The roles of magma and groundwater in the phreatic eurptions at Inyo Craters, Long Valley Caldera, California

Abstract

The Inyo Craters (North Inyo Crater and South Inyo Crater), and a third crater, Summit Crater, are the largest of more than a dozen 650- to 550-yr-B.p. phreatic craters that lie in a 1-km-square area at the south end of the Inyo Volcanic Chain, on the west side of the Long Valley Caldera in eastern California. The three craters are aligned within a 1-km-long northsouth system of fissures and normal faults, and coincide in age with aligned magmatic vents farther north in the Inyo Volcanic Chain, suggesting that they were all produced by intrusion of one or more dikes. To study the sequence and mechanisms of the eruptions, the deposits were mapped, sampled, and compared with subsurface stratigraphy obtained from the core of a slant hole drilled directly below the center of South Inyo Crater from the southwest. The deposits from the two Inyo Craters are fine-grained (median diameter less than 1 mm), are several meters thick at the crater walls, and cover at most a few km² of ground surface. Stratigraphic relationships between the Inyo Craters and Summit Crater indicate that the eruptions proceeded from north to south, overlapped slightly in time, and produced indistinctly plane-parallel bedded, poorly sorted deposits, containing debris derived primarily from within 450 m of the surface. Debris from the deepest identifiable unit (whose top is at 450 m depth) is present at the very base of both Inyo Craters deposits, suggesting that the eruptive vents were open and tapping debris from at least that depth, probably along preexisting fractures, even at their inception. According to ballistic studies, the greatest velocity of ejected blocks was of the order of 100 m/s. All eruptions, particularly the least powerful, selectively removed debris from the finest-grained, most easily eroded subsurface units. Although juvenile fragments have been previously identified in these deposits, they are confined primarily to the grain-size fraction smaller than 0.25 mm dia. and probably did not constitute more than several percent of the deposit. It is therefore suggested that these juvenile fragments were not the main source of heat for the eruptions, and that the eruptions were caused either by: (1) heating of water by fragmented magma that was not ejected before the eruption shut off; (2) slow heating (over months to years) of groundwater under confined conditions without fragmentation of magma, followed by a second process (pressure buildup, seismic faulting, or intrusions) that breached the confinement; or (3) breach of a pre-existing confined geothermal aquifer.