Abyssal Ocean
The deep ocean that lies in water depths of 4,000 meters or deeper.
Acasta Gneiss
The oldest rock dated on Earth, found in the Northwest Territories, Canada.
Accretion
The process by which solid bodies gather together to form a continent or planet.
Aerobic
Growing or thriving only in the presence of oxygen.
Aerosols
Fine particles or liquid droplets suspended in the atmosphere, some of which are byproducts of industrial pollution.
Air Mass
A large, widespread body of air that has the same properties of humidity, temperature, and density (with only slight variations) throughout.
Albedo
The proportion of incoming solar radiation reflected back into space either by the Earth’s surface or by particles in the atmosphere.
Amino Acid
An organic compound containing an amino (-NH2) and a carboxyl (-COOH) that link together to form proteins.
Ammonia
(NH3) A colorless, pungent gas, used to manufacture fertilizers and a wide variety of nitrogen-containing inorganic and organic chemicals.
Ammonium
The chemical ion (NH4+) derived from ammonia that does not appear in a free state, but forms salts and compounds analogous to those of the alkali metals.
Anaerobic
The condition of the absence of free oxygen.
Analog Model
A physical model that reproduces aspects of a specific phenomenon.
Andesite
A dark-colored, fine-grained volcanic rock that is intermediate in composition (with respect to silica, magnesium and iron) between rhyolite (high silica, low magnesium and iron) and basalt (low silica, high magnesium and iron).
Ankylosaurus
A genus of squat, quadrupedal, armored dinosaurs of the Cretaceous Period.
Anorthite
A calcium-oxide-rich plagioclase feldspar mineral, typically white, that occurs in igneous rocks.
Anorthosite
An intrusive igneous rock that consists chiefly of the mineral plagioclase feldspar.
Archean
The interval in Earth’s history between 3.8 billion and 2.5 billion years before present.
Atmosphere
An atmosphere is a layer of gas or layers of gases that envelope a planet, and is held in place by the gravity of the planetary body.
Atmospheric
Circulation movement within the atmosphere caused by differences in air pressure.
Atom
The smallest unit of an element that can still retain all of the characteristics and properties of that element, an atom consists of a dense, central, positively-charged nucleus surrounded by a cloud of electrons.
Atomic Weight
The total number of protons and neutrons in an atom. Also, the mass of an atom relative to the mass of the carbon atom having six protons and six neutrons, which is taken as 12.