Geyser A geyser is a violent ejection of hot water and steam that is ejected under pressure from a vent in the earth's crust. The emission is frequent at regular intervals. Geysers are associated with past or present volcanic activity. The subterranean structure of a geyser comprises a number of waterfilled chambers interconnected with a central pipe. In volcanic regions, groundwater percolates beneath the surface region and is heated by the surrounding hot rock. The hot water comes out due to rise in pressure.
Glacier A mass of ice that moves under the influence of gravity along a confined course away from its surface area is called glacier. Glacier has a variety of forms. It is usually of limited width which distinguishes it from a ice-sheet and a ice cap which have larger dimensions.
Glacier Budget Glacier budget refers to the relationship between the amount of snow and ice that is accumulated by a glacier and that lost through meltating or ablation.
Glauconite Glauconite refers to a bluish-green mineral which consists of the hydrous silicate of iron and potassium. It is indicative of a marine origin for the sedimentary rocks in which occurs.
Gneiss The term gneiss refers to a coarse-grained, crystalline (metamorphic) rock with a banded structure. Most gneisses comprise bands of granular quartz and fledspar. Gneisses are formed by the large scale application of heat and pressure during mountain building and volcanic activity. They are mainly of three categories: (i) paragneiss (produced by magmatic alteraHdft of sedimentary rocks), (ii) orthogneiss (formed by alteration of igneous rocks), and (iii) augengneiss (characterised by eye like swelling of quartz or feldspar crystals).
Gondwana land Gondwana land is the name given to the supercontinent thought to have existed. in the Southern Hemisphere over 200 million years ago following the break up of pangaea. Gondwanaland comprised Antarctic, Australia and parts of South America, Africa and India. According to A. Wegener, Gondwana land separated from the northern continent of Laurasia by the long narrow ocean of Tethys.
Gorge Gorge is a deep and narrow valley with rocky sidewalls. A gorge is formed by the rejuvenation of river stream (e.g., Rhine Gorge in Germany) or by the downcutting of streams through horizontal rock strata (e.g., Grand Canyon).
Graben A German term, graben refers to a block of rock that has sunk between two roughly parallel faults. A graben is synonymous with a rift valley, if a fault-guided trough is occupied by a valley. It contrasts with a horst.
Granite Granite is an igneous rock of coarse grain and plutonic origin. It consists of quartz (20-40 per cent), feldspar (mainly alkali) and mica. Granites are intrusive rocks and occur in many forms, such as sills, dykes and latholiths. On the basis of their grain size and mineral composition, granites are classified as (i) aplite (finest grains), (ii) pegmatite (coarsest grains), (iii) biotite or muscovite (mica predominates), (iv) hornblende or pyroxene (hornblende or pyroxene are among the constituents of rock), (v) quartz diorite or grandiorite
plagioclase predominates, and (vi) adamalite (plagioclase and alkali feldspar are about in equal amount).
Grassland Grassland is a major type of world vegetation. It is characterised by extensive grassy plains, limited precipitation and a season of drought. Natural grasslands occur in both tropical and temperate latitudes.
Great Circle Great Circle is a circle on the earth's surface, the plane of which passes through the centre of the earth. The shortest distance between any J;wo points on the earth's surface is the arc of the Great Circle that passes through them. The equator is also a Great Circle.
Ground Swell The term ground swell refers to waves of great height and length in deep water.
Groundwater Groundwater refers to water that occupies pores, cavities, cracks and other spaces in the rocks beneath the earth. It includes water precipitated from the atmosphere which has percolated through the soil.
Gulf Stream Gulf stream is the name given to a warm current of sea water that originates in the eastern Gulf of Mexico. The southern portion of the Gulf stream is Florida current. The warm Gulf stream combines with the prevailing south-westerly winds to produce the temperate climate of North-West Europe and keeps the Norwegian coast ice-free during the winter months.
Gully A gully is a small but deep channel or raviI\e formed by fluvial erosion. Gullies are not permanently occupied by a stream. They are particularly common in semi-arid areas. Gully erosion is the erosion of soil and rock by the concentration of runoff into gullies.
Guyot A flat-topped variety of a seamount occurring mainly in the Pacific Ocean called guyot. The summits of a guyot may rise up to 3 km from the ocean floor.
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