Unlike catastrophism, states that mountains formed slowly over time.
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gradualism

Explanation:

The gradualism suggest that the geological changes are occuring gradually, slowly, over longer period of time. The same goes for the mountains, and the gradualism is suggesting that they did not formed because of catastrophic events, but because of gradual changes in the Earth;s crust caused by the movement of the tectonic plates. The mountains form because of the uplifting that occurs in the Earth's crust when two tectonic plates collide between each other, be it continental and continental, or oceanic and continental. The land around the boundary is slowly lifting up over the course of millions of years, gradually creating a mountain chain.