G

gabbro

A dark, coarse-grained intrusive igneous rock. Gabbro is made of calcium-rich plagioclase, with amphibole and/or pyroxene, and is chemically equivalent to basalt.

garnet

Family of silicate minerals containing varying amounts of aluminum, iron, magnesium, and calcium. Schist and gneiss often have tiny, glassy red garnet dodecahedrons.

geochemistry

A branch of geology that focuses on the chemical composition of Earth materials.

geothermal energy

Power generation using natural steam derived from the Earth's internal heat.

geomorphology

A branch of geology and geography that studies the development of landforms.

glass (volcanic)

Natural glass (obsidian) that forms when molten lava cools too rapidly to permit crystal growth.

glacier

A long-lived sheet or mass of ice made of recrystallized snow. Glaciers move downhill due to the stress of their own weight.

gneiss

A coarse-grained, foliated metamorphic rock that commonly has alternating bands of light and dark-colored minerals.

Gondwana

A continent formed in the Southern Hemisphere during the Late Paleozoic. It included most of South America, Africa, India, Austrailia, and Antarctica.

graben

An elongate block of rock down-dropped along roughly parallel faults.

graded bed

A sediment layer with a gradation of grain size from large grains to finer grains.

grain size

Refers to the size of individual mineral crystals or particles within a rock or sediment deposit.

granite

A coarse-grained intrusive igneous rock with at least 65% silica. Quartz, plagioclase feldspar and potassium feldspar make up most of the rock and give it a fairly light color. Granite has more potassium feldspar than plagioclase feldspar. Usually with biotite, but also may have hornblende.

granitic

A general term for intrusive igneous rocks that look similar to granite but may range in composition from quartz-diorite to granite. All granitic rocks are light colored; feldspar and quartz are visible in hand specimen.

granodiorite

An intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar.

gravel

All sedimentary particles larger than two millimeters is called gravel. Gravel is subdivided into pebbles, cobbles, and boulders.

greenstone

A metamorphic rock derived from basalt or chemically equivalent rock such as gabbro. Greenstones contain sodium-rich plagioclase feldspar, chlorite, and epidote, as well as quartz. The chlorite and epidote make greenstones green.

gregoryite

An anhydrous carbonate rich in sodium and potassium, found in the carbonatite lava of Ol Doinyo Lengai volcano in Northern Tanzania. Upon contact with moisture in the atmosphere, it reacts quickly, converting the dark lava and ash to white material within a few hours.

ground water

Water found beneath the Earth's surface where all empty space in the rock is completely filled with water.

grus

Coarse sand and gravel that forms from weathering of granitic rocks

gully

A very small channel formed by running water. Gullies hold water for brief periods of time after a rain storm or snow melt.