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Active volcano
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A volcano that has erupted within the past 10,000 years and/or is expected to erupt within the next 10,000 years. |
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Amplitude
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The maximum displacement from a zero value during one complete oscillation. |
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Andesite
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A dark volcanic rock with a silica content of about 60%. Andesite is the second most abundant volcanic rock in Earth's crust. |
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Anomaly
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A pronounced deviation from the accepted normal value or condition. |
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Ash fall
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A layer of volcanic ash that falls from an eruption plume. |
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Asthenosphere
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A soft layer of the Earth's upper mantle lying just below the lithosphere. |
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Atmosphere
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A layer of gases surrounding Earth. |
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Atom
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The smallest component of an element having the chemical properties of the element. Atoms consist of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction. |
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Basalt
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A dark volcanic rock with a silica content of 40 to 50%. Basalt makes up most of the ocean floor and is the most abundant volcanic rock in Earth's crust. |
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Body waves
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Seismic waves that travel through solid rock, including S waves and P waves. |
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Caldera
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A large bowl-shaped crater formed by explosive volcanic eruptions and/or by collapse of a volcano's surface material into the magma chamber. |
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Calibrate
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To determine, check, or rectify the graduation of any instrument or system giving quantitative measurements. |
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CE
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"Common Era" is the most commonly used year-numbering system worldwide. The years are equivalent to the Anno Domini (AD) system, which it replaces. |
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Centrifugal force
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The outward force that draws a rotating body away from the center of rotation. |
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Cinder
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Light pieces of lava, formed from magma that is ejected, cooled, fragmented in the air, and falls back down. Also called "scoria". |
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Cinder cone volcano
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A small, steep-sided volcano composed primarily of cinder. |
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Compass
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An instrument that detects magnetic polarity, used for determining directions by means of a freely rotating magnetized needle that indicates magnetic north. |
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Composite volcano
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A cone-shaped volcano built from layers of pyroclastic material and lava. (It is also called a Stratovolcano.) |
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Compound
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A pure substance composed of two or more elements whose composition is constant. |
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Compression
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Pushing together. |
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Compression waves
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Seismic waves that oscillate parallel to the direction in which the energy is traveling. Commonly called "P waves". |
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Continent
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One of Earth's main landmasses including, Europe, Asia, Africa, North America, South America, Australia, and Antarctica. |
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Continental crust
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The layer of rocks that forms the continents and the areas of shallow seabed close to their shores. |
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Convection
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The transfer of heat by the circulation or movement of the heated parts of a liquid or gas. |
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Convective eruption column
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A shaft of heat and particles rising up from an erupting volcano. |
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Convergent
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Colliding, coming together. |
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Crater
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A steep-sided, bowl-shaped hollow at the top of a volcano. |
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Cretaceous
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The geologic time period spanning roughly 145 to 65 million years ago. |
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Cross section
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A two-dimensional view of a three-dimensional object. A "slice" through an object. |
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Crust
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The thin, outermost layer of Earth, which comprises the continents and ocean floors. |
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Deformation
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Change in size or shape of an object resulting from an applied force. |
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Devonian
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The geologic time period spanning roughly 400 to 350 million years ago. |
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Diffraction
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The apparent bending, spreading out, or other changes in seismic wave direction. |
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Displacement
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The removal of one mass and replacement by another within a given space. |
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Divergent
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Separating, breaking apart. |
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Dome
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A mound of lava and other volcanic material that has built up around the vent of a volcano. |
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Dormant volcano
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An active volcano that is not currently erupting. |
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Earthquake
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Vibrations induced in Earth's crust by the abrupt rupture and rebound of rocks in which strain has been slowly accumulating. |
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EDM
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Electronic Distance Meter |
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Elasticity
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The property of a substance that enables it to change its length, volume, or shape in direct response to an applied force, and to recover its original form upon the removal of the force. A rubber band has elasticity. |
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Emit
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To release or discharge into the environment. |
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Energy
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The capacity to do work. |
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Epicenter
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The point on Earth's surface directly above the origin of disturbance that creates an earthquake. |
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Erratic
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Wandering, not fixed. |
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Eruption
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The ejection of material from a volcano or geyser. |
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Extension
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Pulling apart. |
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Extinct volcano
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A volcano that has not erupted in the past 10,000 years and is not expected to erupt within the next 10,000 years. |
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Fault
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A fracture along which pieces of Earth's crust move relative to one another. |
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Fault trace
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The line where a fault intersects Earth's surface, either on a horizontal plane or in vertical cross-section. |
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Fissure
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A crack through which magma erupts onto Earth's surface. |
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Focus
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The belowground point of origin of an earthquake. Also called a "hypocenter". |
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Fold mountains
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Mountains formed by folding of Earth's crust during the convergence of tectonic plates. |
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Foot wall
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The block of rock below the fault trace or fault plane. |
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Force
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An influence that causes a mass to accelerate. |
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Foreshock
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A relatively small earthquake that precedes a greater one by a few hours, days, or weeks and originates at or near the focus of the larger earthquake. |
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Friction
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Resistance generated by rubbing two surfaces together. |
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Fumerole
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A vent or opening in Earth's surface producing gas or steam. |
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Glacier
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A large mass of ice formed from many years of snow, which flows very slowly over land. |
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Global positioning system
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A space-based global navigation satellite system that provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis. |
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GPS
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Abbreviation for global or geographic positioning system. |
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Granite
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A coarse-grained igneous rock composed chiefly of orthoclase, feldspars, and quartz, usually with lesser amounts of one or more other minerals, as mica, hornblende. |
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Granitic
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Composed chiefly of orthoclase, feldspars, and quartz, usually with lesser amounts of one or more other minerals, as mica, hornblende. |
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Ground deformation
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Change in shape of the ground surface caused by moving magma. |
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Hanging wall
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The block of rock above the fault trace or fault plane. |
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Hot spot
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An upwelling of magma from beneath Earth's crust, caused by a disturbance at the boundary between the solid mantle and the liquid outer core. |
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Hydrophone
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A device for locating sources of sound under water. |
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Hydrous
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Water-bearing. |
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Hypocenter
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The belowground point of origin of an earthquake. Also called a "focus". |
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Hypothesis
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A proposition set forth as an explanation for the occurrence of some specified group of phenomena, either as a provisional conjecture to guide investigation (working hypothesis) ,or accepted as highly probable in the light of established facts. |
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Infrared radiation
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Part of the electromagnetic spectrum with wavelengths longer than visible light and which correspond to temperature. |
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Inner core
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Earth's innermost, hottest portion, which consists of a solid metallic alloy. |
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Instability
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The quality or state of being unstable; a tendency to behave in an unpredictable, different, or erratic manner. |
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Intensity
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The amount of damage experienced at Earth's surface at a given location during an earthquake. |
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Lag time
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The time interval between one specified event and another. |
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Lahar
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A massive mudflow caused by a volcanic eruption. |
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Landmass
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A large, continuous area of land. |
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Landslide
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The downward falling or sliding of a mass of soil, detritus, or rock on or from a steep slope. |
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Laterally
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Proceeding from or directed to the side, moving parallel to. |
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Latitude
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A geographic coordinate that specifies the north-south position of a point on Earth's surface. |
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Lava flow
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Lava that has poured out of a vent and covers the land surface. |
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Lava fountain
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A fountain-like eruption of lava. |
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Lithosphere
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The solid outer shell of the earth, including Earth's crust and the solid outermost layer of the mantle. |
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Logarithmic
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A relationship between sequential numbers in which each number is multiplied by a specified number to equal the next number. The specified number for the Moment Magnitude Scale is 10. |
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Longitude
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A geographic coordinate that specifies the east-west position of a point on Earth's surface. |
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Ma
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Abbreviation for the Latin "mega annum", used in geologic chronology to denote "millions of years ago". |
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Magma
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Very hot molten rock, which is formed deep below ground by partial melting of Earth's crust and mantle. |
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Magma chamber
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A cavity deep below ground where rising magma collects before an eruption. |
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Magnet
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A body that attracts iron. |
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Magnetic
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Having the properties of a magnet. |
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Magnetic reversal
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Change from one polarity to the opposite polarity. |
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Magnetic field
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A region near a magnet in which a magnetic force acts on any other magnet. |
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Magnetometer
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An instrument for measuring the intensity of Earth's magnetic field. |
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Magnitude
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The amount of energy released during an earthquake. |
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Mantle
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The portion of Earth's interior between the crust and the core. |
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Masonry
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Work constructed by a mason, including stonework and concrete. |
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Mercalli Intensity Scale
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A 1 to 12 scale used to measure intensity of shaking experienced during an earthquake. |
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Meteorologist
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A scientist dealing with the atmosphere and its phenomena, including weather and climate. |
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Microscopic
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Able to be seen only with the aid of a microscope. |
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Millennium
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A period of one thousand years. Plural: millennia. |
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Mineral
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A class of naturally occurring substances of a definite chemical composition and crystalline structure, such quartz or feldspar. |
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Minimum age
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The youngest age a feature can be given the data presented. (The feature could be older.) |
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Molecule
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The smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound. |
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Moment Magnitude Scale
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A logarithmic scale for measuring the energy released during an earthquake. The scale has no upper limit. |
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Objective
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Not influenced by personal feelings, interpretations, or prejudice; based on facts; unbiased. |
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Oceanic
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Occurring on or in the ocean. |
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Oceanic crust
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Part of Earth's lithosphere that surfaces in the ocean basins. |
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Orbit
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The curved path, usually elliptical, described by a planet, satellite, or spacecraft, around a celestial body, such as the earth or sun. |
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Ordovician
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The geologic time period spanning roughly 500 to 440 million years ago. |
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Oscillation
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A single fluctuation between maximum and minimum values of a wave that repeatedly and regularly fluctuates above and below some mean value. |
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Outer core
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A liquid layer about composed of iron and nickel which lies above Earth's solid inner core and below its mantle. |
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P waves
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Seismic waves that oscillate parallel to the direction in which the energy is travelling. Also called "Compression waves". |
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Paleomagnetic
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Magnetic polarization acquired by the minerals in a rock at the time the rock was deposited or solidified. |
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Permian
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The geologic time period spanning roughly 300 to 250 million years ago. |
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Pillow lava
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Lava in the form of a rounded, pillow-shaped mass, resulting from a subaqueous volcanic eruption. |
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Pixel
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A square area over which information is averaged. |
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Plate tectonics
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A theory in which the lithosphere is divided into a number of crustal plates, each of which moves on the plastic asthenosphere more or less independently to collide with, slide under, or move past adjacent plates. |
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Plume
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A concentrated cloud of gas or ash. |
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Polarity
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The positive or negative state in which a body reacts to a magnetic field. |
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Progressive
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Going forward or onward; proceeding step by step. |
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Province
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An administrative division or unit of a country. |
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Pumice
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Cinder with so many gas chambers, it can float on water. |
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Pyroclastic flow
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A cloud of superheated gas, ash, and rock that erupts from a volcano and travels quickly down slope. |
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Pyroclastic flow deposit
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A layer of ash and rock deposited by a pyroclastic flow. These can be many meters thick. |
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Pyroclastic material
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Volcanic debris formed by solidification of erupted lava in air, including volcanic dust, ash, cinders, and pieces of rock. |
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Quaternary
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Earth's current geologic period, covering the past 2.6 million years. Recent sediments from the period are generally not yet consolidated into rock. |
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Radiation
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The process in which energy is emitted as particles or waves. |
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Recede
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To go or move away; retreat; withdraw. |
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Remote sensing
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Monitoring conducted at some distance from the object of study. |
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Richter magnitude scale
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A logarithmic scale for measuring earthquake magnitude, developed in the 1930's. This scale is no longer in use. |
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Rift valley
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A portion of Earth's crust, bounded on at least two sides by faults, that has dropped downward in relation to adjacent crust; a "graben". |
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Rupture
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An offset or displacement of the ground surface that may occur as a result of an earthquake. |
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S waves
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Seismic waves that oscillate perpendicular to the direction in which the energy is travelling. Also called "Shear waves". |
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Satellite
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A device designed to be launched into orbit around Earth, another planet, or the sun. |
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Satellite imagery
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Photographs of Earth or other planets made by means of artificial satellites. |
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Seismic activity
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Underground movements including earthquakes and tremors. |
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Seismic signature
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The pattern or shape of a seismic signal recorded by a seismograph. |
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Seismic waves
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Waves of energy that travel through the ground and cause the shaking experienced during earthquakes. |
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Seismograph
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Scientific instruments that measure seismic activity. |
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Seismologist
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A scientist who studies of earthquakes and the propagation of elastic waves through Earth. |
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Semi-solid
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Having a somewhat firm consistency; more or less solid. |
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Sensor
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A mechanical device sensitive to light, temperature, radiation, or specific chemical compounds, that transmits a signal to a measuring or control instrument. |
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Shakemap
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A map of earthquake intensity generated from data collected from a network of seismographs, with interpolated intervening values. |
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Shear waves
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Seismic waves that oscillate perpendicular to the direction in which the energy is travelling. Commonly called "S waves". |
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Shield volcano
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A low, broad, gently sloping, dome-shaped volcano made up of many layers of basaltic lava that build up through repeated eruptions. |
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Silica
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A fundamental component of volcanic rocks, silica content is the most important factor determining magma viscosity. |
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Silicone
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A nonmetallic element occurring in minerals and rocks and constituting more than one fourth of Earth's crust. |
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Silt
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Soil particles with a grain size smaller than sand and larger than clay. |
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Stratosphere
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The layer of the atmosphere above the troposphere, approximately 16 to 48 km above Earth's surface. |
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Stratovolcano
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A cone-shaped volcano built from alternating layers of pyroclastic material and viscous andesitic lava. (It is also called a composite volcano.) |
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Subduction
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The process by which one tectonic plate is drawn down or overridden by another. |
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Subduction zone
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The region where one lithospheric plate descends beneath another plate and dives into the asthenosphere. |
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Subjective
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Existing in the mind; belonging to the observer rather than to what is being observed. |
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Submersible
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A ship capable of submerging and operating under water; submarine. |
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Summit
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The highest point of a mountain, a peak. |
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Surface waves
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High amplitude seismic waves that travel through the material at Earth's surface. |
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Tectonic plate
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A section of Earth's crust and lithosphere that moves above the asthenosphere. |
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Tephra
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Pieces of material produced by a volcanic eruption. Tephra can include ash, cinder, and volcanic bombs or blocks. |
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Tertiary
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The geologic time period spanning roughly 65 to 2.6 million years ago. |
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Tiltometer
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A scientific instrument used to measure changes in the slope of a volcano. |
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Time interval
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A defined span of time. |
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Topography
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Variations in surface elevation. |
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Transform
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Sliding past each other. |
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Tremor
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Low amplitude earthquake activity associated with the movement of magma. |
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Triangulation
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A technique for determining a location (e.g., an earthquake epicenter) by finding the intersection of three arcs. |
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Triassic
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The geologic time period spanning roughly 250 to 200 million years ago. |
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Troposphere
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The lowest layer of the Earth's atmosphere. |
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Tsunami
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A huge ocean wave generated by earthquakes or violent volcanic eruptions. |
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Unconsolidated
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Not yet hardened into rock, e.g. ocean, lake, or river sediments. |
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USGS
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United States Geological Survey. |
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Vent
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An opening in the surface of a volcano through which steam, gas, or lava can flow. |
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Viscosity
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A fluid's resistance to flow. The viscosity of magma is determined by its chemistry and temperature. |
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Viscous
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Having a relatively high resistance to flow; thick and sticky. |
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Visible light
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A small fraction of the radiation wavelength spectrum that we see as colors. |
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Volcano
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A cone that forms above and around a vent as erupted volcanic materials accumulate. |
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Volcanologist
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A scientist who studies volcanoes. |
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Volcanology
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The scientific study of volcanoes. |
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Vulcan
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The ancient Roman god of fire. |
