An oasis is a fertile spot in the middle of a
desert, an island of life in an ocean of temperature extremes. Any oasis always
contains one or more springs (a place where water naturally flows out from the
ground). Oases make it possible to survive long treks through the desert. In
large deserts such as the Sahara, towns cluster around sources of water such as
oases and rivers.
What causes an oasis? An oasis is actually a spot in the desert where the
elevation is low enough that the water table is right underneath the surface,
resulting in the presence of springs. Even in a desert, it rains occasionally,
and this produces a water table just above the bedrock, usually several hundred
feet below the surface. Sand is very porous, so most water runs right through it and
down to the bedrock.
Deserts consist of many millions of tons of sand. There is only one natural
force capable of moving it in appreciable amounts: the wind. Although, in an
average dust storm, ten cubic feet (3.05 cubic meters) of air only holds about
an ounce of sand, a cubic mile (1.6 cubic km) of air can move about 4,600 tons
of it, leading to appreciable erosion. A severe storm is capable of moving as
much as 100 million tons of sand and dust.
In certain areas where large quantities of sand are moved by storms, erosion
burrows all the way down to the water table, putting it just beneath the
surface. Seeds planted in the ground there are capable of sprouting and
extending roots into the moist land, producing an oasis.
Sometimes, the oasis produced by the wind can be very large when vast tracts
of desert are wiped clean by storms. The great Kharga oasis in the Sahara, for
example, is over 100 miles (161 km) long and 12 to 50 miles (19.3 to 80.5 km) in
width. The oasis was produced when erosion caused the margins of a great
depression to sink down to the water table.
porous: describes something that has many small holes, so liquid or air can
pass through, especially slowly 多孔的
（来自：www.wisegeek.com 首都师范大学通讯员刘然供稿 英语点津 Annabel 编辑）