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New light on Mammoth Mountain and Long Valley Caldera 3 March 2010

Posted by admin in Long Valley, Mammoth Mountain, volcanoes.
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New research (link to abstract) in the March 2010 issue of the Geological Society of America Bulletin sheds new light on the volcanism of the Long Valley caldera complex in eastern California, clarifying the eruptive sequence and the character of eruptive activity over the last 190,000 years.

Four eruptive sequences have been identified by Gail Mahood of Stanford University and her co-researchers, who used argon isotopes to study the lavas at Long Valley Caldera and Mammoth Mountain, which straddles the caldera’s south-west rim: the western moat sequence (~190,000–160,000 years ago), the Mammoth sequence (~120,000–58,000 years ago), the northwest caldera sequence (~41,000–29,000 years ago) and the Inyo chain sequence (from ~9,000 years ago to the present). This dating of the silicic lavas in the north-west sector of the caldera to ~41,000-29,000 years ago shows them to be rather younger than previously thought.

Interestingly, mafic and silicic lavas were produced simultaneously during the eruptions: ‘in each eruptive sequence mafic and silicic lavas erupted contemporaneously from spatially associated vents’. The authors suggest that if alkali basalt intruding into the crust caused the silicic eruptions, potentially small-volume silicic eruptions may be produced by this mechanism in future, producing explosive eruptions but within a limited area: they note that ‘In the past 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone subject to volcanic hazards’. This is useful information when it comes to planning for future eruptive activity. The eruption of basalt lava also indicates that no large magma body, which would absorb the basaltic material, is present below the caldera, Professor Mahood told the keen supervolcano fans at Discovery News.

The authors also note that their results provide ‘equivocal support for a suggested anticorrelation between volcanism and glaciation for the past 800,000 years in eastern California’ – that is, the theory that glaciation and volcanism alternated over that period, with maximum volcanism occurring during interglacial periods.

  • Gail A. Mahood, Joshua H. Ring, Simone Manganelli & Michael O. McWilliams, ‘New 40Ar/39Ar ages reveal contemporaneous mafic and silicic eruptions during the past 160,000 years at Mammoth Mountain and Long Valley caldera, California’, Geological Society of America Bulletin, vol. 122, nos. 3-4 (March 2010), pp. 396-407. [doi: 10.1130/B26396.1]. Link to abstract.

California supervolcano has split personalityDiscovery News, 2 March 2010

Global Volcanism Program: Long Valley – information summary for Long Valley Caldera (120314-A)
Global Volcanism Program: Mammoth Mountain – information summary for Mammoth Mountain (1203-15-)
USGS Long Valley Observatory – website for the LVO, which keeps an eye on what the caldera is up to
Geological history of the Long Valley Caldera – from the LVO

The Volcanism Blog