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