Magmatic and hydromagmatic conduit development during the 1975 Tolbachik Eruption,Kamchatka, with implications for hazards assessment at Yucca Mountain,NV

Xenoliths in pyroclastic fall deposits from the 1975 Tolbachik eruption constrain the timing and development of subsurface conduits associated with basaltic cinder cone eruptions. The two largest Tolbachik vents contain xenoliths derived from magmatic and hydromagmatic processes, which can be correlated with observed styles of eruption activity. Although many basaltic eruptions progress from early hydromagmatic activity to late magmatic activity, transient hydromagmatic events occurred relatively late in the 1975 eruption sequence. Magmatic fall deposits contain 0.01–0.3 vol.% xenoliths from <3-km-deep rocks, likely derived from 6–15-m-wide and 1.7–2.8-km-deep conduits. Intervals that supported the highest tephra columns (i.e., droplet flow regime) produced few of these xenoliths; most were derived from intervals with relatively lower columns and active lava flows (i.e., annular 2-phase flow). Several periods of decreased eruptive activity resulted in inflow of groundwater from >500 m depth into the dry-out zone around the conduit, disrupting and ejecting 10⁵–10⁶ m³ of wall-rock through hydromagmatic processes with conduits widening to 8–48 m. Hydromagmatic falls contain 60–75 vol.% of highly fragmented xenoliths, with juvenile clasts displaying obvious magma-water interaction features. During the largest hydromagmatic event, unusual breccia-bombs formed containing a wide range of fresh and pyrometamorphic xenoliths suspended in a quenched basaltic matrix. Hydromagmatic activity during the 1975 Tolbachik eruption occurred below likely fragmentation depths for a basalt containing 2.2 wt.% magmatic water. This activity is more likely related to conduit-wall collapse rather than variations in conduit-flow pressure. In contrast, larger volume silicic eruptions may have transient hydromagmatic events in response to conduit flow dynamics above the magma fragmentation depth. The 1975 Tolbachik volcanoes are reasonably analogous to Quaternary basaltic volcanoes in the Yucca Mountain region and can guide interpretations of their poorly preserved deposits. The youngest basaltic volcanoes near Yucca Mountain have cone deposits characterized by elevated xenolith abundances and distinctive xenolith breccia-bombs, remarkably similar to 1975 Tolbachik deposits. Extrapolation of 1975 Tolbachik data suggests conduits for some Yucca Mountain basaltic volcanoes may have widened locally on the order of 50 m in response to late-stage hydromagmatic events.