active volcano

A volcanic system with current, measurable activity.


Fragments of volcanic rock less than 2 mm in diameter.


To take in.


The envelope of gases surrounding a planet, e.g., Earth's atmosphere consists of approximately 80 percent nitrogen and 20 percent oxygen.

background levels

At typical levels.


A giant crater-like depression in Earth's surface caused when a magma chamber collapses after a rapid, voluminous volcanic eruption.

carbon dioxide

A molecule consisting of one carbon atom and two oxygen atoms (CO2). Its gas is denser than air.

continental crust

The part of Earth's crust that includes and underlies the continents and continental shelves. The continental crust averages about 40 km thick.

continental plate

A tectonic plate that includes continental crust, which is less dense than oceanic crust.


The outermost and thinnest of the solid Earth's layers, which consists of rocky material that is less dense than the rocks of the mantle below.


The average mass per unit volume of a substance.

dome (lava dome)

A mound of viscous, gas-poor lava erupted from a volcanic vent. Because its viscosity prevents the lava from flowing far from the vent, it cools in a domed shape.


A sudden motion or trembling of Earth's crust caused by the passage of seismic waves radiated from a fault along which sudden movement has occurred.

earthquake swarm

A series of earthquakes occurring around the same time, typically near a volcano.


The point on the surface of Earth directly above the subsurface source of an earthquake.

escarpment (scarp)

A steep cliff, either above or below sea level.


A rock fracture or fracture zone along which there has been movement.


A vent on Earth's surface that emits volcanic gas.

gas (volcanic)

Steam (water gas), carbon dioxide, sulfur dioxide, and other gases that are dissolved in magma. When magma erupts onto Earth's surface, it releases these gases. Volcanic gas also can issue directly through fumaroles and soil.


The study of Earth, its history, its composition, its structure, and the dynamic processes that shape it.


The study of Earth's physical properties.


Heat sources under Earth's surface.


A hot spring that can erupt water and steam. Geysers typically occur when geothermal processes keep fluids in a confined area of a fracture or vent at high temperature and under high pressure.

global positioning system (GPS)

A system of satellites, computers, and receivers that can determine the location (latitude and longitude) of a receiver on Earth.


All the water contained in the spaces within the subsurface.

hot spot

A fixed point on the Earth's surface with long-lived volcanism.

hydrogen sulfide

H2S gas. It is toxic, flammable, and smells like rotten eggs.


Of or caused by the circulation of hot water due to thermal processes in Earth's crust.

hydrothermal fluids

Hot water, steam, and other gases trapped in fractured or porous rocks underneath Earth's surface.kilometer (km)A unit of length equal to 1,000 meters, or 0.62 miles.


Magma when it erupts on Earth's surface.

lava flow

Rivers of magma that travel over Earth's surface during a volcanic eruption.


The outer layer of solid rock that includes the crust and uppermost mantle. This layer, up to 100 km thick, forms Earth's tectonic plates.


Molten or partially molten underground rock.

magma chamber

A reservoir of magma.

magnitude (earthquake)

A measure of the total amount of energy released by an earthquake.


The layer within Earth that lies between the crust and the core.


A unit of length in the metric system equal to 3.28 feet.

mud pot

A pool of hot water and fine sediment through which steam, water, or volcanic gas escape.


One of several large, interlocking, mobile pieces of Earth's lithosphere.


Relatively level high ground.


Low energy electromagnetic radiation, with long wavelengths and low frequencies.


Any naturally formed aggregate of one or more minerals, such as granite, shale, or marble.


Relating to or caused by an earthquake or earth vibration.


An instrument that detects and records the vibrations of Earth.


The study of earthquakes and other seismic waves.


The compound silicon dioxide (SiO2). Silica is an important component of many rocks and minerals. It can be found in several forms, including quartz and opals.

subduction zone

The zone of convergence of two tectonic plates, one of which usually overrides the other.


A sudden sinking or gradual downward settling of Earth's surface with little or no horizontal motion.

tectonic plate

One of several large, mobile pieces of Earth's lithosphere adjoining other plates along zones of seismic activity.


The process or result of raising a portion of Earthàs crust through tectonic mechanisms.


An opening such as a fissure, fracture, crack, or hole in Earth's crust through which magma and gas escape.


Having a thick consistency somewhere between a solid and a liquid. The more viscous a material, the more resistant it is to flow.

volcanic center

A region of related volcanoes or volcanic activity.


A vent or fissure in the Earth's surface through which molten lava, ash, and gases are ejected. It is also the name for the structure, usually conical, formed by the materials ejected from the vent or fissure.


A scientist who studies volcanoes.