Menan volcanic complex, Rexburg area, Madison County, Idaho, USAi

Regional Level Types
Menan volcanic complex	Complex
Rexburg area	Area
Madison County	County
Idaho	State
USA	Country

The Menan Buttes are two well-preserved cones of glassy olivine-basalt tuff that rise from the Snake River Plain about 20 miles north of Idaho Falls in southeastern Idaho. The cones were erupted through the water-saturated flood plain of the Snake River, and to this is due their very unusual character. Each cone is about 2 miles long in a northeast direction, but only about two-thirds as wide, and each has a comparably oblate, large summit crater about ½ mile long and 200 to 400 feet deep. The crater rim of the northern cone stands as high as 800 feet above the surrounding plain, and
the southern one 550 feet.

The northeast-trending Snake River Plain is about 50 miles wide at the longitude of Menan Buttes. It has a broad medial upland surface underlain largely by basalt flows of late Quaternary age, and broad marginal alluvial lowlands within which flow most of the streams. The Menan Buttes are at the northwest edge of the southern lowland. The edge of the medial upland is here the river-eroded front of an aa flow of olivine basalt. The flow surface is ragged and nearly free of loess; thus, the flow is probably of Recent age, although it probably is older than the Menan Buttes. Henrys Fork of the Snake River was diverted southward from its general southwest course by the building of the buttes and joins the main fork of the Snake at the east edge of the south cone. Most of the basalt vents of the Snake River Plain are marked by broad, low lava domes. Cinder cones are uncommon, and tuff cones like the Menan Buttes are rare.

The asymmetry of the Menan Buttes cones and their craters clearly demonstrates northeastward drift of pyroclastic ejecta parallel to the prevailing winds. The cones are markedly asymmetric elongate northeastward and their craters lie toward the southwest ends of their midlines. The craters are similarly asymmetric, their northeast walls being the gentlest and the farthest from the vents and standing 100 to 200 feet higher than the rest of the rims.

The cones were built by small fragments of solid glass that were erupted explosively within the craters and then fell to form the inner walls of the craters as well as the outer slopes of the cones. Tuff beds near the angle of repose (35°) only in places near the rim. The crater walls are dip slopes on beds dipping inward at about the angle of repose, and many beds roll over sharply at the rim to dip outward at angles of 20° to 35° on the upper slopes of the cones. Layering is concordant to the gentle slopes around the base of the cones. This is the general case on the steep upper slopes also, where there are however many aberrant dips, both inward and outward, indicating a complex history of overlapping falls. A number of irregular bedding features in the cones can be seen by stereoscopic study of figure 211.1. No fluidal lava features were found on the cones.

Conspicuous layering throughout the tuff is marked by irregular variation in size of glass fragments. Individual layers are a fraction of an inch to several feet thick and are generally thickest high on the cones. The tuff is firmly coherent but porous, and most is soft enough so that it thuds and crumbles when struck by a hammer. The tuff is uniform drab, dark olive gray. It consists largely of sand-size to granule-size fragments of clear, unaltered basaltic glass that is dark in hand specimen but light brown in thin section. There is much pore space, and tiny fragments of similar glass only partly fill the interstices. Palagonite and other amorphous and cryptocrystalline material discontinuously coat most fragments and give them a dull, drab appearance in hand specimen. Glass fragments were solid and too cold to flow when they were erupted. Fragments are subround to angular, are undeformed, and consist of equant granules rather than shards or droplets. Many fragments are microvesicular; a few vesicles are filled, mostly by calcite. Elongate vesicles are common within granules but orientation is random, and neither welding nor secondary flowage was recognized. The glass is of olivine-basalt composition that is probably typical of the region despite the unusual occurrence. The refractive index of the glass in one specimen ranges from 1.610 to 1.616 for yellow light, indicating a silica content of about 4T percent according to the curves presented by Williams and others. The glass contains scattered euhedral crystals of forsteritic olivine ( 2F = 85°-90°) and euhedral laths of labradorite, and sparse octahedra of magnetite. No pyroxene or ilmenite was seen. The Quaternary basalt of the Snake River Plain is olivine-bearing and slightly alkaline, typically contains 45 to 48 percent SiO2, and has late-crystallizing augite and ilmenite (Howard A. Powers, written communication, 1961) and is thus like the Menan Buttes material insofar as they can be compared.