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Glossary
aftershock: An earthquake wave that follows
the main shock of an earthquake.
air mass: A body of air with the same temperature and moisture throughout.
air pre s s u re: The weight of air.
ash: Fragmented volcanic material with particles that measure less than 2 mm in diameter.
ash fall: The ejection of volcanic materials into
the atmosphere and the settling of these materials over a wide area of the surface of the earth.
a s t h e n o s p h e re: The layer of the mantle that
lies directly below the lithosphere and flows,
like taffy.
a t m o s p h e re: The thin blanket of gases that surrounds the earth.
b a rometer: An instrument used to detect and
measure changes in air pressure.
body wave: An earthquake wave that travels
through the body of the earth rather than on its
surface. See P-wave; S-wave. See also surface
wave.
brittle: Describes objects that break easily when
a force is applied to them. See also ductile.
caldera: A large, steep-sided, circular or oval
volcanic depression that forms when magma
retreats or erupts from a shallow underground
magma chamber; with no magma to support the
ground above it, the overlying rock collapses
and the caldera is formed.
cinder cone: A small (less than 400 meters high),
cone-shaped volcano made of broken rocks or
blobs of hardened lava, called “cinders,” that
accumulate around the volcanic vent. See also
composite volcano; shield volcano.
climate: Weather conditions that are characteristic of a region or of a particular place over a
long period of time. See also weather.
cloud: Billions of tiny droplets of water or ice
and dust particles visible from the ground.
cold front: The leading edge of a cold air mass.
It occurs when a cold air mass pushes a warm
air mass ahead of it. See also weather fro n t .
composite volcano: A volcano that forms from
alternating eruptions of viscous lava and broken
rock. It is steep near the summit and flat
toward the base. See also cinder cone; shield
volcano.
condensation: The process by which water
vapor changes from a gas to a liquid.
convection: The process by which heat moves
through air or water.
convection current: A circulating flow of air or
water resulting from temperature differences;
also called a convection cell.
c o re: The earth’s innermost layers, consisting of
a liquid iron outer core and a solid iron-nickel
inner core. See also crust; mantle.
crater: A small, bowl-shaped hole that forms
when rock explodes from a volcano during an
eruption.
c rust: The earth’s outer layer; the coolest and
least dense layer of the earth. See also core;
mantle.
cumulonimbus cloud: An exceptionally dense,
tall cloud that is often accompanied by heavy
rain, lightning, thunder, and sometimes hail.
The top is often shaped like an anvil. It is also
known as a thunderstorm cloud.
current: The movement of a gas or liquid in a
definite direction. An ocean current is one
example.
cyclone: A massive, rotating storm that forms in
the Indian Ocean and off the coast of Australia.
It is equivalent to a hurricane in the Atlantic
Ocean, eastern Pacific Ocean, or Caribbean
Sea. See also hurricane; typhoon.
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GLOSSARY
dam: A barrier built to restrict the flow of
water.
deep-sea trench: A deep, narrow depression in
the seafloor.
ductile: Describes objects that bend, stretch, or
flow when a force is applied to them. See also
brittle.
greenhouse gases: Gases in the earth’s atmosphere, such as water vapor and carbon dioxide,
that absorb energy radiated from the earth and
prevent its escape into space.
Gulf Stream: A warm-water ocean current that
flows north along the East Coast of the United
States.
earthquake: Vibrations in the earth caused by
the sudden release of energy, usually as a result
of the movement of rocks along a fault.
easterlies: Global winds that flow from the east
to the west. See also trade winds; westerlies.
El Niño: An unusually warm flow of surface
water that occurs in the Pacific Ocean about
every 3 to 7 years.
epicenter: The point on the surface of the earth
directly above the focus of an earthquake.
evaporation: The process by which water
changes from a liquid to an invisible gas called
water vapor.
hot spot: A localized zone of melting in the
mantle that is fixed under a plate.
h u rricane: A massive rotating storm that forms
north of the equator in the Atlantic Ocean,
eastern Pacific Ocean, or Caribbean Sea when
warm air rises over tropical waters. Has wind
speeds of 119 kilometers per hour or more. See
also cyclone; typhoon.
fault: A fracture in bedrock, along which blocks
of rock on opposite sides of the fracture move.
flash flood: A sudden and destructive rush of
water, such as along a narrow gully.
focus: The location where the rupture of an
earthquake begins and energy is released.
geologist: A scientist who studies the history
and structure of the earth as it is recorded in
rocks.
global winds: Giant convection currents that
circulate within the Northern and Southern
Hemispheres of the earth. See also easterlies;
trade winds; westerlies.
globe: A spherical model of the earth.
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igneous rock: A solid earth material that forms
when magma or lava cools and crystallizes on
or below the earth’s surface. See also metamorphic rock; sedimentary rock.
intensity: A measure of the damage done by an
earthquake. Determined on the basis of the
earthquake’s effect on people, structures, and
the natural environment.
jet stream: A long, narrow current of very
strong winds in the upper troposphere.
lahar: Mudflow that occurs when rain falls
through clouds of ash or when rivers become
choked with falling volcanic debris.
land breeze: The flow of air from land to water.
See also sea breeze.
landform: A physical feature of the earth’s surface, such as a mid-ocean ridge, a trench, or a
mountain.
G LOSSARY
lava dome: A bulbous, steep-sided dome that
forms at the top of a volcano when thick, relatively “cold” magma emerges from the volcanic
opening.
lava flow: Lava that flows quickly over the surface of the earth and covers a wide area.
levee: A constructed or natural embankment
along the sides of a river meant to protect the
surrounding area from flooding.
lithosphere: The cool, solid outer shell of the
earth. It consists of the crust and the rigid
uppermost part of the mantle and is broken up
into segments, or plates.
mudflow: A powerful “river” of mud that forms
when debris, such as from a volcanic eruption,
moves into a stream or river.
magnitude: A measure of the total amount of
energy released at the source of the earthquake.
mantle: The layer of the earth beneath the
crust. It is about 2900 km thick, and it makes
up about 83 percent of the earth’s interior. See
also c o re; cru s t .
map: A representation of the earth or a part of
the earth, usually on a flat surface.
metamorphic rock: A solid earth material that
forms when any rock type is changed by the
earth’s high temperature and pressure. See also
igneous rock; sedimentary rock.
m e t e o rologist: A scientist who studies the
earth’s atmosphere and who monitors, studies,
and forecasts weather.
Mid-Atlantic Ridge: A zone of intense earthquake and volcanic activity that runs down the
middle of the Atlantic Ocean floor.
mid-ocean ridge: A mountainlike landform that
develops when plates separate and new ocean
lithosphere forms.
mineral: A naturally formed, inorganic solid
composed of one or more elements.
model: A representation that is used to study
objects, ideas, or systems that are too complex,
distant, large, or small to study easily firsthand.
occluded front: A boundary that occurs when
both a cold and a cool air mass collide with a
warm air mass, which becomes trapped and
lifted between them. See also weather fro n t .
ozone layer: A part of the stratosphere that contains a special form of oxygen (O3) that traps
the sun’s harmful ultraviolet radiation and
keeps it from reaching the troposphere.
natural catastrophic event: A powerful and
often dramatic force of nature that changes the
earth’s surface and atmosphere; includes earthquakes, volcanoes, and intense storms such as
hurricanes and tornadoes.
NOAA: National Oceanic and Atmospheric
Administration.
NWS: National Weather Service.
pillow lava: Balloonlike mounds that form when
lava erupts under water or empties into the
ocean.
plate: A large, mobile segment of the earth’s
lithosphere.
plate boundary: A place where pieces of the broken lithosphere meet. Boundary types include
sliding (transform), colliding (convergent), and
spreading (divergent).
plate tectonics: A theory that the lithosphere is
broken into segments, or plates, that “float” on
the asthenosphere, and that interactions among
these plates are associated with earthquakes
and volcanic activity and form mid-ocean
ridges, trenches, mountains, and chains of volcanic islands.
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GLOSSARY
P-wave: A primary (compressional) earthquake
wave that travels through the body of the earth;
so named because it is the first wave to reach a
seismograph station during an earthquake. See
also S-wave.
radiation: The process by which energy is transferred from one object, such as the sun, to
another object without the space between them
being heated.
Ring of Fire: A zone of intense earthquake and
volcanic activity that encircles the Pacific
Ocean basin; also called the Circum-Pacific Belt.
risk: Exposure to the chance of injury or loss.
rock: A solid earth material made of various
minerals. Igneous, metamorphic, and sedimentary are three types of rock.
rock cycle: The process by which earth materials transform from one rock type into another.
sea breeze: The flow of air from water to land.
See also land bre e z e .
sedimentary rock: A solid earth material composed of compacted and cemented sediments or
of particles of various sizes. See also igneous
rock; metamorphic rock.
seismic wave: A wave generated by earthquake
vibrations. See P-wave; S-wave; surface wave.
seismogram: The record made by a seismograph; the paper on which earthquake waves
are recorded.
seismograph: An instrument that detects,
records, and measures the vibrations produced
by an earthquake.
seismologist: A scientist who studies
earthquakes.
seismology: The study of earthquakes.
shield volcano: A wide, broadly sloping volcano
that forms from runny lava. It is the largest of volcanoes. See also cinder cone; composite volcano.
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solar energy: Energy from the sun.
stationary front: A boundary that occurs when
two air masses move close to one another, but
neither has enough force to move the other;
they both remain fixed in place. See also
weather front.
storm surge: An unusually high water level,
caused primarily by strong winds, especially
those associated with a hurricane.
stratosphere: The layer of the earth’s atmosphere above the troposphere. It has very little
water vapor or other gases; protective ozone
layer forms there.
surface wave: An earthquake wave that travels
on or near the surface of the earth.
S-wave: A secondary earthquake wave; so
named because it travels slower than a primary
wave and is the second wave to reach the seismograph station after an earthquake. It travels
through the body of the earth as a series of
crests and troughs. See also P-wave.
temperature: A measure of how hot or cold a
material is; an indication of the amount of heat
energy that has been absorbed by the material.
t h u n d e r s t o rm: A disturbance in the earth’s
atmosphere that involves lightning, thunder,
and sometimes gusty surface winds with heavy
rain and hail.
t o rnado: A violent windstorm that spirals
around a rotating column of air (the vortex)
and moves in a narrow path over land. See also
vortex.
t o rnado warning: A communication to the public that a tornado has been seen by someone or
detected by radar.
t o rnado watch: A communication to the public
that tornadoes are possible. In other words,
thunderstorms with high winds and rain that
may produce a tornado are in the area.
G LOSSARY
trade winds: Global winds that flow toward the
equator, turning west as they go. See also e a s terlies; westerlies.
troposphere: The layer of the atmosphere closest to the earth, in which air moves in all directions. It is where most of the earth’s weather
takes place.
tsunami: A huge sea wave caused by underwater earthquakes or, more rarely, by volcanoes.
typhoon: A massive rotating storm that forms
north of the equator in the western Pacific
Ocean. It is equivalent to a hurricane in the
Atlantic Ocean, eastern Pacific Ocean, or
Caribbean Sea. See also cyclone; hurricane.
weather: The state of the atmosphere at a particular time and place. See also climate.
weather front: A boundary that forms when air
masses meet that have different temperature,
pressure, and humidity conditions. See cold
f ront; occluded front; stationary front; warm
front.
weather satellite: An instrument that orbits
the earth, taking photographs and collecting
measurements.
westerlies: Global winds that flow from the west
to the east. See also easterlies; trade winds.
upwelling: The rising of cold, deep water from
an ocean bottom.
viscosity: The tendency to resist flow; a property of liquids.
volcano: A landform, usually cone-shaped, produced by a collection of erupted material
around a vent, or opening, in the surface of the
earth and through which gas and erupted material pass.
volcanologist: A scientist who studies volcanoes
and volcanic phenomena.
vortex: The movement of liquids or gases in a
spiral around a central axis. In a storm, it is the
calm center area around which clouds spiral.
w a rm front: A boundary that occurs when a
moving, warm air mass overrides a cold air
mass ahead of it. See weather fro n t .
water cycle: The movement and exchange of
water between the earth’s land, atmosphere,
and oceans.
water vapor: Water that has evaporated into a
gas.
waterspout: A rotating column of air over a
large body of water.
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