| A | B |
|---|
| asthenosphere | 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. |
| basalt | 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. |
| buoyancy | The ability of an object to float in a fluid by displacement of a volume of fluid equal to it in mass. |
| continental crust | The solid masses of the continents, composed primarily of granite. |
| continental drift | The theory that the continents move slowly across the surface of Earth. |
| convection | Movement within a fluid resulting from differential heating and cooling of the fluid. Convection produces mass transport or mixing of the fluid. |
| 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. |
| 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. |
| echo sounder | 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. |
| fault | A fracture in a rock mass along which movement has occurred. |
| hot spot | A surface expression of a plume of magma rising from a stationary source of heat in the mantle. |
| lithosphere | The brittle, relatively cool outer layer of Earth, consisting of the oceanic and continental crust and the outermost, rigid layer of mantle. |
| magma | Molten rock capable of fluid flow; called lava above ground. |
| mantle | 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. |
| oceanic crust | The outermost solid surface of Earth beneath ocean floor sediments, composed primarily of basalt. |
| Pacific Ring of Fire | The zone of seismic and volcanic activity that encircles the Pacific Ocean. |
| Pangea | 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. |
| plate tectonics | 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. |
| Richter scale | A logarithmic measure of earthquake magnitude. A great earthquake measures above 8 on the Richter scale. |
| magnetometer | Measures the amount of residual magnetism in a rock. |
| seafloor spreading | 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. |
| seismograph | An instrument that detects and records earth movement associated with earthquakes and other disturbances. |
| subduction | The downward movement into the asthenosphere of a lithospheric plate. |
| surface wave | A wave that surges ashore without breaking. |
| transform fault | A plane along which rock masses slide horizontally past one another. |
