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A coarse-grained, mafic, and intrusive igneous rock. Most oceanic crust contains gabbro.


An abundant sulfide mineral with cubic crystals. It is the most important ore of lead, as well as an important source of silver.


A marine, freshwater, or terrestrial invertebrate animal belonging to the Class Gastropoda of the Phylum Mollusca, and characterized by a single, coiled, calcareous shell, a muscular foot for gliding, and internal asymmetry caused by an embryonic process (torsion). Gastropods include snails and slugs.


A soil order ; these are weakly weathered soils formed in areas that contain permafrost within the soil profile.

gem, gemstone

A mineral that has aesthetic value and is often cut and polished for use as an ornament.


A hollow, roughly spherical node of crystal that forms when minerals precipitate within hardened vesicles (gas bubbles) in volcanic rocks, or within dissolved nodules that leave openings within sedimentary rock.
These geological structures occur in certain sedimentary and igneous rocks.
See also: igneous rock, mineral, sedimentary rock, vesicular

geologic time scale

A standard timeline used to describe the age of rocks and fossils, and the events that formed them. It spans Earth’s entire history, and is often subdivided into four major time intervals: the Precambrian, Paleozoic, Mesozoic, and Cenozoic.


A hot spring characterized by the intermittent explosive discharge of water and steam. Superheated water becomes highly pressurized when it enters underground fractures ; once pressure builds to a certain level, it is released in an eruption of steam and hot water and the process of pressurization begins again.


A terrestrial tree belonging to the plant division Ginkgophyta, and characterized by broad fan-shaped leaves, large seeds without protective coatings, and no flowers. Ginkgos were very common and diverse in the Mesozoic, but today only one species exists, Ginkgo biloba.


A body of dense ice on land that does not melt away annually and has sufficient mass to move under its own weight. Glaciers form when snow accumulates faster than it melts over many years. As long as melt does not exceed accumulation, the ice and snow pile up and become a self-sustaining system.
As glaciers slowly flow, they abrade and erode the landscape around them to create grooves, scratches, moraines, and other distinguishing features. Glaciers form only on land, and are much thicker than ice that forms on the surface of water.
99% of Earth’s glacial ice exists as vast polar ice sheets, but glaciers are also found high in the mountains of every continent except Australia.

glassy rock

A volcanic rock that cooled almost instantaneously, resulting in a rock with tiny crystals or no crystals at all. Obsidian, basalt glass, and pumice are examples of glassy rocks.

global warming

The current increase in the average temperature worldwide, caused by the buildup of greenhouse gases in the atmosphere. With the coming of the Industrial Age and exponential increases in human population, large amounts of gases have been released into the atmosphere (especially carbon dioxide) that give rise to global warming. The term "climate change" is preferred because warming contributes to other climatic changes such as precipitation and storm strength.


A metamorphic rock that may form from granite or layered sedimentary rock such as sandstone or siltstone. Parallel bands of light and dark minerals give gneiss its banded texture.


A soft, yellow, corrosion-resistant element (Au), which is the most malleable and ductile metal on Earth.
Gold has an average abundance in the crust of only 0.004 parts per million. It can be profitably mined only where hydrothermal solutions have concentrated it.

Gondwana, Gondwanaland

The supercontinent of the Southern Hemisphere, composed of Africa, Australia, India, and South America. It combined with the North American continent to form Pangaea during the late Paleozoic.


The near-surface, oxidized portion of a sulfide -rich ore body.


A common and widely occurring type of igneous rock. Granite usually has a medium- to coarse-grained texture, and is at least 20% quartz by volume.


A coarse-grained plutonic rock rich in the elements sodium and calcium, and in the minerals potassium feldspar and quartz.
See also: feldspar, mineral, plutonic, quartz


A mineral, and the most stable form of carbon. Graphite means "writing stone," a reference to its use as pencil lead.
Graphite occurs in metamorphic rocks, igneous rocks, and meteorites.


An extinct colonial invertebrate animal belonging to the Class Graptolithina of the Phylum Hemichordata, and characterized by individuals housed within a tubular or cup-like structure. The soft parts of a graptolite’s body have never been clearly identified.


Unconsolidated, semi-rounded rock fragments larger than 2 millimeters (0.08 inches) and smaller than 75 millimeters (3 inches).

Great Lakes

The largest group of freshwater lakes on Earth (by total surface area and volume), located on the US-Canadian border, and consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario.
Prior to glaciation, the Great Lakes were river valleys that had been scoured and deepened repeatedly by the many ice advances during the Quaternary period. Many sizable glacial lakes were formed at the edge of the melting ice sheet that no longer exist today or have significantly shrunk in size.

greenhouse conditions

Climatic conditions when atmospheric greenhouse gas concentrations are high and global temperatures are elevated. Sea levels are generally higher and glaciers diminish during these conditions.

greenhouse gas

A gas in the atmosphere that absorbs and emits heat. The primary greenhouse gases in the Earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone.

greenstone belt

A series of interlayered volcanic and sedimentary rocks that have been metamorphosed into meta-sedimentary rocks and amphibolite. The rocks are called ’greenstones’ due to the presence of metamorphic minerals that give the rock a greenish-gray color.Many geologists believe these belts are the result of deposition in volcanic arc environments.

Grenville Orogeny

A mountain-building event that took place over the interval of approximately 1.3 to 1 billion years ago, along the southeastern and eastern edges of North America from Mexico to Canada. The Grenville Orogeny played a role in the formation of the supercontinent Rodinia.
See also: orogeny


A variety of dark-colored sandstone that contains angular grains of quartz and feldspar embedded in clay. The presence of greywacke generally reflects an environment in which erosion and deposition occurred too quickly for chemical weathering to fully degrade the parent material.
See also: erosion, feldspar, quartz, weathering


Flat-topped underwater mountains, or seamounts, typically formed after a coral atoll is drowned, subsiding beneath the ocean surface faster than the fringing reef can grow upward,
See also: atoll, erosion


A soft, sulfate mineral that is widely mined for its use as fertilizer and as a constituent of plaster. Alabaster, a fine-grained light colored variety of gypsum, has been used for sculpture making by many cultures since ancient times.