Glossary

‘A’a

A lava surface that is jagged and sharp.

Accidental host

A host that gets sick with a disease from a pathogen, but cannot transmit that disease.

Accretion

The process of creating larger bodies from several smaller ones.

Achondrite

A stony meteorite that does not contain spherical chondrules, or collections of minerals.

Aerial fuels

Fuel for fires found higher than 6 ft (1.8 m) above the surface of the ground.

Aerobiology

A branch of biology in which scientists study the microbes in the air.

Aerosol

A small droplet or gas molecule in the atmosphere.

Aftershock

A smaller earthquake that occurs in the same area, but after a major earthquake. Aftershocks may be nearly as large as the main shock.

Air mass

A body of air with a set of characteristics that is distinct from the air around it.

Albedo

Reflectiveness of a surface. Surfaces with high albedo are strongly reflective.

Algal bloom

Rapid reproduction of algae in a body of water.

Alluvial fan

A fan‐shaped deposit of debris left behind when a steep stream changes slope and starts flowing on flat land. Debris carried in the fast‐moving steep stream can no longer be carried by the slower‐moving stream on flat land, and thus deposits most of its sediment as an alluvial fan.

Alpine glacier

A glacier found in a mountainous region.

Alternative fuels

Fuels other than fossil fuels. This includes nuclear, wind, tidal, hydroelectric, and geothermal energy.

Amplitude

The height of a wave, measured from the midpoint to the top (crest) or bottom (trough) of the wave.

Andesite

An extrusive or volcanic igneous rock. It is typically gray or brown in color and crystals may be visible within the rock. Andesite has between 55% and 69% silica.

Anthropogenic

Created by humans.

Antibiotic resistance

The ability of bacteria to resist antibiotic treatment. Antibiotic resistance is a growing problem because many strains of bacteria are now resistant to most antibiotics, so people infected with these bacteria are very difficult or impossible to treat.

Anticyclone

Rotation around a high pressure system. The rotation is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as a result of the Coriolis effect.

Aphanitic

An igneous rock texture that describes a rock composed primarily of microscopic crystals.

Aphelion

The Earth's farthest distance from the Sun in its elliptical orbit.

Archaea

A group of single‐celled organisms whose cells lack a defined nucleus that have distinct molecular characteristics separating them from bacteria. They are among the most ancient life forms on the planet, and can inhabit a number of environments that are inhospitable to other life forms.

Aspect

The direction a slope faces.

Asteroid

A rocky or stony body in space that is smaller than a dwarf planet.

Asthenosphere

The mechanical layer of the Earth found directly beneath the lithosphere. The asthenosphere is part of the upper mantle compositional layer. It is a plastic solid, which means it is flexible and able to circulate. Circulation in this layer keeps the tectonic plates in motion.

Astronomical high tide

High tide during new Moon and full Moon phases, when the Sun and the Moon have the greatest effect on tidal levels.

Astronomical unit (AU)

The distance from the Earth to the Sun, approximately 93 million mi (149.6 million km).

Atmosphere

A relatively thin layer of air that surrounds a planet. The Earth's atmosphere is composed of about 78% nitrogen, 21% oxygen, 0.9% argon, and 0.04% carbon dioxide.

Atmospheric pressure

The weight of a column of air from the Earth's surface to the edge of the atmosphere. It is measured in millibars or inches of mercury.

Background noise

Small movements of the Earth caused by things such as the wind, thunder, waves crashing on the shoreline, people walking, traffic, animals, or other everyday activity.

Bacteria

The smallest life forms on the planet, they are composed of cells without a defined nucleus.

Basal cavity

A place where an ice shelf is undercut by seawater.

Basalt

An igneous rock composed of microscopic crystals, which has between 45% and 54% silica (SiO2).

Basaltic

An igneous rock composition that has between 45% and 54% silica.

Base flow

The level of sustained flow in a river during dry or fair weather conditions. It usually represents the amount of water supplied to a stream by groundwater, or water that is stored and migrates below the surface of the Earth.

Base isolation

A construction practice in which the weight of a building is carried on flexible pilings or large casters with roller bearings. This allows the building to remain more or less stationary as the ground beneath it moves during an earthquake. Base isolation is a method used to reduce or eliminate damage to a building in an earthquake.

Biogeochemical cycle

A description of a cycle that describes the interaction of organisms, the Earth, and chemical compounds and how they move through the Earth system.

Biosphere

The component of the Earth system that includes all life on Earth.

Black smoker

A hydrothermal vent on the ocean floor that emits hot, mineral laden water. Black smokers are found at midocean ridges in oceans around the world. They were discovered in the 1970s.

Blast deflection

A strategy to change the trajectory of an asteroid so it does not impact the Earth. Blast deflection uses a nuclear explosion, detonated very near the asteroid, causing the asteroid to remain intact but change its trajectory.

Block

Solid piece of rock that is hurled out of a volcano's crater.

Body wave

Seismic waves that travel through the body of the interior of the Earth.

Bolide

An extraterrestrial body that strikes a larger body, such as a meteor striking the Earth.

Bomb

A blob of molten lava or a chunk of rock hurled through the air by an erupting volcano.

Braided stream

A stream that consists of multiple interwoven channels separated by sand or gravel bars. It is common at the base of a mountain range or in any other setting with high sediment supply and nearly flat terrain.

Bread crust bomb

A bomb in which the surface of the rock is cracked like a crusty loaf of bread.

Building codes

A set of laws that govern how a building must be constructed. Some areas of the world require enhanced building codes to ensure that buildings can survive severe shaking from an earthquake or high winds from hurricanes.

Calculated risk

The decision to live with the hazards in a region, even though you know that a natural disaster could happen in that area.

Caldera

A depression on a volcano, caused by the collapse of the ground surface over an area that magma has evacuated.

Calving

A piece of the front of a glacier or ice sheet breaks off of the main body of ice. If the ice falls into the water it becomes an iceberg, or floating block of ice.

Carbon capture

A collection of techniques that allow the removal of carbon dioxide from flue gas and other industrial sources.

Carbon footprint

An individual or country's contribution to excess carbon dioxide in the atmosphere.

Carbon sequestration

A natural or artificial process by which carbon dioxide is removed from the atmosphere and held in solid or liquid form.

Carbon sink

A reservoir that takes up and stores carbon dioxide. One such reservoir is forest biomass, which is the plant material within a forest.

Cellulose

An insoluble substance, which is the main constituent of plant cell walls and of vegetable fibers such as cotton.

Char

Solid material left after vegetation burns.

Chemolithotrophs

Archaea that derive their nutrients directly from rocks. The rock provides them with electrons for energy production.

Chemosynthesis

The process in which bacteria or other organisms produce food from energy derived from oxidation of inorganic chemicals. Carbon dioxide, hydrogen sulfide, and oxygen combine to create sugar, sulfur, and water.

Chondrite

A type of stony meteorite that contains spherical structures called chondrules.

Chondrule

A spherical structure within a stony meteorite, usually composed of mineral crystals.

Cinder

A small block of hardened lava filled with circular air pockets. Cinder is usually black or red in color. It is the main component of a volcanic feature called a cinder cone.

Climate

The prevailing or typical weather patterns for a given region based on...