| Ga |
Billions of years ago (abbreviation). |
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| gabbro |
A coarse-grained, intrusive mafic igneous rock. |
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| Gaia |
The term used for the Earth System, with the implication that it resembles a complex living entity. |
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| galaxy |
An immense system of hundreds of billions of stars. |
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| gene |
An individual component of the DNA code that guides the growth and development of an organism. |
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| genetics |
The study of genes and how they transmit information. |
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| 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. |
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| geochronology |
The science of dating geologic events in years. |
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| geode |
A cavity in which euhedral crystals precipitate out of water solutions passing through a rock. |
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| geographical pole |
The locations (north and south) where the Earth’s rotational axis intersects the planet’s surface. |
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| geologic column |
A composite stratigraphic chart that represents the entirety of the Earth’s history. |
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| geologic history |
The sequence of geologic events that has taken place in a region. |
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| geologic map |
A map showing the distribution of rock units and structures across a region. |
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| geologic time |
The span of time since the formation of the Earth. |
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| geologic time scale |
A scale that describes the intervals of geologic time. |
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| geology |
The study of the Earth, including our planet’s composition, behavior, and history. |
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| geotherm |
The change in temperature with depth in the Earth. |
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| geothermal energy |
Heat and electricity produced by using the internal heat of the Earth. |
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| geothermal gradient |
The rate of change in temperature with depth. |
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| geothermal gradient |
The rate of change in temperature with depth. |
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| 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. |
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| geyser |
A fountain of steam and hot water that erupts periodically from a vent in the ground in a geothermal region. |
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| glacial abrasion |
The process by which clasts embedded in the base of a glacier grind away at the substrate as the glacier flows. |
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| glacial advance |
The forward movement of a glacier’s toe when the supply of snow exceeds the rate of ablation. |
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| glacial drift |
Sediment deposited in glacial environments. |
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| glacial incorporation |
The process by which flowing ice surrounds and incorporates debris. |
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| glacial marine |
Sediment consisting of ice-rafted clasts mixed with marine sediment. |
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| glacial outwash |
Coarse sediment deposited on a glacial outwash plain by meltwater streams. |
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| glacial plucking (or quarrying) |
The process by which a glacier breaks off and carries away fragments of bedrock. |
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| 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. |
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| glacial retreat |
The movement of a glacier’s toe back toward the glacier’s origin; occurs if the rate of ablation exceeds the rate of supply. |
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| glacial subsidence |
The sinking of the surface of a continent caused by the weight of an overlying glacial ice sheet. |
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| glacial till |
Sediment transported by flowing ice and deposited beneath a glacier or at its toe. |
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| glacially polished surface |
A polished rock surface created by the glacial abrasion of the underlying substrate. |
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| glaciation |
A period of time during which glaciers grew and covered substantial areas of the continents. |
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| glacier |
A river or sheet of ice that slowly flows across the land surface and lasts all year long. |
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| glass |
A solid in which atoms are not arranged in an orderly pattern. |
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| glassy igneous rock |
Igneous rock consisting entirely of glass, or of tiny crystals surrounded by a glass matrix. |
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| glide horizon |
The surface along which a slump slips. |
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| global change |
The transformations or modifications of both physical and biological components of the Earth System through time. |
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| global circulation |
The movement of volumes of air in paths that ultimately take it around the planet. |
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| global climate change |
Transformations or modifications in Earth’s climate over time. |
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| global cooling |
A fall in the average atmospheric temperature. |
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| 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. |
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| global warming |
A rise in the average atmospheric temperature. |
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| gneiss |
A compositionally banded metamorphic rock typically composed of alternating dark- and light-colored layers. |
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| Gondwana or Gondwanaland |
A supercontinent that consisted of today’s South America, Africa, Antarctica, India, and Australia. |
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| graben |
A down-dropped crustal block bounded on either side by a normal fault dipping toward the basin. |
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| gradualism |
The theory that evolution happens at a constant, slow rate. |
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| grain |
A fragment of a mineral crystal or of a rock. |
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| 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. |
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| granite |
A coarse-grained intrusive silicic igneous rock. |
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| granulite facies |
A set of metamorphic mineral assemblages formed at very high pressures and temperatures. |
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| 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. |
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| graywacke |
An informal term used for sedimentary rock consisting of sand-size up to small-pebble-size grains of quartz and rock fragments all mixed together in a muddy matrix; typically, graywacke occurs at the base of a graded bed. |
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| greenhouse conditions (greenhouse period) |
Relatively warm global climate leading to the rising of sea level for an interval of geologic time. |
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| 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 the effect of glass in a greenhouse. |
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| greenhouse gases |
Atmospheric gases, such as carbon dioxide and methane, that regulate the Earth’s atmospheric temperature by absorbing infrared radiation. |
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| greenschist facies |
A set of metamorphic mineral assemblages formed under relatively low pressures and temperatures. |
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| greenstone |
A low-grade metamorphic rock formed from basalt; if foliated, the rock is called greenschist. |
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| 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. |
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| 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. |
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| groin |
A concrete or stone wall built perpendicular to a shoreline in order to prevent beach drift from removing sand. |
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| ground moraine |
A thin, hummocky layer of till left behind on the land surface during a rapid glacial recession. |
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| groundwater |
Water that resides under the surface of the Earth, mostly in pores or cracks of rock or sediment. |
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| group |
A succession of stratigraphic formations that have been lumped together, making a single, thicker stratigraphic entity. |
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| growth ring |
A rhythmic layering that develops in trees, travertine deposits, and shelly organisms as a consequence of seasonal changes. |
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| gusher |
A fountain of oil formed when underground pressure causes the oil to rise on its own out of a drilled hole. |
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| guyot |
A seamount that had a coral reef growing on top of it, so that it is now flat-crested. |
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| gymnosperm |
A plant whose seeds are “naked,” not surrounded by a fruit. |
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| gyre |
A large circular flow pattern of ocean surface currents. |
