Mono-Inyo Craters

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One of the Mono craters: an excellent example of a rhyolite dome.

Mono-Inyo Craters in California, U.S.A., are a nearly straight line of small volcanoes that stretch from Mammoth Mountain and Long Valley Caldera in the south to Mono Lake in the north. The Inyo Craters form the southern part of that line and are either phreatic (steam explosion) volcanoes or rhyolite domes. Mono Craters phreatic volcanoes except they have since been either plugged or overtopped by rhyolite domes.

Eruptions at the Mono-Inyo Craters volcanic field occurred at roughly 500-year intervals over the past 2,000-3,000 years. The most recent eruption in the region was at Mono Lake between 1720 and 1850. A dome grew on the lake floor and emerged to make Paoha Island.

The two southernmost Inyo Craters are in a forested area and are open pits about 600 feet (183 m) across, each with small ponds covering their floors. The magma that created these craters never reached the surface, thus there is no fresh ash or solidified lava there. These craters were formed when heat from the raising magma superheated groundwater until the overlying dirt and rock could no longer contain the pressure, resulting in a massive steam explosion. Radiocarbon dating of a log buried in the debris blanket gave an age of 650 years.

North of these craters are five rhyolite domes, including; Deadman Creek Dome, Glass Creek Dome, Obsidian Dome, and Wilson Butte. These domes are composed of gray rhyolite, frothy pumice, and black obsidian. Radiocarbon dating of trees felled and buried by these volcanoes, indicates ages ranging from 500 to 1,000 years.

Mono Craters to the north sit along the eastern edge of Pumice Valley, a large caldera volcano. Radiocarbon dates for the Mono Craters gives ages of 550 years for the youngest dome to 40,000 years for the oldest. All but four of the 24 exposed domes and flows of the Mono Craters are less than 10,000 years old. The most recent eruptive episode occurred between 1325 and 1365, during which time there were several explosive eruptions and five separate lava flows that oozed onto the surface, including Panun Dome and North Coulee flow.

Aerial photo of Panum Crater

Panum Crater is the northernmost volcano in the sequence and is a good example of both a tuff ring (a type of volcano created in a phreatic eruption) and a rhyolite dome. Its structure is two-fold; an outer tuff ring (forming a classic crater) created 1,200 years ago and an inner plug, or dome of rhyolite, pumice and obsidian created from lavas 700 years ago. In this case the magma feeding Panum reached the surface as lava after its heat had already created a steam explosion crater. Other Mono Craters also were formed in this manner, but their plug domes grew larger than their tuff ring craters.

Curiously, Lunar Orbiter spacecraft images reveal fields of volcanic domes that indicate deep-seated, high-silica eruptions on the Moon, possible sources of some tektites found on Earth. These lunar domes are similar to the Mono craters. Researcher Darryl Futrell demonstrated that Mono obsidians resemble some layered tektites on the macroscopic level.

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[edit] References

Alt, David; Donald Hyndman (2000). Roadside Geology of Northern and Central California. Missoula, Montana: Mountain Press Publishing Company. ISBN 0-87842-409-1.
Harris, Stephen L. (2005). Fire Mountains of the West, 3rd edition, Missoula, Montana: Mountain Press Publishing Company. ISBN 0-87842-511-X.