Surprise, Surprise

Story and Photos by Brush Bash

In the forty years I’ve lived in the Idaho Falls area, I’ve often worked and played in the vicinity of the extinct volcanoes North and South Menan Butte. I’ve driven around to the east side of North Menan to show friends and visitors the one-time sprawling elk farm developed on adjoining private land, and I’ve fished the Snake River at the toe of South Menan. But in all that time I never once hiked the trail up North Menan, which is managed by the Bureau of Land Management (BLM).

In mid-July of this year, when a single overcast day slipped in between the persistent ninety-five degree sunny days, I finally made it.

Before my hike, I stood beside a county road and looked across an alfalfa field at the twin buttes. I was excited to try something new, to leap into the unfamiliar. Would I find the Menan trail too steep, the biting insects too numerous? What surprises did the day have in store? I was ready to find out.

My research had revealed that if I had been standing here ten thousand years ago, I could have witnessed the volcanos erupting and the cones forming. I could have watched as hot basaltic magma erupted through the shallow aquifer and the surface water of a wide river.

Unlike the volcano videos often shown on TV, this lava didn’t flow and destroy everything in its path. At Menan, the erupting magma collided with cold water and rapidly cooled into a volcanic glass called tachylite that shattered into ash and fine fragments as the super-heated steam burst from the crater.

The cloud of particles that hovered in the air after each eruption eventually settled to the ground around the vent. Two cones of “tuff” formed, which grew in size with every explosive eruption and now are among the world’s largest [see “Tuff Cones,” IDAHO magazine, February 2015].

It’s difficult for me to think in terms of geologic time. How many eruptions did it take to form tuff cones nearly seven hundred feet tall? How many years did it take for the tuff material to turn into rock? I can’t even begin to comprehend the horse-sized T-Rex-like dinosaurs that roamed southern Idaho a hundred million years ago. But I hadn’t come to the buttes to think about geologic time. I had come to look into an extinct volcano, talk to the ambitious folks who hike to the top, and discover whatever else might materialize.