1. How does an interior desert form? Be sure to include all the necessary geogra

Question

1. How does an interior desert form? Be sure to include all the necessary geographic, atmospheric and oceanic factors.

2. How does a subtropical desert form? Be sure to include all the necessary geographic, atmospheric and oceanic factors.

3. How does a coastal desert form? Be sure to include all the necessary geographic, atmospheric and oceanic factors.

4. How does a rain shadow desert form? Be sure to include all the necessary geographic, atmospheric and oceanic factors.

5. How did Charles Darwin relate fringing reefs, barrier reefs and atolls in his hypothesis for atoll formation? Be sure to include a description of what an atoll is and how it is forming.

Explanation / Answer

1) Interior deserts or inland deserts form in the interior portions of a continent(continental interiors) where no moisture laden winds reach and thus there is little or practically no rainfall. This is because by the time the winds from the coastal areas reach the interior regions, they lose their moisture. They lie hundreds of kilometres away from the ocean where hot summers and cold winters prevail. Eg- Gobi desert.

2) Subtropical deserts form due to the circulation patterns of the air masses. The general atmospheric circulations bring dry, subtropical air into the mid latitudes and these deserts are mostly found between 15 to 30 degrees North of the Equator or along the Tropic of Cancer and between 15 to 30 degrees South of the Equator or along the Tropic of Capricorn. As the hot and moist air rises into the atmosphere near the Equator, it cools and drops its moisture as heavy tropical rainfall. The drier, cooler air mass moves away from the Equator and as it reaches towards the Tropics, the air mass descends and again warms up. Thus this descending air prevents the formation of the clouds and as a result very little or no rain falls on the land below which results in the formation of the subtropical deserts. They include some of the hottest deserts. Eg- Sahara Desert.

3) Coastal deserts form from the contribution of the cold ocean currents. They are complex because thay are at the junction of oceanic, terrestrial and atmospheric systems. The wind blowing towards the shore are chilled by contact with cold water which then produces a layer of fog. This heavy fog then drifts onto land and creates a high humidity. The atmospheric changes that are responsible for causing rainfall are not present and so they are often totally rainless or damp with fog. Geographically they are located near to the shoreline on the western edges of continents near the Tropics of Cancer and Capricorn or in areas where cold upwelling seawater cools the air and decreases its ability to hold moisture. Eg- Atacama desert along the Pacific shoreline of Chile.

4) Rain shadow deserts form near the leeward or rain shadow slopes of high mountain ranges. These regions face away from the prevailing winds and hence when the moisture laden air hits these ranges, they are forced to rise up and form clouds that brings rainfall on the windward slopes of the mountain ranges and the leeward slopes are deprived of rainfall. Eg- Death Valley in the United States.

5) Atolls are ring shaped islands, reefs or chain of islands made up of corals. They are coral reefs encircling a shallow lagoon. They form very slowly when an underwater volcano erupts to form an island and over time the island is eroded away and subsidence occurs causing the island to disappear below the surface leaving behind a circular reef formed of corals. According to Darwin's hypothesis, he stated that in clean, agitated and tropical seas, corals will mainly form fringing reefs just below the low tide level or it may grow along the edges of a newly formed volcanic island to form a frimging reef. If the coastline is stable, gradually the coral will grow out from the shore and a lagoon would open between the reef and the island thus forming a barrier reef and if by any means the coast in sinking or subsiding, the coral might keep pace by growing upwards but as the subsidence occurs beneath the waves, all that would remain behind would be a more or less circular reef or atoll that is a lagoon ringed by coral. Thus, in Darwin's words, his hypothesis states:

"Fringing reefs are thus converted into barrier reefs, and barrier reefs when encircling islands are thus converted into atolls, the instant the last pinnacle of land sinks beneath the surface of the ocean."

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