G

galaxy
A very large group of stars held together by gravity; few million to a few billion stars.
Galilean moons
The four largest moons of Jupiter discovered by Galileo.
gamma ray
A penetrating form of electromagnetic radiation.
gas giants
The four large outer planets composed of the gases hydrogen and helium.
gasification
A technology that cleans coal before it is burned, which increases efficiency and reduces emissions.
gemstone
Any material that is cut and polished to use in jewelry.
geocentric model
Model used by the ancient Greeks that puts the Earth at the center of the universe.
Geographic Information System (GIS)
An information system that links data to a particular location.
geologic map
A map showing the geologic features, such as rock units and structures, of a region.
geologic time scale
A division of Earth’s history into blocks of time distinguished by geologic and evolutionary events.
geostationary orbit
A satellite at just the right distance above Earth so that it orbits at the same rate that Earth spins and stays above a single location.
geyser
A fountain of hot water and steam that erupts onto the surface.
gibbous
Phase of the moon when it is more than half lit but not completely full.
glacial erratic
Large boulder with a different rock type or origin from the surrounding bedrock.
glacial striations
Long, parallel scratches carved into underlying bedrock by moving glaciers.
glacial till
Any unsorted sediment deposited by glacial ice.
glaciers
Large sheets of flowing ice.
Global Positioning System (GPS)
A set of satellites that allows a receiver to know its exact location.
global warming
Warming of Earth’s atmosphere because of the addition of greenhouse gases. The increase in average global temperature is caused by human activities.
globular cluster
Groups of tens to hundreds of thousands of stars held together by gravity.
gradient
The slope of a stream.
Great Red Spot
An enormous, oval-shaped, long-lived storm on Jupiter.
Green Revolution
Changes in the way food is produced since World War II that have resulted in enormous increases in production.
greenhouse effect
The trapping of heat by greenhouse gases in the atmosphere; moderates temperatures.
greenhouse gas
Gases such as carbon dioxide and methane that absorb and hold heat from the sun’s infrared radiation in the atmosphere.
greenstone
A metamorphosed volcanic rock that forms at a subduction zone.
groin
Long, narrow piles of stone or timbers built perpendicular to the shore to trap sand.
ground moraine
Thick layer of sediment deposited under a glacier.
groundwater
Fresh water that moves through pore spaces and fractures in soil and rock beneath the land surface.
gyre
Five loops created by surface ocean currents.