Tectonics

I have recently been polishing off a HUGE Australian history, and you cannot explain Australia, its places, biota and people without the geology, and that needs plate tectonics.

In the early 1960s, we gathered the evidence of sea floor spreading, which showed that the Earth’s surface is made up of plates that were moving, and all of a sudden, the planet’s history made a lot more sense.

First, there was the idea of continental drift, a vague notion that somehow the planet had a changing surface. You only had to look at a map of the Atlantic Ocean to see how Africa would fit in neatly against South America, people said. In 1596, a Dutch map maker called Abraham Ortelius (1527–1598) suggested that the two sides of the Atlantic had been torn apart, but he did not explain what might have done it.

Once people started collecting plants and animals, some interesting parallels showed up, like the presence of monkeys on both sides. You could explain Asian monkeys by assuming they had wandered across from Africa (or vice versa), but the South American monkeys were a puzzle. A close inspection showed that the New World monkeys were quite different, suggesting that a great deal of change had happened since the groups separated. Other plant and animal distributions would also make more sense if continents had originally been joined together.

In 1912, a German meteorologist named Alfred Lothar Wegener (1880–1930) published an account of how continental drift might have happened. He suggested that the supercontinent Pangaea began to split, about 200 million years ago. Alexander Du Toit in Johannesburg supported him and proposed that Pangaea first broke into two large pieces, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. Laurasia and Gondwanaland later broke apart to make today’s continents.

The key find was the distribution of a fossil fern named Glossopteris, found in South America, southern Africa, Australia—and Antarctica. The snag was explaining the huge force needed to move a continent around. While we use the same slab-names today, much of the background is different, and we regard some of today’s land masses as being assembled from several different scraps.

How the mysterious ‘drift’ of the continents was explained, and went from science fiction to science.

Continental drift tried to account for the shapes of the world’s large land pieces and the distributions of animals and plants. Plate tectonics works on the idea that the crust ‘floats’ on the more dense mantle, and that parts are slowly moved around by convection effects. It explains the continent shapes and plant and animal distributions, but it also explains the main mountain regions like the Himalayas and the Alps, the distribution of volcanoes and earthquakes, the location of island groups like Hawaii and the Aleutian islands—and the forces that drive the process.

It all began with the idea of sea floor spreading, and that came from mapping of the sea floor, at first carried out with long weighted lines, lowered to the floor, and later with sonar: sending ultrasonic ‘pings’ at the sea floor and timing their return. This revealed the shape of the seabed. The first chart showing parts of the mid-Atlantic ridge appeared in 1855. Ships laying telegraph cables across the Atlantic also detected parts of it, then in 1947, cores of the seabed showed that the sediment on the floor of the Atlantic was much thinner than it should have been under an ocean that had existed for 4 billion years. Clearly, a rethink was needed.

Before long, other ships were mapping sea floors and tracing the whole of the global mid-ocean ridge, more than 50,000 kilometres long and, sometimes more than 800 km across. This was no mere range of hills, either, because the mountains averaged 4500 metres above the sea floor.

Then there was an oddity that can be found in basalt: the magnetic fields sometimes go the ‘wrong’ way, the reverse of today’s magnetic field. We know now that every so often, there is a polar reversal, where the Earth’s magnetic field ‘flips’, reversing the magnetic north and south poles. As liquid basalt escapes from the Earth, the magnetic field of the moment is printed into the rocks.

If you map the zones of normal and reversed magnetic fields around the mid-Atlantic ridge, you see a pattern of stripes going across the sea floor. The way this is shown in school texts, most people think the sea floor is striped like a zebra. It isn’t like that, at all: the black and white are there to show the two polarities, and all the basalt is black. The ‘stripes’ were of different sizes, reflecting longer and shorter periods between reversals, but the amazing thing was that the two sides of the ridge showed a mirror pattern.

By 1961, people were beginning to hint, rather nervously, that maybe the basalt was oozing from the floor and spreading out to either side. During the 1960s, deep-sea drilling rigs began to bring up cores from the sea floor, and by 1968, fossil and isotope tests on the cores established a proof for the sea floor spreading hypothesis, young rocks near the ridge, old rocks further out.

Now we can explain the more peculiar earthquake areas. Spreading in one place means rocks being buried somewhere else. The subduction zones where one plate slides under another, the deep sea trenches, the position of Wallace’s Line, and even the origins of Africa’s Rift Valley, where many of the earliest human and pre-human fossils are found today, were all explained.

Wallace’s Line divided Asian plants and animals from Australian plants and animals. It is a subduction trench between Australia and Asia.

The Himalayas, the Swiss Alps, and the Andes are all formed as the crust piles up where plates are colliding. The volcano-free earthquakes of Turkey and Greece are explained: the movement between the plates there is not the sort that generates volcanoes. Around the Pacific, the Ring of Fire, the long chain of active volcanoes is explained, while the Hawaiian islands are the result of a plate slipping over a ‘hot spot’ that keeps generating volcanoes.

And that is probably enough for any single theory to have to explain.