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Ignimbrite calderas are large volcanic depressions, often 10-20 miles in diameter, that form when a large-volume, gas-charged viscous magma body ejects huge columns of ash that collapse and inundate the surrounding countryside with a thick blanket of welded ash (called ignimbrite) while the shallow roof of the chamber collapses. Caldera-forming eruptions are relatively rare catastrophic events, second only in scale to large asteroid impacts. Deep magma systems that feed these "supervolcanoes" appear to march to the beat of there own drummer; eruptions are both episodic and irregular in timing and intensity.

Kilometer thick piles of "dirty" lithic-rich welded tuff (fused ash) commonly mark the collapsed cores of ignimbrite calderas in the highly faulted and extended terrane of the central Rio Grande rift near Socorro. A detailed account of the eruptive history of the 31.9 Ma Socorro caldera is now possible through high-precision 40Ar/39Ar dating of numerous eruptive units previously defined by detailed geologic mapping (Chamberlin, McIntosh and Eggleston, in press). Our research indicates the Socorro caldera became unusually quiet shortly after the primary ignimbrite eruption. Waning-stage pulsating eruptions and the crystal-rich character of the ignimbrite imply that the magma body under the Socorro caldera literally ran out of gas to drive the explosive eruptions; also the magma body became immobilized and locked by crystals soon after the caldera collapsed (Chamberlin, 2001).

Photo: Comagmatic lag breccias in the upper caldera-facies Hells Mesa Tuff near the Bursum mine in the northeastern Chupadera Mountains. Outcrop shows weathered top of steeply east-dipping breccia zone.



  1. Chamberlin, R.M., McIntosh W.C., and Eggleston T.L., 2004, 40Ar / 39Ar Geochronology and Eruptive History of the Eastern Sector of the Oligocene Socorro Caldera, Central Rio Grande Rift, New Mexico: in Tectonics, Geochronology and Volcanism in the Southern Rocky Mountains and Rio Grande Rift, S.M. Cather, W.C. McIntosh and S.A. Kelly (eds.), New Mexico Bureau of Mines and Mineral Resources Bulletin.
  2. Chamberlin, R.M., 2001, Waning-stage Eruptions of the Oligocene Socorro Caldera, Central New Mexico: in Volcanology in New Mexico, L.S. Crumpler and S.G. Lucas (eds.), New Mexico Museum of Natural History and Science Bulletin 18, p. 69-77.
  3. Chamberlin, R.M., 2001, Modal Mineralogy, Textural Data and Geochemical Data for Caldera-facies Hells Mesa Tuff and a Comagmatic Lava Dome, with Selected Data Plots and Supporting Map Data: New Mexico Bureau of Geology and Mineral Resources Open-file Report 458, 41 p.
  4. Chamberlin, R. M., 1994, Indications of a faulted and mineralized Late Triassic caldera margin near Eumundi, southeastern Queensland; Queensland Government Mining Journal, v. 95, no. 1108, p. 20–37.