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Eolian Deposits
Sand accumulates in a desert through eolian, or wind blown, deposits. These deposits form in arid regions after dry air masses create wind patterns that pick up and carry loose sediments into these regions. Eolian sand deposits are also found along seas and lakes, in addition to deserts. An erg, or sand sea, is an area that receives deposits of more than 125 square kilometers of eolian sand. The single largest erg in the world is the Sahara Desert in North Africa. Eolian sand deposits cover more than 2,700,000 square miles. Sand dunes are smaller areas of eolian sand.
Alluvial Deposits
Rocks and minerals eroded into fine particles and transported downstream in a water source form alluvial deposits. Sand created in this manner typically originates from a mountain stream or river and is deposited into a valley floor. Alluvial deposits are composed of fine layers and form deep, rich soils. Closer to a riverbank or natural levee, sand is the dominant particle. Farther away, the deposit is composed mostly of clay rather than sand. Alluvial deposits eventually create a layer of nutrient rich top soil and are greatly valued for being fertile land for growing crops.
Glacial Deposits
When a glacier melts and retreats, it erodes the bedrock beneath the glacier and can leave behind a rich diversity of deposits. Sand is one such material left behind in areas once covered by glaciers. Glacial debris consisting of sand, silt and clay comes down to floodplains when melt water from retreating glaciers drains into nearby rivers and streams. Some of the sand will end up in outwash plains. Other places where it accumulates are kettles (depressions), kames (small mounds) and eskers (narrow ridges). Finer sand particles from glaciers can become airborne, and travel several miles to form loess deposits. The majority of material in those deposits, however, is silt and clay.
Volcanic Deposits
Molten rock enriched by various minerals feeds volcanoes. When a volcano erupts, what is left behind depends entirely on the type of magma that becomes lava. Some volcanic deposits are in the form of dark colored sand. Lava plains or plateaus, especially in oceanic environments, typically contain sand and other finer particles. This is because they form from thinner lava that spreads out and hardens into a mostly flat surface. Lava that emerges from continental volcanoes typically forms into jagged rocks because it is much thicker in composition coming from its original magma state.