Kvasness, Astri

The Menan Buttes, Madison County-Jefferson County border, southeastern Idaho

The Menan Buttes are two basaltic tuff cones (Creighton, 1987). The southern butte is .3 km3, whereas the northern butte is .7 km3, giving them a total volume of the same scale as the 1980 Mt. St. Helens. The two buttes are stretched towards the N-E, suggesting the wind direction at the time of eruption. They rise up to 250 meters above the plain and 100 meters deep craters in the middle. The ages of the buttes are unknown, but are described as "young", and middle to late Pleistocene in age. The buttes are created out of the same kind of basalt as the snake-river plain. Looking at a profile, it is obvious that something is radically different between the underlying basalt-shield and the buttes. This has been explained to be the result of incorporation of meteoric water from the Snake River Plain aquifer and water from the river Henrys fork (or equivalent) that runs nearby. The water first got incorporated at a shallow level, creating a phreatomagmatic explosion that caused the tuff-rings to form. The high water-content contributed to the alteration of the volcanic material, and the ash fall is consolidated by a lot of clays from alteration. The sideromelane-ash with scoriaceous lapilli makes the deposits strongly consolidated. The eruption is believed to have tapered until a late stage where water got incorporated at a deeper level. This second phreatomagmatic explosion caused a gun-barrel-effect. A high column of ash and water rose from the ring and settled at a much steeper angle near the summit. This created a tuff-cone. The tuff-cone had an angle of deposit that was higher than the angle of repose (up to 55°) thus the material started to flow slowly down the sides of the hill. This created large slump-blocks and internal folding. There are less than 3% of xenoliths in this cone. The ones that are found are rounded pebbles from the gravel of the underlying aquifer.

References:
Creighton, D. N., 1987. Menan Buttes, southeastern Idaho. Geological Society of America Centennial Field Guide- Rocky mountain Section. 109-111.