DEFINITION OF SAND SEA

Deserts covering vast expanses covered by thick

sands, including sand dunes of several types and by an

absence of other geographic features are known as sand seas,

or locally as ergs in the North African Sahara. Interdune

areas may be covered by relatively flat tabular sand sheets, or

even evaporite basins (sabkhas). Sand seas are abundant in

parts of the Sahara of North Africa, the Namib of southern

Africa, the Rub‘ al-Khali (Empty Quarter) of Arabia, the

Great Sandy Desert of Australia, the Gobi Desert of Asia, and

in the Nebraska Sand Hills of Nebraska.

Sand seas form where the velocity of the transporting

wind decreases, dropping its load. The decreased velocity

may be caused by a number of factors including their location

in topographic lows, or adjacent to topographic barriers

such as mountains that cut across the direction of sand trans

port. A striking example of this process is found in the Wahiba

Sand Sea of Oman. Here, the Eastern Hajar Mountains

terminate the northward-flowing Wahiba sands, and an intermittent

river system at the base of the mountains removes

sand that gets close to the mountain front, carrying it to the

coast of the Arabian Sea. Longshore transport then carries

this sand southward where winds pick it up from beaches

and cause it to reenter the Wahiba sand sheet in the south,

forming a sort of sand gyre. Sand seas may also form where a

large body of water intercepts drifting sand, or where the

sand is carried into shifting climate zones where the wind

strength decreases.

Surface features in sand seas include bed forms of a variety

of scales ranging from several different types of ripples that

may be up to an inch (several cm) high, to dunes that are typically

up to 300 feet (100 m) tall, to huge bedforms called draa

that are giant dunes up to 1,650 feet (500 m) tall, with wavelengths

of up to several kilometers. These bedforms are typically

superimposed on each other, with dunes migrating over draa, and several different sets of ripples migrating over the

dunes. The wind directions inferred from the different sets may

also be different, with ripples reflecting the most recent winds,

dunes the dominant winds over different seasons, and draa

reflecting the very long-term direction of wind in the basin.

See also DESERT.

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