| A | B |
|---|
| pressure | the amount of force pushing on a surface or area |
| crust | the layer of rock that forms Earth's outer surface |
| mantle | the layer of hot, solid material between Earth's crust and core |
| lithosphere | a rigid layer made up of the uppermost part of the mantle and the crust. One of the four spheres into which scientists divide Earth |
| asthenosphere | the soft layer of the mantle on which the lithosphere floats |
| outer core | a layer of molten iron and nickel that surrounds the inner core of Earth |
| inner core | a dense sphere of solid iron and nickel in the center of Earth |
| convection | the transfer of heat bymovements of a fluid |
| convection current | the movement of a fluid, caused by differences in temperature, that transfers heat from one part of the fluid to another |
| Pangaea | the name of the single landmass that broke apart 225 million years ago and gave rise to today's continents |
| continental drift | the hypothesis that the continents slowly move across Earth's surface |
| fossil | the preserved remains or traces of living things |
| mid-ocean ridge | the undersea mountain chain where new ocean floor is produced; a divergent plate boundary |
| sonar | a system that determines the distance of an object under water by recording echoes of sound waves; gets its name from sound navigation and ranging |
| sea-floor spreading | the process by which molten material adds new oceanic crust to the ocean floor |
| deep-ocean trench | a deep valley along the ocean floor through which oceanic crust slowly sinks toward the mantle |
| subduction | the process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary |
| plate | a section of the lithosphere that slowly moves over the asthenosphere, carrying pieces of continental and oceanic crust |
| scientific theory | a well-tested concept that explains a wide range of observations |
| plate tectonics | the theory that pieces of Earth's lithosphere are in constant motion, driven by convection currents in the middle |
