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

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VOLKER LORENZ Center for Volcanology, University of Oregon, Eugene, Oregon 97403 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 north- east 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 under- lain 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.
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