Crater Mountain is carved from greenstone of the Hozomeen Terrane, once the basaltic floor of the ancient Methow Ocean.

The mountain's name probably derives from the craterlike shape of its summit, which was carved from the ancient metamorphosed basalt by modern cirque glaciers.

The rugged battlements of the greenstone viewed from the highway rest above phyllite of the Little Jack terrane, which is exposed in the lower slopes of the mountain.

As a result, the west side of the North Cascades experiences high precipitation, especially during the winter months in the form of snowfall.

[5] Due to its temperate climate and proximity to the Pacific Ocean, areas west of the Cascade Crest very rarely experience temperatures below 0 °F (−18 °C) or above 80 °F (27 °C).