Sunday, November 19th, 2017

* Gabbro-Igneous rock that is mineralogically identical to basalt, but that has a medium- to coarse-grained texture because of its plutonic origin.
* Gaia Hypothesis -Named for the Greek Earth goddess Gaea, this hypothesis holds that the Earth should be regarded as a living organism. British biologist James Lovelock first advanced this idea in 1969.
* Gaining stream-A stream that receives water from groundwater because its channel lies below the water table. (syn: effluent stream)
* Galactic halo-A spherical cloud of dust and gas that surrounds the Milky Way's galactic disk.
* Galaxy- A cluster of stars (millions, or even billions), dust, and gas held together by gravity.
* Galaxy-A large volume of space containing many billions of stars, held together by mutual gravitational attraction.
* Galilean moons -Jupiter's four largest moons- Io, Europa, Ganymede and Callisto; discovered independently by Galileo and Marius.
* Gem-A mineral that is prized for its rarity and beauty rather than for industrial use.
* Geocentric universe-A model of the universe with the Earth at the center.
* Geocentric-A model that places the Earth at the center of the celestial bodies.
* Geologic column-A composite columnar diagram that shows the sequence of rocks at a given place or region arranged to show their position in the geologic time scale.
* Geologic structure-Any feature formed by deformation or movement of rocks, such as a fold or a fault. Also, the combination of all such features of an area or region.
* Geologic time scale-A chronological arrangement of geologic time subdivided into units.
* Geologic timetable- a system by which the major events of Earth's history are arranged in the order in which they occured. It is based on rock and fossil evidence.
* Geologists - people who study rocks and the Earth.
* Geology-The study of the Earth, the materials that it is made of, the physical and chemical changes that occur on its surface and in its interior, and the history of the planet and its life forms.
* Geosphere-The solid Earth, consisting of the entire planet from the center of the core to the outer crust.
* Geosynchronous orbit -A direct, circular, low-inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface.
* Geothermal Energy- Energy derived from the internal heat of the earth.
* Geothermal energy-Energy derived from the heat of the Earth.
* Geothermal gradient-The rate at which temperature increases with depth in the Earth.
* Geothermal Power- Power generated by using the heat energy of the earth.
* Geyser- a boiling hot spring which erupts periodically.
* Geyser-A type of hot spring that intermittently erupts jets of hot water and steam. Geysers occur when groundwater comes in contact with hot rock.
* Glacial abrasion- A common mechanical weathering process where rock and debris frozen into the sides and bottom of a glacier act like sandpaper and wear down the bedrock the glacier is moving across.
* Glacial ice- Naturally occurring ice which exhibits internal plastic flow and deformation.
* Glacial polish- Polished bedrock surfaces left behind after melting of glacial ice. The polishing is probably due to very fine grained rock flour carried at the base of the ice.
* Glacial polish-A smooth polish on bedrock created when fine particles transported at the base of a glacier abrade the bedrock.
* Glacial quarrying (plucking)- A common mechanical weathering process in alpine glaciated terrain where glacial ice frozen into cracks in the bedrock literally "pluck" rock material from the valley floor.
* Glacial striation-Parallel grooves and scratches in bedrock that form as rocks are dragged along at the base of a glacier.
* Glacial Valley (Glacial trough)- a U-shaped valley formed by glacial erosion.
* Glacier- A large mass of freshwater ice originally of atmospheric origin (snow) that forms on land over many years. There are two main types of glacier- mountain glaciers (alpine glaciers) which form at altitude among mountain peaks and valleys; and continental glaciers (ice sheets), which form over continents at high latitude. Glaciers formed over much of the high latitudes of continents during Ice Ages. Today only Greenland and Antarctica have continental glaciers.
* Glacier-A massive, long-lasting accumulation of compacted snow and ice that forms on land and moves downslope or outward under its own weight.
* Glaze-An ice coating that forms when rain falls on surfaces that are below freezing.
* Globular cluster-A spherically symmetrical collection of stars that shared a common origin. Numerous globular clusters lie within the Milky Way's galactic halo.
* Gneiss-A foliated rock with banded appearance formed by regional metamorphism.
* Graben-A wedge-shaped block of rock that has dropped downward between two normal faults.
* Graded bedding-A type of bedding in which larger particles are at the bottom of each bed, and the particle size decreases towards the top.
* Graded stream-A stream with a smooth concave profile. A graded stream is in equilibrium with its sediment supply; it transports all the sediment supplied to it with neither erosion nor deposition in the stream bed.
* Gradient- The change in field value (elevation, temperature, pressure, etc.) between two points divided by their distance apart.
* Gradient-The vertical drop of a stream over a specific distance.
* Gradualism-A theory of evolution that proposes that species change gradually in small increments.
* Granite-A medium- to coarse-grained, sialic, plutonic rock made predominately of potassium feldspar and quartz.
* Gravel-Unconsolidated sediment consisting mainly of rounded particles with a diameter greater than 2 millimeters.
* Gravity- The force of attraction between two objects that is influenced by the mass of the two objects and the distance between the two objects.
* Greenhouse Effect- A popular term used to describe the effects of gases such as water vapor, carbon dioxide and other trace gases (collectively known as the greenhouse gases) in keeping the Earth's temperature warmer than it would be otherwise. These gases absorb long-wave (heat) radiation and reradiate some of that energy back to the surface and the atmosphere. In the process, less of the heat from the sun is lost to space.
* Greenhouse effect-An increase in the temperature of a planet's atmosphere caused when infrared-absorbing gases are introduced into the atmosphere.
* Groin-A narrow wall built perpendicular to the shore to trap sand transported by currents and waves.
* Ground moraine-A moraine formed when a melting glacier deposits till in a relatively thin layer over a broad area.
* Ground water-Water contained in soil and bedrock. All subsurface water.
* Groundwater- Water stored beneath the surface in open pore spaces and fractures in rock.
* Guyot-A flat-topped seamount.
* Gypsum-A mineral with the formula (caso4.2H2O). It commonly forms in evaporite deposits.
* Gyre-A closed, circular current either in water or air.

* Hadean Eon-The earliest time in the Earth's history.
* Hail- precipitation in the form of irregular balls or lumps made of concentric layers (like an onion) of ice. Hail forms when strong winds repeatedly blow falling ice back up again. Each time, more ice sticks to them, forming new layers.
* Hail-Ice globules varying from 5 millimeters to a record 14 centimeters in diameter that fall from cumulonimbus clouds.
* Half-life - The time required for half of the atoms in a radioactive substance (unstable parent) to decay to a stable end product (daughter).
* Half-life-The time it takes for half of the nuclei of a radioactive isotope in a sample to decompose.
* Halo- A ring or arc of colored or white light that encircles the sun or moon when seen through a cloud of ice crystals. Halos are produced by the refraction of light. The most commonly observed halo forms at a 22 degrees radius from the sun/moon. One at 46 degrees radius may also be seen.
* Hanging valley-A tributary glacial valley whose mouth lies high above the floor of the main valley.
* Hanging wall-The rock above an inclined fault.
* Hardness- The resistance of a mineral to scratching.
* Hardness-The resistance of the surface of a mineral to scratching.
* Heat Lightning- A popular term for lightning that is visible but for which no thunder is heard. It usually occurs in scattered thunderstorms/showers on hot nights when storm are at a distance from the observer.
* Heat transfer- Movement of heat from one place to another.
* Heliocentric solar system-A model of the universe with the sun at the center.
* Heliocentric-A model that places the Sun at the center of the Solar System.
* Hertzsprung-Russell diagram (H-R diagram)-A plot of absolute stellar magnitude (or luminosity) against temperature.
* Holocene- The time period from 10,000 years ago to the present. Also, the rocks and deposits of that age.
* Horn-A sharp, pyramid-shaped rock summit where three or more cirques intersect near the summit.
* Hornblende-A rock-forming mineral. The most common member of the amphibole group.
* Hornfels-A fine-grained rock formed by contact metamorphism.
* Horse Latitudes- belts of high pressure and very dry descending air, located at about 30 degrees north and south of the equator. On land, these are usually deserts. At sea, they are dry, windless regions where sailors used to get stranded for long periods of time (no wind). To conserve water, they threw their horses overboard. Passing ships would see the horses floating in the middle of the ocean.
* Horse latitudes-A region of the Earth lying at about 30 degrees north and south latitudes, in which air is falling, forming a vast high-pressure region. Generally dry conditions prevail, and steady winds are rare.
* Horst-A block of rock that has moved relatively upward and is bounded by two faults.
* Hot Spot- A volcanic center, 60 to 120 miles (100 to 200 km) across and persistent for at least a few tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs and may not be associated with ocean ridges.
* Hot spot-A persistent volcanic center thought to be located directly above a rising plume of hot mantle rock.
* Hot spring-A spring formed where hot groundwater flows to the surface.
* Hot-spot Volcanoes- Volcanoes related to a persistent heat source in the mantle.
* Hubble's Law-A law that states that the velocity of a galaxy is proportional to its distance from Earth. Thus the most distant galaxies are travelling at the highest velocities.
* Humidity- Generally, some measure of the water vapor content of the air. See also, absolute humidity, specific humidity, relative humidity.
* Humidity-A measure of the amount of water vapor in the air. Absolute humidity is the amount of water vapor in a given volume of air. Relative humidity is the ratio of the amount of water vapor in a given volume of air divided by the maximum amount of water vapor that can be held by that air at a given temperature.
* Humus-The dark organic component of soil composed of litter that has decomposed sufficiently so that the origin of the individual pieces cannot be determined.
* Hurricane- a tropical cyclone with sustained winds exceeding 120 km/h (74 mph)
* Hurricane-See "tropical cyclone."
* Hydration-The chemical combination of water with another substance.
* Hydraulic action-The mechanical loosening and removal of material by flowing water.
* Hydride-A compound of hydrogen and one or more metals. Hydrides can be heated to release hydrogen gas for use as a fuel.
* Hydrologic cycle-The constant circulation of water among the sea, the atmosphere, and the land.
* Hydrological Cycle- The vertical and horizontal transport of water through the global environment in all its states between the earth, the atmosphere and seas, lakes, rivers and other water bodies. Often also called the water cycle
* Hydrolysis-A decomposition reaction involving water.
* Hydrosphere- all of the waters of the Earth, both above and below ground.
* Hydrosphere-All of the Earth's water, which circulates among oceans, continents, and the atmosphere.
* Hydrothermal metamorphism-Changes in rock that are primarily caused by migrating hot water and by ions dissolved in the hot water.
* Hydrothermal ore deposit-An ore deposit formed by precipitation of dissolved metals from hot water solutions.
* Hydrothermal vein-A sheet-like mineral deposit that fills a fault or other fracture, precipitated from hot water solutions.
* Hygrometer- An instrument that measures the water vapor content of the atmosphere
* Hypothesis- an informed guess that tries to explain how or why an event occurs.