Plate tectonics
Earth's outer shell is broken into plates that slowly move, pulling apart at ridges and rifts and colliding elsewhere.
Earth's rigid outer shell is broken into a few dozen large and small pieces, the tectonic plates, which move slowly over hotter, softer rock beneath. Most of the planet's earthquakes and volcanoes happen where plates meet. Plate tectonics is the theory that explains this movement and the landscapes it builds.
Plates can pull apart, push together or slide past each other. Where they pull apart under the ocean, molten rock rises to fill the gap and freezes into new crust along mid-ocean ridges, and the new floor is carried away on both sides. This is seafloor spreading, proposed by Harry Hess and Robert Dietz in the early 1960s and confirmed by matching magnetic stripes in the rock on either side of the ridges. Slow ridges such as the Mid-Atlantic Ridge spread at less than 40 millimetres a year; parts of the East Pacific Rise reach about 145 millimetres a year.
New oceans begin when a continent starts to split. In continental rifting the crust stretches and thins, magma pushes up into vertical cracks and a rift valley forms, as in East Africa today. In 2005, in the Afar region of Ethiopia, a 60-kilometre fissure opened up to eight metres wide. If rifting carries on, the sea eventually floods in, as it did to form the Red Sea.
A common misconception is that continents plough through the ocean floor. In fact continents ride along as passengers on plates that include the sea floor around them.
Coming up in future stories
- Africa Ripped Open in Three Weeks 2005 · Dabbahu, Afar, Ethiopia
- The Earthquake That Gave Napier an Airport 1931 · Napier, Hawke's Bay, New Zealand
- The Map Her Boss Called 'Girl Talk' 1952-1977 · Lamont Geological Observatory, New York