The Menan Buttes

Rising abruptly from the flat Snake River Plain northwest of Idaho Falls are the Menan Buttes, two of the largest volcanic tuff cones on Earth. They look like a matched pair of steep, grass covered hills, each about two miles long with a deep crater at the summit, standing hundreds of feet above the plain. What makes them remarkable is how they formed.

Roughly ten thousand years ago, rising basalt magma hit the water soaked ground and shallow aquifer of the Snake River flood plain. The water flashed instantly to steam and blew the molten rock apart into tiny glassy fragments that piled up and welded into these cones. Because the magma met fresh river water rather than seawater, North Menan Butte is recognized as a National Natural Landmark, celebrated as one of the very few volcanoes in the country known to have erupted through fresh water.

The quick chilling turned much of the lava straight to volcanic glass. A public trail climbs North Menan Butte for a sweeping view across the plain to the Tetons. If you are driving between Teton Valley and points west, these twin buttes are a reminder that the same restless geology that raised the mountains also built quieter wonders out on the plain.