The hot, plastic layer of the upper mantle below the lithosphere, extending some 350 to 650 kilometers (220 to 400 miles) below the surface. Convection currents within the asthenosphere power plate tectonics.
The relatively heavy crustal rock that forms the seabeds, composed mostly of oxygen, silicon, magnesium, and iron. Its density is about 2.9 g/cm3.
The ability of an object to float in a fluid by displacement of a volume of fluid equal to it in mass.
The theory that Earth's surface features are formed by catastrophic forces such as the biblical flood. Catastrophists believe in a young Earth and a literal interpretation of the biblical account of Creation.
The transfer of heat through matter by the collision of one atom with another.
The solid masses of the continents, composed primarily of granite.
The theory that the continents move slowly across the surface of Earth.
Movement within a fluid resulting from differential heating and cooling of the fluid. Convection produces mass transport or mixing of the fluid.
A single closed-flow circuit of rising warm material and falling cool material.
convergent plate boundary
A region where plates are pushing together and where a mountain range, island arc, and/or trench will eventually form; often a site of much seismic and volcanic activity.
The innermost layer of Earth, composed primarily of iron, with nickel and heavy elements. The inner core is thought to be a solid 6,000°C (11,000°F) sphere, the outer core a 5,000°C (9,000°F) liquid mass. The average density of the outer core is about 11.8 g/cm3, and that of the inner core is about 16 g/cm3.
The outermost solid layer of Earth, composed mostly of granite and basalt; the top of the lithosphere. The crust has a density of 2.7- 2.9 g/cm3 and accounts for 0.4% of Earth's mass.
The temperature above which a material loses its magnetism.
The mass per unit volume of a substance, usually expressed in grams per cubic centimeter (g/cm3).
The formation of layers in a material, with each deeper layer being denser (weighing more per unit of volume) than the layer above.
divergent plate boundary
A region where plates are moving apart and where new ocean or rift valley will eventually form. A spreading center forms the junction.
A sudden motion of Earth's crust resulting from waves in Earth caused by faulting of the rocks or by volcanic activity.
A device that reflects sound off the ocean bottom to sense water depth. Its accuracy is affected by the variability of the speed of sound through water.
A fracture in a rock mass along which movement has occurred.
The relatively light crustal rock—composed mainly of oxygen, silicon, and aluminum—that forms the continents. Its density is about 2.7 g/cm3.
A surface expression of a plume of magma rising from a stationary source of heat in the mantle.
Balanced support of lighter material in a heavier, displaced supporting matrix; analogous to buoyancy in a liquid.
The brittle, relatively cool outer layer of Earth, consisting of the oceanic and continental crust and the outermost, rigid layer of mantle.
The rigid portion of Earth's mantle below the asthenosphere.
Molten rock capable of fluid flow; called lava above ground.
A device that measures the amount and direction of residual magnetism in a rock sample.
The layer of Earth between the crust and the core, composed of silicates of iron and magnesium. The mantle has an average density of about 4.5 g/cm3 and accounts for about 68% of Earth's mass.
Ascending columns of superheated mantle originating at the core-mantle boundary.
The rigid inner mantle, similar in chemical composition to the asthenosphere.
The outermost solid surface of Earth beneath ocean floor sediments, composed primarily of basalt.
An assemblage of subducting oceanic lithosphere scraped off (obducted) onto the edge of a continent.
Primary wave; a compressional wave that is associated with an earthquake and that can move through both liquid and rock.
Pacific ring of fire
The zone of seismic and volcanic activity that encircles the Pacific Ocean.
The "fossil," or remanent, magnetic field of a rock.
Name given by Alfred Wegener to the original "proto- continent." The breakup of Pangaea gave rise to the Atlantic Ocean and to the continents we see today.
Name given by Alfred Wegener to the ocean surrounding Pangaea.
One of about a dozen rigid segments of Earth's lithosphere that move independently. The plate consists of continental or oceanic crust and the cool, rigid upper mantle directly below the crust.
The theory that Earth's lithosphere is fractured into plates that move relative to each other and are driven by convection currents in the mantle. Most volcanic and seismic activity occurs at plate margins.
The disintegration of unstable forms of elements, which releases subatomic particles and heat.
The process of determining the age of rocks by observing the ratio of unstable radioactive elements to stable decay products.
A logarithmic measure of earthquake magnitude. A great earthquake measures above 8 on the Richter scale.
Secondary wave; a transverse wave that is associated with an earthquake and that cannot move through liquid.
The theory that new ocean crust forms at spreading centers, most of which are on the ocean floor, and pushes the continents aside. Power is thought to be provided by convection currents in Earth's upper mantle.
A low-frequency wave generated by the forces that cause earthquakes. Some kinds of seismic waves can pass through Earth. See also P wave; S wave.
An instrument that detects and records earth movement associated with earthquakes and other disturbances.
(1) The wide band at Earth's surface 105° to 143° away from an earthquake in which seismic waves are nearly absent. P waves are absent because they are refracted by Earth's liquid outer core; S waves are absent from this band and the zone immediately opposite the earthquake site because they are absorbed by the outer core. (2) In sonar, the volume of ocean from which sound waves diverge and in which a submarine may hide.
The junction between diverging plates at which new ocean floor is being made; also called spreading zone.
The downward movement into the asthenosphere of a lithospheric plate.
An area at which a lithospheric plate is descending into the asthenosphere. The zone is characterized by linear folds (trenches) in the ocean floor and strong deep-focus earthquakes; also called a Wadati-Benioff zone.
A very large mantle plume.
An isolated segment of seafloor, island arc, plateau, continental crust, or sediment transported by seafloor spreading to a position adjacent to a larger continental mass; usually different in composition from the larger mass.
A plane along which rock masses slide horizontally past one another.
transform plate boundary
Places where crustal plates shear laterally past one another. Crust is neither produced nor destroyed at this type of junction.
The theory that all of Earth's geological features and history can be explained by processes occurring today and that these processes must have been at work for a very long time.
(1880-1930) German scientist who proposed the theory of continental drift in 1912.
Wilson, John Tuzo
(1908-1993) Canadian geophysicist who proposed the theory of plate tectonics in 1965.