An 861-m-long hole has been cored on a slanted trajectory that passed directly beneath South Inyo Crater in the west moat of Long valley Caldera, California. The purpose of the hole was to investigate the magmatic behavior that led to surface deformation and phreatic activity during the 600-year-old eruption of the Inyo vent chain. The hole was sited 216 m southwest of the crater, passed beneath its center at a depth of 566 m, and terminated 79 m northeast of the crater center at a depth of 810 m. Metamorphic basement was encountered at a depth of 779 m. The volcanic and sedimentary sequence consists solely of post-Bishop Tuff caldera fill, including 319 m of moat basalt and 342 m of early rhyolite, and is nearly 900 m thinner than in a Unocal Corporation well 900 m to the southeast. Apparently, a major fault lies between the two holes and forms part of the western structural boundary of the caldera, 3--4 km inboard of its topographic boundary. Breccia zones that intrude the caldera fill were intersected at 12.0--9.3 m and 1.2--0.8 m SW and 8.5--25.1 m NE of the crater center.

The largest breccia unit is symmetrically zoned from margins rich in vesicular rhyolite and locally derived rhyolite wall rock, with most clasts ≤0.1 m, to a center of up to 50 vol % basalt, with individual basalt clasts to 1 m in intersected length. The basalt appears to be an early feeder for the moat basalt sequence. The vesicular rhyolite is chemically distinct from both the rhyolite wall rock and previously recognized 600-year-old Inyo eruptives but matches fresh-appearing pyroclasts in the crater ejecta. This component of the breccia is therefore interpreted as representing juvenile magma that vesiculated and fragmented as it rose along the margins of an older basalt intrusion. During this process, wall rock fragments were ejected from depths extending to at least 800 m. Progression to a magmatic eruption was probably prevented by rapid influx of groundwater into the feeder. The chemical data do not support the hypothesis that a single, simple dike exists under the entire segment of the Inyo chain that was active 600 years ago. If such a dike exists, it apparently tapped different magma chambers along its ~10-km length. The excess of surface extension over that needed to accommodate the intruded juvenile material may reflect the presence of a larger intrusion at depth, below the level of fragmentation, or the contribution of tectonic activity to the surface deformation. ¿ American Geophysical Union 1988