Explore the Deep Sea
Volcanoes & Vents
Rock rafts—tectonic plates
The ground beneath our feet is not as solid and unchanging as it might appear. In fact, the earth's surface (crust) consists of more than a dozen huge rafts of rock floating on a "sea" of soft rock in the mantle beneath. These rafts, or tectonic plates, fit together like the pieces of a jigsaw. But they are not stationary. They move slowly but inexorably—at about the same speed our fingernails grow. This may not sound fast, but over the course of years, it leads to some dramatic consequences: volcanoes, mountain ranges, tsunamis and earthquakes.
Diverging, colliding, sliding sideways
- In places where tectonic plates are moving apart, the crust is stretched: cracks widen and are filled with molten rock from the mantle beneath. The Great Rift Valley of East Africa is one area where two plates are diverging, forming a gigantic crack, or rift, so big it can be seen from space. But most areas where plates are moving apart cannot be seen easily, because they are found in the depths of our oceans.
- Each tectonic plate moves on its own trajectory. Different plates have different trajectories, so in many parts of the world, plates are moving towards each other: colliding. Plate collisions have created some of the most spectacular mountain ranges on our planet: the Alps, the Himalayas and the Andes.
- The San Andreas Fault in California is one of many zones where two plates are neither colliding nor moving apart, but grinding past each other. In this case, the fault marks the boundary beween the Pacific Plate and the North American Plate.
In some places, hot rock rises upwards in the mantle away from the edge of a plate. Volcanos can form over these "hotspots". The Hawaiian Islands are one example: Kauai was the first island to form, when the hotspot was young; as the Pacific Plate moved relative to the hotspot, the volcano which makes up Kauai went extinct, and the volcanoes which constitute Oahu and Maui formed in turn; the island of Hawaii is currently located over the hotspot—it contains the world's highest volcano (Mauna Loa), as well as the most active (Kilauea), and several others. Other examples of volcanic areas overlying hotspots include Peeps Seamount in the Bering Sea and Pitcairn Island in the Pacific.
Tectonic plates are thought to have been moving for billions of years. The continents they carry have collided and split apart throughout geological time. For instance, about 250 million years ago, around the time the dinosaurs were starting to appear on the earth, the map of the earth looked very different from today. There was just one super-continent (called "Pangaea"). But by the time the first birds and mammals evolved, the super-continent had started to break up, spreading apart along a rift that eventually became the Atlantic Ocean. India, which was attached to the eastern side of Africa, began to move northwards, eventually crashing into Asia to create the Himalayas. North and South America did not become linked until about 1.5 million years ago—after the earliest humans began using stone tools.