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Glossary G-H

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.
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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.
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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.
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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.
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granodiorite

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

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


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
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gully

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



H


half-life

The time required for one-half of the atoms of a radioactive substance to decay.
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hardness

A measure of a mineral’s resistance to scratching. The hardness of a mineral is measured by scratching it against another substance of known hardness.

headland

Headlands are projections of land that stick out into a sea or lake.
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Holocene

An epoch of the Quaternary Period beginning 10,000 years ago and continuing today.
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horst

An elongate block of rock uplifted along roughly parallel faults.

hornblende

See amphibole.


hornblende schist

A schist rich in hornblende. Generally with abundant plagioclase feldspar as well. Grades into amphibolite.

hornfels

A dark, very fine-grained metamorphic rock produced by the recrystallization of a fine-grained rock by heat from a nearby igneous intrusion. From the German, meaning horn rock.


hot spot

An area of concentrated heat in the mantle that produces magma that rises to the Earth’s surface to form volcanic islands. The volcanic activity of the Hawaiian Islands is one example. Hot spots generally persist for millions of years.

hydrology

The science that deals with water on and beneath the Earth surface.


hydrous

Literally, "with water". Refers to minerals or other materials which have water as a primary constituent.

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