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Why are the Appalachian Mountains lower than younger mountain ranges?

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A car drives along the Trans-Canada Highway with Mount Sir Donald's sharp, snow-capped peak rising above forested slopes in the Selkirk Mountains, British Columbia. — mountain Mount Sir Donald in the Selkirk Mountains, British Columbia, looms above a segment of the Trans-Canada Highway.

The Appalachian Mountains are lower than younger mountain ranges primarily because they are among the oldest mountains on Earth, having been subjected to hundreds of millions of years of continuous erosion.1

While tectonic processes build mountains by elevating terrain, erosional processes—such as the action of water and ice—work ceaselessly to destroy them. For a mountain range to maintain high elevations, tectonic uplift must occur faster than erosion can wear the land down. Because the major mountain-building events that formed the Appalachians, such as the Alleghenian orogeny, concluded hundreds of millions of years ago, the forces of erosion have had vast amounts of time to carve away the peaks and reduce the overall elevation of the range.12345

In contrast, younger mountain ranges have not yet been subjected to such prolonged erosional reduction, allowing them to retain more of their original height. Over geologic time, this ongoing battle between tectonic building and erosional destruction determines the final topography of any mountain belt.23