| A | B |
|---|
| aa | a slow-moving type of lava that hardens to form rough, jagged chunks |
| active | said of a volcano that is erupting or has shown signs of erupting in the near future |
| aftershock | an earthquake that occurs after a larger earthquake in the same area |
| asthenosphere | the soft layer of the mantle on which the lithosphere floats |
| basalt | a dark, dense igneous rock with a fine texture, found in oceanic crust |
| caldera | the large hole at the top of a volcano formed when the roof of a volcano's magma chamber collapses |
| cinder cone | a steep, cone-shaped hill or mountain made of volcanic ash, cinders and bombs piled up around a volcano's opening |
| composite volcano | a tall, cone-shaped mountain in which layers of lava alternate with layers of ash and other volcanic materials; also known as a stratovolcano |
| convergent boundary | a plate boundary where two plates move TOWARD each other |
| crater | a bowl-shaped area that forms around a volcano's central opening |
| crust | the layer of rock that forms Earth's outer surface |
| divergent boundary | a plate boundary where two plates move AWAY from each other |
| dormant | said of a volcano that does NOT show signs of erupting in the near future |
| earthquake | the shaking that results from the movement of rock beneath Earth's surface |
| epicenter | the point on Earth's surface directly above an earthquake's focus |
| extinct | said of a volcano that is unlikely to erupt again |
| fault | a break in Earth's crust where slabs of rock slip past each other |
| geology | the study of the structure and composition of planet Earth |
| hot spot | an area where magma from deep within the mantle melts through the crust above it |
| igneous rock | a type of rock that forms from the cooling of molten rock at or below Earth's surface |
| inner core | a dense sphere of solid iron and nickel in the center of Earth |
| lava | liquid magma that reaches the surface; also the rock formed when liquid lava hardens |
| liquefaction | the process by which an earthquake's violent movement suddenly turns loose soil into liquid mud |
| lithosphere | a rigid layer made up of the uppermost part of the mantle and the crust |
| magma chamber | the pocket beneath a volcano where magma collects |
| mantle | the layer of hot, solid material between Earth's crust and core |
| Mercalli scale | a scale that rates earthquakes according to their intensity and how much damage they cause |
| metamorphic rock | a type of rock that forms from an existing rock that is changed by heat, pressure or chemical reactions |
| mid-ocean ridge | the undersea mountain chain where new ocean floor is produced; a divergent plate boundary |
| ore | rock that contains a metal or economically useful mineral |
| outer core | a layer of molten iron and nickel that surrounds the inner core of Earth |
| P wave | Primary wave; a type of seismic wave that compresses and expands the ground |
| pahoehoe | a hot, fast-moving type of lava that hardens to form smooth, ropelike coils |
| pangaea | the name of the single landmass that broke apart about 200 million years ago and gave rise to today's continents |
| plate | a section of the lithosphere that slowly moves over the asthenosphere, carrying pieces of the continental and oceanic crust |
| pyroclastic flow | the expulsion of ash, cinders, bombs and gases during an explosive volcanic eruption |
| Richter scale | a scale that rates seismic waves as measured by a particular type of mechanical seismograph |
| S wave | Secondary wave;a type of seismic wave that moves the ground up and down and side to side |
| sediment | small, solid particles of material from rocks and organisms which are moved by water or wind |
| sedimentary rock | a type of rock that forms from particles from other rocks or the remains of plants and animals that are pressed and cemented together |
| shield volcano | a wide, gently sloping mountain made of layers of lava and formed by quiet eruptions |
| silica | a material that is formed from the elements oxygen and silicon; silica is found in magma and sand |
| subduction | the process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary |
| transform boundary | a plate boundary where two plates move past each other in opposite directions; an example is the San Andreas Fault |
| trench | a deep canyon in the ocean floor |
| volcano | a weak spot in the crust where magma has come to the surface |
