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How much of the Sahara Desert is covered by sand?

http://www.100md.com   2004-6-27 chinadaily
     Mention the Sahara Desert and many people think of sand dunes stretching from horizon to horizon. While enormous fields of sand cover some parts of the Sahara, that sand only covers about 20% of the world's largest desert. The rest consists of rocky plateaus, open plains of gravel, and barren rocks.

    From space, the Sahara is a mottled patchwork of browns and yellows. The bright yellow-white sand fields are especially prominent in the eastern and western regions, while the wrinkled, brown Ahaggar and Tibesti mountain ranges dominate the central area.

    In the west, the great El Djouf basin ("The Belly") holds one of the largest expanses of sand in the world. This almost completely lifeless "empty quarter" of Mauritania is more than 1000 kilometers (600 miles) across. In the east there is even more sand, mostly concentrated in windblown trails running southwest across Egypt and Libya.

    Sahara Deser:撒哈拉沙漠

    sand dunes : 沙丘

    plateaus: 高原

    gravel: 沙砾

    mottled: 斑驳的

     Mauritania: 毛利塔尼亚(北非古国)