Geology topics
Source-linked reports with geographic coverage including Valley of Ten Thousand Smokes.
Three fossil fumaroles in rhyolite-rich ash-flow tuff from the lower Valley of Ten Thousand Smokes (VTTS) have been studied to ascertain chemical losses and gains and mineralogical evolution during fumarolic alteration and subsequent cooling and weathering processes. Major oxide and trace element data were interpreted using the isocon method of Grant (1986).
The June 1912 eruption of Novarupta filled nearby glacial valleys on the Alaska Peninsula with ash-flow tuff (ignimbrite), and post-eruption observations of thousands of steaming fumaroles led to the name ‘Valley of Ten Thousand Smokes’ (VTTS). By the late 1980s most fumarolic activity had ceased, but the discovery of thermal springs in mid-valley in 1987 suggested continued cooling of the ash-flow sheet. Data collected at the mid-valley springs between 1987 and 2001 show a statistically significant correlation between maximum observed chloride (Cl) concentration and temperature. These data also show a statistically significant decline in the maximum Cl concentration. The observed variation in stream chemistry across the sheet strongly implies that most solutes, including Cl, originate within the area of the VTTS occupied by the 1912 deposits. Numerous measurements of Cl flux in the Ukak River just below the ash-flow sheet suggest an ongoing heat loss of ∼250 MW. This represents one of the largest hydrothermal heat discharges in North America. Other hydrothermal discharges of comparable magnitude are related to heat obtained from silicic magma bodies at depth, and are quasi-steady on a multidecadal time scale. However, the VTTS hydrothermal flux is not obviously related to a magma body and is clearly declining. Available data provide reasonable boundary and initial conditions for simple transient modeling. Both an analytical, conduction-only model and a numerical model predict large rates of heat loss from the sheet 90 years after deposition.
The 1912 Valley of Ten Thousand Smokes (VTTS) ignimbrite was constructed from 9 compositionally distinct, sequentially emplaced packages, each with distinct proportions of rhyolite (R), dacite (D), and andesite (A) pumices that permit us to map package boundaries and flow paths from vent to distal extents. Changing pumice proportions and interbedding relationships link ignimbrite formation to coeval fall deposition during the first ∼16 h (Episode I) of the eruption. Pumice compositional proportions in the ignimbrite were estimated by counts on ≥100 lapilli at multiple levels in vertical sections wherever accessible and more widely over most of the ignimbrite surface in the VTTS. The initial, 100% rhyolite ignimbrite package (equivalent to regional fall Layer A and occupying ∼3.5 h) was followed by packages with increasing proportions of andesite, then dacite, emplaced over ∼12.5 h and equivalent to regional fall Layers B1-B3. Coeval fall deposits are locally intercalated with the ignimbrite and show parallel changes in R:D (rhyolite:dacite) proportions, but lack significant amounts of andesite. Andesite was thus dominantly a low-fountaining component in the eruption column and is preferentially represented in packages filling the VTTS north of the vent. The most extensive packages (3 and 4) occur in B1 and early B2 times where flow mobility and volume were optimized; earlier all-rhyolite flows (Package 1) were highly energetic but less voluminous, while later packages (5-9) were both less voluminous and emplaced at lower velocities. Package boundaries are expressed as one or more of the following: sharp color changes corresponding to compositional variations; persistent finer-grained basal parts of flow units; compaction swales filled by later packages; erosional channels cut by the flows that fill them; lobate accumulations of one package; and (mostly south of the vent) intercalated fall deposit layers. Clear flow-unit boundaries are best developed between ignimbrite of non-successive packages, indicating time breaks of tens of minutes to hours. Less well-defined stratification may represent rapidly emplaced successive flow units but often changes over short distances and indicates variations in localized depositional conditions.
Samples from a fossil fumarole originating in the 1912 ash-flow tuffin the Valley of Ten Thousand Smokes have been analyzed to ascertain chemical changes resulting from high-temperature fumarolic alteration and subsequent cooling and weathering of the protolith. Samples of the underlying, dominantly leached, dacite-rich portion of the ash-flow tuff adjacent to the fumarolic conduit and samples of encrusted fallout from the shallow part of the fossil fumarole were interpreted using the isocon method of Grant (1986). The results show that, relative to unaltered l9l2 dacite, chosen as a standard composition for the protolith in this fossil fumarole, mass was conserved during the alteration reactions for most of the system, but mass gains of l4–2D% were determined for three samples in the leached ash-flow tuff Relative to unaltered dacite protolith, significant enrichments occurred in SO 3 , LOI (~H 2 O), Cl, F, Zn, Pb, Cu, Sn, Cr, Ni, As, Sb, Au, Br in various parts of the fossil fumarole. Some of these were during the high-temperature part of the alteration, and some were during cooling processes when acid alteration becomes prominent. The REEs indicate some depletion in highly altered samples relative to dacite protolith and differential mobility of Eu 2+ relative to trivalent REEs. This is manifested by positive Eu anomalies in REE patterns normalized against REE in the dacite protolith. Mineral phases introduced in the alteration assemblages include alunite reflecting high SO 3 , activity, hydrated aluminum hydroxy-fluoride (a ralstonite-like phase) and fluorite reflecting high F activity, smectite, magnetite, hematite, and goethite reflecting oxidation and hydration reactions. Opal and a portion of the α-cristobalite reflect SiO 2 , mobility; however, the abundance of α-cristobalite is formed from pumice leached during high-temperature vapor-phase processes and devitrification of the altered glass.