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Wednesday, March 24, 2010

geography short notes -10


WADI


It is traditionally a valley. In some cases it can refer to a dry river bed that contain water only during times of heavy rain. The wadi is very widely found in arabic to ponyms were streams in a desert environment & generally are dry year round except after rain. The deposition of wadi is rapid because of the sudden loss of the velocity & absorption of the water into the ground.

Why was the Prime Meridian established at Greenwich

In 1675, the Royal Observatory at Greewich, outside of London was created to study the determination of longitude (a vital comonent to oceanic navigation). The United Kingdom and, by association, the United States, used Greenwich as the reference point for longitude.
Thus, by the time of an international conference in 1884 to establish fixed degrees of longitude on the planet, many countries had been using Greenwich as the Prime Meridian for decades so the tradition became internationally accepted



World Largest Coral Reef!

GREAT BARRIER REEF - WORLD'S LARGEST CORAL REEF SYSTEM

it is the worlds largest coral reef system, composed of roughly 3000 individual reefs & 900 islands stretching for 2600 km over an area of approximately 3,44,400 square kilometers. the reef is located in the coral sea, off the coast of Queensland in northeast australia. Great Barrier Reed can be seen from outer space & is worlds biggest single structure made by living organism. this is build by billions of tiny organisms , known as coral polyps. when coral sea basin was formed, coral reef began to grow in the basin until about 25 million years ago, they can increase in diameter from 1-2 cms per year & grow vertically any where from 1-15 cms per year. they are limited to growing above a depth of 150 meters due to their need for sunlight & cannot grow above sea level. the land formed the subtrate of the current great barrier reef was a coastal plain formed from the eroded sediments of the great dividing range with some larger hills from 20,000 years ago until 60,000 years ago, the sea level rose steadily. by around 13,000 years ago the sea level was 60 meters lower than the present day, corals began to grow around the hills of the costal plain , which were by then, continental islands. this GBR supports a diversity of life, including many vulnerable & endangered species, some of them are endamic to the reef system. there are thirty species of whales, dolphines , porpoises have been recorded in the GBR, including the dwarf minke whale, indo-pacific humpback dolphin & the humpback whale. large populations of dugongs live there. six species of a sea turtles come to the reef to breed-the green sea tutle, leather back the sea turtle, hawksbill turtle, loggeshead sea turtle, olive ridly. salt water crocodiles live near the reef.

Xenolith!

XENOLITH

it is rock fragment which becomes enveloped in a larger rock during the latter's development & hardening. in geology xenolith is almost used to describe igneous rock during magma emplacement & eruption. xenolith may be engulfed along the margins of magma chamber, torn loose from the walls of an erupting lava conduct / explosive diatreme along the base of a flowing lava on earth's surface. xenolith provide information about the composition of the innacessible mantle, basalts, kimberlites, lamproites which have their source in the upper mantle. often contain fragments & crystals assumed to be a apart of the originating mantle minualogy.

Yardang!

YARDANG

It is a wind-abraded ridge found in cohesive material. The word is derived from turkish meaning, 'steep bank'. Some are found in dried-up river beds. These are elongate features typically 3 or more times longer than they are wide, when viewed from above , resemble the hull of a boat.

TYPES:
a) Mega-yardang :
can be several kilometers long & hundreds of meters high.

b) Meso-yardang : these are generally a few meters high & 10-15 meters long.

c) Micro-yardang : these are only few centimeters high.

Yardang form in environment where water is scarce. The prevailing winds are strong, unidirectional & carry an abrasive sediment load. A large concentration of mega are found near the tibesti mountains in the central sahara. There is a famous yardang are 'hole in the rock' in papayo parla in phoenix, arzona , a rock formation with a roughly another yardang arizona is window rock, near the town of window rock. It is a 200 feet sandstorm hill with a very large circular rock in the middle of it.

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