VOLKER LORENZ Center for Volcanology, University of Oregon, Eugene, Oregon 97403

Some Aspects of the Eruption Mechanism

of the Big Hole Maar, Central Oregon

ABSTRACT At the Big Hole maar in central Oregon, rising permits calculation of the apparent density and basalt magma came in contact with abundant velocity of the fluid system. Values for the final ground water at a depth of more than 200 m below strong eruptions are 0.01 g/cm3 and 226 m/sec and the surface, causing phreatomagmatic eruptions, for some of the earlier strong eruptions 0.01 g/cm3 possibly 20,000 yrs ago. Late-stage subsidence along and 200 m/sec. The kinetic energy of some of the a ring fault accounts for the large crater cut into strongest eruptions was approximately 1.33 X 1021 older rocks. Subsequent erosion of pyroclastic ergs. debris in the crater wall increased the diameter of Vesiculated tuffs, that is, tuffs with smooth-walled the crater while decreasing its depth. bubbles between the particles, are present between Investigation of the distribution pattern of large 1 and 2.5 km from the center of the crater; they ejected blocks and application of a model of block appear to be the deposits of base surges. acceleration in a dense rising two-phase system

INTRODUCTION southern rim about 1660, and the northern rim only 1540 m. The maximum depth of the crater Big Hole is a large forest-covered maar on the is, therefore, about 160 m, and the maximum northwest margin of Fort Rock basin, in Lake depth below the original ground level is ap- County, central Oregon. It lies approximately proximately 100 m. The original surface was 33 km southeast of La Pine, just south of the rather flat in the southwestern part of the maar State Highway 31, 33 km south of Newberry and dropped sharply, possibly along a fault or Caldera, and 10 km west of Hole-in-the-Ground flexure, toward the northeast part. The pyro- maar. It was mentioned briefly in a report by clastic deposits are about 60 m thick on the Peterson and Groh (1961 and 1963), and de- eastern rim, and gradually diminish in thickness scribed in more detail by Heiken (1970) in his away from the crater. The deposits generally general study of the tuff-rings and maars of dip gently away from the crater except in the central Oregon. northeast wall and on the southwest slope of In view of Heiken's report, only a few data Big Hole Butte, where inward dips reflect the are presented here on the origin and mode of pre-eruption morphology. formation of the maar, the main purpose of the North of the crater and northeast of Big Hole present paper being to discuss the distribution Butte, younger lava from the Paulina Moun- of the ejected blocks and the origin of the tains, which surround Newberry Caldera, vesiculated tuffs. covers part of the ejecta of Big Hole (Fig. 1). This lava may be approximately 10,000 yrs old BIG HOLE (Lorenz, 1970). The eruption of Mt. Mazama The diameter of Big Hole measured at the which produced the caldera of Crater Lake rim varies from 2.1 to 2.4 km. However, the about 7000 years ago (Bedwell, 1969), laid diameter is only 1.6 km at the level of the down a sheet of pumice, 60 to 80 cm thick, at original surface, which is given by the top of Big Hole, so that exposures are restricted to the older lavas in the maar wall. No good topo- steep interior walls and part of the northeast graphic map is available; hence, the elevations outer slope of the maar. Erosion has proceeded given.are only approximate. The flat floor of the much further at Big Hole than at Hole-in-the- maar lies about 1500 m above sea level, the Ground maar, the age of which is believed to be

Geological Society of America Bulletin, v. 81, p. f 823-1830, 5 figs., June 1970 1823

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/6/1823/3428393/i0016-7606-81-6-1823.pdf by guest on 25 September 2021 1824 V. LORENZ—ERUPTION MECHANISM, BIG HOLE, MAAR, CENTRAL OREGON N ! convolute bedding, accretionary lapilli, and vvv v v v v v y.. y v v v v v impact sags indicates phreatomagmatic erup- V tions, condensation of water vapor in the erup- =a>y V V y .. 'ff^'^'^^ ' V; V V vVv v tion clouds, and accumulation of wet or muddy v..>-^r^_--^.j^°tr , ..(o^^ix%ix •.^' v v v x^.- yv v v deposits. Contact of rising-basaltic magma with (^ V V \ X i-u ^4s V ,. abundant ground water accounts for the nature of the eruptions. The faults in the Big Hole area, one of which apparently crosses the maar, are oriented northeast-southwest and are spaced at intervals of 1.2 to 1.7 km. Magma appears to have risen along a fault, the displacement along which was not more than 10m. Since Big Hole was formed, erosion has changed its shape considerably, having cut into the debris especially on the east and northeast outer slopes. The upper part of the crater VV Younger basalt XX Older basalt wall has receded (Fig. 2). The older lavas have ""- Ignimbrite resisted erosion much more than the softer j^- Fault pyroclastic debris. All of the erosion products ^ Cliff inside crater from the upper part of the crater wall have -• 'T/O Isopleth: I m Largest diameter of accumulated on the floor, the present flatness of © blacks of earlier eruptions: 0.4m which is thus only of secondary origin. The X Vesioulated tuffs original floor may be 100 m deeper. The pyroclastic ejecta, as noted already, Figure 1. Geologic map of Big Hole and surround- contain only a small amount of country rock ing area showing isopleths of large blocks ejected during final eruptions, maximum diameter of krge blocks fragments; hence, the question arises as to what ejected during earlier eruptions, and location of happened to the older rocks that formerly vesiculated tuffs. B.H.B.: Big Hole Butte. occupied the volume of the crater below the level of the original surface. Slumping of wall rocks into an open vent would change the shape between about 13,000 and 18,000 yrs B.P.; of the crater, but would not affect its volume. accordingly, Big Hole may be about 20,000 yrs Therefore, subsidence along a ring fault due to old. withdrawal of magmatic support seems to be A ledge on the southwestern half of the the only adequate explanation. Big Hole is in crater wall marks the original surface. It is fact a small collapse caldera or a maar of "type formed by basaltic lavas of varying thickness, b" (Lorenz and others, 1970). underlain by a sheet of ignimbrite. The ignimbrite marks the top of Hampton's (1964) Peyerl Tuff unit, the thickness of which is approximately 130 m. This tuff unit is underlain by porphyritic basalt flows, some of which were penetrated in a drill hole at Hole-in-the- Ground (Lorenz, 1970), where an 8-m-thick porphyritic olivine basalt overlies several highly porphyritic basalt flows, at least 82 m thick, blocks of which are also found in the ejecta of Big Hole. Assuming similar thicknesses of the strata at Big Hole, the eruption focus must have been at a depth of more than 220 m. The pyroclastic ejecta, which have been well + + Younger basolt described by Heiken (1970), are characterized :••'.•'.'-''•''• Tuffs eroded from rim v v Older bat a It

by abundant juvenile lapilli and a relatively "1 Bedded luffs '•" —' Peyerl tuff small amount of country-rock fragments. They Tuff fillln weni A A Porphyritic basalt are bedded, individual layers varying from 1 to T~\}\ 9 10 cm thick. The presence of sideromelane, Figure 2. Schematic cross section through Big Hole.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/6/1823/3428393/i0016-7606-81-6-1823.pdf by guest on 25 September 2021 NOTES AND DISCUSSIONS 1825

EJECTED BLOCKS AND MAGNITUDE addition, blocks of a certain size thrown to their OF ERUPTIONS respective isopleths were assumed to have been ejected at an angle of 45° to reach the maximum Ejected blocks are scattered throughout the distance. Blocks of the same size found inside fine-grained pyroclastic debris in the rim of the the isopleth were ejected at higher or, less prob- maar but are concentrated mainly in two ably, at lower angles, or with lower velocities. horizons, the upper and lower "blocky" beds, The initial velocities, Fo, of the blocks ejected each of which is underlain by fine-grained beds at an angle of 45° can be calculated from that contain fewer and smaller blocks. The upper "blocky" bed, which apparently repre- sents the final products of eruption, is approx- where D is the distance from the crater center imately 5 to 10 m thick at the rim and contains and g is the acceleration due to gravity. Results many large blocks, some of which are rounded are given in Figure 3. The blocks furthest away and reach a maximum diameter of 2.3 m. from the crater, at a distance of 3 km, measure Rounding of the blocks took place, prior to 20 to 30 cm in diameter and, hence, had initial their ejection, while they were in a fluidized velocities of 172 m/sec. bed (Lorenz and others, 1970) at a depth of The terminal velocity, Vt, of a block falling 200 m or more below the surface. They were through a fluid is also the velocity of the fluid rounded during the weaker eruptions that required to suspend this block. For this purpose produced the upper fine-grained, almost block- the fluid velocity, V'/, is then assumed to be free beds immediately prior to the final strong eruptions. At the end of their ballistic trajec- Vt = Fo + Vt, (2) tories the large blocks produced deep and asym- which is the velocity of the eruption cloud metric impact sags in the underlying beds. The rising from the crater. The terminal velocity of considerable size and depth of the impact sags a round particle depends largely on its radius, r, clearly indicates a large momentum of the air- the densities of the fluid, pi, the particle, pt, borne blocks and thus testifies to strong erup- and a drag coefficient C, and can be expressed tions. with the equation: The decrease of the maximum diameter of the largest blocks with increasing distance from M P. - Pi) (3) the crater was studied and an isopleth map was V 3-C-/>, compiled in order to work out the magnitude of the strongest eruptions (Fig. 1). The ac- curacy of the map is best in the northern and eastern sector where exposures are best. The isopleths are slightly elongated toward the northeast, suggesting inclined eruptions or an 3 - / Earlier elongated feeder, aligned along a northeast- Eruptions southwest-trending fissure or fault. Final Evaluation of the isopleth map is based only 0 2 on distances and sizes along a line drawn toward (km) the northeast and a model that has already been applied to the evaluation of block-distribution I - at Hole-in-the-Ground maar (Lorenz, 1970). All blocks in the size-range 0.1 to 2.3 m 200 - reached their ejection velocities by drifting in Vo (m/sec) . the rising gas and small particles inside the vent |50 and were not in contact with each other. The 100 - flow pattern around all blocks of this size range _l I L was assumed to be the same, that is, Reynolds 0.2 04 1.0 1.5 numbers were taken to have been uniform for 0.5 2 all blocks within this range. At the center of the max.(m) crater, a point source was assumed from which Figure 3. Diagram showing relationship between the blocks were ejected and followed parabolic maximum diameter, ^max, of large ejected blocks, ejec- trajectories, unrestricted by air resistance. In tion distance, D, and ejection velocity, Fo.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/6/1823/3428393/i0016-7606-81-6-1823.pdf by guest on 25 September 2021 1826 V. LORENZ—ERUPTION MECHANISM, BIG HOLE MAAR, CENTRAL OREGON

In turbulent flow, a typical value of C is taken to a certain extent. The fluid velocity at the as 0.4. A constant can then be derived: crater, however, will be larger than that calculated, perhaps 240 to 250 m/sec, and the 81 (4) density will be lower because an equilibrium 3-C condition cannot have been reached in the For irregular fragments of quartz where the rapidly changing two-phase system. If the drag coefficient is larger, experiments in air minimum ejection angle was 55° to 65°, a have shown the constant to be 71 (Hardinge and slightly higher fluid velocity would result. Prankish, 1945). Applying this constant, Calculation under the above assumptions for equation (3) becomes: a block 10 cm across gives 26 m/sec as the terminal velocity, an initial velocity of about P, - Pi -r 200 m/sec, and a possible maximum ejection V, = 71 (5) distance of 4 km. It is also possible to calculate the size of blocks that were suspended in this Both Vt and pi are unknown in this equation, 3 fluid system. The condition for calculation of p, is taken as 2.7 g/cm . Using various pi values the "equilibrium" size is and block diameters, values of Vt can be calcu- V = V . lated. The only set of Vt values that will be t f (7) useful is the one which can be added to the Hence, the diameter of a block suspended in respective Vt values to give a constant value of the fluid system is 7.5 m. The largest blocks in the lower 'blocky' beds in the northeast wall, 1 km away from the center of the crater, have a diameter of about As can be seen in Table 1, at a fluid density of 1.5 m; those found in presumably equivalent 0.01 g/cm8, the fluid velocity Vj is nearly the beds 2 km away from the center of the crater same for different block sizes, that is, about on the northeast side of Big Hole Butte have 226 m/sec. Small irregularities in the values a maximum diameter of 40 cm; and those at 2.6 can easily be attributed to uncertainties of the km reach 20 cm across. Application of the same mapping. At larger or smaller values of the model, as given above, leads to initial velocities fluid density, the values of the fluid velocity of these blocks of 99.4, 140.5 and 160.2 m/sec, spread much more. Within the limits of the to an apparent fluid density of 0.01 g/cm3 and assumptions made above, a rising fluid system an apparent fluid velocity of 200 m/sec (Table 8 with an apparent density of 0.01 g/cm and a *)• velocity of 226 m/sec could produce the differ- Blocks that rose vertically in the eruption ent initial velocities of blocks of various size clouds and had a diameter of about 10 cm must carried by the fluid. have reached heights of more than 2 km. The In nature, blocks follow ballistic trajectories eruption clouds however, owing to the thermal instead of parabolic ones, and are released from effect of hot expanding water vapor, must have an eruption source of a certain cross section and reached even greater heights, perhaps 5 km or height, the effects of which balance each other more.

TABLE 1. VALUES OF EJECTION DISTANCES D' Fo* 2r* Ft* Ff* km m/sec m m/sec m/sec 2.8 166.1 0.5 58.2 224.3 2.16 146.0 1.0 82.5 228.5 A 1.7 129.5 1.5 101.0 230.5 1.15 106.5 2.0 116.3 222.8 2.6 160.2 0.2 36.8 197.0 B 2.0 140.5 0.4 52.3 197.8 1.0 99.4 1.5 101.0 200.4 3 * D, initial velocities, V0, terminal velocities, Vt, at an apparent fluid density of 0.01 g/cm , and apparent fluid velocities, Vf, for blocks of different maximum diameter, 2 r, ejected under 45° from horizontal. A: final eruptions; B: earlier eruptions.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/6/1823/3428393/i0016-7606-81-6-1823.pdf by guest on 25 September 2021 NOTES AND DISCUSSIONS 1827

At Hole-in- die-Ground, 10 km to the cast, places formed impact sags without disrupting the calculated maximum fluid velocity was the bedding. They are also characterized by about 200m/scc, that is, slightly less than at Big layers of accretionary lapilh, by cross-bedding, Hole. However, the apparent fluid density was and by local undulations with a wave length of 0.04 g/cm3, and, hence, larger blocks are to be 5 to 7 m and a maximum amplitude of 1 m. found at the same distance irom the crater Analysis of the cross-bedding indicates that the center. At Hole-in-thc-Ground, the equilibrium currents responsible for their formation were si/.c was calculated to be approximately 23 m. the result of "base surges" from the crater of These variations may reflect variations in Big Hole. Asymmetric impact sags also point the diameters of the vents and craters, varia- to Big Hole as the source of the impacting tions in the amounts of gas and solid phase blocks. available, and different parameters of the gas In addition, there are two beds of vesiculated phase. More investigations of this kind, and tuff, that is, tuff containing cavities formed by others of recent eruptions, where comparison gas. The upper bed lies 35 cm above the lower can be made with actual measurements of the bed; both are between 3 and 10 cm thick and initial velocities of blocks, the fluid velocity, are easy to recognize, because they are harder and even the fluid density arc required before than the beds above and below. Both beds are any valid implications can be drawn. graded in their lower parts and have very Eruption pressures at the orifice can be cal- sharp contacts with the overlying beds. The culated by application ol Bernoulli's theorem: upper surfaces of both beds show ripples with a wavelength of 5 to 10 cm and an amplitude of 1 to 3 cm. The dip of the beds varies between where P is the velocity-producing pressure and 5 ° and 20°; as it increases, so do the wavelength V the maximum velocity of the solid phase. ps and amplitude of the ripples. The ripples are is taken as 2.7 g/cm3 and Fas 230 m/sec for the elongated perpendicular to the slope (Fig. 4). final eruptions and as 200 m/sec for the earlier The steeper the dip, the more asymmetric arc strong ones. Hence, eruption pressures of some the ripples; the downslopc side of each ripple is of the final and earlier eruptions were about shorter and steeper than the upslope side. The 715 bars and 540 bars, respectively. lower face ol each bed is always even. These If we assume that the strongest eruptions features, taken together, indicate that the covered a circular area of 8 km diameter with ripples resulted from flowage and are not ol pyroclastic debris of a density of 2.0 g/cm3 and eolian or base-surge origin. They cannot have an average thickness of 5 cm, the kinetic energy been caused by recent erosion, because they of these eruptions can be calculated using: were formed prior to deposition of the overlying beds. We conclude, therefore, that the (9) flow structures developed in layers of water- soaked pyroclastic debris. Then the kinetic energy of the strongest eruptions was approximately 1.33 X 1021 ergs or the equivalent of 31.7 kilotons trinitrotoluene. Hence, the strongest of these phreatomagmatic eruptions at Big Hole are comparable to medium-sized nuclear events. VESICULATED TUFFS Pyroclastic beds are well exposed at the northeast end of the slightly elongated ridge of Big Hole Butte, 2 km northeast of the center of the crater of Big Hole. They extend from the early eruption deposits, which are palagon- itized at Big Hole Butte, into the deposits of the final eruptions that exhibit impact sags made by falling blocks up to 1.5 m in diameter. The well-bedded tuffs just below these upper Figure 4. Photograph of vesiculated tuff from "blocky" beds are characterized by mcdium- northeastern end of Big Hole Butte ridge showing si/,ed blocks, 5 to 20 cm across, which in some gravity-flow ripples.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/6/1823/3428393/i0016-7606-81-6-1823.pdf by guest on 25 September 2021 1828 V. LORKNZ—ERUPTION MECHANISM, BIG HOLE MAAR, CENTRAL OREGON

The average grain size of the graded vesic- posited by airfalls, all the gas would have been ulated tuffs is less than 0.3 mm. Fragments separated from the solid ejecta before they from 1 to 4 mm across may be found at their landed from flight. It seems far more likely base, along with accretionary lapdli, 0.3 to that the tuffs were laid down by base surges, 1.0 cm across. Most vesicles in the tuffs measure the transporting medium of which was water between 0.5 and 1 mm in diameter, a few exceed vapor (Fisher and Waters, 1969; Moore, 1967). 4 mm, and some reach 1 cm across (Fig. 5). Some water vapor can be assumed to have con- They are irregular in the lower graded part of the densed in the spreading clouds. During passage beds, but tend to be ovoid or spheroidal in the of the base surges, a considerable amount of upper finer grained part of the tuffs. Their vapor may have been trapped in the simul- walls arc smooth and are lined with particles taneously deposited muddy fine-grained debris, that are considerably smaller than those in even far from the vent. Continued expansion of other parts of the tuif. Large vesicles may be this entrapped vapor produced the smooth- concentrated at certain levels, and some of them walled vesicles. The accretionary lapilli in the may be connected with each other to form ir- lower graded part of the vesiculated tuff beds regular openings. Elongated vesicles tend to be either may have been picked up from under- inclined downslope, indicating late-stage shear lying beds or were newly lormed in the base movements, the tops of the beds having moved surges. A third possibility is that they were de- downslope with respect to the bottoms. These rived from simultaneous airfalls. shear movements also are expressed by the Vesiculated tuffs are also exposed in the east ripples already described, and were caused by wall of Big Hole where some hard beds, small-scale gravity flow. 5-10 cm thick, contain many vesicles; some Formation of the vesicles necessitated an hard beds contain only a few vesicles, and many expanding gas phase surrounded by a viscous, contain none at all. They are also seen at the containing medium. Dry particles of the given east side of Big Hole Butte, 2.7 km from the size could not have retained a gas phase; hence, center of Big Hole (Fig. 1). They are also de- the tuff must have been slightly wet or muddy. veloped in the north rim of Hole-in-the-Ground Molecular forces exerted by adsorbed water, maar (Lorenz, 1970) and in the east rim of binding the small particles together, produced North Twin Lake, west of La Pine, also in cen- a coherent medium permitting the gas phase tral Oregon. to expand and form vesicles. Vesiculated tuffs seem to have been described The fine-grained vesiculated tuffs extend at previously only from the Hverfjall and Ludent least 2 km from the vent. Had they been de- tuff-rings in northern Iceland (Einarsson, 1965; J. B. Murray, 1967; Lorenz and others, 1970). An upward increase in the number of vesicles in the tuffs in the rims of these craters has been attributed to the diminishing heights of the eruption columns as activity continued, so that more and more gas was trapped in the ejecta as they were deposited (J. B. Murray, 1967). No wet surrounding was postulated. Vesiculated tuffs may accumulate on the rims of craters either from base surges or from rapid downfall of ejecta out of small cauliflower eruption clouds, but only base surges can account for their occurrence far from the eruptive vents. ACKNOWLEDGMENTS The author gratefully acknowledges stim- ulating discussions in the field with Grant Heiken, Alexander R. McBirney, Norman V. Peterson, and Howel Williams. Howel Williams Figure 5. Photograph of vesiculated tuff from also aided by editing the manuscript. The work northeastern end of Big Hole Butte ridge showing was supported by NASA Grant NCR vesicles; size of slab is 2.5 X 3 cm. 38-003-012.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/6/1823/3428393/i0016-7606-81-6-1823.pdf by guest on 25 September 2021 NOTES AND DISCUSSIONS 1829

REFERENCES CITED Bedwell, S. F., 1969, Prehistory and environment Lorenz, V., 1970, An investigation of volcanic of the pluvial Fort Rock Lake area of South depressions. Part IV. Origin of Hole-in-the- Central Oregon: Ph.D. thesis, Oregon Univ., Ground, a maar in Central Oregon: Natl. Eugene, Oregon, 392 p. (available on micro- Aeronautics and Space Agency Prog. Rept., film or by interlibrary loan). NGR-38-003-012, (in press). Einarsson, T., 1965, The ring-mountains Hverfjall, Lorenz, V., McBirney, A. R., and Williams, H., Ludent, and Hrossaborg in Northern Iceland: 1970, An investigation of volcanic depressions. Visindafelag Islandinga (Societas Scientiarum Part III. Maars, tuff-rings, tuff-cones, and Islandica), Greinar IV, p. 1-28. diatremes. NASA Progress report (NGR-38- Fisher, R. V., and Waters, A. C., 1969, Bed forms 003-012), (in press). in base-surge deposits: Lunar implications: Moore, J. G., 1967, Base surge in recent volcanic Science, v. 165, p. 1349-1352. eruptions: Bull. Volcano!., Tome 30, p. 337- Hampton, E. R., 1964, Geologic factors that control 363. the occurrence and availability of groundvvater Murray, J. B., 1967, A comparative study of the in the Fort Rock Basin, Lake County, Oregon: craters Hverfjall and Ludent, Iceland, and U.S. Geol. Survey Prof. Paper 383-B, 29 p. craters in the interior of Alphonsus on the Hardinge, H., and Prankish, T. A., 1945, Aii moon. (Privately circulated manuscript avail- sizing and dust collection, sect. 9 in Taggart, able from author.) A. F., Handbook of Mineral Dressing: New Peterson, N. V., and Groh, E. A., 1961, Hole-in- York, John Wiley & Sons, Inc., p. 1-37. the-Ground, Central Oregon. Meteorite crater Heiken, G., 1970, Tuff-rings and "maars" of the or volcanic explosion? Ore Bin, v. 23, p. 95- Fort Rock-Christmas Lake Valley Basin: 100. Unpubl. Ph.D. thesis, University of California, 1963, Maars of South-Central Oregon: Ore Santa Barbara. Bin, v. 25, p. 73-88.

MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 14, 1970

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/6/1823/3428393/i0016-7606-81-6-1823.pdf by guest on 25 September 2021