Continental Drift and Plate Tectonics

The continents we see today have not always been where they are. One of the great revolutions in science was the discovery that the Earth's outer shell is broken into giant slabs that slowly move, carrying the continents with them. The story runs from continental drift through sea-floor spreading to the modern theory of plate tectonics. Together these ideas explain earthquakes, volcanoes and mountains alike.

In 1912, Alfred Wegener proposed the theory of continental drift in his book The Origin of Continents and Oceans. He argued that the continents have moved sideways over time. According to him:

• about 250 million years ago there was a single supercontinent, Pangaea, surrounded by one ocean, Panthalassa,
• around 200 million years ago Pangaea broke into two: Laurasia (drifting north) and Gondwanaland (drifting south), separated by the Tethys Sea.

He supported this with evidence:

• the jigsaw fit of coastlines (South America and Africa),
• matching rock structures across oceans,
• identical fossils, coal deposits and glacial striations on now-separated continents, and
• matching placer deposits, the mineral grains that streams concentrate in their beds.

The placer evidence is worth spelling out, because it is a favourite exam point. Placer deposits are heavy mineral grains, such as gold, that running water washes out of source rocks and piles up downstream. The coast of Ghana in West Africa holds rich gold placers, yet Ghana itself has no gold-bearing veins to supply them. The matching gold-bearing source rocks lie across the ocean in the plateau of Brazil. The grains could only have reached Ghana when Brazil and West Africa lay side by side as one landmass, so the placers were later split apart by drift.

The plant fossils point the same way. The Gondwana sediments of India, a thick rock sequence bearing a distinctive flora, have near-identical counterparts in six other Southern Hemisphere landmasses: Africa, South America, Antarctica, Australia, Madagascar and the Falkland Islands. One shared sediment system spread across all of them argues that they once formed a single block, Gondwanaland.

His weakness was that he could not explain the forces driving the drift. Even so, his idea laid the foundation for what followed.

Continental drift did more than rearrange the map. It shaped the evolution of organisms. As Pangaea split and the fragments drifted apart, populations of plants and animals that had once lived together were isolated on separate continents. Cut off from one another, each isolated population then evolved along its own path. This is why identical fossils of the same ancient species appear only in the older rocks of the now-separated continents, the rocks laid down before the split, and not in the younger ones.

Repeated glacial cycles influenced evolution as well. Each advance and retreat of the ice sheets changed climate, sea level and habitats, and these swings created fresh environmental pressures that selected for new traits. So both the drifting of continents and the cycle of glaciations helped drive the evolution of life.

The missing mechanism came from Harry Hess, who proposed sea-floor spreading. He showed that:

• the youngest rocks lie at the centre of the oceans (the mid-ocean ridges) and the oldest near the continental margins,
• new crust is continually formed at the ridges from rising magma, and
• this new crust spreads outward, pushing the older crust towards the margins.

So the ocean floor itself is being created and moved, strong proof that continents drift.

These ideas were unified into the theory of plate tectonics (the term "plate" coined by J. T. Wilson, 1965). The Earth's rigid outer shell (lithosphere) is broken into several plates about 100 km thick. These plates float on the semi-molten asthenosphere and are moved by convection currents in the mantle.

There are seven major plates (Eurasian, Antarctic, North American, South American, Pacific (the largest), African and Indo-Australian) plus several minor ones.

Where plates meet, there are three types of boundary:

• Divergent (constructive): plates move apart, magma rises and forms new crust (mid-ocean ridges, rift valleys like the Red Sea).
• Convergent (destructive): plates collide. The denser plate is subducted (the Benioff zone), forming trenches and volcanoes. This also builds fold mountains: the Himalayas formed where the Indian plate collided with the Eurasian plate.
• Transform (conservative): plates slide past each other, causing earthquakes (the San Andreas Fault).