Abstract - Long Valley caldera, California, formed during the cataclysmic Pleistocene eruption of the Bishop Tuff (c. 750 km3 of magma erupted). Previous stratigraphic and petrologic studies of this eruption deciphered an intriguing, but ill-explained, pattern of vent migration, thought to mirror the lateral propagation (‘unzipping’) of magma-tapping ring-fractures during caldera collapse. We present new physical modeling data that systematically illustrate how a range of magma chamber plan-view ellipticities may affect patterns of ring-fracture localization and propagation during magma chamber roof subsidence and caldera collapse. From these models, we show that the pattern of vent migration and ring-fracture unzipping at Long Valley was intrinsically related to two interlinked factors: 1) the high ellipticity of the pre-collapse magma chamber roof; and 2) the occurrence, upon the onset of chamber depressurization and roof sagging, of maximum pre-failure shear strain along the elliptical roof’s short axis. Other elliptical calderas also host major vents near the ends their short axis and are inferred to have developed with similar unzipping patterns to those in experiment and at Long Valley. The plan-view ellipticity of a magma chamber roof may thus decisively influence the location and dynamics of elliptical ring fracturing and eruption at all scales in nature.
Abstract of talk given to:
International Geological Congress, Oslo, August 2008.