綠洲是浩瀚沙漠中的片片沃土，它就像是沙漠瀚海上美麗的珍珠，鑲嵌在沙漠裡，閃爍著神奇的色彩。那麼綠洲是怎樣形成的呢？就讓下面這篇文章來告訴你答案吧。An oasis is a fertile spot in the middle of a desert, an islandoflife in an ocean of temperature extremes. Any oasis alwayscontainsone or more springs (a place where water naturally flowsout fromthe ground). Oases make it possible to survive long treksthroughthe desert. In large deserts such as the Sahara, townsclusteraround sources of water such as oases and rivers.
What causes an oasis? An oasis is actually a spot in thedesertwhere the elevation is low enough that the water table iscenterunderneath the surface, resulting in the presence of springs.Evenin a desert, it rains occasionally, and this produces a watertablejust above the bedrock, usually several hundred feet belowthesurface. Sand is very porous, so most water runs center throughitand down to the bedrock.
Deserts consist of many millions of tons of sand. There isonlyone natural force capable of moving it in appreciable amounts:thewind. Although, in an average dust storm, ten cubic feet(3.05cubic meters) of air only holds about an ounce of sand, acubicmile (1.6 cubic km) of air can move about 4,600 tons of it,leadingto appreciable erosion. A severe storm is capable of movingas muchas 100 million tons of sand and dust.
In certain areas where large quantities of sand are movedbystorms, erosion burrows all the way down to the watertable,putting it just beneath the surface. Seeds planted in thegroundthere are capable of sprouting and extending roots into themoistland, producing an oasis.
Sometimes, the oasis produced by the wind can be very largewhenvast tracts of desert are wiped clean by storms. The greatKhargaoasis in the Sahara, for example, is over 100 miles (161 km)longand 12 to 50 miles (19.3 to 80.5 km) in width. The oasiswasproduced when erosion caused the margins of a great depressiontosink down to the water table.
porous
: describes something that has many small holes, so liquid oraircan pass through, especially slowly 多孔的
(實習編輯:顧萍)