A mountain is an elevated portion of the surface of a planet, generally with steep sides that show significant exposed bedrock. Although there are no universally accepted definitions, a mountain is usually considered higher than a hill. Minimum height requirements range from 300 m (984 ft) above the surrounding land, 610 m (2,001 ft) above sea level and average elevations between 1,000 m (3,281 ft) and 3,500 m (11,483 ft) or higher. It may either have a limited summit area or be a smaller plateau with high elevation and steep sides. A few mountains are isolated summits, but most occur in mountain ranges.