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Glossary of Terms

Centrifugal Force

The apparent force that causes a spinning object to move away from the center of rotation.


A class of microorganisms that grows in colonies in the ocean. One important class of coral secretes the compound calcium carbonate, forming a hard outer shell. These corals create reefs.


Earth's outer layer. Earth's crust is much thicker on land, where it can be up to 30 miles thick. Under the ocean, the crust is only about 6 miles thick. The crust is generally less dense than the mantle, immediately below.

Decay (Radioactive)

Isotopes of elements are defined as either stable or unstable. Unstable isotopes change over time at a rate called the half-life change, or decay, to other, lighter isotopes. In the process they emit atomic particles and radiation.

Electromagnetic Spectrum

Light and related forms of energy exhibit properties of both waves and particles. In its wave-like manifestation, light energy is characterized by a wave-length. The electromagnetic spectrum is the range of wavelengths over which light energy occurs. The visible spectrum, light that is visible to the human eye, is a small portion of the entire electromagnetic spectrum. The spectrum also includes infrared energy and ultraviolet light, portions of the spectrum humans can't see.


A negatively charged particle contained in the outer shells of atoms.


A class of single-celled marine microorganism. Foraminifera produce a durable shell that is often preserved in sediments. Foraminifera with distinctive shells grow in distinct habitats. This characteristic makes it possible for researchers to pinpoint the location where fossil foraminifera lived. Researchers often study past climate by analyzing isotopes of oxygen preserved unchanged in the shells of fossil foraminifera.

Fossil Beach

The preserved remains of an ancient beach.


A large, permanent mound of ice that flows under its own weight.

Greenhouse Gas

A gas in the atmosphere that absorbs energy in the infrared portion of the electromagnetic spectrum. Such gasses absorb energy that would otherwise pass from Earth's surface into space. Some of this captured energy is eventually transferred back to Earth's surface.

Ice Age

During the last 2.5 millions years, glaciers in the northern hemisphere have advanced and retreated at regular intervals. The periods of maximum advance, when the glaciers have covered large parts of North America, Europe and Asia, are known as ice ages. In between ice ages, glaciers retreat to a remnant in Greenland or disappear completely. These warm periods are known as interglacials.

Ice Sheet

The largest kind of glacier on Earth, also called a continental glacier, reflecting its huge size. Currently there are only three ice sheets, one in Greenland and two in Antarctic. There have been others during ice ages.


Atomic elements are defined by how many protons they contain in their nuclei. Atoms with the same number of protons but a different number of neutrons are isotopes of the same element. Isotopes of the same element behave identically in most chemical reactions. But they have different physical properties. These differences are used to distinguish between isotopes.


The shell of viscous rock between Earth's outer crust and its core. The mantle flows slowly under the the overlying crust causing the Earth's surface to subside or uplift over time. The mantle is nearly 3,000 miles thick.


A mathematical description of a natural phenomena or process. Complicated models with many variables and a lot of data specifying physical conditions require super-computer processing.


The innermost part of an atom.

Parts Per Million

A fraction of a whole constituting one millionth, or one ten thousandth of a percent. The constituents of the atmosphere are frequently described in parts per million by volume.

Peripheral Bulge

When a glacier advances, such as during an ice age, the land underneath subsides due to the mass loading of the ice. Areas nearby, but not directly under the ice, rise up, forming a rim of elevated land called the peripheral bulge.


The geological epoch from the end of the Pliocene, 2.6 million years ago, to the end of the last ice age, 11,700 years ago.


The geologic epoch between 5.3 and 2.6 million years ago. The Pliocene was preceded by the Miocene and followed by the Pleistocene.


An NSF-funded research program to determine accurately the level of the sea during the mid-Pliocene, about 3 million years ago.


Energetic particles or waves that can travel through a vacuum.


Land returning to an earlier state after the retreat or melting of a glacier that caused the land to subside.


Detritus, such as inert sand and clay or dead organisms, that falls to the bottom of a body of water and accumulates over time. In places where sediment is undisturbed by currents or burrowing organisms, the detritus can compact into distinguishable layers. These layers can be counted or chemically tested to determine their age. Chemical and physical analyses of material in the layers can provide information about conditions locally or globally when the sediment dropped to the bottom.

Tectonic Plate

Earth's crust is made of distinct segments, or tectonic plates, that move across the surface of the planet, over the top of the mantle.  These crustal plates, some of which have continents embedded in them, meet along plate boundaries where earthquakes and volcanoes typically occur.