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Active volcano

A volcano that has erupted within the past 10,000 years and/or is expected to erupt within the next 10,000 years.


The maximum displacement from a zero value during one complete oscillation.


A dark volcanic rock with a silica content of about 60%. Andesite is the second most abundant volcanic rock in Earth's crust.


A pronounced deviation from the accepted normal value or condition.

Ash fall

A layer of volcanic ash that falls from an eruption plume.


A soft layer of the Earth's upper mantle lying just below the lithosphere.


A layer of gases surrounding Earth.


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.


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.

Body waves

Seismic waves that travel through solid rock, including S waves and P waves.


A large bowl-shaped crater formed by explosive volcanic eruptions and/or by collapse of a volcano's surface material into the magma chamber.


To determine, check, or rectify the graduation of any instrument or system giving quantitative measurements.


"Common Era" is the most commonly used year-numbering system worldwide. The years are equivalent to the Anno Domini (AD) system, which it replaces.

Centrifugal force

The outward force that draws a rotating body away from the center of rotation.


Light pieces of lava, formed from magma that is ejected, cooled, fragmented in the air, and falls back down. Also called "scoria".

Cinder cone volcano

A small, steep-sided volcano composed primarily of cinder.


An instrument that detects magnetic polarity, used for determining directions by means of a freely rotating magnetized needle that indicates magnetic north.

Composite volcano

A cone-shaped volcano built from layers of pyroclastic material and lava. (It is also called a Stratovolcano.)


A pure substance composed of two or more elements whose composition is constant.


Pushing together.

Compression waves

Seismic waves that oscillate parallel to the direction in which the energy is traveling. Commonly called "P waves".


One of Earth's main landmasses including, Europe, Asia, Africa, North America, South America, Australia, and Antarctica.

Continental crust

The layer of rocks that forms the continents and the areas of shallow seabed close to their shores.


The transfer of heat by the circulation or movement of the heated parts of a liquid or gas.

Convective eruption column

A shaft of heat and particles rising up from an erupting volcano.


Colliding, coming together.


A steep-sided, bowl-shaped hollow at the top of a volcano.


The geologic time period spanning roughly 145 to 65 million years ago.

Cross section

A two-dimensional view of a three-dimensional object. A "slice" through an object.


The thin, outermost layer of Earth, which comprises the continents and ocean floors.


Change in size or shape of an object resulting from an applied force.


The geologic time period spanning roughly 400 to 350 million years ago.


The apparent bending, spreading out, or other changes in seismic wave direction.


The removal of one mass and replacement by another within a given space.


Separating, breaking apart.


A mound of lava and other volcanic material that has built up around the vent of a volcano.

Dormant volcano

An active volcano that is not currently erupting.


Vibrations induced in Earth's crust by the abrupt rupture and rebound of rocks in which strain has been slowly accumulating.


Electronic Distance Meter


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.


To release or discharge into the environment.


The capacity to do work.


The point on Earth's surface directly above the origin of disturbance that creates an earthquake.


Wandering, not fixed.


The ejection of material from a volcano or geyser.


Pulling apart.

Extinct volcano

A volcano that has not erupted in the past 10,000 years and is not expected to erupt within the next 10,000 years.


A fracture along which pieces of Earth's crust move relative to one another.

Fault trace

The line where a fault intersects Earth's surface, either on a horizontal plane or in vertical cross-section.


A crack through which magma erupts onto Earth's surface.


The belowground point of origin of an earthquake. Also called a "hypocenter".

Fold mountains

Mountains formed by folding of Earth's crust during the convergence of tectonic plates.

Foot wall

The block of rock below the fault trace or fault plane.


An influence that causes a mass to accelerate.


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.


Resistance generated by rubbing two surfaces together.


A vent or opening in Earth's surface producing gas or steam.


A large mass of ice formed from many years of snow, which flows very slowly over land.

Global positioning system

A space-based global navigation satellite system that provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis.


Abbreviation for global or geographic positioning system.


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.


Composed chiefly of orthoclase, feldspars, and quartz, usually with lesser amounts of one or more other minerals, as mica, hornblende.

Ground deformation

Change in shape of the ground surface caused by moving magma.

Hanging wall

The block of rock above the fault trace or fault plane.

Hot spot

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.


A device for locating sources of sound under water.




The belowground point of origin of an earthquake. Also called a "focus".


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.

Infrared radiation

Part of the electromagnetic spectrum with wavelengths longer than visible light and which correspond to temperature.

Inner core

Earth's innermost, hottest portion, which consists of a solid metallic alloy.


The quality or state of being unstable; a tendency to behave in an unpredictable, different, or erratic manner.


The amount of damage experienced at Earth's surface at a given location during an earthquake.

Lag time

The time interval between one specified event and another.


A massive mudflow caused by a volcanic eruption.


A large, continuous area of land.


The downward falling or sliding of a mass of soil, detritus, or rock on or from a steep slope.


Proceeding from or directed to the side, moving parallel to.


A geographic coordinate that specifies the north-south position of a point on Earth's surface.

Lava flow

Lava that has poured out of a vent and covers the land surface.

Lava fountain

A fountain-like eruption of lava.


The solid outer shell of the earth, including Earth's crust and the solid outermost layer of the mantle.


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.


A geographic coordinate that specifies the east-west position of a point on Earth's surface.


Abbreviation for the Latin "mega annum", used in geologic chronology to denote "millions of years ago".


Very hot molten rock, which is formed deep below ground by partial melting of Earth's crust and mantle.

Magma chamber

A cavity deep below ground where rising magma collects before an eruption.


A body that attracts iron.


Having the properties of a magnet.

Magnetic reversal

Change from one polarity to the opposite polarity.

Magnetic field

A region near a magnet in which a magnetic force acts on any other magnet.


An instrument for measuring the intensity of Earth's magnetic field.


The amount of energy released during an earthquake.


The portion of Earth's interior between the crust and the core.


Work constructed by a mason, including stonework and concrete.

Mercalli Intensity Scale

A 1 to 12 scale used to measure intensity of shaking experienced during an earthquake.


A scientist dealing with the atmosphere and its phenomena, including weather and climate.


Able to be seen only with the aid of a microscope.


A period of one thousand years. Plural: millennia.


A class of naturally occurring substances of a definite chemical composition and crystalline structure, such quartz or feldspar.

Minimum age

The youngest age a feature can be given the data presented. (The feature could be older.)


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.

Moment Magnitude Scale

A logarithmic scale for measuring the energy released during an earthquake. The scale has no upper limit.


Not influenced by personal feelings, interpretations, or prejudice; based on facts; unbiased.


Occurring on or in the ocean.

Oceanic crust

Part of Earth's lithosphere that surfaces in the ocean basins.


The curved path, usually elliptical, described by a planet, satellite, or spacecraft, around a celestial body, such as the earth or sun.


The geologic time period spanning roughly 500 to 440 million years ago.


A single fluctuation between maximum and minimum values of a wave that repeatedly and regularly fluctuates above and below some mean value.

Outer core

A liquid layer about composed of iron and nickel which lies above Earth's solid inner core and below its mantle.

P waves

Seismic waves that oscillate parallel to the direction in which the energy is travelling. Also called "Compression waves".


Magnetic polarization acquired by the minerals in a rock at the time the rock was deposited or solidified.


The geologic time period spanning roughly 300 to 250 million years ago.

Pillow lava

Lava in the form of a rounded, pillow-shaped mass, resulting from a subaqueous volcanic eruption.


A square area over which information is averaged.

Plate tectonics

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.


A concentrated cloud of gas or ash.


The positive or negative state in which a body reacts to a magnetic field.


Going forward or onward; proceeding step by step.


An administrative division or unit of a country.


Cinder with so many gas chambers, it can float on water.

Pyroclastic flow

A cloud of superheated gas, ash, and rock that erupts from a volcano and travels quickly down slope.

Pyroclastic flow deposit

A layer of ash and rock deposited by a pyroclastic flow. These can be many meters thick.

Pyroclastic material

Volcanic debris formed by solidification of erupted lava in air, including volcanic dust, ash, cinders, and pieces of rock.


Earth's current geologic period, covering the past 2.6 million years. Recent sediments from the period are generally not yet consolidated into rock.


The process in which energy is emitted as particles or waves.


To go or move away; retreat; withdraw.

Remote sensing

Monitoring conducted at some distance from the object of study.

Richter magnitude scale

A logarithmic scale for measuring earthquake magnitude, developed in the 1930's. This scale is no longer in use.

Rift valley

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".


An offset or displacement of the ground surface that may occur as a result of an earthquake.

S waves

Seismic waves that oscillate perpendicular to the direction in which the energy is travelling. Also called "Shear waves".


A device designed to be launched into orbit around Earth, another planet, or the sun.

Satellite imagery

Photographs of Earth or other planets made by means of artificial satellites.

Seismic activity

Underground movements including earthquakes and tremors.

Seismic signature

The pattern or shape of a seismic signal recorded by a seismograph.

Seismic waves

Waves of energy that travel through the ground and cause the shaking experienced during earthquakes.


Scientific instruments that measure seismic activity.


A scientist who studies of earthquakes and the propagation of elastic waves through Earth.


Having a somewhat firm consistency; more or less solid.


A mechanical device sensitive to light, temperature, radiation, or specific chemical compounds, that transmits a signal to a measuring or control instrument.


A map of earthquake intensity generated from data collected from a network of seismographs, with interpolated intervening values.

Shear waves

Seismic waves that oscillate perpendicular to the direction in which the energy is travelling. Commonly called "S waves".

Shield volcano

A low, broad, gently sloping, dome-shaped volcano made up of many layers of basaltic lava that build up through repeated eruptions.


A fundamental component of volcanic rocks, silica content is the most important factor determining magma viscosity.


A nonmetallic element occurring in minerals and rocks and constituting more than one fourth of Earth's crust.


Soil particles with a grain size smaller than sand and larger than clay.


The layer of the atmosphere above the troposphere, approximately 16 to 48 km above Earth's surface.


A cone-shaped volcano built from alternating layers of pyroclastic material and viscous andesitic lava. (It is also called a composite volcano.)


The process by which one tectonic plate is drawn down or overridden by another.

Subduction zone

The region where one lithospheric plate descends beneath another plate and dives into the asthenosphere.


Existing in the mind; belonging to the observer rather than to what is being observed.


A ship capable of submerging and operating under water; submarine.


The highest point of a mountain, a peak.

Surface waves

High amplitude seismic waves that travel through the material at Earth's surface.

Tectonic plate

A section of Earth's crust and lithosphere that moves above the asthenosphere.


Pieces of material produced by a volcanic eruption. Tephra can include ash, cinder, and volcanic bombs or blocks.


The geologic time period spanning roughly 65 to 2.6 million years ago.


A scientific instrument used to measure changes in the slope of a volcano.

Time interval

A defined span of time.


Variations in surface elevation.


Sliding past each other.


Low amplitude earthquake activity associated with the movement of magma.


A technique for determining a location (e.g., an earthquake epicenter) by finding the intersection of three arcs.


The geologic time period spanning roughly 250 to 200 million years ago.


The lowest layer of the Earth's atmosphere.


A huge ocean wave generated by earthquakes or violent volcanic eruptions.


Not yet hardened into rock, e.g. ocean, lake, or river sediments.


United States Geological Survey.


An opening in the surface of a volcano through which steam, gas, or lava can flow.


A fluid's resistance to flow. The viscosity of magma is determined by its chemistry and temperature.


Having a relatively high resistance to flow; thick and sticky.

Visible light

A small fraction of the radiation wavelength spectrum that we see as colors.


A cone that forms above and around a vent as erupted volcanic materials accumulate.


A scientist who studies volcanoes.


The scientific study of volcanoes.


The ancient Roman god of fire.