Other Volcanic Rocks In and Near the Monument
Not all of the volcanic rock in the Monument area belongs to the Postpile basalt flow, although much of it has been grouped into a single geologic unit on most published maps of the area. This has led to confusion both as to the composition of the Postpile flow (called andesite on some maps) and as to its source (said to be Mammoth Pass in some reports). Recent studies have given us a better understanding of the variation in rock types and the complex relationships between the different volcanic units. Individual volcanic units previously grouped with the Postpile flow, but here considered separately, are: andesite of Mammoth Pass; rhyodacite of Rainbow Falls; and basalt of The Buttresses (basalt of the Devils Postpile is the youngest unit; the others are listed in order of increasing age).
Lava that erupted from a vent near Mammoth Pass once was thought to be part of, and indeed the source of, the Postpile basalt. This was simply because this lava was erupted from an obviously higher elevation and cascaded into the Middle Fork valley toward the Postpile. The Mammoth Pass rock, however, is andesite, a rock with somewhat more silica than basalt, and resulted from a separate volcanic event. In appearance the andesite lacks the small, but visible, crystals of feldspar that characterize the Postpile basalt.
The oldest volcanic unit formerly included with the basalt of the Devils Postpile is a basalt that makes up The Buttresses, southwest of the Postpile. In appearance this basalt superficially resembles the Postpile basalt, but the abundant visible crystals are mostly pyroxene rather than feldspar, a subtle but important difference between the two rock types. The basalt of The Buttresses was erupted from one or more vents west of the Monument and flowed eastward into the Middle Fork valley.
A totally different type of volcanic rock that occurs near the Monument is a pyroclastic rock or welded-tuff. This tuff formed from volcanic ash that was hot and plastic when it fell and fused into a generally cohesive mass. Exposures of this tuff can be seen behind the old ranger cabin east of Reds Meadow. At this locality the tuff has columnar joints, but the columns are nearly horizontal rather than vertical. The exact source of the volcanic ash is not known, but it is probably related to the extensive deposits of the Bishop Tuff, a welded tuff that erupted from the Long Valley caldera, and can be seen in road cuts near Toms Place on Highway 395.
The most recent volcanic event in the Devils Postpile area, the only one younger than that which produced the Postpile flow, built the Red Cones about one and one-half miles southeast of the Monument. The two basalt cinder cones, with well-preserved summit craters, illustrate what the eroded pile of cinders near the Upper Soda Springs Campground might once have looked like. A basalt lava flow that issued from the base of the southern cone extends down Crater Creek to within one mile of the Middle Fork, perhaps similar to the way that the Postpile flow might have issued from the ancient cinder cone that once existed near the Upper Soda Springs Campground. The Red Cones and their lava flow are so well preserved because they are less than 10,000 years old, and thus escaped the powerful excavating force of glacial erosion that so drastically modified the older volcanic rocks.
Pumice, a frothy volcanic glass, is a porous lightweight material of pastel shades that covers the .ground at various places in the area. Its unweathered, loosely compacted nature indicates it was deposited recently. This pumice was formed in postglacial time when molten rock of high silica content and abundant dissolved gasses erupted from the Mono and Inyo Craters northeast of the Monument.
The recency of the Red Cones and widespread blanket of pumice, the presence of hot springs here and to the east, and the recent volcanic eruptions and earthquakes in Long Valley provide dramatic evidence that volcanoes in this region cannot be labeled "extinct." Within the framework of geologic time, which we tend to think of in terms of millions of years, the dormancy of local volcanism is a barely measurable pause that might, at any time, be abruptly terminated by the onset of new eruptions.