Sand is a naturally occurring granularmaterial composed of finely divided rock and mineral particles. The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropicalcoastal settings is silica (silicon dioxide, or SiO2), usually in the form of quartz.
The second most common form of sand is calcium carbonate, for example aragonite, which has mostly been created, over the past half billion years, by various forms of life, like coral and shellfish. It is, for example, the primary form of sand apparent in areas where reefs have dominated the ecosystem for millions of years like the Caribbean.
The Sahara (Arabic: الصحراء الكبرى, aṣ-Ṣaḥrāʾ al-Kubrā , 'the Greatest Desert') is the world's hottest desert, the third largest desert after Antarctica and the Arctic. At over 9,400,000 square kilometres (3,600,000 sq mi), it covers most of North Africa, making it almost as large as China or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean. To the south, it is delimited by the Sahel, a belt of semi-arid tropical savanna that composes the northern region of central and western Sub-Saharan Africa.
Some of the sand dunes can reach 180 metres (590 ft) in height. The name comes from the plural Arabic language word for desert, (صحارىṣaḥārā [ˈsˤɑħɑ:rɑ:];