Crater Rings

A volcanic subsidence crater (also called a pit crater or collapse crater) is formed when subsurface magma, supporting the rock overhead, drains away for whatever reason. The unsupported rock "ceiling" over the void left by the draining magma then collapses. (Much the same mechanism accounts for the formation of summit caldera craters, but those are typically at a larger scale.)

A subsidence crater can be distinguished from other craters, such as maars (e.g., Soda Lakes in Nevada), or impact craters, because they have no built-up rim formed by debris ejected out of the crater. They also typically have flat floors, reflecting the former presence of a lava lake.

Crater Rings are two collapse craters on a low, broad shield volcano that is thought to be geologically young (less than about two million years old). The east crater is about half a mile across and 300 feet deep, while the west one is slightly smaller and shallower (approximately 200 feet deep).

This shield volcano lies out in the Snake River Plain north of Mountain Home, Idaho, an area that has seen abundant volcanic activity in the recent geologic past. Despite the ubiquitous volcanism, however, subsidence craters like this are rare in these volcanic fields.