Ring-fracture eruption of the Bishop

WES HILDRETH U.S. Geological Survey, Menlo Park, 94025 GAIL A. MAHOOD Department of Geology, Stanford University, Stanford, California 94305

ABSTRACT ring-fault propagation and incorporation of INTRODUCTION fragments of locally distinctive, caldera-mar- Lithic fragments of precaldera basement gin lithologies into successive outflow sheets. Recent modeling of ash-flow formation by rocks in the Plinian fallout deposit of the The change from a single-vent Plinian mode collapse of columns (for exam- Bishop Tuff indicate that the eruption began of eruption to multiple vents along the ring ple, Sparks and others, 1978) is based upo:i the in what is now the south-central part of Long fault took place after emplacement of as little concept of single-vent eruptions, influenced Valley caldera, along or adjacent to the Hil- as 20% of the total Bishop ejecta. Approxi- largely by data from the deposits of eruptions of ton Creek fault. The earliest ash flows origi- mately two-thirds of the total eruptive vol- small-to-moderate volume. Wide acceptant of nated there as well, but contrasting lithic ume was trapped within the subsiding such models has promoted their over-apjilica- contents in seven! later ash flows indicate caldera. tion and has tended to obscure the multiple-vent

Figure 1. Present outcrop of Bishop Tuff, lithic sample locations (dots), and places mentioned in text. Drilling indi- Adobe cates >1,500 m of unexposed Bishop 2045 Valley Tuff beneath intracaldera fill. Eleva- x 2003 Mono tions (in metres) give an impression of caldera and outflow relief. Large intra- Basin L Mono l caldera graben (simplified) crosses Craters s crest of resurgent dome, and small western one marks late Holocene erup- tive alignment (Bailey and Koeppen, 1977). CDM= Casa Diablo Mountain, LC = Lake Crowley, RM= Reds Mea- dow, SL = Sotcher Lake.

^ Bishop Geological Society of America Bulletin, v. 97, p. 396-403, 5 figs., April 1986. # 1265

396

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nature and ring-fracture mechanism of truly voluminous ash-flow eruptions. By combining petrochemical evidence for a zoned eruptive se- quence and lithologic evidence that compares circumcaldera assemblages of basement rocks with the accidental lithic fragments in successive pyroclastic subunits, we here document an ex- ample of a large eruption that had multiple vents, changes in vent location with eruption progress, and a sectorial sequence of ash-flow emplacement lobes. Eruption of the Bishop Tuff 0.73 m.y. ago was accompanied by subsidence of the 400-km2 in eastern California (Gil- bert, 1938; Bailey and others, 1976). More than 50 km3 of rhyolitic magma dispersed widely as ashfall (Izett and others, 1970); at least 200 km3 produced variably welded outflow sheets origi- nally >2,200 km2 in area; and -500 km3 ponded and welded within the synchronously collapsing caldera (Fig. 1). The intracaldera tuff is nowhere exposed, but geothermal drill holes have penetrated -1,500 m of it, the deepest hole to date (2,100 m) hav- ing bottomed in Bishop Tuff without reaching the subsided precaldera floor beneath it (Sorey, 1985). Postcaldera fill atop the intracaldera tuff is as thick as 700 m, and exposed relief on the caldera walls is today still >1,000 m in the northeast sector, >700 m on the south wall, and 250-600 m on the west and northwest. Total subsidence of the cauldron block was, therefore, at least 2-3 km, which suggests that caldera-wall slumping and grinding on ring faults could have greatly supplemented explosive reaming at vents as the means of introducing precaldera rock fragments into the erupting pyroclastic ejecta. Lithic contents of 1% to 3% are typical of out- flow exposures and, if representative of the total eruptive volume, indicate that the Bishop Tuff 3 contained no less than 7 km and probably >20 Figure 2. Generalized distribution of precaldera rocks. None are exposed inside the deeply 3 km of accidental lithic fragments. The fraction downfaulted caldera. (A) Pre-Cenozoic granitoid plutons, Mount Morrison pendant metased- of lithics in the intracaldera tuff could well be imentary rocks (Pz ms), and Ritter pendant metavolcanic rocks (Mz mv). Other granitoids significantly greater, as in the case of deposits include: diorite and gabbro (di), granite of Lee Vining Canyon (lvc), granite of June Lake (jl), described by Lipman (1976,1984) and by Hil- granite of Casa Diablo Mountain (cdm), granite of Mono Recesses (mr), and granite of Deer dreth and others (1984). Spring (ds). (B) Quaternary and Tertiary volcanic rocks (QTv). Mafic and intermediate rocks are as old as 3.2 m.y. (Bailey and others, 1976); Glass Mountain are 2.1-0.8 m.y. old ERUPTIVE SEQUENCE (Metz and Mahood, 1985). Geology adapted from Rinehart and Ross (1957,1964), Huber and Rinehart (1965), Kistler (1966), Crowder and Sheridan (1972), Krauskopf and Bateman The Bishop Tuff can be divided into eight (1977), and Bailey and Koeppen (1977). easily distinguishable emplacement units. Their sequence, mineralogy, and penological zonation have been discussed elsewhere (Hildreth, 1979); three of which are recognized in the southeast- the Mono Basin lobe. In all sectors, there is an so, only a synopsis is given here. As much as 5 m ern sector and two, in the southwestern. Simple upward change toward less-evolved of inversely graded, Plinian pumice fall (Bate- cooling units that extend north into Adobe Val- compositions and higher Fe-Ti-oxide tempera- man, 1965, Fig. 65), which is exposed only east ley and northwest into Mono Basin consist pre- tures, which indicates progressive tapping of a and southeast of the caldera (see Fig. 4 below), dominantly of tuff that was emplaced late in the zoned magma reservoir. A little mixing of pum- had accumulated before it was overrun by the over-all sequence, but, at proximal locations, ice from different levels of the reservoir or from earliest ash flows. Partial cooling breaks locally highly evolved tuff identical to that emplaced successive phases of the eruption did take separate successive ash-flow emplacement units, earliest in other sectors crops out at the base of place (Hildreth, 1985), late in the emplacement

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sequence especially, but, on the whole, there was pre-Cenozoic metamorphic and granitoid rocks Crest Quartz Monzonite, and the granites of a remarkably orderly compositional-stratigraph- (Fig. 2A) and Pliocene-Pleistocene volcanic June Lake and Casa Diablo Mountain ic progression. Changes in the composition and rocks (Fig. 2B). The latter are divisible into (1) (Fig. 2A). variety of phenocrysts correlate with increasing widely scattered basalts and andesites; (2) quartz magma temperature (Hildreth, 1979), and, al- latites, mostly in the northwestern quadrant; and LOCATION OF THE PLINIAN VENT though neither the first appearance of pyroxenes (3) high-silica rhyolites of Glass Mountain, in nor the later disappearance of allanite coincided the northeastern quadrant. All of these rocks Fallout deposits beneath the ash-flow sheets strictly with emplacement-unit boundaries, such were truncated during caldera subsidence and are exposed at only six locations (Fig. 4), Most changes are helpiiil indicators of the eruptive occur as readily identifiable lithic fragments in of the ashfall remnants identified beyond this sequence. various subunits of the Bishop Tuff. protecti ve capping (Izett and others, 1970) are at Emplacement probably required only hours Equally distinctive asymmetries exist in the least partly reworked, and nearly all are >300 or days and, in any event, was rapid enough that distribution of the pre-Cenozoic basement lith- km from the source. Neither the total vok.me of no erosion or significant reworking took place ologies (Fig. 2 A). The eastern half of the caldera fallout nor the proportion of Plinian vis-a-vis between successive units, which share the same is surrounded almost exclusively by Mesozoic coignimbrite components in the fallout is, there- compound cooling zonation wherever they are granitic-to-dioritic plutonic rocks, whereas those fore, well established. Scarcity of exposure also superimposed. For example, the partial cooling around the western half are about equally di- renders very imprecise any attempt to determine breaks between the three southeastern em- vided among plutonic rocks, Mesozoic metavol- either an isopach focus or a dispersal axis lor the placement units are well expressed only distally canic rocks, and Paleozoic metasedimentary initial Plinian phase of the eruption. Hand pick- where much of the tuff is nonwelded or partially rocks of the Mount Morrison pendant (Rinehart ing of accidental lithic fragments from stratified welded; in thicker sections closer to the caldera, and Ross, 1957, 1964). The latter are predomi- fallout deposits at the six near-source locations the three cooled together to form a single, nantly metasiltstones and metapelites but also (Fig. 4), however, has permitted rather precise densely welded sheet. include marble, calc-hornfels, and distinctive location of the Plinian vent, that is, where erup- light-gray quartzite (Fig. 3). The plutonic lithol- tion of the Bishop Tuff began, prior to the be- PRECALDERA BASEMENT ROCKS ogies truncated by the caldera include a few ginning of subsidence and the opening o." ring minor granitic and hypabyssal rocks but are vents. The precaldera basement exposed at the mar- principally the distinctive Round Valley Peak All exposed lithic fragments larger than ~3 gins of Long Valley caldera consists entirely of Granodiorite, the strongly porphyritic Wheeler mm were picked from outcrop areas of typically

j 1 i i km Figure 3. Map of part of the Mount Morrison pendant, simplified from Rinehart and Ross (1957,1964), showing probable eruption site of the airfall and earliest a«;h-flow phases of the Bishop Tuff. Symbols: wc - Wheeler Crest Quartz Monzonite, rvp = Round Valley Peak Granodiorite, gr = granite of McGee Mountain, Q = quartzite, S = slate, M = marble, C = calc-hornfels, H = hornfefsed metasiltstones and metapelites, Qv = post caldera silicic volcanic rocks.

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5-10 m2. About 100-400 lithics from each lo- Suites of lithics from all fallout locations the eruption did not break out in what is now cation were returned to the laboratory, washed, (Fig. 4) are dominated by dark colored metasilt- the northwestern part of the caldera (compare identified by hand lens (and by microscope stone and metapelite, but they also contain 9-27 with Fig. 2B). Similarly, the absence of frag- when necessary), tallied according to lithology, wt % granitoids and 5-30 wt % light gray quartz- ments of the obsidian and phenocryst-poor fel- and weighed. A few bulk samples of fallout and ite. Numerically, quartzite constitutes only 2% site of precaldera Glass Mountain (Metz and of nonwelded ash-flow deposits were sieved in to 6% of the lithic fragments picked, but its Mahood, 1985) in the fallout excludes the cal- order to examine still smaller lithics, but the greater mean size (commonly 3-5 cm) enhances dera's northeastern quadrant as a likely source proportions changed little. Because disintegra- its weight fraction. Granitoid lithics in the fallout of the Plinian outburst (Fig. 2B). About 4-9 wt tion of weathered granitoid and mafic-volcanic that are large enough to identify are mostly % of the airfall lithics are porphyritic glassy lithics is commonly severe during sieving, hand Wheeler Crest Quartz Monzonite. The virtually , which is commonly flow foliated, picking may actually provide more representa- complete absence in fallout deposits of basaltic mostly slightly vesicular or breadcrusted, and tive data for deposits as old as the Bishop Tuff. and intermediate-volcanic lithics indicates that thought to represent cognate fragments of Bish- op Tuff magma that had extensively degassed in transient conduits (dikes) prior to disruption. The predominance of metasedimentary lithics in the fallout clearly places the Plinian vent

upper

lower

Figure 4. Weight proportions of granitoid (G), quartzite (Q), other metasedimentary (M), rhyolitic (R), quartz-latitic (L), basaltic to andesitic (B), and cognate vitrophyric (V) lithic fragments found in the Plinian fallout and in the northern and southwestern ash-flow sectors at representative locations. The bolder circles show data for the Plinian fallout deposit, which is exposed only east and southeast of the caldera. Upper, middle, and lower refer to stacked cooling units where more than one is present; top and base refer simply to the local limits of vertical exposure. Outlines of Bishop Tuff and Long Valley caldera as in Figure 1.

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somewhere along ¡in intracaldera projection of (Fig. 1) are older than the caldera (Bailey and deposits. Prevalence of dense welding, which re- the Mount Morrison pendant. In combination others, 1976; Bailey and Koeppen, 1977), and quired chipping of lithics from many outcrops, with the apparent absence of Round Valley their en echelon arrangement may have helped together with clayey weathering of basaltic lith- Peak Granodiorite. the importance of quartzite to focus growth of the magmatic system here in ics. and granular decomposition of plutonic ones, and Wheeler Crest Quartz Monzonite further the first place. The medial graben across the commonly makes quantitative extraction impos- restricts possible v<;nt locations to an intracal- postcaldera, structurally resurgent dome (Bailey sible. Reproducibility is consequently no 1 letter dera extension of the narrow zone where these and others, 1976) also lies within this en echelon than about ±10%, although lithic propoitions lithologies coexist, immediately west of the Hil- zone. The lithic evidence indicates that it was determined by combined collection and field ton Creek fault (Fig. 3). A vent location at the within this zone, probably in the south-central counting show such striking differences among southern end of this zone (Figs. 2 and 3), in part of the present caldera—where much of the emplacement units, both vertically and sœtor- distinction to sites near the center of the caldera, 1980-1985 seismicity has also taken place ially (Figs. 4 and 5), that the uncertainties are is clearly favored by (1) the absence from fallout (Sanders, 1984; Cockerham and Pitt, 1984)— unimportant to our conclusions. deposits of lithic fragments from the precaldera that the Bishop eruption began with a Plinian Another problem, locally troublesome, is en- volcanic rocks that strike so widely into the outburst. trainment by the ash flows of lithic debris from north margin of the caldera (Fig. 2B) and (2) the the overrun surface. This is identifiable princi- highly asymmetric distribution of presubsidence SOURCES OF SUCCESSIVE pally where ash flows poured through rugged ash flows, deposits of which are thick and wide- ASH-FLOW LOBES terrain or down steep slopes, as well as in near- spread to the southeast but thin, nonwelded, and basal exposures adjacent to topographic obsta- only locally present north of the caldera. The Collection of reliable lithic data for the ash- cles. Such occurrences have been excluded from Hilton Creek and other range-front faults flow sheet is more difficult than for the fallout the present analysis. Throughout most of the

upper

N A

O 2 4 6 8 10 middte

lower

Little Round Valley upper

lower

Figure 5. Weight proportions of lithic fragments at representa' tive locations in southeastern and eastern ash-flow sectors. Sym bols as in Figure 41.

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Bishop Tuff, however, surficial entrainment aphyric rhyolitic felsite and obsidian of Glass tally, in near-basal exposures where basaltic lith- appears not to have introduced a recognizable Mountain. As summarized in Figure 5, many ics are correspondingly fewer (Fig. 4). fraction of the lithics present; lithic populations, westerly outcrops of the uppermost sheet have even in many basal ash-flow exposures, gener- higher fractions of granitoid lithics (principally Southwestern Sector ally correspond to the caldera-margin basement Wheeler Crest Quartz Monzonite) than do east- lithologies in each outflow sector, not to those of erly outcrops. In addition, the top sheet exhibits Relationships between the different lithic the overrun surface. Furthermore, when the a generally eastward-increasing proportion of suites, emplacement sequences, and caldera- later ash-flow subunits (which exhibit the great- Glass Mountain rhyolite, a lithology scarce or margin basement rocks of each sector are further est sectorial variability in lithic populations) absent in the sheets beneath it. We interpret illustrated in the canyon of the San Joaquin were emplaced, the fallout and as much as 100 these shifts as indicating northeastward propaga- River (Fig. 1), where glaciated remnants of two m of earlier ash-flow deposits had already sealed tion of a ring-fault vent system, presumably in conformable cooling units are locally preserved off most of the ground surface. Finally, virtually response to the onset of subsidence. Proportions (Huber and Rinehart, 1967; Hildreth, 1979). In all of the lithics are angular. Rounded pebbles of the various lithic fragments in the ash flows this sector, neither the pyroxene-free earlier are rare in the tuff despite being widespread in reflect the precaldera lithologies that were sheet nor the overlying, higher-temperature, glacial and alluvial deposits of the pre-Bishop progressively disrupted along the developing pyroxene-bearing sheet contains any rhyolitic substrate (Bateman, 1965; Sharp, 1968). Where caldera ring vent. lithics. East of Reds Meadow and Sotcher Lake they do occur, surficially entrained lithics are (Fig. 1), near-basal exposures of tuff banked generally confined to the lowest 3-5 m of the Northern Sectors against the foot of steep valley walls contain tuff, they tend to be larger than those transported unusually high proportions of basaltic and grani- from the vents, and, if not rounded, they are In the southeasterly outflow sector, the toid lithics, many of them rounded and evidently commonly angular blocks plucked from nearby higher-temperature, pyroxene-bearing pumice picked up from the overrun ground surface. jointed basement. We conclude that, with a few that erupted late in the zoned sequence (Hil- Away from the canyon walls, however, meta- easily recognizable exceptions, nearly all of the dreth, 1979) is largely restricted to the top unit. sedimentary and granitoid lithics (in a ratio of lithics at the 60-odd ash-flow locations studied North of the caldera, however, the outflow lobes ~3:1) are predominant in both cooling units, were entrained at the vents during eruption. in Adobe Valley and Mono Basin (Fig. 1) con- and only small fractions of basalt and quartz sist predominantly of such material. Bulk and latite lithics are present (Fig. 4). The latter suite, Southeastern Sector mineral chemistry of juvenile ejecta in these thought to be syneruptively rather than surfi- lobes (Hildreth, 1979) indicate that they overlap cially entrained, indicates eruptive sources for Three emplacement units separated by partial and extend to higher temperatures and less both sheets within the southwesternmost part of cooling breaks are present just southeast of the evolved compositions the magmatic zonation the caldera, where rhyolite is absent, quartzite is caldera margin in Little Round Valley, where represented in the three southeasterly subunits. scarce, and metapelite and metasiltstone are each is 25-40 m thick. They thicken southeast- Emplacement relatively late in the outflow se- abundant (Figs. 2 and 3). The apparent absence ward and together compose the 150- to 200-m- quence is confirmed by a few proximal expo- of metavolcanic lithics from the Ritter pendant thick composite sheet that underlies the exten- sures of the earlier-erupted, more-evolved, py- (Fig. 2A) suggests that vents for the Bishop Tuff sive plateau called the "Volcanic Tableland," roxene-free tuff at the very base of the Mono lay at least a few kilometres inboard of the into which are incised several gorges (Fig. 1). Basin lobe. southwestern topographic rim of the now ero- sionally enlarged caldera. The three units were emplaced in rapid succes- The suite of lithics in this basal tuff (Fig. 4) sion, probably within hours or days, as there is resembles that of the early ejecta in other sectors no sign of erosion, fallout, or significant rework- but additionally includes ~8 wt % of the basalt Eastern Sector ing between sheets, and in thick sections they and quartz latite prominent along the caldera's cooled and welded as one compound cooling north rim (Fig. 2B). Throughout most of the Due east of the caldera there lies an extensive unit. The upper two extend all the way to the Mono Basin lobe, these volcanic lithologies are apron of unconsolidated pyroclastic and alluvial distal scarp near Bishop, but the lowest disap- more prevalent still, constituting 25-70 wt % of debris, largely Glass Mountain rhyolite, which pears beneath the level of exposure in central the lithics present. Rhyolitic lithics, in contrast, was formerly overlain by a sheet of Bishop Tuff Owens Gorge. The topmost unit has been are important only along the eastern fringe of that is erosionally receding southward (Fig. 1). stripped from many proximal parts of the sheet, the Mono Basin lobe, where ash flows that East of this apron there lies the Benton Range, a but it forms the plateau surface south of Casa erupted along the Glass Mountain segment of 6- to 13-km-wide rugged terrain of granitoid Diablo Mountain and lower Chidago Canyon, the ring fault were able to surmount laterally the and strongly jointed metasedimentary rocks and it extends in the subsurface (Bateman, 1965) north-south basement high between the Adobe (Rinehart and Ross, 1957). Beyond the Benton several kilometres south of Bishop. Valley and Mono Basin lobes (Figs. 1 and 4). In Range there is an extensive sheet of Bishop Tuff Lithic populations in the lower two em- the Adobe Valley lobe, on the other hand, lithic (Crowder and Sheridan, 1972) thought, on pet- placement units (Fig. 5) are similar to those in populations everywhere include 43-64 wt % rological grounds, to have been emplaced early the underlying fallout (Fig. 4), everywhere dom- Glass Mountain rhyolite, as well as significant in the eruptive sequence (Hildreth, 1979). inated by metasedimentary lithologies from the proportions of basalt and quartz latite, which are Between the Benton Range and Lake Crow- Mount Morrison pendant. The topmost unit, very scarce in the southeasterly subsheets. Large ley, Bishop Tuff exposures are continuous with however, commonly contains more granodiorite fractions of metasedimentary lithics in the the lower-temperature, southeastern emplace- and a much greater proportion of the nearly Adobe Valley lobe have been noted only dis- ment units and, accordingly, are virtually free of

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rhyolitic lithics. The higher-temperature, top- lite, quartzite, and Wheeler Crest Quartz Mon- sented as much as 100 km3 of magma anil the most unit, rich in rhyolitic lithics, was probably zonite coexisting in the Plinian deposits points intracaldera tuff as little as 400 km3, then the present originally, but no remnants like that in strongly to an initial outbreak in the south- ring-vent phase would have started after release Little Round Valley have been found north of central part of the caldera, along or just west of of -28% of the total ejecta. To the extent that Owens Gorge. East of the Benton Range and the Hilton Creek fault (Fig. 3). The earliest ash the intracaldera tuff is thicker than the proven north of Chidago Canyon, the tuff is likewise flows, principally emplaced southeastward, car- 1,500 m or that some fraction of the downwind almost entirely an sarly emplacement unit, but ried lithic suites similar to those of the imme- fallout is post-subsidence, coignimbrite ash, its lithic population (Fig. 5) is unusual and diately preceding fallout, metasedimentary lith- however, the transition could have taken place quite variable, containing 21-60 wt % rhyolite ics predominating in all sectors. The data hint at proportionately earlier. Conversely, the tiansi- and 34-54 wt % metasedimentary rocks, many slight sectorial differences among the early ash tion would have been proportionately later to larger than 20 cm a nd evidently of local deriva- flows (for example, the presence of basalt and the degree that some fraction of the tufl had tion. It seems likely that many of the lithics in scarcity of quartzite in the earliest San Joaquin ponded within a sagging depression prior to this sector were picked up in crossing the Glass subunit, Figs. 4 and 5), but it is hard to distin- onset of ring faulting and opening of the ring Mountain debris apron and during turbulent guish vent control of such second-order differ- vents. However one deals with the uncertainties, passage through rugged terrain in the Benton ences from surface contamination during the data indicate that more than two-thirls of Range. outflow. the Bishop Tuff was erupted from ring vents, The uppermost southeastern unit contains the during caldera collapse, after extinction of the DISCUSSION first unequivocal evidence for the shift to multi- single-vent Plinian column. ple or ring-fissure vents along a propagating ring Voluminous silicic pyroclastic eruptions no Theoretical modeling of the formation of py- fault. Presumably, the shift coincided with the doubt exhibit wide ranges of eruption-col umn roclastic flows by collapse of single-vent Plinian onset of caldera subsidence. The concurrent first behavior and of mechanical response by roof columns (Sparks ai d Wilson, 1976; Sparks and appearance of higher-temperature, pyroxene- rocks to progressive magma withdrawal. Col- others, 1978; Wilson and others, 1980) has re- bearing ejecta may reflect a change from lapse of overloaded vertical eruption columns cently induced some geologists to over-general- chamber-wide drawdown of volatile-rich mag- was suggested as a general mechanism for ize this otherwise useful and stimulating work ma to forced ejection of hotter magma from generation of ash flows, large and small, by to account for empl acement of virtually all ash- somewhat deeper levels, possibly sectorially re- Smith (1960, p. 802-806) and was first devel- flow deposits. Even though the cited papers spe- stricted, owing to asymmetric subsidence of the oped theoretically by Sparks and Wilson (1976). cifically discuss the common occurrence of cauldron block. The ring fault propagated coun- Various authors have subsequently used the that lack preceding fall deposits, terclockwise around to the northern margin of term "column collapse" to invoke at least three some have sought to apply the single-vent, the foundering caldera, where the highest- different processes: (1) ash-flow generation by column-collapse scenario universally, even to temperature magma was ejected and emplaced fractional collapse at the margins of a persistent caldera-related sheets as voluminous as several last. The relative time of emplacement of the Plinian eruption column; (2) drastic reduction in hundreds or even th ousands of cubic kilometres. high-temperature, upper cooling unit in the San height of a formerly high converting colamn, Many caldera-forming eruptions apparently Joaquin canyon is not well known, but, in pet- generally attributable to a marked increase in undergo a transition, shifting at some stage from rochemical zonation, it overlaps the northern the mass-discharge rate and leading to genera- a single-vent, Plinian phase (with or without ac- outflow lobes (Hildreth, 1979). During this ring- tion of voluminous ash flows from fairly low, companying, collapse-fed ash flows) to a subsi- vent phase of the eruption, the lithic suite in each sustained or pulsatory, fountaining columns; and dence-related, ring-vent phase (Bacon, 1983; outflow sector was clearly derived from the pre- (3) immediate collapse at the onset of eruption, Lipman, 1984; Heiken and McCoy, 1984; caldera rocks syneruptively downfaulted along from a low height above the vent, resulting in Druitt and Sparks, 1984; Druitt, 1985) which, the adjacent caldera margin. lateral dispersal of nearly all of the pyroclastic in some cases, can account for most of the total The transition from a single vent to multiple mass as ash flows, only a small fraction of the volume of ejecta released. Eruption of the Bish- circumcaldera vents took place after eruption of ejecta ever being convectively transported by the op Tuff followed such a pattern, as indicated -50 km3 of magma as fallout and ~ 100 km3 as eruption column itself to great elevations. The by both petrologic and lithic-fragment data that outflow sheets. During the subsequent ring-vent third process accounts for the abundance of permit recognition of multiple vents, of changes phase, an additional 100 km3 was emplaced caldera-related ash-flow sheets that lack evi- in vent location during eruption progress, and of outside the caldera, and at least 400 km3 (and dence for any associated Plinian airfall deposits. a lobate, sectorial emplacement sequence. No conceivably >600 km3) accumulated inside the The initial conditions that promote proofs 3 nonbasal Bishop airfall deposits are known that foundering depression. Assuming the intracal- were outlined by Wilson and others (1980) in could suggest a resumption of Plinian activity dera welded tuff to be 500 km3, these relation- terms of relatively higher mass-discharge rate, associated with the ring-vent phase. ships indicate that caldera subsidence began greater vent dimensions, and lower gas content and eruption velocity. The lack of precaldera volcanic lithics in the after emplacement of -60% of the extracaldera basal airfall and earliest ash-flow deposits indi- ejecta but after eruption of only -20% of the Considering the Bishop Tuff in this frame- cates that the eruption did not begin near the total. work, a few thin, nonwelded ash-flow units in- northern or eastern margins of the present cal- The volume estimates are least certain for the tercalated within the Plinian pumice-fall deposit dera (compare Figs. 2 and 4). On the other fallout and intracaldera components. To illus- can be attributed to process 1, and most cf the hand, the abundant of metasiltstone, metape- trate an extreme variation, if the fallout repre- 100 km3 of pre-subsidence ash-flow deposits

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eruption, Thira, Cyclades, Greece: Journal of Geophysical Research, probably reflect process 2. During the ring-vent promoting immediate collapse from a ring-fault v. 89, p. 8441-8462. phase, however, process 3 was predominant, as fissure vent. Hildreth, W., 1979, The Bishop Tuff: Evidence for the origin of compositional zonation in silicic magma chambers: Geological Society of America many new vent segments opened and released Multiple arcuate ring-fault vents, direct col- Special Paper 180, p. 43-75. 1985, The Bishop Tuff: Trace contents of dark pumice: Geological sectorially restricted ash flows from the ring- lapse of eruptive curtains from such vents, rela- Society of America Abstracts with Programs, v. 17, p. 361. fault zone during caldera subsidence. This final tively early onset of subsidence, sectorially Hildreth, W„ and Mahood, G. A., 1985, Correlation of ash-Dow tuffs: Geologi- cal Society of America Bulletin, v. 96, p. 968-974. and volumetrically predominant phase of the complex eruptive sequences, entrainment of Hildreth, W„ Grander, A. L., and Drake, R. E, 1984, The Loma Seca TufT and the Calabozos caldera: A major ash-flow and caldera complex in the Bishop eruption could have been quite complex, great volumes of lithic fragments produced by southern Andes of central Chile: Geological Society of America Bul- potentially involving fitful subsidence, many syneruptive ring faulting and caldera-wall letin, v. 95, p. 45-54. Huber, N. K., and Rinehart, C. D., 1965, Geologic map of the Devils Postpile concurrent vents, mixing of pumice and lithics slumping, and entrapment of a large fraction of quadrangle, Sierra Nevada, California: U.S. Geological Survey Quad- rangle Map GQ-437, scale 1:62,500. from adjacent vent segments, and transient open- the eruptive volume inside the caldera may all 1967, Cenozoic volcanic rocks of the Devils Postpile quadrangle, east- ing and closing of various vents as a result of be common features of large continental, ash- ern Sierra Nevada, California: U.S. Geological Survey Professional Paper 554-D, p. D1-D21. subsidence along irregular fault planes and of flow calderas (Lipman, 1984; Hildreth and Izett, G. A., Wilcox, R. F., Powers, H. A., and Desborough, G. A., 1970, The Bishop ash bed, a Pleistocene marker bed in the western United States: caldera-wall slumping into active vent segments. Mahood, 1985). Quaternary Research, v. 1, p. 121-132. Conceptually, the number, location, and Kistler, R. W., 1966, Geologic map of the Mono Craters quadrangle, Mono and Tuolumne Counties, California: U.S. Geological Survey Quad- geometry of vents are questions distinct from ACKNOWLEDGMENTS rangle Map GQ-462, scale 1:62,500. Krauskopf, K. B., and Bateman, P. C., 1977, Geologic map of the Glass that of eruption-column dynamics. We suggest, Mountain quadrangle, Mono County, California, and Mineral County, nonetheless, that many of the ring-fault vents Nevada: U.S. Geological Survey Quadrangle Map GQ-1099, scale We thank R. A. Bailey, P. C. Bateman, D. C. 1:62,500. active during subsidence of a large caldera Ross, and M. F. Sheridan for helpful discussions Lipman, P. W., 1976, Caldera-collapse breccias in the western San Juan Moun- tains, Colorado: Geological Society of America Bulletin, v. 87, would be elongate fissures that release not quasi- and C. R. Bacon, T. H. Druitt, C. D. Miller, p. 1397-1410. cylindrical columns but complex eruptive cur- 1984, The roots of ash flow calderas in western North America: Win- C. D. Rinehart, and R.S.J. Sparks for perceptive dows into the tops of granitic batholiths: Journal of Geophysical Re- tains that are unlikely ever to entrain sufficient reviews. Mahood was supported by National search, v. 89, p. 8801-8841. Metz, J., and Mahood, G., 1985, Precursors to the Bishop Tuff eruption: Glass air to sustain a convecting column. Foundering Science Foundation Grant EAR 84-07822. Mountain, Long Valley, California: Journal of Geophysical Research, of the cauldron block into the magma reservoir v. 90, no. B13, p. 11,121-11,126. Rinehart, C. D„ and Ross, D. C„ 1957, Geology of the Casa Diablo Mountain could open fissures hundreds of metres wide, quadrangle, California: U.S. Geological Survey Quadrangle Map GQ- 99, scale 1:62,500. promoting enormous discharge rates and sus- 1964, Geology and mineral deposits of the Mount Morrison quadrangle, taining such curtains in segments, perhaps dis- Sierra Nevada, California: U.S. Geological Survey Professional Paper REFERENCES CITED 385, 106 p. continuous, each many kilometres long. Local- Sanders, C. O., 1984, Location and configuration of magma bodies beneath Bacon, C. R., 1983, Eruptive history of Mount Mazama and Crater Lake Long Valley, California, determined from anomalous earthquake sig- ization of point vents, as along a basaltic fissure, caldera, Cascade Range, U.S. A.: Journal of Volcanology and Geother- nals: Journal of Geophysical Research, v. 89, p. 8287-8302. would be unlikely owing to ring faulting and to mal Research, ». 18, p. 57-115. Sharp, R. P., 1968, Sherwin Till-Bishop Tuff geological relationships. Sierra Bailey, R. A., and Koeppen, R. P., 1977, Preliminary geologic map of Long Nevada, California: Geological Society of America Bulletin, v. 79, sustained subsidence of the chamber roof into Valley caldera. Mono County, California: U.S. Geological Survey p. 351-364. Open-File Map 77-468. Smith, R. L., 1960, Ash flows: Geological Society of America Bulletin, v. 71, the large, gas-rich magma body. Because such a Bailey, R. A., Dalyrymple, G. B., and Lanphere, M. A.. 1976, Volcanism, p. 795-842. curtain would have only half as great a ratio of structure, and geochronology of Long Valley caldera. Mono County, So rey, M. L., 1985, Evolution and present state of the hydrothermal system in California: Journal of Geophysical Research, v. 81, p. 725-744. Long Valley caldera: Journal of Geophysical Research, v. 90, no. B13, lateral surface area to cross-sectional area as Bateman, P. G, 1965, Geology and tungsten mineralization of the Bishop p. 11,219-11,228. district, California: U.S. Geological Survey Professional Paper 470, Sparks, R.S.J., and Wilson, L„ 1976, A model for the formation of would a cylindrical column of identical width, 208 p. by gravitational column collapse: Journal of the Geological Society of far less air could be entrained from the surround- Cockerham, R. S., and Pitt, A. M., 1984, Seismic activity in Long Valley London, v. 132, p. 441-451. caldera area, California: June 1982 through July 1984: U.S. Geological Sparks, R.S.J., Wilson, L., and Hulme, G„ 1978, Theoretical modeling of ing atmosphere for each mass unit erupted. As it Survey Open-File Report 84-939, p. 493-526. the generation, movement, and emplacement of pyroclastic flows by Crowder, D. F., and Sheridan, M. F„ 1972, Geologic map of the White column collapse: Journal of Geophysical Research, v. 83, p. 1727-1739. is the heating of such entrained air, turbulently Mountain Peak quadrangle, Mono County, California: U.S. Geological Wilson, L„ Sparks, R.S.J., and Walker, G.P.L., 1980, Explosive volcanic mixing in large proportions with the erupting jet, Survey Map GQ-1012, scale 1:62,500. eruptions—IV. The control of magma properties and conduit geometry Druitt, T. H., 1985, Vent evolution and lag breccia formation during the on eruption column behavior Geophysical Journal of the Royal Astro- that drives the buoyant rise of high convecting Cape Riva eruption of Santorini, Greece: Journal of Geology, v. 93, nomical Society, v. 63, p. 117-148. p. 439-454. columns (Sparks and Wilson, 1976), this simple Druitt, T. H., and Sparks, R.SJ., 1984, On the formation of calderas during ignimbrite eruptions: Nature, v. 310, p. 679-681. geometric contrast may play as great a role as do Gilbert, C. M., 1938, Welded tuff in eastern California: Geological Society of MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 6,1985 the larger vent dimensions and discharge rate in America Bulletin, v. 49, p. 1829-1862. REVISED MANUSCRIPT RECEIVED OCTOBER 10,1985 Heiken, G., and McCoy, F., 1984, Caldera development during the Minoan MANUSCRIPT ACCEPTED OCTOBER 14, 1985

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