"Constraints on the Potential Subsurface Area of Disruption

CONSTRAINTS ON THE POTENTIAL SUBSURFACE AREA OF DISRUPTION ASSOCIATED WITH YUCCA MOUNTAIN REGION BASALTIC VOLCANOES

Prepared for

Nuclear Regulatory Commission Contract NRC-02-93-005

Prepared by Brittain Hill

Center for Nuclear Waste Regulatory Analyses San Antonio, Texas

November 1996 3/193

ABSTRACT

Determining the subsurface area that a basaltic volcano can disrupt is a key parameter in calculating the amount of high-level radioactive waste that conceivably can be transported into the accessible environment. Although there is evidence that some Quaternary Yucca Mountain region (YMR) volcanoes disrupted anomalously large amounts of subsurface rock, the volume of disruption cannot be calculated directly. An analog volcanic eruption at Tolbachik volcano in Kamchatka, however, provides critical constraints on the subsurface area potentially disrupted by basaltic cinder cones. During a 12 hr period of the 1975 Tolbachik eruption, 2.8x10 6 m3 of subsurface rock was brecciated and ejected from the volcano. Using geologic and geometric constraints, this volume represents a circular diameter of disruption of 49±7 m. An unusual type of volcanic bomb was produced during this disruptive event, which contains multiple subsurface rock types that each show a range in thermal effects. These bombs represent the mixing of different stratigraphic levels and significant distances outward from the conduit during a single event. The same type of unusual volcanic bomb is found at Lathrop Wells and Little Black Peak volcanoes in the YMR. In addition, Lathrop Wells and to a lesser extent Little Black Peak have anomalously high subsurface rock-fragment abundances and are constructed of loose, broken tephra. These characteristics strongly suggest that subsurface disruptive events analogous to that at 1975 Tolbachik occurred at these YMR volcanoes. Rock fragment types, stratigraphic relationships, and geophysical data indicate Lathrop Wells disrupted a 0.5-2-km-thick crustal section, which is comparable to the thickness disrupted at 1975 Tolbachik. The volume of Lathrop Wells is nearly identical to 1975 Tolbachik. Because the volume of disrupted subsurface rock is probably directly related to eruption volume, Lathrop Wells likely disrupted a subsurface volume similar to 1975 Tolbachik. Thus, the subsurface conduits of future YMR basaltic eruptions may have the potential to widen on the order of 49±7 m in diameter. Conduit response to stress anisotropy in the disturbed geologic setting of the proposed repository site may result in an ellipsoidal cross sectional area with major axis length greater than 47±7 m.

ii * m

CONTENTS

Section Page

FIGURES ...... iv TABLES ...... v ACKNOWLEDGMENTS ...... vi QUALITY OF DATA ...... vi

1 INTRODUCTION ...... 1

2 TOLBACHIK-YUCCA MOUNTAIN REGION VOLCANO ANALOGY ...... I

3 1975 TOLBACHIK ERUPTION ...... 4

4 WHITE-ASH DEPOSIT ...... 6

5 WHITE-ASH EVENTS IN THE YUCCA MOUNTAIN REGION ...... 9

6 SUBSURFACE AREA OF DISRUPTION ...... 9

7 CONCLUSIONS ...... 11

8 REFERENCES ...... 13

iii FIGURES

Figure Page

I Location map for the 1975 Tolbachik eruption. Shaded 1975 volcanoes are Cone I (1), Cone 11 (2), and Southern Cone (S) ...... 2

2 A) Comparison of 1975 Tolbachik Cone I with Lathrop Wells volcano, Yucca Mountain Region. Lathrop Wells lavas are thicker than Cone I whereas Tolbachik lavas flowed down a steeper topographic gradient ...... 5

3 Xenolith bombs for 1975 Tolbachik Cone I (A-C), Lathrop Wells (D-E), and Little Black Peak (F) .... 8

4 Diagram of subsurface disruption associated with white-ash eruption at Tolbachik Cone I. Note horizontal scale is twice vertical scale ...... 11

iv TABLES

Table Page

I Average composition and viscosity of basaltic magmas from the Yucca Mountain Region and 1975 Tolbachik eruption ...... 3

2 Calculation of the subsurface area disrupted during the 1975 Tolbachik white-ash phase of the Cone I eruption ...... 12

v * *0 d/3

ACKNOWLEDGMENTS

Philip Doubik, State University of New York, Buffalo, generously provided initial results from his research on the petrology of Tolbachik white-ash deposits and contributed significantly to the 1994 field studies in Kamchatka. Yuri Doubik and Alexander Ovsyannikov of the Institute for Volcanic Geology and Geochemistry, Petropavlovsk-Kamchatsky, also made substantial contributions to the field research reported herein. Discussions with Charles Connor, Jim Luhr, Steve Self, and George Walker also helped to solidify many of the relationships between white-ash, xenolith bombs, and Yucca Mountain Region eruption processes. Detailed reviews by Charles Connor, Pat Mackin, and Wes Patrick improved the technical content and presentation of this report. This report was prepared to document work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) for the Nuclear Regulatory Commission (NRC) under Contract No. NRC-02-93-005. The activities reported here were performed on behalf of the NRC. The report is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.

QUALITY OF DATA

DATA: CNWRA-generated original data contained in this report meets quality assurance requirements described in the CNWRA Quality Assurance Manual. Sources for other data should be consulted for determining the level of quality for those data.

vi 1 INTRODUCTION

Determining the subsurface area that a volcano can disrupt is a key parameter in calculating the amount of high-level radioactive waste (HLW) that can be conceivably transported into the accessible environment. This parameter cannot be measured at Yucca Mountain region (YMR) volcanoes, although there is direct evidence that anomalously large amounts of subsurface rock have been disrupted in some Quaternary eruptions (e.g., Crowe et al., 1986). Studies of historically active basaltic volcanoes that are analogous to YMR volcanoes can provide critical data and insights into igneous processes that can affect repository performance.

This report summarizes results of Center for Nuclear Waste Regulatory Analyses (CNWRA) research at Tolbachik volcano in Kamchatka, Russia, where the process of subsurface disruption was well documented during the 1975 eruption. The 1975 Tolbachik eruption is closely analogous to YMR volcanism and this analogy is also documented in this report. Deposits from the 1975 Tolbachik eruption are compared to similar deposits at Lathrop Wells and Little Black Peak volcanoes in the YMR, and the implications for the area of subsurface disruption are explored.

2 TOLBACHIK-YUCCA MOUNTAIN REGION VOLCANO ANALOGY

The 1975 Tolbachik eruption occurred in a Holocene cinder cone field on the distal southern flanks of Ostry and Plosky Tolbachik stratovolcanoes, which are part of the active Klyuchevskoy volcanic complex in the Kamchatka volcanic arc (Figure 1). Although the Kamchatka volcanic arc is regionally distinct from the tectonic setting of the Basin and Range Province, these regional tectonic differences do not affect the local processes of basaltic magma ascent and eruption. Like the YMR, this part of the Kamchatka volcanic arc is dominated by local extensional tectonic features. Tolbachik volcanoes are located in the central part of an intra-arc graben containing predominantly northeast-trending faults and volcano alignments (Figure 1). Graben subsidence during the Plio-Quatemary resulted in deposition of approximately 1.2 km of mafic volcanic rock upon several kilometers of volcanically derived sediments underneath the Tolbachik volcanoes (Balesta et al., 1983). Resistivity studies (Balesta et al., 1984) and the distribution of springs indicate depth to the water table of 500-800 m beneath the site of the 1975 eruption.

Equilibration depth and source material can directly affect the degassing history of the magma, which relates to eruption explosivity. The basaltic magma from the 1975 Tolbachik eruption was derived from melting of lithospheric mantle, which then ponded near the base of the crust at approximately 40 km (Balesta et al., 1984; Volynets et al., 1983; Flerov et al., 1984). Quatemary basalts of the YMR also were derived from compositionally similar mantle material and last equilibrated at 30-40 km, at or slightly below the base of the crust (Vaniman et al., 1982; Brocher et al., 1993). Thus, the 1975 Tolbachik and YMR magmas most likely had similar equilibration depths, volatile contents, and ascent histories.

The Theological characteristics of magma directly affect volatile release and thus explosivity of volcanic eruptions (e.g., Williams and McBirney, 1979). Basalts from the 1975 Tolbachik eruption and Quaternary YMR have very similar chemical composition and mineral abundances. These data are summarized in Table 1. Magma densities calculated with partial molar volume methods of Lange and Carmichael (1987) and Lange (1994) are nearly identical for these basalts (Table 1). Using the methods of Shaw (1972) and Pinkerton and Stevenson (1992), viscosities of these magmas also are remarkably similar (Table 1). For comparison, the dacitic magma from the 1980 Mount St. Helens eruption (Cashman and Taggart, 1983)

I Tolbachik1 975

Budinkov et al. (1983)|

* Basaltic vent f jg

Figure 1. Location map for the 1975 Tolbachik eruption. Shaded 1975 volcanoes are Cone I (1), Cone II (2), and Southern Cone (S). Plosky Tolbachik shown by (P). lsopachs from 19 75 fall deposits are shown by thick black lines. Note strong NE-trend to vent alignment, which represents regional trend to normal faults. Inset map shows location of Tolbachik and some major Quaternary volcanoes of Kamchatka.

2 0 0

Table 1. Average composition and viscosity of basaltic magmas from the Yucca Mountain Region and 1975 Tolbachik eruption. Avg LW = average Lathrop Wells, CF = Quaternary Crater Flat, CI Early = early stage Cone I, CI Late = late stage Cone I. Yucca Mountain Region data from Hill and Luhr (unpublished research); 1975 Tolbachik from Volynets et al. (1983).

Avg LW Avg CF Cl Early CI Late

#of Analyses 20 32 21 11 Wt. % oxides

SiO2 48.8±0.5 50.2±1.2 49.80 50.00

TiO 2 1.95±0.05 1.59±0.27 1.02 1.30

A120 3 16.81±0.19 17.3±0.4 13.48 15.32

Fe203 2.33±0.03 2.15±0.11 3.06 3.47 FeO 8.37±0.11 7.72±0.40 6.99 6.88 MnO 0.17±0.01 0.17_0.01 0.16 0.17 MgO 5.86±0.05 5.09±0.15 9.88 7.69 CaO 8.57±0.17 8.71±0.28 11.60 9.83

Na2O 3.62±0.06 3.41±0.23 2.44 3.14

K20 1.80±0.05 1.76±0.15 1.03 1.62 P205 1.22±0.02 1.10±0.10 0.25 0.35

Estimated H20(%) 2 2 2 2 Estimated T ("C) 1100 1100 1200 1200

Density (kg m-3) 2590 2570 2600 2640 Viscosity (Pa s) 17 30 4 6 Crystal Fraction 0.03 0.03 0.03 0.03 Viscosity with 20 34 5 7 crystals (Pa s)

3 has a viscosity of 1.8x106 Pa s. Thus, the rheological characteristics of the 1975 Tolbachik and Quatemary YMR basalts are nearly identical, and there is reason to believe that basalts in these two systems were similarly explosive.

In summary, the 1975 Tolbachik eruption is demonstrably analogous to Quatemary YMR volcanoes based on close similarities in (i) extensional tectonic setting, (ii) crustal thickness, (iii) depth to water table, (iv) source composition, (v) magma composition, and (vi) magma rheology. Eruption volumes also are very similar between these two systems. Estimated magma volumes for Quatemary YMR volcanoes are 0.14 km3 for Lathrop Wells, 0.06 km3 for Sleeping Butte, and 0.23 km3 for Crater Flat (Crowe et al., 1995). The 1975 Cone I eruption at Tolbachik produced 0.14 km3 of magma. 3 1975 TOLBACHIK ERUPTION

The 1975 Tolbachik eruption was monitored by more than 200 Russian scientists and has been the subject of many detailed investigations. The chronology of this eruption is summarized to present the sequence and character of events that may typically accompany a basaltic cinder cone eruption. Information in this section is compiled from Tokarev (1978; 1983), Jayaweera et al. (1976), Budnikov et al. (1983), Faberov (1983), Fedotov (1983), Fedotov et al. (1976; 1984; 1991), and Maleyev and Vande-Kirkov (1983).

The 1975 eruption was heralded by a swarm of more than 300 earthquakes near the eruption site, which began on 27 June 1975. Sporadic ash and gas emissions occurred at the summit caldera of Plosky Tolbachik (Figure 1) on 28 June and continued to 7 July 1975. Intense earthquake activity continued until 4 July, when activity rapidly declined over a 24 hr period. On 5 July, a 300-m-long fissure opened at the site of Cone I and formed 3 small cinder and spatter cones. By 7 July, activity had coalesced into a single vent, Cone I, and eruption intensity began to increase from small pulsed events into sustained convective discharge. Column heights averaged 5-8 km by 10 July, with the eruption plume extending 200-500 km to the south. Activity continued essentially unchanged until 23 July, when pauses in the eruption of several seconds to hours began. Although the eruption was discontinuous, explosivity increased and the ash column rose at times to 12 km. The first lava was erupted from a bocca immediately south of Cone I on 27 July (Figure 2a), accompanied by a marked decrease in eruption explosivity. A second lava flow was erupted from a bocca on the north flank of Cone I on 2 August 1975, when convective column heights were 2-3 km. Short pauses in the eruption of tephra continued throughout this interval, but lava flows continued to emanate from boccas during these pauses.

Late in the evening of 7 August, a 9 hr pause in the eruption occurred. Although the eruption of tephra recommenced at 08:00 on 8 August, lava from the southern Cone I bocca ceased. Following a several hour pause in the eruption in the afternoon of 8 August, the character of tephra eruption changed dramatically. For 12 hr, ejecta dominantly consisted of pulverized subsurface rock intermixed with juvenile magma, instead of wholly juvenile basaltic magma. This period of the eruption is referred to as the "white-ash" event and represents brecciation and dispersal of several millions of cubic meters of relatively shallow subsurface rock. The white-ash event marked the end of the Cone I eruption on 9 August 1975.

Although at the time most observers believed the eruption had ceased with the end of activity at Cone I, 13 hr later a second, 300-m-long fissure opened about I km north of Cone I late on 9 August. Cone II began to form over that fissure with the eruption of both tephra and lava from a rapidly centralized vent. The Cone II eruption continued through 15 September, with several lava boccas formed at the base and vicinity of the main vent. Cone II was less explosive than Cone I, with convective column heights

4 0 I/95

A CONE

Lathrop Wells, Nevada Lava volume: 0.035 km3 Cone volume: 0.02 km3

LAVA 1975 Tolbachik Cone 1 Lava volume: 0.023 km3 Cone volume: 0.06 km3

1975 Tolbachik Cone 1 White ash eruption 8-9 August, 1975

Isopachs in cm from field observations in Budnikov et al. (1983) and B. Hill 1994 field work, Cone 1 and lavas shown in grey

0 5 10

Figure 2. A) Comparison of 1975 Tolbachik Cone I with Lathrop Wells volcano, YMR. Lathrop Wells lavas are thicker than Cone I whereas Tolbachik lavas flowed down a steeper topographic gradient. Including tephra, eruption volumes are the same for these volcanoes. B) Interpreted isopach map for Cone I white ash deposit. Cone I and lavas shaded, isopachs in cm.

5 * *1 i3/93

generally less than 4 km and an eruption plume extending to about 300 km of the vent. Several periods of "white-ash" formation also occurred during the Cone II eruption, generally in association with formation of larger boccas. These "white-ash" events were much smaller than the 8 August Cone I white-ash and formed thin, irregularly dispersed beds within Cone II tephra deposits. An additional small cone, Cone III, formed immediately north of and erupted simultaneously with Cone II between 17 and 25 August. A compositionally distinct magma was mixed into the magma system during the last week of the Cone II eruption. All activity ceased at Cone II on 15 September 1975. Following a 3 d hiatus in activity, a new cone formed 10.5 km south of Cone II. The Southern Cone erupted a different composition magma, which closely resembled magmas that characterize the main Plosky and Ostry Tolbachik stratovolcanoes and some Holocene basalts in the area. Activity at Southern Cone was very low energy and formed a spatter cone with only locally distributed ash. Most of the Southern Cone and lavas formed during late 1975, but a low level of activity continued to December 1976.

The total volume of basaltic magma from the Northern Cones eruption is 0.45 km3. The cinder cones at Cone I and Cone II each represent 0.06 km3 of basaltic magma. Cone I erupted 0.023 km3 of lava and 0.06 km3 of tephra, whereas Cone II erupted 0.20 km3 of lava and 0.05 km3 of tephra. These volumes and eruption durations can be used to calculate the power of an eruption (cf. Jarzemba, 1996), which is a critical parameter in models of igneous activity effects on repository performance. Wilson et al. (1978) calculate the power or steady rate of thermal energy release (Q) as

Q = tvs(T (1) where t is the magma density in kg m-3 , v is the tephra mass-flow rate in m3 sol, s is the specific heat for basalt (I.1x10 3 J kg-' K-l), T, is the temperature of the erupting magma in 'K, Tf is the final temperature of the deposit (270 'K), and F is a dimensionless variable related to eruption efficiency. Using the parameters in table 1, calculated volumes, and observed durations for the 1975 Tolbachik eruption, Cone I has a power of l.lx1l1t W and Cone II is 3.2x10 10 W. By analogy, these values of power represent likely values for YMR basaltic volcanoes of similar size and eruption style. 4 WHITE-ASH DEPOSIT

Several million cubic meters of shallow subsurface rock was pulverized and erupted during the last 12 hr of activity at Cone I. Although volcanologically spectacular, this event has received only sparse study in the available literature. This section describes initial results of ongoing petrology studies of the Cone I white-ash.

The volume of the white-ash deposit was calculated at about 7x106 m3 by Budnikov et al. (1983) based on field measurements. Isopach maps or data used to derive this volume, however, have not been presented. An isopach map of the white-ash deposit (Figure 2b) can be constructed from field measurements and descriptions in Budnikov et al. (1983) and Maleyev and Vande-Kirkov (1983). Using the two-slope extrapolation method of Fierstein and Nathenson (1992) and the isopach map in Figure 2b, the volume of the white-ash is calculated as 9.1x106 m3. This volume is reasonably consistent with the volume calculated by Budnikov et al. (1983) immediately after the eruption. The Budnikov et al. (1983) volume of 7x106 m is probably more accurate because the distal extent of the deposit was relatively intact during their study. Most of the less than 5 cm isopachs in Figure 2b north of the lavas are interpolated due to erosion of the deposit and may thus overestimate the actual volume of material.

6 * X

The 7x10 6 m3 of white-ash represents a lesser volume of solid rock because the deposit consists of void spaces in addition to small rock fragments. Based on initial laboratory measurements, white-ash has a bulk density of about 1,500 kg m-3 . Initial density of the wall rock can be interpreted from types and abundances of rock fragments within the deposit. Approximately 30 percent of the white-ash (Budnikov et al., 1983) is juvenile basalt that has a density of 1,500 kg m-3 . Subtracting the juvenile component from the white-ash leaves a deposit volume of 4.9x106 M3. The remainder of the white-ash consists of 50 volume percent fresh to moderately altered basaltic rock fragments that are not juvenile and about 20 volume percent lithic sandstone, argillite, and tuffaceous sedimentary rock. Based on general measurements of rock density in Carmichael (1990), generic basalt and volcanically derived sedimentary rock each have a density of about 2,600 kg m 3. Correcting for expansion, the amount of subsurface rock contained in the white-ash deposit is 2.8x 106 M3. This volume is used in subsequent calculations.

There is a moderate degree of uncertainty regarding the abundances of rock types within the white-ash deposit. Specific rock types are often difficult to identify with the small (i.e., <1mm) fragments that characterize the deposit. In addition, many of the volcanic and volcanically derived sedimentary rocks in the area contain crystals larger than 5 mm, which occur as small fragments in the deposit. The initial results of ongoing research (P. Doubik, personal communication, 1996) are that the Cone I white-ash subsurface rock types are 30 percent sedimentary and 70 percent volcanic fragments. The volcanic rock fragments consist primarily of fresh to moderately altered basaltic lava, with subordinate amounts of more silicic lava and pyroclastic rock. Sedimentary fragments consist primarily of lithic sandstone with subordinate amounts of argillite, tuffaceous sandstone, and arkosic sandstone.

Near-vent deposits created during the white-ash phase of Cone I further bound proportions of erupted subsurface rock types. Coarse-grained (i.e., >1 cm) rock fragments accumulate rapidly on the flanks of basaltic cinder cones, whereas finer-grained fragments are transported away from the vent and are deposited with tephra. The proportions of rock types ejected during the white-ash eruption should be the same for both near-vent and more distal deposits. Rock fragments near the base of Cone I, which are from the last stage of the Cone I eruption, are 20 percent sedimentary and 80 percent volcanic. The higher proportion of volcanic rock fragments at Cone I relative to white-ash is likely due to better identification of larger fragments in near-vent deposits, although it is possible that some juvenile basalt was incorrectly identified as older, subsurface rock.

Unusual volcanic bombs found on the flanks of both Cone I and Lathrop Wells provide the critical link to YMR volcanoes. These volcanic bombs consist of multiple types of subsurface rock suspended within a quenched basaltic matrix (Figure 3). The suspended rock fragments in these bombs are the same types and show the same range of thermal histories as subsurface rock fragments that erupted individually. For any specific rock type, fragments range from fresh and angular, without any thermal effects, to partially melted and deformed. Small areas of some fragments are glassy, which indicates these areas melted and quenched during ascent. The polythermal history is important because it demonstrates that rocks adjacent to and for some distance outward from the magma conduit were entrained by the same event. In addition, the mixture of rock types within single volcanic bombs could only occur if these rock types were ascending simultaneously in the conduit. Rock types are distributed randomly with distance from the bomb margin, whereas zonation would be expected if the rock fragments accreted sequentially. The bombs are massive and not layered, which would necessarily occur if bomb formation represented multiple fragmentation and entrainment events, such as reworking in the conduit. Thus, the polylithologic, polythermal volcanic bombs preserve an event that abruptly widened the subsurface conduit and erupted relatively large volumes of rock. These bombs only occur on the outer flank of Cone I and were produced during the last stage of the Cone I eruption. Although rock fragments are preserved on the upper surfaces

7 * d

______I

~~~~~~-.--]ZiEE i-f o

(F) frgetRoc coss of diffren '.ithoogie, eac wit a rag of thrml efetsseddi qunce ba- 1[_RIR...... _*IIHII'Isalticmatix

8 0 0

of Cone I lavas, polylithologic, polythermal volcanic bombs were not found after several dedicated searches. These bombs are thus the near-vent facies of the white-ash phase of Cone I eruption and will be referred to as "xenolith bombs." 5 WHITE-ASH EVENTS IN THE YUCCA MOUNTAIN REGION

Xenolith bombs remarkably similar to those of Cone I at Tolbachik are preserved in late-stage cone deposits at Lathrop Wells volcano in the YMR (Figure 3). In addition, several smaller fragments of similar bombs occur on the southern flanks of Little Black Peak volcano in the YMR (Figure 3). The Lathrop Wells xenolith bombs have been found only on the outer southeastern flanks of the cone. Inner cone deposits exposed by quarrying lack these xenolith bombs. The Lathrop Wells xenolith bombs contain fragments of silicic tuff, which appears to be dominantly Tiva Canyon Tuff, and volcanic sediments. Fragments of tuff range from angular and unaltered to highly deformed and glassy (Figure 3). The rock fragments are suspended in a quenched basalt matrix that has the same degree of vesiculation as the matrix in Tolbachik xenolith bombs. Xenolith bombs like those at Lathrop Wells and Tolbachik Cone I have not been found or reported at other western Basin and Range basaltic volcanoes (e.g., Valentine and Groves, 1996). Although several pieces of subsurface rock occasionally are found in some volcanic bombs, these bombs lack the unusually high abundance and range of thermal histories of rock fragments in xenolith bombs.

In addition to the xenolith bombs, both Lathrop Wells and Little Black Peak volcanoes have unusually high subsurface rock-fragment abundances relative to other Quaternary YMR volcanoes and other basaltic volcanoes in the western Basin and Range and elsewhere. Rock fragments <1 mm average 1 percent by volume at Lathrop Wells (Crowe et al., 1986). Visual estimates for larger rock fragments also are around I percent for Lathrop Wells, and roughly 0.5 percent at Little Black Peak. In contrast, other typical Basin and Range basaltic volcanoes have less than 0.01 volume percent rock fragments (e.g., Valentine and Groves, 1996). Juvenile cone scoria at Lathrop Wells, Tolbachik Cone I, and to a lesser extent Little Black Peak, consists of angular, broken pieces of larger fragments that were cool on impact with the cone slope. Typically, cinder cone eruptions do not eject material high enough to cool sufficiently to permit brittle fragmentation. A common cinder cone feature is beds of agglutinated tephra that accumulated at temperatures high enough to deform plastically and form a highly cohesive bed. Lathrop Wells, Tolbachik Cone I, and to a lesser extent Little Black Peak, consist of loose, nonagglutinated tephra, which indicates these eruptions were more explosive than typical basaltic volcanoes.

Lathrop Wells was an unusually explosive basaltic volcanic eruption, as evinced by anomalously high rock-fragment abundances and loose accumulations of broken tephra. Little Black Peak also exhibits these characteristics, albeit to a lesser degree. The latest, potentially most explosive stages of these eruptions, however, are only preserved on the cone flanks. Erosion has removed all but trace amounts of Lathrop Wells tephra-fall deposits, and fall deposits have been completely eroded at Little Black Peak. Any white- ash-type deposits away from the cone base have been removed by erosion at those volcanoes. The xenolith bombs, however, provide direct evidence that an unusual brecciation event occurred during the latest stage of eruption at these two cinder cones. 6 SUBSURFACE AREA OF DISRUPTION

The well-preserved white-ash at Tolbachik Cone I allows calculation of the subsurface area necessarily fragmented and erupted. Although this calculation is not possible for YMR volcanoes, analogy with

9 * 0

Tolbachik Cone I suggests similar magnitude disruptive events occurred at the youngest YMR volcano, Lathrop Wells, and a smaller event was likely at Little Black Peak.

There are three primary sources of uncertainty that affect calculations of subsurface area disrupted during the Cone I white-ash eruption (i) geometry of the conduit, (ii) depth to major changes in rock type, and (iii) abundance of rock fragment types. Variations in rock fragment abundances are discussed in Section 4. Conduit geometry is highly variable for basaltic volcanoes within the upper I km of the surface. Basaltic volcanoes generally are fed by tabular dikes, which can locally widen to several meters to tens of meters in response to local changes in rock strength (e.g., Delaney et al., 1986; Gartner and Delaney, 1988). These widened zones focus vertical flow in the magma system into crudely cylindrical conduits that feed basaltic volcanoes, whereas magma away from these zones solidifies rapidly (e.g., Delaney and Pollard, 1982). For calculations of the subsurface area removed during the Cone I white-ash eruption, the form of this area is assumed cylindrical. Conduit geometry, however, may be dependent on the distribution of stress at shallow crustal depths. Stress anisotropy may result in conduits that are elongated parallel to the direction of maximum horizontal compression and thus have major axis lengths greater than modeled by a cylinder. A circular cross sectional area thus should be viewed as an estimate of the minimum diameter disrupted during a white-ash-type event. Volume calculations also do not account for the initial conduit diameter, which is likely less than 2 m.

The depth to the major lithology change from dominantly volcanic to dominantly sedimentary provides the critical constraint on area of disruption calculations. The Quaternary Tolbachik volcanoes formed in a subsiding basin, bounded about 25 km to the east by several large-displacement normal faults. Bedrock exposures east of this fault zone show the distal Quaternary volcanic section is about 400-500 m thick. Volcanic rocks have a gradational transition to interbedded sandstone and siltstone, which are volcanically derived and deposited in an apparent fore-arc basin (Shanster, 1983). Fore-arc sediments rest upon volcanically derived sandstone and siltstone deposited in a littoral marine environment (Shanster, 1983). This entire sequence of sedimentary rocks is about 700 m thick in eastern exposures and is Late Miocene or younger (Flyorov, 1979; Shanster, 1983). The Neogene sedimentary section was deposited on a Paleogene flysch sequence that formed when a Cretaceous and older crustal section was accreted to Kamchatka (e.g., Geist et al., 1994). Although the volcanic section is about 500 m thick 25 km east of the 1975 Tolbachik cones, seismic reflection studies show a major lithology change occurs at 1,200-1,300 m beneath the cones (Balesta et al., 1983). This change likely represents the transition between dominantly volcanic and dominantly sedimentary rocks. In addition, the local water table is around 800 m deep beneath the Tolbachik area, which may coincide with a major lithology change (Balesta et al., 1984). Thus, the volcanic section beneath the 1975 Tolbachik volcanoes is at most 1,300 m thick, but may be thinner due to the gradational nature of the volcanic-sedimentary contact. Calculations of the subsurface area of disruption will need to consider that the volcanic section may range from about 800 m to 1,300 m thick (Figure 4).

The cylinder diameters in table 2 and Figure 4 were calculated by first selecting a thickness for the volcanic section then using the volume of volcanic rock fragments to constrain the diameter. This diameter is then used to calculate the disrupted thickness for the sedimentary section. These calculations are repeated for three thickness intervals and three proportions of volcanic-to-sedimentary rock fragments in the deposit. As described in section 4, the total volume of solid rock ejected during the white-ash phase of the Cone I eruption is 2.8x106 m3. Based on the uncertainties discussed previously, this volume of subsurface material represents widening of the conduit to an average diameter of 49±7 (table 2).

1o * * IcJ?

0 _ -______

200-__ d=3i7mum,~ .io~~~~~~~~V . Ica

200- - ~~ d=60-7 m ,

1400-

Figure 4. Diagram of subsurface disruption associated with white-ash eruption at Tolbachik Cone I. Note horizontal scale is twice vertical scale. Depths to major geologic boundaries discussed in text, Cone I dimensions from Tokarev (1983).

7 CONCLUSIONS

The 1975 Tolbachik eruption provides important constraints on the character of volcanic processes that may affect repository performnance. This eruption is demonstrably analogous to the types of eruptions that characterize late Quaternary activity in the YMR. Late stage activity at Tolbachik Cone I culminated in fragmentation and dispersal of 2.8x 106 m3 of subsurface rock, which represents widening of the conduit to an average diameter of 49±7 m. Most deposits from the later stages of YMR Quaternary eruptions are not preserved. Subsurface rock fragments, however, are unusually abundant on the flanks of Lathrop Wells and Little Black Peak volcanoes. In addition, an unusual type of xenolith bomb occurs at these volcanoes, which represents the mixing of several thermally and lithologically distinct types of rock fragments in a single event. The same type of xenolith bombs and unusual rock-fragment abundances occur at Tolbachik Cone I and were produced during the climactic disruptive phase of the eruption. Thus, Lathrop Wells and

I1I Table 2. Calculation of the subsurface area disrupted during the 1975 Tolbachik white-ash phase of the Cone I eruption

Rock Proportions Volume Volcanic | Thickness Cylinder Thickness 3 3 2.8x 106 M Rock (mi ) Volcanics (m) Diameter (m) Sediments (m) Voic 80%, Sed 20% 2.26x 106 800 60 200

Voic 80%, Sed 20% 2.26x 106 1000 54 250 VoIc 80%, Sed 20% 2.26x 106 1300 47 325

VoIc 70%, Sed 30% 1.98x 106 800 56 343 VoIc 70%, Sed 30% 1.98x106 1000 50 429 VoIc 70%, Sed 30% 1.98x10 6 1300 44 557

Volc 50%, Sed 50% 1.41X 106 800 47 800 Volc 50%, Sed 50% 1.41x 106 1000 42 1000 VoIc 50%, Sed 50% 1.41x10 6 1300 37 1300

Average 49 1 Std. Dev. 7

Little Black Peak in all likelihood had late-stage subsurface disruption events analogous to Tolbachik Cone I. The volume of YMR disruptive events cannot be calculated, as most of the deposits from these events have been eroded. Rock fragments at Lathrop Wells consist of Tertiary ignimbrites and interbedded volcaniclastic sediments. Simple stratigraphic relationships demonstrate at least 550 m of Miocene ignimbrite underlie Lathrop Wells (Swadley and Carr, 1987). Gravity data show that Lathrop Wells is located on the eastern boundary of a Paleozoic basement low, which is overlain by 1-2 km of Tertiary volcanic and sedimentary rock (e.g., Langenheim and Ponce, 1995). Rock fragments at Lathrop Wells likely came from a depth interval comparable to 1975 Tolbachik. Lathrop Wells, however, is nearly identical in volume to Tolbachik Cone I. The amount of subsurface rock disrupted should be directly related to the volume of rock near the conduit that was heated during the eruption. Because Lathrop Wells and Tolbachik Cone I are similar types of volcanoes with nearly identical eruptive volumes, it is reasonable to expect that similar subsurface areas were disrupted. Little Black Peak has roughly an order of magnitude smaller eruption volume than Lathrop Wells and Tolbachik Cone I. Thus, the subsurface area disrupted at Little Black Peak was likely an order of magnitude smaller. The anisotropic distribution of crustal stress in the repository setting may affect the shape of the conduit, possibly forming more ellipsoidal cross-sectional areas with major axis lengths greater than the diameter of a circle.

12 *S *%3

8 REFERENCES

Balesta, S.T., A.A. Kargapol'tsev, and G.B. Grigoryan. 1983. The structure of the New Tolbachik Volcanoes from seismic data. S.A. Fedotov and Ye.K. Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 316-326.

Balesta, S.T., M.M. Zubin, A.A. Kargopoltsev, and I.A. Fedorchenko. 1984. Deep structure of eruption region. S.A. Fedotov, ed. Large Tolbachik Fissure Eruption, Kamchatka 1975-1976. Moscow, Russia: Science: 514-536.

Brocher, T.M., M.D. Carr, K.F. Fox, and P.E. Hart. 1993. Seismic reflection profiling across Tertiary extensional structures in the eastern Amargosa Desert, southern Nevada, Basin and Range province. Geological Society of America Bulletin 105: 30-46.

Budnikov, V.A., Ye.K. Markhinin, and A.A. Ovsyannikov. 1983. The quantity, distribution and petrochemical features of pyroclastics of the great Tolbachik fissure eruption. S.A. Fedotov and Ye.K. Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 41-56.

Carmichael, R.S. 1990. CRC PracticalHandbook of Physical Properties of Rocks and Minerals. Boca Raton, FL: CRC Press, Inc. 162-173.

Cashman, K.V., and J.E.Taggart. 1983. Petrologic monitoring of 1981 and 1982 eruptive products from Mount St. Helens. Science 221(4618): 1,385-1,387.

Crowe, B.M., K.H. Wohletz, D.T. Vaniman, E. Gladney, and N. Bower. 1986. Status of Volcanic Hazard Studies for the Nevada Nuclear Waste Storage Investigations. Los Alamos National Laboratory Report LA-9325-MS, Vol. II. Los Alamos, NM: Los Alamos National Laboratory.

Crowe, B.M., F.V. Perry, J. Geissman, L. McFadden, S. Wells, M. Murrell, J. Poths, G.A. Valentine, L. Bowker, and K. Finnegan. 1995. Status of Volcanic Hazard Studies for the Yucca Mountain Site Characterization Project. Los Alamos National Laboratory Report LA-12908-MS. Los Alamos, NM: Los Alamos National Laboratory.

Delaney, P.T., and D.D. Pollard. 1982. Solidification of basaltic magma during flow in a dike. American Journal of Science 282: 856-885.

Delaney, P.T., D.D. Pollard, J.I. Ziony, and E.H. McKee. 1986. Field relations between dikes and joints: Emplacement processes and paleostress analysis. Journal of Geophysical Research 91(B5): 4,920-4,938.

Faberov, A.I. 1983. Activity of the Ploskiy Tolbachik volcano, June-July 1975. S.A. Fedotov and Ye.K. Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 36-40.

13 Fedotov, S.A. 1983. Chronology and features of the southern breakout of the Great Tolbachik fissure eruption, 1975-1976. The Great Tolbachik Fissure Eruption, Geological and Geophysical Data 1975-1976. S.A. Fedotov, and Ye.K. Markhinin, K. eds. Cambridge, MA: Cambridge University Press: 11-26.

Fedotov, S.A., V.I. Gorel'chik, and V.V. Stepanov. 1976. Seismological data on magma chambers, mechanisms, and evolution of the 1975 Tolbachik basalt fissure eruption on Kamchatka. Translated from Doklady Akademii Nauka SSSR 228-6: 1,407-1,410.

Fedotov, S.A., A.M. Chirkov, and A.A. Razina. 1984. Northern Breakout. S.A. Fedotov, ed. Large Tolbachik Fissure Eruption, Kamchatka 1975-1976. Moscow, Russia: Science: 11-74.

Fedotov, S.A., S.T. Balesta, V.N. Dvigalo, A.A. Razina, G.B. Flerov, and A.M. Chirkov. 1991. The New Tolbachik Volcanoes. S.A. Fedotov and Yu.P. Masurenkow, eds. Active Volcanoes of Kamchatka. Moscow, Russia: Nauka Press: 275-279.

Fierstein, J., and M. Nathenson. 1992. Another look at the calculation of fallout tephra volumes. Bulletin of Volcanology 54: 156-167.

Flerov, G.B., V.N. Andreev, V.A. Budnikov, and A.I. Tsyurupa. 1984. Petrology of the eruption products. S.A. Fedotov, ed. Large Tolbachik Fissure Eruption, Kamchatka 1975-1976. Moscow, Russia: Science: 223-284.

Flyorov, G.B. 1979. Crustal xenoliths in products of the large 1975-1976 fissure Tolbachik eruption. Academy of Sciences of the USSR: Volcanology and Seismology 2: 43-54.

Gartner, A.E., and P.T. Delaney. 1988. Geologic Map Showing a Late Cenozoic Basaltic Intrusive Complex, Emery, Sevier, and Wayne Counties, Utah. Miscellaneous Field Studies Map MF-2052. Reston, VA: U.S. Geological Survey.

Geist, E.L., T.L. Vallier, and D.W. Scholl. 1994. Origin, transport, and emplacement of an exotic island- arc terrane exposed in eastern Kamchatka, Russia. Geological Society of America Bulletin 106: 1,182-1,194.

Jayaweera, K.O.L.F., R. Seifert, and G. Wendler. 1976. Satellite observations of the eruption of Tolbachik volcano. EOS, Transactions of the American Geophysical Union 57: 196-200.

Jarzemba, M.S. 1996. Stochastic radionuclide distributions after a basaltic eruption for performance assessments of Yucca Mountain. Nuclear Technology. In press.

Lange, R.A. 1994. The effect of H20, CO2 and F on the density and viscosity of silicate melts. Reviews in Mineralogy, Volume 30: Volatiles in Magmas. M.R. Carroll and J.R. Holloway, eds. Washington, DC: Mineralogical Society of America: 331-369.

Lange, R.A., and I.S.E. Carmichael. 1987. Densities of Na2O-K20-CaO-MgO-FeO-Fe 2O3 -AI203 -TiO2 - SiO2 liquids: New measurements and derived partial molar properties. Geochimica et Cosmochimica Acta 51: 2,931-2,946.

14 Langenheim, V.E., and D.A. Ponce. 1995. Depth to pre-Cenozoic basement in southwest Nevada. Proceedings of Sixth Annual International Conference on High-Level Radioactive Waste Management. La Grange Park, IL: American Nuclear Society: 129-131.

Maleyev, Ye.F., and Yu.V. Vande-Kirkov. 1983. Features of pyroclastics of the northern breakthrough of the Great Tolbachik Fissure Eruption and the origin of its pale-grey ash. S.A. Fedotov and Ye.K. Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 57-71.

Pinkerton, H., and R.J. Stevenson. 1992. Methods of determining the rheological properties of magmas at sub-liquidus temperatures. Journal of Volcanology and Geothermal Research 53: 47-66.

Shanster, A.Ye. 1983. Basement xenoliths in the eruption products of the New Tolbachik Volcanoes and the problem of the fom.Oati31n of magma conduits in the upper crust. S.A. Fedotov and Ye.K. Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 72-82.

Shaw, H.R. 1972. Viscosities of magmatic silicate liquids: An empirical method of prediction. American Journal of Science 272: 870-893.

Swadley, W.C, and W.J. Carr. 1987. Geologic Map of the Quaternary and Tertiary Deposits of the Big Dune Quadrangle, Nye County, Nevada, and Inyo County, California. U.S. Geological Survey Miscellaneous Investigations Series Map 1-1767. Reston, VA: U.S. Geological Survey.

Tokarev, P.I. 1978. Prediction and characteristics of the 1975 eruption of Tolbachik volcano, Kamchatka. Bulletin Volcanologique 41-3: 251-258.

Tokarev, P.I. 1983. Calculation of the magma discharge, growth in the height of the cone and dimensions of the feeder channel of Crater I in the Great Tolbachik Fissure Eruption, July 1975. S.A. Fedotov and Ye.K. Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 27-35.

Valentine, G.A., and K.R. Groves. 1996. Entrainment of country rock during basaltic eruptions of the Lucero Volcanic Field, New Mexico. Journal of Geology 104: 71-90.

Vaniman, D.T., B.M. Crowe, and E.S. Gladney. 1982. Petrology and geochemistry of Hawaiite lavas from Crater Flat, Nevada. Contributions to Mineralogy and Petrology 80: 341-357.

Volynets, O.N., G.B. Flerov, V.N. Andreyev, E.I. Popolitov, V.A. Abramov, L.L. Petrov, S.A. Shcheka, and G.I. Selivanova. 1983. Geochemical features of the rocks of the Great Tolbachik Fissure Eruption 1975-1976 in relation to petrogenesis. S.A. Fedotov and Ye.K. Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 116-140.

Williams, H., and A.R. McBirney. 1979. Volcanology. San Francisco, CA: Freeman, Cooper and Company.

15 S~~~~~~~~~~~~~

Wilson, L., R.S.J. Sparks, T.C. Huang, and N.D. Watkins. 1978. The control of volcanic column heights by eruption energetics and dynamics. Journal of Geophysical Research 83(B4): 1,829-1,836.