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abiotic
Non-living features of an ecosystem include space, nutrients, air, and water.
abrasion
A form of mechanical weathering that occurs whenever one rock hits another.
absolute age
The actual age of a material in years.
abyssal plains
Very flat areas that make up most of the ocean floor.
acid rain
Rain that has a pH of less than 5.0.
active volcano
A volcano that is currently erupting or is just about to erupt.
adaptation
A trait that an organism inherits that helps it survive in its natural environment.
adaptive radiation
An explosion in the diversity of species as vacant niches are filled. This often occurs after a mass extinction.
advection
Horizontal movement of a fluid or the transport of a substance in the flow.
air mass
A large mass of air with the same temperature and humidity characteristics.
air pressure
The force of air pressing on a given area.
albedo
The amount of light that reflects off a surface; snow and ice have high albedo.
alkaline
Also called basic. Substances that have a pH of greater than 7.0.
alluvial fan
Curved, fan-shaped, coarse-sediment deposit that forms when a stream meets flat ground.
alpine (valley) glacier
A glacier found in a valley in the mountains.
altitude
Distance above sea level.
amber
Fossilized tree sap.
amino acid
Organic molecules that are the building blocks of life.
amniote egg
An egg that contains all the nutrients needed for the developing embryo and is protected by a shell.
amplitude
The height of a wave from the center to the top of the crest (or the bottom of the trough).
anticline
A fold that arches upward; older rocks are in the center and younger rocks are at the outside.
anticyclone
Wind system that rotates around a high pressure center.
aphotic zone
The zone in the water column deeper than 200 m where sunlight does not penetrate.
apparent polar wander
The path on the globe showing where the magnetic pole appeared to move over time.
aquaculture
Agriculture of the sea; farmed fish, seafood and aquatic plants.
aquifer
A layer of rock, sand, or gravel that holds large amounts of ground water.
arch
An erosional landform that is produced when waves erode through a cliff.
asterism
A group or cluster of stars that appear close together in the sky.
asteroid
Rocky objects larger than a few hundred meters that orbit the Sun.
asteroid belt
Region between the orbits of Mars and Jupiter where many asteroids are found.
astronomer
Scientists who study the universe beyond Earth.
atmosphere
The layer of gases that surrounds a planet.
atom
The smallest unit of a chemical element.
atomic mass
The number of protons and neutrons in an atom.
aurora
A spectacular light display that occurs in the ionosphere near the poles.
avalanche
Mass of snow that suddenly moves down a mountain under the influence of gravity.
axis
An imaginary line that runs from the North Pole to South Pole, and it includes the center of Earth.