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Glossary

active volcano

A volcanic system with current, measurable activity.ashFragments of volcanic rock less than 2 mm in diameter.

assimilate

To take in.

atmosphere

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

background levels

At typical levels.

caldera

A giant craterlike depression in Earthàs 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 Earthàs 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.

crust

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

density

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.

earthquake

A sudden motion or trembling of Earthàs 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.

epicenter

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.

fault

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

fumarole

A vent on Earthàs 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 Earthàs surface, it releases these gases. Volcanic gas also can issue directly through fumaroles and soil.

geology

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

geothermal

Heat sources under Earthàs surface.

geyser

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.

groundwater

All the water contained in the spaces within the subsurface.hot spotA fixed point on the Earthàs surface with long-lived volcanism.hydrogen sulfideH2S gas. It is toxic, flammable, and smells like rotten eggs.

hydrothermal

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

hydrothermal fluids

Hot water, steam, and other gases trapped in fractured or porous rocks underneath Earthàs surface.kilometer (km)A unit of length equal to 1,000 meters, or 0.62 miles.lavaMagma when it erupts on Earthàs surface.

lava flow

Rivers of magma that travel over Earthàs surface during a volcanic eruption.

lithosphere

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.

magma

Molten or partially molten underground rock.magma chamberA reservoir of magma.

magnitude (earthquake)

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

mantle

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

meter

A unit of length in the metric system equal to 3.28 feet.mud potA pool of hot water and fine sediment through which steam, water, or volcanic gas escape.

plate

One of several large, interlocking, mobile pieces of Earthàs lithosphere.

plateau

Relatively level high ground.

radio

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

rock

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

seismic

Relating to or caused by an earthquake or earth vibration.

seismograph

An instrument that detects and records the vibrations of Earth.

seismology

The study of earthquakes and other seismic waves.

silica

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.

subsidence

A sudden sinking or gradual downward settling of Earthàs surface with little or no horizontal motion.tectonic plateOne of several large, mobile pieces of Earthàs lithosphere adjoining other plates along zones of seismic activity.

uplift

The process or result of raising a portion of Earthàs crust through tectonic mechanisms.ventAn opening such as a fissure, fracture, crack, or hole in Earthàs crust through which magma and gas escape.

viscous

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.

volcano

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.

volcanologist

A scientist who studies volcanoes.

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