There are many volcanic mountains in southern Idaho, particularly within the Snake River Plain Province. These mountains generally consist of individual cones of cinder and extrusive igneous rock. The volcanic material was extruded through a central vent in the earth's crust and piled up on the surface to form a cone. In Idaho, volcanic mountains tend to be smaller than other types of mountains, generally less than 1,000 feet high. They also tend to be isolated and erratically distributed, although they are commonly aligned along rifts or fissures such as the Great Rift of the Snake River Plain. Volcanic mountains are generally dome to conical shaped and are symmetrical in plan view. As a general rule, volcanic mountains consisting mostly of cinders and tuffaceous material are the most susceptible to erosion of all mountains.
Erosional mountains are found in regions of crustal uplift such as the central Idaho uplands. They are characterized by steep gorges, precipitous slopes and youthful streams. Idaho's erosional mountains have primarily been carved by glaciers and running water and are the result of hundreds of thousands of years of erosion in the intervening valleys.