An oasis is a green patch of land formed around a water source. This makes possible the growth of vegetation and the development of an ecosystem, albeit small, around it. If it is large enough, the environment also provides living conditions for humans, helping to evolve as a civilization.
There are a number of factors that contribute to the formation of oases that individually or in combination with other factors facilitate the formation. For an oasis to form there should be a water table beneath the land. Depending on the geographical features, the water source can be an underground river or an aquifer or an above ground river in the proximity of the desert. Lakes and seas lying near the deserts are also able to trigger the creation. The water can seep to the surface with enough pressure and form an oasis. Man-made wells are also utilized to access the water in many places.
The oasis formation is also helped by heavy rains and storms in the desert. Since the sand is porous, the rainwater would ooze down the pores and it might be trapped by the various layers of rocks and stones, thus creating a water source for the oasis. Powerful storms displace huge amounts of sand which lowers the level of land and brings the waterbed closer to the surface and makes it easily accessible.
When enough water has risen to the surface it forms a pool. And though this pool is continuously losing water due to evaporation during the day, its surface level more or less remains the same as more water rises up and replaces the lost water. As a fraction of this water gets soaked up by the surrounding sand it creates a fertile patch of land, ready to facilitate the growth of plant matter.
The vegetation growth is often aided by the migratory birds who drop seeds which sprout in the presence of water. Date palms, figs, peaches and apricots are amongst the most common plants found in oases. These plants, particularly date palms, help trap the water further and better the conditions for organic life.
Oases have played important roles in the course of trading and cultural exchange between various civilizations, and the progress of human lives. The notable ones include Kharga in Egypt, Tutua in Algeria and Kufra in Libya.