Glossary
active volcano
A volcanic system with current, measurable activity.
ash
Fragments of volcanic rock less than 2 mm in diameter.
assimilate
To take in.
atmosphere
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.
caldera
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.
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.
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 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.
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 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.
geology
The study of Earth, its history, its composition, its structure, and the dynamic processes that shape it.
geophysics
The study of Earth's physical properties.
geothermal
Heat sources under Earth's 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 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.
hydrothermal
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.
lava
Magma when it erupts on Earth's surface.
lava flow
Rivers of magma that travel over Earth's 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 chamber
A 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 pot
A pool of hot water and fine sediment through which steam, water, or volcanic gas escape.
plate
One of several large, interlocking, mobile pieces of Earth's 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 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.
uplift
The process or result of raising a portion of Earthàs crust through tectonic mechanisms.
vent
An opening such as a fissure, fracture, crack, or hole in Earth's 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.