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 240,000 people. 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.   Let's Get Down to Earth! Understanding how and where hydrothermal vents and other high-energy phenomena occur on the seafloor requires a closer look at the Earth's structure and the forces at work deep within the planet. The Earth's inner core is a solid sphere composed mostly of iron. It is about 2,400 kilometers (1,500 mi) in diameter and is believed to be as hot as 6650° C (12000° F). This heat is probably generated by the radioactive decay of uranium and other elements. The inner core is bordered by a liquid outer core that is 4700° C (8500°F). Surrounding the outer core is the mantle, which is composed of hot, molten rock called magma. The churning of the magma, caused by the heat rising from the core, generates pressure on the Earth's surface layer, or crust. The crust is very thin compared to the other layers, ranging in thickness from only about 3.2 kilometers (2 mi) in some areas of the ocean floor to some 121 kilometers (75 mi) deep under mountains. The crust is composed of plates on which the continents and oceans rest. Like giant rafts, these plates move slowly on the magma beneath them. The plates may move apart, collide, and slide past each other, resulting in such high-energy phenomena as hydrothermal vents, volcanoes, and earthquakes.