| A | B |
|---|
| atoll | A string of coral islands that form a circle (around a sunken islands.) |
| barrier reef | A coral reef that grows about 25 kilometers offshore, separated by a channel from the mainland. |
| continental drift | Theory that the continents once formed one large landmass, then broke apart and drifted to their present positions. |
| continental rise | Slightly elevated region formed at base of continental slope by accumulation of mudslide sediments. |
| continental slope | Area where seafloor drops steeply at outer edge of continental shelf. |
| convection current | Continual movement of gas or liquid in a cycle as the heated part rises and cooler park sinks; inside Earth, magma rises to crust; in atmosphere it causes breezes. |
| crust | Earth’s surface layer (above mantle), about 40 kilometers thick. |
| delta | Fan-shaped feature formed by sediments that pile up at a river’s mouth. |
| fault | A crack in Earth’s crust that occurs at the margin of two plates. |
| fjord | A steep, deep, narrow inlet from the sea, formed by the action of glaciers. |
| fringing reef | A coral reed that grows a few kilometers offshore, parallel to the mainland. |
| guyots | Flattened undersea structures, formed when tops of seamounts are eroded by waves and currents. |
| hot spot | An area of intense geologic activity in the crust where a seamount forms. |
| hydrothermal vent | An area in the rift zone at which hot springs emerge. |
| island arcs | Groups of volcanic islands that form an arc near an ocean trench. |
| key (cays) | Small islands that form when chunks of coral stone break off from reefs and accumulate on seafloor. |
| magma | Molten material within Earth’s mantle (called lava at Earth’s surface). |
| mantle | Regions of geologic activity between the Earth’s core and crust. |
| mid-ocean ridge | Undersea volcanic mountain range that encircles the globe. |
| plate tectonics | Theory that Earth’s crustal plates float on the mantle. |
| plates | Segments of Earth’s crust that float (with the continents on top) on the mantle. |
| rift valley | A depression that runs along the crest of the mid- ocean ridge. |
| seafloor spreading | The moving apart of Earth’s plates, caused by the upward movement of magma under mid-ocean ridge. |
| seamounts | Small undersea mountains formed by lava piling up on seafloor; they form over hot spots in the mantle. |
| subduction | Occurs when one crustal plate plunges down under another plate. |
| Submarine canyons | Steep, v-shapes depressions that cut through the continental shelf; extensions of sunken river valley. |
| topography | Study of Earth’s surface features, on land and on the ocean floor. |
| trenches | Deepest, steepest depression son ocean floor formed by subduction. |
