L. J. P. MUFFLER D. E. WHITE U.S. Geological Survey, Menlo Park, California 94025 A. H. TRUESDELL
Hydrothermal Explosion Craters in Yellowstone National Park
ABSTRACT proposed mechanism is reasonable. The sizes of craters expected in various rock types Hydrothermal explosions are produced correspond with those observed. when water contained in near-surface rock at temperatures as high as perhaps 250°C flashes INTRODUCTION to steam and violently disrupts the confining While preparing a geologic map of Lower rock. These explosions are due to the same Geyser Basin in 1966, we recognized two instability and chain reaction mechanism as large craters, each about 0.4 mi in mean geyser eruptions but are so violent that a diameter. Subsequently, other U.S. Geologi- large proportion of solid debris is expelled cal Survey personnel recognized similar craters along with water and steam. at a number of localities in Yellowstone Park Hydrothermal explosions are not a type of (Fig. 1). We have studied these other craters volcanic eruption. Although the required only in reconnaissance, and almost all the energy probably comes from a deep igneous data in this paper were obtained from the source, this energy is transferred to the sur- two craters in Lower Geyser Basin. None of face by circulating meteoric water rather than these craters has been mentioned previously by magma. The energy is stored as heat in in the geologic literature, although the small hot water and rock within a few hundred feet lakes within the Twin Buttes crater of Lower of the surface. Geyser Basin are labeled "crater lakes" on At least ten hydrothermal explosion craters, the Lower Geyser Basin map of the Hayden ranging in diameter from a few tens of feet to Sutvey (Hayden, 1883). about 5000 ft, have been recognized in Yel- We are grateful to R. L. Christiansen, G. lowstone National Park. Eight of these craters M. Richmond, and H. A. Waldrop for data are in hydrothermally cemented glacial de- on craters in the northern and eastern parts posits; two are in Pleistocene ash-flow tuff. of the Park and for many helpful discussions Each is surrounded by a rim composed of of the volcanic and glacial aspects of the debris derived from the crater. Juvenile vol- problem. We also acknowledge the mapping canic ejecta are absent, and there is no evi- and collaboration of our colleague R. O. dence of impact. Fournier, and the reviews of R. S. Fiske, D Geologic relations at the Pocket Basin J. Milton, R. F. Roy, and D. L. Blackstone. crater establish that the explosion there took place during the waning stages of early Pine- HYDROTHERMAL EXPLOSIONS dale Glaciation. This association with ablat- The craters discussed here were not formed ing ice suggests that an ice-dammed lake by volcanic activity or by impact but by a existed over a hydrothermal system at the mechanism we term "hydrothermal explo- Pocket Basin site and that the hydrothermal sion." A hydrothermal explosion takes place explosion was triggered by the abrupt de- when water contained in near-surface rocks crease in confining pressure consequent to at temperatures as high as perhaps 250°C sudden draining of the lake. Most of the flashes to steam and violently disrupts the other explosion craters in Yellowstone Park confining rocks, expelling solid material as could have been triggered in the same manner. well as water and steam. Craters formed by Calculations of energy available in Yellow- hydrothermal explosions range in diameter stone hot-spring systems and of energy re- from a few tens of feet to at least 4000 ft. quired to form craters indicate that the Hydrothermal explosions are relatively un-
Geological Society of America Bulletin, v. 82, p. 723-740, 10 figs., March 1971 723
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common; thermal systems commonly dissi- tent) (White, 1967, p. 651-652). This insta- pate their heat by geyser and hot-spring bility produces hot-spring flow, geyser activ- action rather than by major eruptions or rock ity, and hydrothermal explosions, depending and water. on the physical characteristics of the system. In hydrothermal systems such as those of If near-surface permeability is relatively Yellowstone National Park, temperatures high, the instability of the hot-water column much higher than surface boiling can be is counteracted by convection, circulation, achieved at very shallow depths, because of surface boiling, and surface discharge as hot the increase of boiling point with pressure. springs. But if near-surface permeability is In many high-temperature water systems, the decreased by deposition of hydrothermal temperatures are controlled by the two-phase minerals or by a cap rock, steady-state proc- boundary between water and steam (Fig. 2). esses are not effective in dispersing the excess Such systems are inherently unstable because energy of high-temperature inflow, and geyser relatively low-temperature water (high in activity will result. density) is situated above high-temperature A geyser periodically erupts a turbulent water (low in density but high in energy con- mixture of water and vapor (White, 1967,
JIO/OO'
Figure 1. Map of Yellowstone National Park. Locations of hydrothermal explosion craters indicated by stars.
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Temperature in °C take place at the same locality, but much time 50 100 150 is necessary to replenish the expended energy. We choose to designate this cratering mechanism as "hydrothermal explosion" rather than "hydrothermal eruption" in order to emphasize the distinctions from geyser Boiling point eruptions. "Hydrothermal eruption" is used curve under as a general term encompassing both hydro- hydrostatic thermal explosions and geyser eruptions. pressure Hydrothermal explosions have been re- ported from Lake City, California (White, 1955), Steamboat Springs, Nevada (White, 1955; White and others, 1964), Waiotapu, Temperature measured ot hole bottom during drilling New Zealand (Lloyd, 1959), and Tuscany, Italy (Marinelli, 1969). In addition, the 1957 eruption on Iwo Jima (Corwin and Foster, 1959) and the 1951-1952 activity at Nobon- betsu, Japan (Fukutomi and Fujiki, 1953), YELLOWSTONE No. 3 may also have been hydrothermal explosions. At Lake City, California, a previously incon- spicuous group of hot springs suddenly erup- ted on March 1, 1951, as violent mud vol- Fugure 2. Graph showing temperatures in canoes that involved over 300,000 tons of U.S. Geological Survey research drill hole Y-3 mud (White, 1955). Approximately 20 acres (just west of the Pocket Basin explosion crater) were intensely cratered and disturbed, and controlled by the boiling point curve. The curve shows the boiling point of pure water fine debris was showered as far as 4 mi away. under the hydrostatic pressure of liquid water During the declining activity that continued everywhere at the boiling point, assuming water for several days, five centers of activity evolved level to be at the ground surface. During drill- from what initially had been a single major ing at depths greater than 250 ft, the hole center. exhibited positive well-head pressures, per- A hydrothermal explosion is not a volcanic mitting temperatures to be slightly above the eruption, because no magma is directly in- two-phase curve for hydrostatic pressure alone. volved. The energy required is brought from depth by upflowing hot water and is stored p. 642). The violent nature of a geyset erup- as heat in water and rocks within a few tion is due to flashing of water to steam hundred feet of the surface. In contrast, rhe throughout a column of water everywhere at energy for a volcanic eruption is carried the boiling point. When water at the top of directly from depth to the surface by magma such a column is removed (for example, by or by fluidized rock. The explosive nature of bailing) the effective weight of the column some volcanic eruptions may be due either decreases, steam forms and displaces water, to rapid exsolution and expansion of gas further reducing the confining pressure at from magma or to vaporization of sea water depth, and the resulting chain reaction leads or cool ground water contacted by the magma. immediately to geyser eruption. If geyser When ground water is affected, the eruption eruptions are sufficiently violent, they can is termed "phreatic" (Stearns and Macdonald, expel rock fragments and can, in time, gradu- 1946, p. 16). ally destroy the geyser channels and termi- nate geyser activity (White, 1967, p. 681 - 682). CRATERS IN LOWER GEYSER BASIN Hydrothermal explosions are due to the Lower Geyser Basin is a broad, nearly flat same instability and chain reaction mecha- valley about 7200 ft above sea level, sur- nism as are geyser eruptions, but are so violent rounded by rhyolite plateaus that rise 400 to that they disrupt the confining rocks and 1000 ft above the valley floor (Fig. 3). expel a large proportion of solid debris. Be- Rhyolite flows ranging in age from 120,000 cause of this disruptive action, hydrothermal to nearly 600,000 yrs (R. L. Christiansen and explosions do not have the short-term peri- J. D. Obradovich, 1969, written commun.) odicity of geysers. Successive explosions may crop out east of the basin and extend west-
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ward under the basin. The rhyolite flow that yrs B.P., according to Table 2 of Richmond, partly encircles Lower Geyser Basin on the 1965) are predominantly kame gravel and north, west, and south is even younger, prob- sand, with minor till and lacustrine deposits, ably about 90,000 yrs. Volcanic rocks in and and are cemented only locally. Extensive de- surrounding the basin were erupted from posits of gravel and sand at low elevations vents 6 to 12 mi away on the Central and in Lower Geyser Basin comprise both early Madison Plateaus (R. L. Christiansen, 1969, Pinedale outwash deposits and subsequent oral commun.). There are no known volcanic alluvial deposits. Glaciers of middle and late vents in or immediately adjacent to Lower Pinedale age did not reach Lower Geyser Geyser Basin. Basin (H. A. Waldrop, 1969, oral commun.). Lower Geyser Basin is filled to its modern Three main types of chemical deposits sill level by glacial and alluvial deposits. A from hot springs overlie the glacial and allu- late stade of Bull Lake Glaciation is repre- vial deposits in Lower Geyser Basin. Sinter sented by ice-contact sandstone and con- (amorphous silica deposited on the ground glomerate cemented by hydrothermal opal surface by flowing hot water) is dominant. and zeolites. Owing to this cementation, Silica mud, a soft, gray deposit composed these rocks were preserved as resistant topo- primarily of diatoms (Weed, 1889) reaches a graphic highs through the subsequent early thickness of 6 ft under the marshy flats. stade of Pinedale Glaciation. Deposits of Travertine (CaCO3) is deposited at Hot Lake this early Pinedale stade (20,000 to 25,000 and at a few springs along Fairy Creek.
Figure 3. Map of Lower Geyser Basin. Stars denote sites of major hydrothermal explosions. Stippled areas are covered by hydrothermal explosion breccia.
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Lower Geyset Basin is on the southeast ever, is similar to some of the core recovered projection of the Cenozoic fault zone notth from Y-3 drill hole, just west of Pocket of Hebgen Lake, Montana (Witkind and Basin (Fig. 4). Core samples recovered from othets, 1964). Although no northwest- depths of less than 94 ft are cemented in part ttending faults have been found in Lower by quartz, and conglomerate from 75 to 94 ft Geyser Basin, northwest-trending lineaments contains subangular clasts similar to those in are indicated by alignment of hot springs and the breccia fragments. However, the thin- hot-spring groups and by northwest-trending bedded sandstone and silstone so character- cracks on sinter mounds. istic of the breccia fragments are not found A major lineament trends southwest across in the drill core, which contains only medium- Lower Geyser Basin from Nez Pierce Creek to thick-bedded sediments. Almost no core to Twin Buttes. This lineament is marked by was recovered from the depths of 95 to 134 hot-spring groups and by the highest sub- ft, and this interval may have contained thin- surface temperatures, as inferred from chemi- bedded sediments. Alternatively, there may cal indicators (R. O. Fournier and A. H. have been abrupt facies changes between Truesdell, unpub. data). The hydrothermal thin-bedded sediments directly under Pocket explosion craters at Pocket Basin and at Twin Basin and thick-bedded sediments just to the Buttes are also located on this lineament. west. The breccia ridge was formed during the Pocket Basin Crater waning stages of early Pinedale glaciation. Pocket Basin (Figs. 3 and 4) is an oval The ridge is unglaciated, and unconsolidated area 1200 ft by 2600 ft enclosed by a low explosion breccia overlies opal-cemented ridge of explosion debris. Where unmodified early Pinedale kame gravel just east of Pocket by hot-spring action or erosion, the inner Basin. On the other hand, both the inner and slopes of the ridge are 20° to 25° and the outer slopes of the ridge are overlapped by outer slopes 10° or less. Height of the ridge sand and gravel that could be either early above the basin floor ranges from 63 ft on Pinedale outwash material or subsequent al- the east side to 12 ft on the south side. The luvium. Precise dating of the ridge is pro- ridge is breached at two points by the Fire- vided by its relationships to the course of hole River. the Firehole River, described below. The debris ridge is an unconsolidated and Most of the floor of Pocket Basin is cov- unsorted deposit composed predominantly ered by silica mud, which ranges in thickness of angular fragments of tough yellow-stained from 4 to 6 ft in three auger holes and is sandstone, siltstone, and conglomerate, all underlain by sand and gravel. Sinter is domi- probably representing hydrothermally ce- nant at the surface near the river, where mented Bull Lake deposits. The rocks are there are a number of boiling springs and cemented by quartz and contain disseminated several geysers. Elsewhere in the crater, the pyrite, much of which has been oxidized to present thermal features are warm pools that hydrous iron oxides. The few rounded rhyo- do not deposit sinter. Parts of the debris lite cobbles in the debris were probably ridge, especially to the east and south, are derived from unconsolidated glacial deposits leached by sulfuric acid formed by oxidation (early Pinedale?) that overlay the hydro- of H 28. Mud pots and mud volcanoes are thermally altered Bull Lake sediments prior abundant between Pocket Basin and Rush to explosive disruption and formation of the Lake and just northeast of Rush Lake (Fig. 4). debris ridge. Fragments in the explosion de- Bedrock beneath the Pocket Basin area is bris generally are less than 1 ft in diameter, a rhyolite flow which had its source on the but a few blocks are 6 to 8 ft in diameter. Central Plateau (Fig. l). This flow was en- Bombs, pumice blocks, and other volcanic countered at depths greater than 138.5 ft in ejecta, as well as angular fragments of bed- U.S. Geological Survey research drill hole rock rhyolite, are absent. Y-3, located on the southwest flank of the The sedimentary rock that makes up the Pocket Basin debris ridge (Fig. 4). breccia fragments is dissimilar to the glacial An explosive event centered in Pocket deposits found at the surface in Lower Geyser Basin is indicated by (1) the presence of a Basin in its cementation by quartz rather than debris rim around the basin, (2) the asym- by opal and in the subangular shapes of many metrical shape of the rim along all radii from clasts within ejected blocks. The rock, how- the center of the basin, with steep inner
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Figure 4. Annotated aerial photograph of deposits and subsequent alluvial deposits); s, acid alteration is occurring. Locations of Y-3 Pocket Basin explosion crater, k, early Pine- sinter; m, silica mud; mp, mud pots; al, kaolinite drill hole and point A, probably a stubby mud- dale kames; b, hydrotherinal explosion breccia; alluvium. Dotted pattern denotes areas where flow, are also shown. g, sand and gravel (early Pinedale outwash
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slopes and gentle outer slopes, and (3) the west of Pocket Basin at the time of the ex- absence elsewhere in Lower Geyser Basin of plosion, a lake would soon have formed rocks similar to the debris fragments. The south of the breccia ridge. When the rising absence of volcanic ejecta argues against a lake level reached the lowest point of the volcanic origin, and there are no field or breccia rim, it would overflow into the crater petrographic features suggesting impact. A and, after filling the crater, would then dis- hydrothermal origin is indicated by the spa- charge over the lowest part of the rim on the tial relation to modern hot-spring activity, opposite side. Rapid entrenchment of the the dominance of silicified, iron-stained, river into the unconsolidated debris would hydrothermally altered sedimentary rock as then permit the course through Pocket Basin angular fragments in the explosion debris, to be maintained after complete melting of and the location of Pocket Basin along a the adjacent ice. major zone of intense thermal activity. The absence of angular rhyolite fragments Hydrothermal Explosions in the explosion breccia indicates that the near Rush Lake explosion took place entirely in sediments Hydrothermal explosions took place in the overlying rhyolite bedrock, which probably Rush Lake area (Figs. 3 and 4) prior to the lies at a depth roughly similar to that in drill Pocket Basin explosion, but the craters pro- hole Y-3 (138.5 ft). The maximum depth of duced have been modified beyond recogni- the explosion may have been determined by tion by subsequent glacial and fluviatile ac- depth to rhyolite, which could have been too tion. Evidence for these explosions consists tough and too low in porosity to be dis- of angular breccia fragments contained in the rupted by the explosion. early Pinedale kame sands and gravels that The explosion debris was mapped as much now cover the Rush Lake area. as 0.75 mi from the center of Pocket Basin. The predominant clasts of these sediments Most of the debris was emplaced by air fall, are (l) sand grains and granules of obsidian but a lobe of breccia on the northwest side and (2) rounded pebbles and cobbles of (point A in Fig. 4) may be a shott, stubby lithoidal rhyolite. These constituents were mudflow similar to that of the Lake City, eroded from the Central Plateau (Fig. 1), California, hydrothermal explosion (White, transported by ice and running water to 1955, p. 1118). _ Lower Geyser Basin, and deposited adjacent Course of Firehole River. The breccia to ice by running water. The kames also con- ndge around Pocket Basin is 12 to 63 ft tain angular fragments of yellow-stained, higher than the younger sand and gravel quartz-cemented sedimentary rock identical plain of the northern part of Lower Geyser to the breccia fragments of Pocket Basin. On Basin, yet the Firehole River flows through some slopes, the angular clasts comprise Pocket Basin via two gaps in this ridge. Sub- more than 75 percent of the clasts; on other sequent to the hydrothermal explosion, the slopes, they are absent. This irregular dis- Firehole River cut through this impeding tribution and the angularity of the fragments breccia ridge rather than establishing a chan- indicate derivation from a nearby source area. nel at a lower elevation on the gravel plain. The explosions that produced these angu- The most reasonable explanation of the chan- lar fragments took place during kame deposi- nel through Pocket Basin is that all potential tion, not afterwards, and were thus earlier channels to the west and east were blocked than the Pocket Basin explosion. Where by ice still remaining adjacent to the breccia kame bedding is exposed, the angular frag- ridge. ments are seen to be within the sands and The Pocket Basin explosion took place gravels, and thus were not deposited on top after deposition of the early Pinedale kames of an already formed kame surface. At several but prior to deposition of the sand and gravel localities we can show that the variation from unit that may include early Pinedale outwash place to place in size distribution of the material (p. 727). If the explosion took place angular fragments is identical to the variation during the recessional stages of early Pine- in size distribution of the rounded rhyolite dale gladation, stagnant ice could have filled clasts. Consequently, the angulat fragments much of Lower Geyser Basin but have been were sorted and deposited by running water absent over major hot-spring areas such as in the same mannet as were the rounded Pocket Basin. If ice still remained east and clasts.
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The angular fragments must have been rhermally cemented sandstone and conglom- derived from the Rush Lake area rather than erate of the late Bull Lake Glaciation. Hydro- the Pocket Basin area. Fragments up to 2 ft thermal cementation by opal and xeolites in diameter were found in kame deposirs as (clinoptilolite, mordenite, and analcime) al- much as 1800 fr from the edge of the Pocket lowed the deposits to be preserved through Basin crater and more than 20 ft above the rhe subsequent early Pinedale Glaciation. highest point of the Pocket Basin rim. The zeolites that were originally present are Small hydrothermal explosions may also not stable in acid conditions and have there- have taken place in the Rush Lake area sub- fore been leached from many outcrops on the sequent to the Pocket Basin explosion. Rush slopes of Twin Buttes, where warm vapor is Lake and a small, heavily forested pit just still being discharged and the deposits are southwest of the lake are circular, cratedike being altered by sulfuric acid. depressions in unconsolidated early Pinedale Gravel and sand of early Pinedale age cover kames. Their occurrence in an area of high several square miles west of Twin Buttes heat flow between two major mud pot areas (Fig. 2), probably as both outwash and kame makes it unlikely that they are glacial kettles deposits (G. M. Richmond, 1966, oral com- resulting from the melting of stagnant blocks mun.). These deposits are overlain by spo- of ice. Direct evidence for an explosion radic fragments of air-fall breccia from the origin, however, is lacking, owing to the un- Twin Buttes crater, and thus are older than consolidated nature of the glacial deposits in the explosion. which the depressions are situated. Breccia from the Twin Buttes crater was emplaced mainly to the east and south of the Twin Buttes Crater crater, and the southeast rim is composed The second major explosion crater in Lower primarily of breccia. In contrast to the Pocket Geyser Basin is a circular, closed depression Basin breccia, which lies on a surface of low O.-i mi in diameter located just southeast of relief, the breccia from Twin Buttes was em- Twin Buttes (Figs. 3 and 5). The floor of the placed in great parr by mudflows and land- crater is approximately 300 ft below the tops slides. Blocks as much as 400 ft across moved of Twin Buttes and 270 ft above the floor of downslope on the steep slopes southeast of Lower Geyser Basin. The lowest part of the the crater. The breccia is composed almost rim (on the southeast side) rises 10 ft above entirely of opal-cemented sandstone and con- the crater floor. glomerate identical to that of Twin Buttes, The explosion rook place within hydro- many samples of breccia contain the same
Figure 5. Oblique aerial photograph of Twin sandstone and conglomerate of late Bull Lake Buttes and the explosion crater, looking north- age. The near rim is composed of explosion west. The far wall is composed of bedded breccia.
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assemblage of zeolites. Juvenile volcanic explosion feature. It is mantled by breccia ejecta and angular rhyolite fragments are from the Twin Buttes crater and is therefore absent. slightly older. At the center of this crater is a The floor of the Twin Buttes crater, where low hillock made up of breccia. This breccia not obscured by swamps and lakes, is covered consists not only of the opal-cemented Bui! with breccia. Five small depressions within Lake sedimentary rock but also of angular the crater are filled with water. The circular blocks of lithoidal rhyolite as much as 1 ft in form of each suggests that they are sites of diameter that may have been derived from small hydrothermal explosions that followed the rhyolite bedrock that presumably under the main explosion, comparable to the late- lies the area. Angular fragments of rhyohre stage activity of the Lake City explosion also were found at the edge of the breccia (White, 1955). Although there is no thermal mass approximately 2500 ft northeast. The activity on the crater floor, warm vapor vents nearest outcrop of bedrock rhyolite is more occur near the top of south Twin Duttes, and than 0.8 mi to the south of the crater. there are warm, acid-altered areas on both peaks arid on the breccia rim to the east. Out- OTHER CRATERS IN side the- crater to the south are two geysers, YELLOWSTONE NATIONAL PARR Imperial and Spray, both of which issue from A group of explosion craters on the east breccia. The former was one of the major side of Roaring Mountain was noted in 1966 geysers of Yellowstone Park in 1928 anti by R. L. Christiansen (1966, oral cornmun.! 1929 (Allen and Day, 1935, p. 294-303). These craters occur in a cluster covering an One-fourth mile northeast of the Twin area of about 0.5 sq mi (A on Fig. 61 and as Buttes crater, there is a smaller topographic a single isolated large crater (B on Fig. 6). depression (A on Fig. 3). This depression is Although there is acid-sulfate activity on the- heavily forested, but the scanty data that walls of most of these craters, the craters art could be obtained suggest that it, too, is an not collapse features formed by acid leaching.
Figure 6. Stereo pair of vertical aerial photographs of group of explosion craters on the east side of Roaring Mountain. Letters are keyed to text.
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as are the irregularly shaped depressions in explosion crater 0.65 mi by 1.0 mi across. The the intensely altered ground just to the west subcircular rim rises 160 ft to more than 200 and south (C and D on Fig. 6). ft above the lake except where breached by The Roaring Mountain craters were formed Sedge and Bear Creeks. The slopes toward in bedrock rather than in glacial sediments Turbid Lake are 30° to 35°; a constructional as in Lower Geyser Basin. Bedrock in the outer slope is developed only on the north- Roaring Mountain area consists of the Yel- west side and dips away from the lake at 5° lowstone Tuff, which is overlain about 0.25 or less. A section through the rim along Bear mi to the northeast by basalt (Boyd, 196l). Creek shows unsorted explosion breccia 5 to Both bedrock units are partly covered by till 20 ft thick overlying poorly cemented con- of late Bull Lake age (K. L. Pierce, 1968, glomerate and till of probable middle Pine- oral commun.), which is probably about dale age (G. M. Richmond, 1970, written 50,000 to 100,000 yrs old. Pinedale glacial commun.). The breccia everywhere around deposits do not occur on Roaring Mountain. the rim is composed of angular fragments of Geologic relations are reasonably clear pumice-rich granule to pebble conglomerate only in the largest crater (B on Fig. 6), a and sandstone. Acid hot springs occur along subcircular pit 850 ft across and 110 to 200 Bear Creek where it empties into Turbid ft from floor to rim. The crater walls slope Lake, and Turbid Lake itself is acidic. 35° inward and are composed of outcrops of Squaw Lake, located just north of Yellow- Yellowstone Tuff except on the south side stone Lake (Fig. l), also lies in an explosion where the upper 100 ft is silica-cemented crater. The nearly circular crater is 1300 ft in conglomerate. This sedimentary rock is most diameter and is in deposits of late Bull Lake readily interpreted as material of late Bull and middle Pinedale age (G. M. Richmond, Lake age deposited in a. paleo-valley cut into 1970, written commun.). The crater rim rises the tuff. Both tuff and sedimentary rock are 30 to 35 ft above Squaw Lake. The inner acid-altered, and moist warm ground and slope is approximately 25°, whereas the outer warm vapor vents are found throughout the slopes to the east, south, and west dip 2° to crater. Explosion breccia, probably in great 3° away from the crater. Angular blocks of part derived from this crater, is exposed on cemented conglomerate and sandstone con- the northwest wall, where 15 ft of red-stained taining clinoptilolite are found on the north- unconsolidated breccia overlies Yellowstone eastern inner slope of the crater. Elsewhere, Tuff. Both breccia and tuff are being altered diagnostic lithologic evidence of mode of by continuing acid thermal activity. emplacement is absent. Small seeps of water Explosion breccia was deposited by air fall with pH's near 6 and temperatures as high as around the crater group to a distance of per- 54°C occur on the eastern side near lake level. haps 0.5 mi. Many breccia fragments were Duck Lake, located just west of Yellow- bleached by acid alteration before eruption stone Lake (Fig. l), lies in an oval depression and display cavities filled with a delicate box- 2500 ft by about 1200 ft that probably also work of silica. These cavities are pseudo- is an explosion crater. The lake has no surface morphs of sanidine phenocrysts that were outlet. Its level is 40 to 110 ft below the leached after silica was deposited along the crater rim and 57 ft above the level of Yellow- cleavage planes. Beyond the immediate vicin- stone Lake. The crater rim is composed of ity of the craters, the breccia mantle is dis- unsorted breccia made up of pumice frag- continuous, and its outer limit is poorly ments, obsidian blocks, and a few rounded defined. Throughout much of its extent, the cobbles of exotic igneous rocks. The frag- breccia (composed of tuff fragments) was ments range to as much as 2 ft across. In deposited on tuff of similar lithology. Con- pits dug on the inner slope of the northeast- sequently, the relationship of breccia to bed- ern rim, the breccia overlies bedded pumice- rock is unequivocal only to the northeast rich sediments of probable Bull Lake age and where breccia fragments rest on basalt. De- till as young as middle Pinedale age (G. M. lineation of the breccia extent is further com- Richmond, 1970, written commun.). Pumice plicated by the pre-breccia till and by the fragments from the breccia are rich in clinop- heavy forest cover. tilolite and mordenite, the zeolites charac- Turbid Lake, located about 2 mi northeast teristic of near-surface hot-spring alteration of Yellowstone Lake (Fig. l), lies within an in Lower Geyser Basin (Honda and Muffler,
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1970). No thermal activity is visible within ments of the breccia consist of sinter and the Duck Lake crater, but extensive hot- opal-cemented sandstone and gravel, and spring activity occurs in a nearly continuous range up to about 3 ft in diameter. This ex- strip along Yellowstone Lake, 900 ft to the plosion feature appears quite young, for in east. most places, no sinter has been deposited on Two explosion craters were recognized by or around the breccia fragments. It may even R. L. Christiansen and G. M. Richmond dur- have formed during this century, for Black ing their mapping of bedrock and glacial Opal Pool is not shown on the Hague Survey geology east of the Yellowstone River. We map of Upper Geyser Basin (Hague, 1904) have not visited these craters, and the follow- but is noted as Black Pearl Geyser by Allen ing descriptions are from Christiansen, Rich- and Day (1935, p. 134). mond, and R. O. Fournier (1969, oral com- Other circular depressions in Yellowstone mun.). National Park were initially considered to be One of these craters is a closed basin ap- possible explosion craters but later proved to proximately 700 ft across in the southern be better explained by other mechanisms. part of Josephs Coat Springs (Fig. 1). An Goose and Feather Lakes in Lower Geyser arcuate rim on the western and southern Basin (Fig. 3) and a lake at the south end of sides of this crater rises about 160 ft above the the Mud Volcano area (Fig. l) proved to be basin and is composed of explosion breccia. kettles produced by melting of isolated mas- The floor of the crater and the low rim to the ses of stagnant glacial ice. A depression in east and north are made up of silica-cemented, Upper Geyser Basin (A on Fig. 7) also prob- pre-explosion glacial deposits, and the ex- ably is a kettle, but the equivocal evidence plosion debris is formed of angular fragments does not preclude a hydrothermal explosion of these deposits. The crater floor and the origin. A number of depressions in acid- rim to the north and east are extensively altered areas appear not to be explosion acid-altered. craters but to have been produced by acid The other crater is at the eastern end of the leaching and consequent subsidence. Among Hot Spring Basin Group (Fig. 1). It is ap- these are the depressions west and south of proximately 1000 ft across and is surrounded the explosion craters at Roaring Mountain by an explosion breccia rim that rises to as (C and D on Fig. 6), a small depression 2000 much as 50 ft above the crater floor. The ft southwest of The Reservoir at Norris crater floor is bedrock Yellowstone Tuff, and Geyser Basin, and the depression at Clear the fragments within the breccia are ex- Lake (3000 ft south of Artist's Point on the clusively of Yellowstone Tuff. The whole south side of the Grand Canyon of the area is subjected to continuing acid alteration. Yellowstone). G. M. Richmond (1969, oral commun.) 0) - also recognized an explosion crater at Fern \\0°50' Lake, 9 mi north-northeast of Yellowstone •>f-- Opal fSapphire Pool Lake (Fig. 1). Fern Lake lies in a craterlike 1 depression 0.5 by 0.8 mi across, the rim of \ Gey serf ( which (except where breached by the drain- ing stream) rises 40 to 120 ft above the lake. "W-- According to Richmond, unconsolidated de- •3 ' :i bris erupted from the crater lies on cemented ^J **' lacustrine deposits of probable Bull Lake age 4 4° 28'- and on uncemented kame gravels of Pinedale age. A small hydrothermal explosion also oc- ""*% Y'^ Old Faithful. curred at Black Opal Pool in Upper Geyser Geyser \ Basin (Fig. 7). This pool discharges quietly from old sinter, and sinter continues to be deposited along the discharge channel. Sur- rounding the pool for as much as 100 ft to I the south and 400 ft to the north is a breccia Figure 7. Map of Upper Geyser Basin, show- ing location of Black Opal Pool and location mantle resting on the sinter. Angular frag- (A) of a depression of uncertain origin.
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CRATERING MECHANISM (5) occurrence of eight of the ten major Most natural terrestrial craters were formed crater sites (all but Roaring Mountain and by one of two general mechanisms, impact Hot Spring Basin) in glacial deposits of ap- and volcanism. Hydrothermal explosions do preciable thickness (that is, not merely thin not fit into either category. They are clearly ground moraine), although such thick gla- unrelated to impact, yet do not involve the cial sediment covers less than 15 percent of direct, immediate action of magma. Although the Park area; (6) the dominance of hydro- the heat source at depth in many hot-spring thermally altered and cemented glacial sedi- areas is probably a cooling igneous body, ment in the explosion fragments at these near-surface magma need not play any direct eight sites; among these, only the obscure role in hydrothermal explosions. The energy feature east of Twin Buttes contains frag- for a hydrothermal explosion is derived en- ments that could be from immediately under- tirely from the hot water and hot rock in the lying bedrock; (7) localization of the Pocket upper few hundred feet of a hot-spring sys- Basin and Twin Buttes craters along the tem, and several mechanisms other than direct lineament of highest subsurface temperature effects of magma can trigger hydrothermal (as deduced from chemistry of hot springs) explosions. that extends northeast-southwest across Lower Geyser Basin; (8) clustering of craters Among volcanic craters are included maars at several sites; and (9) demonstration from and diatremes (Shoemaker, 1962, p. 290-292, drilling that adequate temperatures (175°C) 295), and phreatic explosion craters (Stearns and energy characterize most of Yellow- and Macdonald, 1946, p. 16). Phreatic ex- stone's thermal areas at shallow depths. plosions are similar to hydrothermal explo- sions in that the explosive character is due to the rapid expansion of water into steam. TRIGGERING MECHANISMS Phreatic explosions, however, are produced Owing to the instability of a high-temper- by the injection of magma or hot fluidized ature hydrothermal system, any sudden de- rock into ground water of any temperature, crease of pressure on the system is potentially are characterized by unaltered ejecta, and the trigger for a hydrothermal explosion. This commonly are accompanied by juvenile vol- trigger may be an obvious external event like canic ejecta or associated extrusions. Some an earthquake or may be a subtle, apparently eruptions that have been called phreatic may insignificant change such as rapid fall in have been hydrothermal explosions, utilizing barometric pressure or slight increase in heat stored energy that had been transferred by supplied to the system. There was no obvious circulating hot water. triggering mechanism for the 1951 Lake City The Yellowstone craters discussed in this hydrothermal explosion (White, 1955). Ap- paper show no evidence of an impact origin. parently, the system was so unstable that it Shatter cones were not found, high-pressure was triggered by an event too minor to be minerals were not identified, and thin sec- noted separately. We now know that mineral tions of the breccia fragments show no fea- deposition and rock alteration can result in tures that can be attributed to shock (Short, "self sealing" and that water pressures of as 1966). Likewise, there is no evidence such as much as 30 percent over hydrostatic can de- juvenile volcanic ejecta to support a volcanic velop in self-sealed systems. Such overpres- origin. sures may have been significant at Lake City. On the other hand, the aggregate of geo- Earthquake activity is an obvious external logic relations detailed in previous sections mechanism that could trigger a hydrothermal of this paper strongly support a hydrothermal explosion, particularly if water overpressures explosion origin. Pertinent items include: existed. Yellowstone is in a seismically active (1) concentration of the craters in thermal zone (Ross and Nelson, 1964, Fig. 14), and areas of the world's premier hot-spring region; nearby earthquakes have caused extensive (2) close association of most of the known changes in the hydrothermal activity of Yel- craters with modern thermal activity; (3) ab- lowstone Park (Marler, 1964). In addition, sence of any volcanic activity in Yellowstone fluid overpressures are common in Yellow- National Park at the time of formation of at stone thermal areas. It should be noted, how- least eight of the craters (early Pinedale or ever, that there were no major hydrothermal later); (4) shallow focus of all the explosions; explosions following the 1959 Hebgen Lake
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earthquake, although the disruption of Sap- several places in the northern Rocky Moun- phire Geyser in Upper Geyser Basin (Marler, tains (Richmond and others, 1965). 1964) was essentially a small hydrothermal Rapid draining of a lake that had formed explosion. behind early Pinedale ice would drastically The close spatial and temporal relation of reduce the confining pressure on any hydro- many of the Yellowstone explosion craters to thermal system under the lake (Fig. 8). If we glacial deposits suggests another triggering assume the lake level to be controlled by an mechanism, namely the rapid draining of a ice level at 7600 ft, the temperature distribu- glacially dammed lake. The geologic evidence tion in and beneath the lake at Pocket Basin for this mechanism is permissive for all of the is represented on Figure 8 by curves such as craters and is particularly appealing for Pocket A (active convection) or B (conductive heat Basin. flow from aquifer to bottom of lake). These We have shown that the Pocket Basin hy- are only two of a family of temperature-depth drothermal explosion took place during the profiles in a glacial lake and an underlying waning stages of early Pinedale Glaciation. hot-spring system. Curve H0 is the boiling The hydrothermally cemented fragments in point curve for water assuming a free water the explosion breccia indicate that the hot- level at 7600 ft. Below ground level, the spring system did exist prior to the explosion. temperatures are controlled to a major extent Early Pinedale ice spread westward from the by this boiling point curve, in a manner anal- Central Plateau across the Firehole River ogous to USGS drill hole Y-3 (Fig. 2). drainage (Richmond, 1964), but because of Draining of this glacially dammed lake the hot-spring activity, it is unlikely that ice would reduce the effective confining pressure occupied all parts of Lower Geyser Basin, on the sublacustrine hot-spring system by particularly during the waning stages of glaci- the equivalent of 400 ft of water (approxi- ation. Using the data of Fournier and others mately 12 atm), and the controlling boiling (1967), one can calculate that the modern point curve would be relative to a free-water average heat flux of Lower Geyser Basin (in- level at 7200 ft. Water on a curve such as "A" cluding Midway Geyser Basin, which is not a along a channel of upflowing hot water would separate geologic or hydrologic entity) is be greatly superheated relative to either the 2 1 about 700 microcal cm~ sec^ , enough to Temperature 0 50 10 melt 10 ft of ice per year over the entire - - T J basin. Since most of this heat flux is con- centrated in less than 10 percent of the area of Lower Geyser Basin, more than 100 ft of ice per year could be melted over the hot- spring areas. It is probable that the heat flux during Pinedale time was the same or greater and that the areal distribution was similar. Accordingly, lakes may have existed over hot-spring areas in Lower Geyser Basin dur- ing the waning stages of early Pinedale Glaci- ation; ice would have remained in the non- thermal areas far longer than in the intensely thermal parts of Lower Geyser Basin. Glacially dammed lakes are common in Alaska, western Canada, Iceland, and Nor- way. Rapid release of great quantities of water by lifting, collapse, or erosion of ice dams has been demonstrated in scores of instances (for example, Thorarinsson, 1939; Stone, 1963a; Mathews, 1965). Lake George, Alaska, each summer releases !/2 to 2 million acre-feet of water in a few days, and the level Figure 8. Graph illustrating how rapid of the lake can drop as much as 175 ft (Stone, draining of a glacially dammed lake could 1963b). Glacially dammed lakes of Bull Lake trigger a hydrothermal explosion. See text for and Pinedale age have been recognized at explanation.
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new hydrostatic curve (Hn) or the lithostatic in pressure was small enough or slow enough curve, and vapor pressures at all depths less (or both) that the increased boiling was than approximately 300 ft would be in excess balanced by increased hot-spring and geyser of lithostatic. For example, at 100 ft below activity. Only at Pocket Basin and at Twin ground level, temperature of the water and Buttes was draining so rapid and of such an the rock would be 200°C and the vapor pres- extent that the strength of the hydrothermally sure of water 15.3 atm. However, lithostatic cemented rocks was overcome and a cata- pressure at this same depth would be only strophic hydrothermal explosion resulted. 7.7 atm. ENERGY RELATIONSHIPS Sudden draining of a glacially dammed lake commonly results in a major flood and dis- The model proposed here for the forma- tinctive flood deposits (Thorarinsson, 1939). tion of hydrothermal explosion craters in Deposits that can be related with certainty to Yellowstone National Park can be tested by draining of a lake over Pocket Basin are comparing the energy required to form the absent along the Firehole and Madison Rivers observed craters with the computed energy downstream from Lower Geyser Basin (H. A. stored in the rocks prior to disturbance. The Waldrop, 1969, oral commun.). However, calculations are necessarily approximate since any such deposits may well have been ex- the properties of the rock before the explo- tensively modified by later fluviatile action sion and the depth of the craters are imper- or may have been covered by later outwash fectly known. deposits from the melting early Pinedale Energy Available for Cratering glaciers. Furthermore, it should be noted that the area of a lake at Pocket Basin need only A reasonable thickness for the glacial have been about 1A sq mi. Assuming a depth gravels and lake sediments in the areas of the of 400 ft, such a lake would have a volume of Yellowstone hydrothermal explosion craters only 64,000 acre-feet, compared to Yi to 2 is about 200 + 50 ft. The temperature at the million acre-feet discharged annually from lake bottom at points of thermal water up- Lake George. This relatively small volume of flow is assumed to be 190°C, the boiling water might not form distinctive flood point at the appropriate pressure; at the deposits. rhyolite-sediment interface, it is assumed to Our model of Lower Geyser Basin during be 210°C (Fig. 8). Parts of the affected sys- late glacial time implies that most of the tem may not have been at maximum permis- thermal systems created melt-water lakes that sible temperatures, and so the average tem- drained from time to time. These systems perature is conservatively assumed ro be presumably all contained sufficient energy for 190°C. a hydrothermal explosion. Why, then, are The hydrothermal explosion process that there not hydrothermal explosion features we assume thus involves water at an initial associated with all of the hot-spring systems temperatureof 190°C and pressureof 12.4 atm of Lower Basin instead of just two? absolute irteversibly flashing to steam and The answer to this question is analogous cooling in the process to about 92°C, the to the explanation for the rarity of geyers. average boiling point for air pressure at the Although many hot-spring systems have am- prevailing elevation. At the same time, addi- ple energy for geyser eruption, in most in- tional heat is transferred from the rock debris dividual springs this energy is dissipated by to the water, with further production of steam. steady-state processes of convection, conduc- The energy available for cratering may be tion, boiling, evaporation, and mixing with calculated from the properties of an isolated cooler water. Most hot-spring systems can system (Denbigh, 1957, p. 17). A system is tolerate low rates of upflow and steam forma- isolated if there is no exchange of mass, heat, tion and can even adjust to slight increases in or work with the surroundings. In Figure 9, rates of boiling without triggering a geyser for the system in the initial state, the high- eruption. pressure (12.4 atm) hot water and rock are The hydrothermal systems of Lower Geyser represented as being in one compartment of a Basin generally must have adjusted in a simi- rigid insulated box separated by a sliding lar manner to drainage of overlying meltwater partition (representing the glacial lake) from and the consequent decrease in confining a large volume of air at a low pressure (0.7 pressure. In all but two cases, this reduction atm). The removal of the partition (draining
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The internal energies of saturated stea m and water at 190°C and 92°C are available from the steam tables (Keenan and Keyes, 1936). The decrease in internal energy of the rock is essentially equal to the loss of heat, mrcrAt, since AU = q—PAY, q = mrcrAt, and the volume change of the rock (AV) is negligible. Figure 9. The explosion of a geothermal We therefore can write, for times ti and t2, system represented as occurring within a rigid 1 2 <£^ 2-4>^ i = mr'c rAt + Xmw ^(I/ wate r - U'stea m )' insulated box due to the rapid removal of a tl t2 + (l-X)mv (U - U ). (4) partition. The volume of air is so large that / ww \ water water' compression is negligible. 3 For one cm of our system, <£2 —0i = 4.0 cal, which is sufficient to mechanically lift the of the lake) allows the water and rock to 3 expand and cool to the final state which is one cm of rock and residual water to an taken to be steam, water, and rock at 92°C. elevation of 2500 ft above its starting point if To determine the fraction of the water that no other energy absorbing processes occurred. will be vaporized, the water and rock may be For the water alone, with no heat transfer considered to be first cooled at constant from the rock, X = 0.18 and <£s—0i = 1.5 volume to 92°C and then allowed to expand cal. Band A in Figure 10 shows the range of isothermally to 0.7 atm. In the first process, available energy with variable amounts of heat is given off; in the second process, heat heat transfer from the rock. The left edge is 1.5 cal cm"3, with no heat from the rock, and is absorbed. Since the system exchanges no 3 heat with the surroundings (due to the rapidity the right edge is 4 cal cm" , with all of the of the process), these heats are equal. There- rock heat transferred. The actual situation is fore, for each cm*, we can write likely to be intermediate. m c At + m c At = Xm L. (l) Calculations by Goguel (1956) of the avail- w w r t w able energy in phreatic explosions, using a In this equation, mw and mt are the masses of method based on entropy, resulted in some- water and rock, cw and cr their specific heats, what higher figures due to the neglect of At the temperature change, L the heat of water density and due to the assumption of a vaporization of water, and X the fraction of specific heat of 0.5 cal cm"3 for the rock no water vaporized. For our system with At = matter what the porosity. A recalculation of 98°C, mw = (initial water density) (pore vol- the available energy per cm3 by Goguel's ume) = (.875 gm cm"3) (.2 cm3) = 0.175 1 1 1 method with corrections for fluid density and gm, cw = 1 cal grrr °C~ , cr = 0.2 cal gm" 1 for the change of rock specific heat with "C" (Ingersoll and others, 1948), mr = 2.0 1 porosity yield the same results as the internal gm, and L = 544 cal gm" , we calculate that energy method used here. X = 0.59. In an isolated system, the change in total Energy Required for Cratering energy, E, is zero. However, there may be a White (1955) calculated the mechanical transfer of energy between the internal energy, energy required to produce the Lake City U (which is a function only of the internal craters and concluded that as little as 12 per- state of the material in the system and which cent of the stored energy would suffice. His may be interpreted as measuring the random calculation neglected comminution and en- kinetic motion of the constituent molecules), ergy expended in accelerating the material. and other forms of energy. In our case, these The comminution enetgy was probably small are the potential energy of uplift, kinetic for the mud involved in this explosion. energy of acceleration, and the comminution Roddy (1966, 1968) calculated the me- (or rock-breaking) energy. Denoting the sum chanical energy of lifting and acceleration of these cratering energies by
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rocks (Fig. 10). The potential energy change debris falling inside and outside (Carlson and was calculated from the mean change in Jones, 1965). elevation of the ejected mass, but, instead of The calculation of the comminution energy adopting an arbitrary figure for mean acceler- was made by the use of the energy-size reduc- ation, a calculation was made of the accelera- tion relationship of Charles (1957), with the tion necessary to eject the debris at optimum size modulus (the size which divides the trajectory. This was done by integration of debris into equal weights of larger and smal- the acceleration of the mass in a cylindrical ler particles) estimated to be one centimeter. annulus (of radius, r + dr) necessary to pro- Since much of the hydrothermal explosion pel the mass to a final position in a ring with debris is quartz cemented, the size modulus- a radius equal to twice that of the crater. This energy relation was assumed to be equal to distance is an average observed in experimen- that of quartz as determined experimentally tal cratering, with about equal amounts of by Hukki (quoted in Bergstrom and others, 1961). The resulting comminution energy was 0.5 cal cm"3. Using comminution energy measurements for pyrex glass by Charles (1957) and Bergstrom and others (196l) yields 0.1 cal cm~3. This may approximate the opal- and zeolite-cemented debris from the Twin Buttes craters. Presumably unce- mented material would have zero comminu- tion energy. The potential, kinetic, and com- minution energies were summed for each crater size to arrive at band B, which repre- sents the range of possible cementation from unconsolidated (left-hand side) to quartz- cemented (right-hand side). Because of the uncertainties mentioned earlier, this type of calculation yields approximate values. Cratering Experiments Experimental cratering with chemical high explosive and nuclear devices has yielded scaling laws that allow predictions of the sizes of craters produced by different amounts of explosives (Vaile, 1961). Chemical and nuclear explosions have different efficiencies u due to their different explosive mechanisms calories (peak temperatures and pressures, rapidity, and so on). For this reason, the experimental Figure 10. Calculated and experimental scaling laws apply only approximately to energy-size relationships for explosion craters. hydrothermal explosions. Band A: the energy available at various crater sizes from 200 ft of water-saturated sediments Size-energy curves are shown in Figure 10, with 20 percent porosity, 2.2 bulk density calculated from scaling laws for dry sand, initially under a 400-ft-deep lake and with alluvium, and sandstone. The scaling laws temperatures on the boiling curve. Band B: are taken from Vaile (1961) for explosions at energy required to form craters of various sizes 100 ft depth with nuclear and chemical de- assuming an excavation to 200 ft with material vices. The energy available for cratering was lifted a mean height of 50 ft above ground with assumed to be 10 percent of the total energy, acceleration sufficient to move material laterally and the conversion 1 kiloton TNT equals to an average position 2 crater radii from the 1012 calories was used (Ornellas, 1968). Since center. Left side is for unconsolidated material; a constant depth of explosion (200 ft) has right side includes comminution energy for quartz cementation (see text). Dashed lines: been assumed in calculating the available size-energy relationship for nuclear explosions energy, the experimental curves were ad- at 100-ft depth in various materials, calculated justed to maintain a constant crater depth from experimental scaling laws (Vaile, 1961). equal to that of the one kiloton explosion.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/723/3432828/i0016-7606-82-3-723.pdf by guest on 29 September 2021 REFERENCES CITED 739
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