This map shows the major tectonic plates that make up the Earth's crust and the directions in which they
are moving. Map adapted from NOAA.
Simply defined, the term plate tectonics refers to how the Earth's surface is made up of plates. In geology,
a plate is a large slab of rock, while tectonics is a word of Greek origin meaning "to build."
The theory of plate tectonics became widely accepted by scientists in the 1960s and 1970s. It revolutionized
our understanding of the Earth and unified the Earth sciences, from the study of fossils (paleontology) to the
study of earthquakes (seismology).
According to this theory, the Earth's crust is made up of about a dozen plates on which the continents and oceans
rest. These plates are continually shifting because the surface beneath them - the hot, soft mantle - is moving
slowly like a conveyor belt, driven by heat and other forces at work in the Earth's core. The plates are moving
about a centimeter (0.5 in) to 15 centimeters (6 in) per year in different directions.
Vents, Volcanoes & Quakes
The Earth's tectonic plates can move apart, collide, or slide past each other. The Mid-Ocean Ridge system -
the Earth's underwater mountain range - arises where the plates are moving apart. As the plates part, the seafloor
cracks. Cold seawater seeps down into these cracks, becomes super-heated by magma, and then bursts back out into
the ocean, forming hydrothermal vents.
As the plates move farther apart, magma from the Earth's interior percolates up to fill the gap, sometimes leading
to the eruption of undersea volcanoes. This process, called seafloor spreading, is how new seafloor is formed.
Conversely, when tectonic plates meet, the force causes mountains to rise and deep trenches to form. When the
edge of one plate is forced under another - a process called subduction - it causes intense vibrations in
the Earth's crust, producing an earthquake. One of the most violent earthquakes related to plate tectonics struck
northeast China in 1976. The disastrous Tangshan quake, registering 7.8 on the Richter scale, killed more than
Undersea earthquakes and volcanic eruptions can generate catastrophic ocean waves called tsunamis (meaning
"harbor wave" in Japanese). During a major quake, the seafloor can move several meters, setting into
motion a huge amount of water. The resulting waves may race across the ocean at speeds up to 800 kilometers (500
mi) per hour. The largest tsunami recorded measured 63 meters (210 ft) above sea level when it slammed into Siberia's
Kamchatka Peninsula in 1737.