A cracked shell on a slowly moving interior
Earth’s outer layer, the lithosphere, is made of the crust together with the rigid uppermost part of the mantle. It is broken into several large pieces and many smaller ones, called tectonic plates. These plates rest on the asthenosphere, a hotter and softer part of the upper mantle that can flow very slowly over geologic time, although it is not liquid. The plates move relative to one another at speeds typically of a few centimeters per year, often compared with the rate at which fingernails grow.
This small speed adds up. Over millions of years, plates travel thousands of kilometers, opening oceans and closing others. The theory of plate tectonics describes these movements and explains why earthquakes, volcanoes and mountain ranges occur where they do.
Plate boundaries
Most geological activity is concentrated along the edges of plates. There are three basic kinds of boundary.
- Divergent boundaries, where plates move apart. New crust forms as magma rises and solidifies. The Mid-Atlantic Ridge is a well-known example, and on land the East African Rift shows the same process beginning.
- Convergent boundaries, where plates move toward each other. If one plate is dense oceanic crust, it usually sinks beneath the other in a process called subduction, forming a deep ocean trench and often a chain of volcanoes. If two continental plates collide, neither sinks easily, so the crust crumples and thickens into mountains. The Himalayas formed this way, as the Indian plate pushed into Asia.
- Transform boundaries, where plates slide past each other horizontally. The San Andreas Fault in California is a famous example.
The sudden slips along these boundaries are the focus of the discussion of earthquake causes, and the melting above subduction zones and beneath ridges helps explain volcanic eruptions.
Continental drift: an idea ahead of its time
In 1912 the German scientist Alfred Wegener proposed that the continents had once been joined in a single landmass, later called Pangaea, and had since drifted apart. He pointed to several kinds of evidence:
- The coastlines of South America and Africa look as if they could fit together.
- Fossils of the same ancient plants and animals, such as the freshwater reptile Mesosaurus and the plant Glossopteris, occur on continents now separated by wide oceans. Similar reasoning applies to dinosaur fossils of related species found on different continents.
- Matching rock types and mountain belts appear on opposite sides of the Atlantic.
- Glacial deposits from the same ancient ice age are found in places now in the tropics.
Most geologists at the time rejected the idea, largely because Wegener could not offer a convincing force that could move continents through solid ocean floor. Some of his specific explanations were also incorrect.
The evidence that changed minds
After the Second World War, improved mapping and surveying of the seafloor revealed a continuous mountain range running through the oceans, with a rift valley along its crest. In the early 1960s, the American geologist Harry Hess suggested that new seafloor forms at such ridges and spreads outward, a concept now called seafloor spreading.
Key support came from magnetism. Rocks containing iron-rich minerals record the direction of Earth’s magnetic fields as they cool. Earth’s field has reversed polarity many times in the past, and surveys showed symmetrical stripes of alternating magnetic polarity on either side of mid-ocean ridges, as expected if new crust is added and carried away from the ridge. Other evidence includes the finding that ocean floor is much younger than the continents, with the oldest ocean crust being about 180 million years old or somewhat older, compared with continental rocks several billion years old. Patterns of earthquakes tracing plate boundaries and the matching of deep ocean drilling samples added more weight. By the late 1960s, plate tectonics had become the accepted framework.
What moves the plates
Scientists are still working out how the pieces fit together, but the main contributors are widely discussed. Heat from Earth’s interior drives slow circulation in the mantle. At subduction zones, the cold, dense edge of a sinking plate pulls the rest of the plate along, often called slab pull. At ridges, elevated crust may slide away from the ridge, sometimes called ridge push. Which of these dominates, and how mantle flow couples to the plates, is an area of ongoing research.
Measuring the motion today
Plate motion is no longer inferred only from geology. Satellite positioning systems, similar to those used for GPS navigation, can measure the position of ground stations to within a few millimeters, and repeated measurements over years show them drifting at rates consistent with the geological record. Examples include the widening of the Atlantic by a few centimeters each year.
Why it matters
Plate tectonics links together many things that once seemed unrelated: the location of mountains, the pattern of earthquakes, the distribution of fossils, and the long-term cycling of rock and carbon through the Earth. Over very long periods, the arrangement of continents also influences ocean circulation and climate, a connection that features in the story of ice ages. It helps explain, too, why the rocks in the rock cycle keep being recycled, buried and lifted again.