JOURNALOF GEOPHYSICALRESEARCH, VOL. 91, NO. B12, PAGES12,539-12,571, NOVEMBER10, 1986 MOST RECENT ERUPTION OF THE MONO CRATERS, EASTERN CENTRAL CALIFORNIA Kerry Sieh and Marcus Bursik Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena Abstract. The most recent eruption at the gradually diminishing explosiveness of the North Mono Craters occurred in the fourteenth century Mono eruption probably resulted from a decrease A.D. Evidence for this event includes 0.2 km3 of of water content downward in the dike. The pyroclastic fall, flow, and surge deposits and pulsating nature of the early, explosive phase of 0.4 km3 of lava domesand flows. These rhyolitic the eruption may represent the repeated rapid deposits emanated from aligned vents at the drawdown of slowly rising vesiculated magma to northern end of the volcanic chain. Hence we about the saturation depth of the water within have named this volcanic episode the North Mono it. eruption. Initial explosions were Plinian to sub-Plinian events whose products form Introduction overlapping blankets of air fall tephra. Pyroclastic flow and surge deposits lie upon Several dozen silicic eruptions have occurred these undisturbed fall beds within several along the tectonically active eastern flank of kilometers of the source vents. Extrusion of the central Sierra Nevada within the past 35,000 five domes and coulees, including Northern Coulee years; many of these eruptions have, in fact, and Panurn Dome, completed the North Mono occurred within the past 2000 years. This eruption. Radiocarbon dates and violent, exciting, and youthful past suggests an dendrochronological considerations constrain the interesting future and encouraged us to undertake eruption to a period between A.D. 1325 and this study of the timing and nature of one of the 1365. The lack of lacustrine laminae or aeolian most recent eruptions. and fluvial beds between individual pyroclastic The most recent period of volcanic activity in beds suggests that the explosive phases of the this region began about 3 m.y. ago with eruption took place over a period of not more widespread outpourings of basalt and andesite and than several months. Within the resolution of more local extrusions of quartz latite [Bailey et the available radiocarbon and dendrochronologic al., 1976]. Between 2.1 and 0.8 m.y. •go, many dates, the North Mono eruption is contemporaneous cubic kilometers of high-silica rhyolite were with the latest eruption of the Inyo volcanic erupted from centers to the southeast [Metz and chain, about 20 km to the south. However, the Mahood, 1985]. The most voluminous and explosive Inyo tephra blanket clearly overlies, and thus eruption in the region occurred about 0.7 m.y. postdates, all North Mono tephra. Minor ago. At that time, the evacuation of some disturbance of the North Mono tephra prior to 600 km• of high-silica rhyolite from a deposition of the Inyo tephra indicates that the subterranean chamber produced subsidence of •he period of time between the North Mono and Inyo Long Valley caldera (Figure 1) and the widespread eruptions was probably no more than a year or dispersal of air fall and pyroclastic flow two. This near contemporaneity of the two deposits [Bailey et al., 1976; Izett and Naeser, eruptions suggests a genetic relationship. 1976; Hildreth• and Mahood, 1986]. Liquefaction of North Mono sands on the floor of In the 0.7 m.y. subsequent to the climactic Mono Lake occurred twice during the waning stages caldera-forming event, many smaller eruptions of of the North Mono eruption and 3 times basaltic to rhyolitic magma have occurred within immediately before and after pulses of the Inyo the western half of the caldera [Bailey et al., eruption. This is evidence that five earthquakes 1976]. Seismic and geodetic activity within and of ML • 5.5 occurred during the North Monoand south of the caldera since 1978 appears, in fact, Inyo eruptions. The chemical and textural to be related to modern movement of magma or similarity of the erupted products and their other fluids beneath the caldera (see summary of nearly simultaneous evacuation from aligned vents recent investigations by Hill et al. [1985]). indicates that the North Mono eruption resulted The youngest manifestation of volcanic from intrusion of a dike beneath the northern activity on the eastern flank of the central 6 km of the volcanic chain. Several observations Sierra Nevada is a chain of craters and silicic suggest that dike intrusion beneath the Mono flows that extends 40 km northward from Long Craters has replaced normal faulting as the Valley caldera to Mono Lake. This zone of recent mechanism for elastic rebound and permanent activity comprises the volcanos of Mono Lake, the extension of the crust at this latitude. domes and flows of the Mono Craters, the Inyo However, dike widths compatible with relief of domes and craters, and the phreatic explosion purely tectonic strains (•3 m) are probably too pits of Mammoth Mountain (shown in black in narrow to have allowed the North Mono magma to Figure 1). These youthful features have erupt. Overpressurtzation of the Mono Craters attracted the attention of geologists for over a magma reservoir by another mechanism, perhaps century. Earlier investigations include those of magma mixing, appears necessary as well. The Russell [1889], Mayo and others [1936], Putnam [1938], Rinehart and Huber [1965], Kistler [1966] Copyright 1986 by the American Geophysical Union. and Huber and Rinehart [1967]. The oldest known pyroclastic products from the Paper number 6B5929. chain are the rhyolitic air fall beds found 0148-0227 / 86/006B-5929 $05.00 throughout 35,000- to 12,000-year-old lacustrine 12,539 12,540 Sieh and Bursik: Eruption of MonoCraters AboutA.D. 1350 O 10 KM , , , , , , , ,,, , l190W MONO CRATERS CRATERS LONG VALLEY CALDERA Fig. 1. The Mono and Inyo craters and volcanoes of Mono Lake (in black) form a 40-km-long chain of Late Pleistocene and Holocene craters, domes, and flows which have been emplaced along the tectonically active eastern flank of the Sierra Nevada. Pattern of latest Pleistocene and Holocene faults is from our unpublished mapping. beds around Mono Lake [Lajoie, 1968; Lajoie and phase, progressed through a period dominated by Robinson, 1982]. Wood [1977a, 1984] estimated pyroclastic flows and surges, and ended with a the oldest exposed domes to be about 35,000 years period during which viscous, block-encrusted lava old. domes and flows were extruded. The products of Most of the young domes, flows, and craters the earliest of these three phases are air fall between Mammoth Mountain and Mono Lake are beds that blanket at least 8000 km2 of the region Holocene in age. Using obsidian hydration-rind surrounding Mono Lake. The pyroclastic flow and thicknesses calibrated by one radiocarbon date, surge deposits of the second phase are limited to Wood [1977a, 1984] estimated ages of 10,000 to an area of approximately 100 km2 around the 640 years for all but four of the 24 currently source vents, and the lava domes and flows cover exposed domes and flows of the Mono Craters. All only 6 km2 in the immediatevicinity of the the Inyo domes and craters have formed during the eruptive vents. These deposits are described Holocene epoch [Miller, 1985], and Stine [1984] below, in the general order of their eruption. has shown that the volcanic islands of Mono Lake are less than 1800 years old. The dacitic lavas Air Fall Beds and minor pyroclastic beds found on the northern flank of Paoha, the larger of the two principal Shallow natural and hand-dug exposures in the islands, were erupted during the youngest known region of Mono Lake commonly reveal a distinctive eruption in the region, no more than about 200 packet of planar, well-sorted ash and lapilli years ago. beds. Figures 2 and 3 are photographs of two such exposures; a locality characterized by thick Products of the Eruption and coarse beds deposited near the source vents is illustrated in Figure 2; a site characterized We have divided the products of the most by a thin set of fine-grained beds deposited far recent eruption of the Mono Craters into three from the source vents is illustrated in groups, on the basis of their timing and areal Figure 3. Although this series of air fall beds extent. As is commonto silicic eruptions (see, varies markedly in thickness and coarseness, we for example, Williams and McBirney [1979, p. 75] have been able to correlate the series throughout and Fisher and Schmincke [1984, p. 63]), this the region by tracing distinctive individual episode began with its most explosive (Plinian) beds. Steh and Burstk: Eruption of Mono Craters About A.D. 1350 12,541 The air fall blanket is well preserved where it is thick (• 50 cm) and where it was buried soon after deposition. Fortunately, rapid burial (by pyroclastic flows on the plains south of Mono Lake, by clays beneath Mono Lake, by windblown sand northeast of the lake, and by debris flows in the canyons of the Bodie Hills) protected these beds from erosion and bioturbation at numerous informative localities. Thin blankets of the ash left unburied at the surface have been bioturbated by plant roots and organisms or eroded by nonbiological agents. At sites within several kilometers of the source vents the various air fall beds are composed predominantly of angular, aphyric white lineated pumice, with lesser amounts of gray microvesicular glass and black obsidian. Analyses of obsidian clasts by electron microprobe (S. Newman et al., personal communication, 1985) indicate that all of the air fall beds are high-silica rhyolite. In thin section, opaque microphenocrysts and anhedral grains of biotite and feldspar are rare but present.