the Himalayas were thrust skywards. They formed a barrier between the plants and animals of the north and those of the south, a barrier crossed only by high-flying bird migrants.
When the edge of one plate dives below its neighbour, the Earth’s crust is so churned up that an active volcanic zone is created. In South America, the Nazca Plate is sliding below the South American plate, forming the volcano-rich Andes. Their formation reversed the flow of the Amazon River. Later,
when the river broke through to the Atlantic, it gave rise to the greatest rainforest on Earth, one of the most biologically diverse habitats on the planet.
Mass movement
In North America, the major mountain ranges, the Rockies in the west and the Appalachians in the east, both run north–south so, with nothing to prevent it, polar air can reach the deep south of the USA. In winter, alligators can be seen frozen into the surface ice of lakes, while in summer, Arctic air collides with warm Gulf air and the result is atmospheric mayhem – tornadoes.
The speed with which the continents move varies considerably. North America is moving away from Europe at such a rate that your flight to New York increases by 2.5cm a year. But the whizz-kid of plate tectonics is Australia. Since separating from Antarctica, it has been heading north at a staggering 7cm a year, a little less than twice the rate that your fingernails grow, and will shortly (on a geological timescale) collide with South-East Asia, pushing up a new mountain chain. In fact, it’s moving so fast, that GPS can’t keep up with it. The last calibration was made in 1994, when it was 200m further north than on the previous correction. Since then, it has travelled another 1.5m and rotated clockwise. One day in the distant future, Australia’s wildlife will be experiencing major changes in climate – maybe towards a wetter island continent – and another surge in biological diversity.
“Australia’s rapid northward trajectory shows that the continents have drifted apart as far as they’ll ever be, and they are now getting back together again,” says Jonny. “There have been five supercontinents during the 4.6 billion years of Earth’s history, and in about 250 million years’ time, there will be a sixth – Pangaea Ultima. As the new continent and its climate stabilises, the evolutionary brakes will come on. For a long while, Earth will not experience the extraordinary biological diversity that we have seen in the past 200 million years.”
Australia’s northward trajectory shows that the continents have drifted apart as far as they can.