Glossary
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Ga
Billion years ago (abbreviation).
gabbro
A coarse-grained, intrusive mafic igneous rock.
Gaia
The term used for the Earth system, with the implication that it resembles a complex living entity.
galaxy
An immense system of hundreds of billions of stars.
gem
a cut and finished stone ready to be used in jewelry.
gemstone
a mineral that has special value because it is rare and some people consider it beautiful.
gene
An individual component of the DNA code that guides the growth and development of an organism.
genetics
The study of genes and how they transmit information.
geocentric Universe concept
An ancient Greek idea suggesting that the Earth sat motionless in the center of the Universe while stars and other planets and the Sun orbited around it.
geochronology
The science of dating geologic events in years.
geode
A cavity in which euhedral crystals precipitate out of water solutions passing through a rock.
geographical pole
The locations (north and south) where the Earth’s rotational axis intersects the planet’s surface.
geologic column
A composite stratigraphic chart that represents the entirety of the Earth’s history.
geologic history
The sequence of geologic events that has taken place in a region.
geologic map
A map showing the distribution of rock units and structures across a region.
geologic time
The span of time since the formation of the Earth.
geologic time scale
A scale that describes the intervals of geologic time.
geology
The study of the Earth, including our planet’s composition, behavior, and history.
geotherm
The change in temperature with depth in the Earth.
geothermal energy
Heat and electricity produced by using the internal heat of the Earth.
geothermal gradient
The rate of change in temperature with depth.
geothermal region
A region of current or recent volcanism in which magma or very hot rock heats up groundwater, which may discharge at the surface in the form of hot springs and/or geysers.
geyser
A fountain of steam and hot water that erupts periodically from a vent in the ground in a geothermal region.
glacial abrasion
The process by which clasts embedded in the base of a glacier grind away at the substrate as the glacier flows.
glacial advance
The forward movement of a glacier’s toe when the supply of snow exceeds the rate of ablation.
glacial drift
Sediment deposited in glacial environments.
glacial incorporation
The process by which flowing ice surrounds and incorporates debris.
glacial marine
Sediment consisting of ice-rafted clasts mixed with marine sediment.
glacial outwash
Coarse sediment deposited on a glacial outwash plain by meltwater streams.
glacially polished surface
A polished rock surface created by the glacial abrasion of the underlying substrate.
glacial plowing
The process by which flowing ice bulldozes and moves loose sediment.
glacial plucking (or quarrying)
The process by which a glacier breaks off and carries away fragments of bedrock.
glacial rebound
The process by which the surface of a continent rises back up after an overlying continental ice sheet melts away and the weight of the ice is removed.
glacial retreat
The movement of a glacier’s toe back toward the glacier’s origin; glacial retreat occurs if the rate of ablation exceeds the rate of supply.
glacial subsidence
The sinking of the surface of a continent caused by the weight of an overlying glacial ice sheet.
glacial till
Sediment transported by flowing ice and deposited beneath a glacier or at its toe.
glaciation
A period of time during which glaciers grew and covered substantial areas of the continents.
glacier
A river or sheet of ice that slowly flows across the land surface and lasts all year long.
glass
A solid in which atoms are not arranged in an orderly pattern.
glassy igneous rock
Igneous rock consisting entirely of glass, or of tiny crystals surrounded by a glass matrix.
glide horizon
The surface along which a slump slips.
global change
The transformations or modifications of both physical and biological components of the Earth system through time.
global circulation
The movement of volumes of air in the paths that ultimately take it around the planet.
global climate change
Transformations or modifications in Earth’s climate over time.
global cooling
A fall in the average atmospheric temperature.
global positioning system (GPS)
A satellite system people can use to measure rates of movement of the Earth’s crust relative to one another, or simply to locate their position on the Earth’s surface.
global warming
A rise in the average atmospheric temperature.
gneiss
A compositionally banded metamorphic rock typically composed of alternating dark and light layers.
Gondwana
A supercontinent that consisted of today’s South America, Africa, Antarctica, India, and Australia. Also called Gondwanland.
graben
A down-dropped crustal block bounded on either side by a normal fault dipping toward the basin.
gradualism
The theory that evolution happens at a constant, slow rate.
graded bed
a bed that contains coarser sediment (sand) at the base and finer sediment (mud) at the top.
grain
A fragment of a mineral crystal or group of crystals.
grain rotation
The process by which rigid, inequant mineral grains distributed through a soft matrix may rotate into parallelism as the rock changes shape owing to differential stress.
granite
A coarse-grained intrusive silicic igneous rock.
gravitational spreading
A process of lateral spreading that occurs in a material because of the weakness of the material; gravitational spreading causes continental glaciers to grow and mountain belts to undergo orogenic collapse.
graywacke
An informal term used for sedimentary rock consisting of sand-sized up to small-pebble-sized grains of quartz and rock fragments all mixed together in a muddy matrix; typically, graywacke occurs at the base of a graded bed.
greenhouse conditions (greenhouse period)
Relatively warm global climate leading to the rising of sea level for an interval of geologic time.
greenhouse effect
The trapping of heat in the Earth’s atmosphere by carbon dioxide and other greenhouse gases, which absorb infrared radiation; somewhat analogous to glass in a greenhouse.
greenhouse gases
Atmospheric gases, such as carbon dioxide and methane, that regulate the Earth’s atmospheric temperature by absorbing infrared radiation.
greenschist facies
The lowest metamorphic grade, in which chlorite has formed.
greenstone
A low-grade metamorphic rock formed from basalt; if foliated, the rock is called greenschist.
Greenwich mean time (GMT)
The time at the astronomical observatory in Greenwich, England; time in all other time zones is set in relation to GMT.
Grenville orogeny
The orogeny that occurred about 1 billion years ago and yielded the belt of deformed and metamorphosed rocks that underlie the eastern fifth of the North American continent.
groin
A concrete or stone wall built perpendicular to a shoreline in order to prevent beach drift from removing sand.
ground moraine
A thin, hummocky layer of till left behind on the land surface during a rapid glacial recession.
groundwater
Water that resides under the surface of the Earth, mostly in pores or cracks of rock or sediment.
group
A succession of stratigraphic formations that have been lumped together, making a single, thicker stratigraphic entity.
growth ring
A rhythmic layering that develops in trees, travertine deposits, and shelly organisms as a consequence of seasonal changes.
gusher
A fountain of oil formed when underground pressure causes the oil to rise on its own out of a drilled hole.
guyot
A seamount that had a coral reef growing on top of it, so that it is now flat-crested.
gymnosperm
A plant whose seeds are “naked,” not surrounded by a fruit.
gyre
A large circular flow pattern of ocean surface currents.