Plate Tectonics Glossary: Key Terms Explained Simply

A

Accretionary wedge: A pile of sediment and rock scraped off a sinking oceanic plate and stacked against the edge of the overriding plate at a subduction zone. It grows as folded and faulted slices, and parts of it can rise above sea level as islands. See: Convergent boundaries
Aftershock: A smaller earthquake that follows a larger one in the same area, as the rocks around the ruptured fault adjust. Aftershocks can continue for months or years after a large earthquake. See: Earthquake map
Andesite: A gray volcanic rock with a medium silica content, typical of volcanoes above subduction zones. It is named after the Andes, where it is common. See: Volcano map
Asthenosphere: The hot, weak layer of the upper mantle beneath the lithosphere. It is solid but can flow very slowly, which allows the rigid plates above it to move. See: Plate boundaries

B

Back-arc basin: A basin that opens behind a volcanic arc, on the side away from the trench, when the overriding plate is stretched. The Mariana Trough and the Sea of Japan are examples. See: Convergent boundaries, Philippine sea plate
Basalt: A dark, fine-grained volcanic rock low in silica. Basalt forms most of the oceanic crust and erupts at mid-ocean ridges, hotspots such as Hawaii and continental rifts. See: Divergent boundaries
Benioff zone: A sloping band of earthquakes that traces a slab of oceanic lithosphere sinking into the mantle at a subduction zone. It can reach depths of about 700 km and is also called the Wadati-Benioff zone. See: Convergent boundaries, Earthquake map

C

Caldera: A large basin formed when the ground collapses into an emptied magma chamber after a big eruption. Calderas can be several kilometers to tens of kilometers across, and many later fill with lakes. See: Volcano map
Continental crust: The thick, relatively light crust that forms the continents, made mostly of rocks rich in silica such as granite. It averages around 35 km thick and is too buoyant to sink easily into the mantle. See: All plates
Continental drift: Alfred Wegener's early twentieth century hypothesis that the continents were once joined and have since moved apart. It was replaced by plate tectonics, which explains how the continents move. See: Pangaea
Continental rift: A place where a continent is being stretched and pulled apart, forming a long valley bounded by faults. If rifting continues, it can split the continent and open a new ocean, as in the Red Sea. See: Divergent boundaries
Convergent boundary: A plate boundary where two plates move toward each other. It produces subduction zones, trenches, volcanic arcs and mountain belts. See: Convergent boundaries
Core: The metal center of Earth, made mostly of iron and nickel. The outer core is liquid and generates Earth's magnetic field, and the inner core is solid.
Craton: An old, stable interior part of a continent that has escaped major deformation for hundreds of millions of years or more. Cratons often have thick lithospheric roots, as beneath much of Canada and Australia. See: North american plate, Australian plate
Crust: The thin outermost rock layer of Earth. It comes in two kinds, thin dense oceanic crust and thick lighter continental crust, and it forms the top of the lithosphere. See: All plates

D

Diffuse plate boundary: A boundary where deformation is spread across a wide zone rather than concentrated on one fault. The broad zone between the Indian and Australian plates is a well known example. See: Plate boundaries, Indian plate
Divergent boundary: A plate boundary where two plates move apart and new lithosphere forms in the gap. Mid-ocean ridges and continental rifts are divergent boundaries. See: Divergent boundaries

E

Earthquake: Shaking of the ground caused by the sudden slip of rock along a fault. Most earthquakes happen at plate boundaries, where stress from plate motion builds up over time. See: Earthquake map
Epicenter: The point on Earth's surface directly above the place where an earthquake starts. News reports and earthquake maps usually locate quakes by their epicenter. See: Earthquake map
Euler pole: The point on Earth's surface about which one plate rotates relative to another. Relative speed is zero at the pole and increases with distance from it, which is why speeds change along a single boundary. See: Plate boundaries

F

Fault: A fracture in rock along which the two sides have moved past each other. Faults are classed by their motion as normal, reverse (thrust) or strike-slip. See: Earthquake map
Focal depth: The depth below the surface at which an earthquake starts. Quakes shallower than 70 km are called shallow, those from 70 to 300 km intermediate, and those deeper than 300 km deep. See: Earthquake map
Foreshock: A smaller earthquake that comes before a larger one in the same place. A quake can only be recognized as a foreshock after the larger event happens. See: Earthquake map
Fracture zone: A long, inactive scar on the ocean floor that continues beyond the end of a transform fault. Both sides of a fracture zone belong to the same plate. See: Transform boundaries

G

Gondwana: The southern half of Pangaea, made of what are now South America, Africa, Arabia, Madagascar, India, Antarctica and Australia. It existed as a large continent before Pangaea formed. See: Pangaea

H

Hotspot: An area of long-lived volcanic activity thought to sit above a plume of hot rock rising from deep in the mantle. As a plate moves over a hotspot it can leave a chain of volcanoes, as in Hawaii. See: Volcano map
Hypocenter: The point inside Earth where an earthquake rupture begins, also called the focus. The epicenter lies directly above it on the surface. See: Earthquake map

I

Intensity: A measure of how strongly an earthquake is felt and how much damage it causes at a particular place. It is usually reported on the Modified Mercalli scale, in Roman numerals from I to XII, and differs from place to place for the same quake. See: Earthquake map
Island arc: A curved chain of volcanic islands that forms where one oceanic plate subducts beneath another. The Aleutian, Mariana and Tonga islands are island arcs. See: Convergent boundaries, Ring of Fire
Isostasy: The balance in which the crust floats on the denser mantle beneath it, with thick or light crust standing higher. Mountains have deep roots, and land pressed down by ice sheets slowly rises after the ice melts.

L

Laurasia: The northern half of Pangaea, made of what are now North America, Greenland, Europe and most of Asia. See: Pangaea
Lava: Molten rock that has erupted onto Earth's surface, and the rock that forms when it cools. Runny basalt lava flows far, while silica-rich lava is thick and slow. See: Volcano map
Lithosphere: The rigid outer shell of Earth, made of the crust and the uppermost mantle. It is broken into the tectonic plates. See: All plates

M

Magma: Molten or partly molten rock beneath Earth's surface. Magma forms where the mantle melts at spreading ridges, above subduction zones and at hotspots. See: Volcano map
Magnetic striping: The pattern of parallel bands of normal and reversed magnetism in ocean floor rocks, mirrored on each side of a mid-ocean ridge. It records reversals of Earth's magnetic field as new crust formed and was key evidence for seafloor spreading. See: Divergent boundaries
Magnitude: A single number for the size of an earthquake at its source. Each whole step up means about 10 times more ground motion on a seismograph and about 32 times more energy released. See: Earthquake map
Mantle: The thick layer of hot rock between the crust and the core, making up most of Earth's volume. It is solid but flows very slowly over millions of years.
Mantle convection: The slow circulation of rock in the mantle as Earth loses heat from its interior. Plates form the cold top layer of this system, and sinking plates are part of the flow. See: All plates
Mantle plume: A column of unusually hot rock rising from deep in the mantle. Plumes are thought to feed hotspots such as Hawaii and Iceland, though their depth and origin are still debated. See: Volcano map
Megathrust: The large, gently dipping fault where one plate slides beneath another at a subduction zone. Megathrust earthquakes are the largest on Earth, including the 1960 Chile (M9.5) and 2011 Japan (M9.1) quakes. See: Convergent boundaries, Earthquake map
Microplate: A small tectonic plate, often found where larger plates meet in complex ways. In the PB2002 model, 32 of the 52 plates are microplates, from the North Andes Plate down to the tiny Manus Plate. See: All plates, Manus plate
Mid-ocean ridge: An undersea mountain chain where two plates move apart and new oceanic crust forms. The Mid-Atlantic Ridge and the East Pacific Rise are the best known. See: Divergent boundaries
Moment magnitude: The standard scale for the size of medium and large earthquakes, written Mw. It is based on the area of the fault that slipped, how far it slipped and the stiffness of the rock, so it does not saturate for very large quakes. See: Earthquake map

N

Normal fault: A fault where the rock above the fault plane slides down relative to the rock below, as the crust is stretched. Normal faults are typical of rifts and mid-ocean ridges. See: Divergent boundaries

O

Oceanic crust: The thin, dense crust beneath the ocean basins, about 7 km thick and made of basalt and gabbro. It forms at mid-ocean ridges and is recycled into the mantle at subduction zones. See: Divergent boundaries
Ophiolite: A slice of oceanic crust and upper mantle that has been pushed up onto land during plate convergence. Well known examples are the Troodos Mountains of Cyprus and the mountains of Oman. See: Cyprus, Oman
Orogeny: A mountain-building event, usually caused by plate convergence or continental collision. The belt of deformed rock it creates is called an orogen. See: Convergent boundaries

P

P wave: The fastest seismic wave, which travels by squeezing and stretching rock in the direction it moves. P waves pass through solids, liquids and gases and are the first to arrive at a seismograph. See: Earthquake map
Paleomagnetism: The record of Earth's past magnetic field preserved in rocks. It shows where continents lay when the rocks formed and provided strong evidence that continents have moved. See: Pangaea
Pangaea: The supercontinent that joined almost all of Earth's land, assembled by about 335 million years ago and breaking apart from about 200 million years ago. See: Pangaea
Plate tectonics: The theory that Earth's lithosphere is broken into plates that move over the asthenosphere, interacting at their boundaries. It explains the locations of earthquakes, volcanoes, mountain belts and ocean basins. See: All plates, Plate boundaries
Pyroclastic flow: A fast, ground-hugging avalanche of hot gas, ash and rock fragments from an explosive eruption. It is one of the deadliest volcanic hazards. See: Volcano map

R

Relative plate speed: The speed at which two neighboring plates move with respect to each other along their shared boundary. The plate speeds on this site are relative speeds from the PB2002 model. See: All plates, Plate boundaries
Richter scale: The first widely used earthquake magnitude scale, developed by Charles Richter in 1935 for earthquakes in southern California. It underestimates very large quakes, so seismologists now use moment magnitude instead for them. See: Earthquake map
Ridge push: A force that helps move plates, caused by gravity acting on the high, young lithosphere at mid-ocean ridges. The plates slide gently down and away from the ridge crest. See: Divergent boundaries
Rift valley: A long valley formed where the crust drops down between parallel faults as a region is stretched. The East African Rift and the central valley of slow mid-ocean ridges are rift valleys. See: Divergent boundaries
Ring of Fire: The belt of subduction zones, volcanoes and earthquakes that almost encircles the Pacific Ocean. It is commonly cited as holding about 75% of Earth's active volcanoes. See: Ring of Fire

S

S wave: A seismic wave that moves rock from side to side, at right angles to its direction of travel. S waves are slower than P waves and cannot pass through liquids, which showed that Earth's outer core is molten. See: Earthquake map
Seafloor spreading: The process by which new oceanic crust forms at mid-ocean ridges and moves away to each side as the plates separate. It explains why the ocean floor is young and why its age increases away from ridges. See: Divergent boundaries
Seismic waves: Waves of energy that travel through Earth from an earthquake or explosion. Body waves (P and S waves) travel through the interior, and surface waves travel along the surface and cause much of the shaking damage. See: Earthquake map
Seismometer: An instrument that detects and records ground motion from earthquakes. The record it produces is a seismogram, and networks of seismometers locate and measure earthquakes worldwide. See: Earthquake map
Shield volcano: A broad, gently sloping volcano built from many flows of runny basalt lava. Mauna Loa in Hawaii is a classic shield volcano. See: Volcano map
Slab pull: The force exerted by a cold, dense slab of oceanic lithosphere as it sinks into the mantle at a subduction zone, dragging the rest of the plate behind it. It is generally considered the strongest force driving plate motion. See: Convergent boundaries
Stratovolcano: A steep, cone-shaped volcano built from layers of lava, ash and rock debris, also called a composite volcano. Stratovolcanoes are typical of subduction zones and often erupt explosively; Mount Fuji is an example. See: Volcano map, Ring of Fire
Strike-slip fault: A fault where the two sides slide horizontally past each other. Transform plate boundaries such as the San Andreas Fault are strike-slip faults. See: Transform boundaries
Subduction: The process in which one plate sinks beneath another into the mantle at a convergent boundary. Subduction zones produce trenches, volcanic arcs and the largest earthquakes on Earth. See: Convergent boundaries, Ring of Fire
Supercontinent: A landmass containing most or all of Earth's continents. Pangaea is the most recent; Rodinia is an older one, and future supercontinents have been proposed as hypotheses. See: Pangaea
Surface waves: Seismic waves that travel along Earth's surface rather than through its interior. They arrive after P and S waves and usually cause the strongest shaking far from the epicenter. See: Earthquake map

T

Tectonic plate: A large, rigid piece of the lithosphere that moves as a unit over the asthenosphere. The PB2002 model divides Earth's surface into 52 plates. See: All plates
Thrust fault: A low-angle reverse fault where rock is pushed up and over other rock as the crust is squeezed. Thrust faults are common in collision belts, and the megathrust of a subduction zone is the largest kind. See: Convergent boundaries
Transform fault: A strike-slip fault that forms a plate boundary, along which two plates slide past each other without making or destroying crust. Most connect offset segments of mid-ocean ridges. See: Transform boundaries
Trench: A long, deep, narrow depression in the ocean floor where one plate bends down into a subduction zone. The Mariana Trench holds the deepest point in the oceans, nearly 11 km down. See: Convergent boundaries, Mariana plate
Triple junction: A point where three plate boundaries, and three plates, meet. The Afar region of Ethiopia and Djibouti is a triple junction where the Arabian, Somali and African plates come together. See: Divergent boundaries, Djibouti
Tsunami: A series of ocean waves caused by a sudden movement of the seafloor or water, most often a large undersea earthquake. Tsunamis can cross entire oceans, as after the 2004 Sumatra and 2011 Japan earthquakes. See: Earthquake map, Ring of Fire

V

Volcanic arc: A chain of volcanoes that forms above a subduction zone, parallel to the trench. It can be an island arc in the ocean or a mountain range on a continent, such as the Andes or the Cascades. See: Convergent boundaries, Volcano map