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What Is the Difference Between Mountains & Hills?

Even to the naked eye, there is a clear distinction between a hilly area and a mountainous one. High areas of the earth's land mass arising abruptly above their surroundings are generally referred to as mountains; hills are merely elevated portions of a plain. The general geological distinction between a hill and a mountain is based on height -- a local relief lower than 2,000 feet is considered a hill, while one that goes higher is classified as a mountain. There are several other differences besides height.

Slope
- Mountains have slope gradients that vary from steep to moderately steep to very steep. Hills are delineated in slope gradient from steep to rolling to gentle undulating. Hill slopes generally have rounded crests; mountains usually have high, rugged ones.
Formation
- Mountains are formed in various ways -- from the buildup of volcanic rock as a result of volcanic eruptions; structural denudation -- erosion carved by wind or water; and from the movement of the earth's tectonic plates. Major mountain systems generally form from tectonic plate movement, where two plates are colliding, causing the land area to rise little by little each year. Hills often occur through weathering and are rarely a result of tectonic plate collision.
Arrangement
- Mountains can come in a "range" format, where many mountains remain clustered together. In some cases, a single mountain exists. A range consists of mountains laid out in a long narrow belt with many peaks. A mountain group is composed of closely associated mountains lacking the "belt-like" structural characteristic of a range. Hills generally aren't found in similar arrangements.
Variation in Structure and Composition
- Mountains show great variety in structure. The Alps of Europe have one of the most complex structures of any mountain range in the world. Their rocks have been subjected to tremendous forces, resulting in intrusions of dome-like formations of igneous rock. They possess ridges with sharp crests. In contrast, hills have a very simple structure. Much of the sedimentary rock has been stripped away by erosion, exposing the underlying igneous and metamorphic rocks, which form a central core surrounded by a series of concentric ridges and broad crests.

Nature