IGNIMBRITE CALDERAS

Richard Chamberlin

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.

References: