Glossary main content.



The interval in Earth's history between 3.8 billion and 2.5 billion years before present.


Photosynthetic, oxygen-producing microbes.


A negatively charged atomic particle.


A substance composed of atoms having an identical number of protons in each nucleus. An element cannot be separated into simpler substances by chemical means. A trace element is one present in only small quantities, less than 0.1 percent by weight.


The study of physical properties of Earth.

Huronian Supergroup

A rock formation built up originally of sediments deposited between about 2.5 and 2.2 billion years ago


One or more atoms of the same chemical element that differ in atomic weight because they have different numbers of neutrons. The atomic weight of the isotope is written in superscript to the left of the chemical symbol, such as 14C.


Single-celled, microscopic organisms, which include bacteria, algae and fungi.


A naturally-occurring, homogeneous inorganic element or compound having a definite chemical composition and orderly internal structure, crystal form, and characteristic chemical and physical properties.


The smallest unit of matter into which an element or a compound can be divided and still retain its chemical and physical properties. It consists of a single atom or group of like or different atoms bonded together by chemical forces.

ozone layer

A layer in the stratosphere that contains about ninety percent of Earth's ozone. The ozone layer occurs approximately twenty-five kilometers (sixteen miles) above the surface of Earth.


The chemical process in which green plants (and blue-green algae) make carbohydrates from carbon dioxide and water using sunlight or light as an energy source. Most forms of photosynthesis release oxygen as a byproduct, the chief source of atmospheric oxygen.

sedimentary rock

A rock formed by the consolidation or cementation of sediment particles, or chemically precipitated at the depositional site.

ultraviolet radiation (UV)

A portion of the electromagnetic spectrum. Ultraviolet radiation has a shorter wavelength than visible light.


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.