Un oasis es un paraje de un desierto en el cual se pueden encontrar agua y vegetación. Suelen constituir porciones más o menos extensas de terrenos fertilizados por una fuente de agua en medio de los arenales. En estos lugares puede haber pequeños asentamientos habitados, como los de la región del Sahara, la península arábiga o pueblos como los de Pica, en Chile, Ica en Perú o Parras de la Fuente, en México. Además de dar cultivos a los pobladores cercanos, sirven para abastecer a los viajeros y caravanas. Algunas veces éstos eran confundidos por ilusiones ópticas llamadas espejismos. La palabra oasis proviene del griego ὄασις [oasis], tomado del egipcio uḥ3t, en demótico uḥỉ y en copto uaḥe.

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In geography, an oasis (plural: oases) is an isolated area of vegetation in a desert, typically surrounding a spring or similar water source, such as a pond or small lake. Oases also provide habitat for animals and even humans if the area is big enough. The location of oases has been of critical importance for trade and transportation routes in desert areas; caravans must travel via oases so that supplies of water and food can be replenished. Thus, political or military control of an oasis has in many cases meant control of trade on a particular route. For example, the oases of Awjila, Ghadames, and Kufra, situated in modern-day Libya, have at various times been vital to both North-South and East-West trade in the Sahara Desert. Oases are formed from underground rivers or aquifers such as an artesian aquifer, where water can reach the surface naturally by pressure or by man-made wells. Occasional brief thunderstorms provide subterranean water to sustain natural oases, such as the Tuat. Substrata of impermeable rock and stone can trap water and retain it in pockets, or on long faulting subsurface ridges or volcanic dikes water can collect and percolate to the surface. Any incidence of water is then used by migrating birds, which also pass seeds with their droppings which will grow at the water's edge forming an oasis. It can also be used to plant crops.

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