Why are some mountains much older and lower than others?
Quick Answer
Older mountains have had more time to erode, so they are usually lower and rounder. The Appalachians once rivalled modern high ranges but were worn down over hundreds of millions of years.
The Full Story
A mountain range can start very tall and still end up modest in height if erosion has had enough time to remove rock. Rain, rivers, glaciers, wind, and chemical weathering steadily lower the landscape, especially when tectonic uplift slows or stops. The Appalachian Mountains formed during collisions over 300 million years ago and were once much higher than they are today. By contrast, younger ranges like the Himalayas are still actively being uplifted, so they remain extremely high.
Key Facts
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"Why are ancient mountains gentle while young mountains are towering?"