Geologists create volcanic hazard maps using scientific data to portray potential future geological events; the end users are principally public safety officials. Typical maps use a few simple polygons to outline areas of potential inundation or cover by a few categories of flows based on past frequency and size. Uncertainties in data regarding flow characteristics complicate the construction of accurate hazard maps. Generally, there are inadequate exposures of good sections, poorly known extents of units, and imprecise volumes for deposits. Crisis conditions limit the time available for field and laboratory work. Computer models can simulate possible scenarios, but the volumes, styles of emplacement, and source starting locations are poorly known in many cases. The large uncertainty in initial conditions is seldom taken into account in the construction of hazard maps, and these uncertainties are rarely passed on to the end users of the maps. TITAN2D is a computational model for volcanic block-and-ash flows and rock avalanches of various types and scales, and it forms the core of the TITAN toolkit for volcanic hazard analysis, which can integrate high-performance computing, database management, and visualization to a very sophisticated level. TITAN provides a solution to mapping problems by providing a probabilistic calculation of inundation depth that takes into account many of the critical uncertainties.

Silicic volcanism in the central Aeolian Islands of Lipari and Vulcano has followed a consistent pattern during the past 22,000 years. Active eruptive cycles generally began with hydrovolcanic breccias, surge beds, and ash-fall deposits. They ended with magmatic effusions that formed lava domes and short coulees. Long repose periods separated shorter active cycles. Eruptions occurred from both isolated vents located along fissures (e.g., domes of southern Lipari) and central vents with a long history of activity and repose (e.g., Fossa cone of Vulcano). The compositions of the juvenile products include leucite tephrite, trachyte, and rhyolite. The average volume of silicic products in an eruptive cycle on Lipari and Lentia was about 5 × 10 8 m 3 of juvenile magma. The repose period between major active periods was about 4,000 years. The production rate for the period of 22,000 years ago to the present was 10 5 m 3 per year. The average volume of erupted material in an active cycle at Fossa (other than the Punte Nere cycle) was about 2 × 10 7 m 3 of juvenile magma. Repose times between cycles range from 300 to 800 years. The rate of magma production for the entire Fossa cone during its 6,000-year growth was 5 × 10 4 m 3 per year. Vulcanello produced about 3 × 10 7 m 3 of tephritic to trachytic magma in the past 2,100 years, a production rate of 1.5 × 10 4 m 3 per year.

Several types of mineralization appear to be related to the emplacement of fluorine-rich silicic lava flows and domes. An important example is the beryllium deposit at Spor Mountain, west-central Utah, where bertrandite, fluorite, amorphous silica, and Mn-Fe oxides replace dolomite fragments in tuffaceous surge deposits just beneath a topaz-bearing rhyolitic lava flow. The Be-mineralized zone is also highly enriched in F, Sn, W, Nb (and presumably Ta), Zn, Pb, and several other metals (but not in Mo). The uniform lateral character of the mineralization, the restriction of Be mineralization to the upper-most few meters of tuff, and the lack of mineralization in fluorite-bearing breccia pipes (tuffaceous vent breccias, in some cases) in underlying dolomite suggest (Bikun, 1980) that the beryllium mineralization resulted from the devitrification of the overlying lavas (a “steam iron” model). Mass-balance calculations based on comparisons of the chemical compositions of glassy and devitrified rhyolite are consistent with this unconventional interpretation. A second important example is provided by “Mexican-type” fumarolic tin deposits, characterized by cassiterite in carapace breccias of rhyolitic domes. Low-temperature dissolution and reworking of early fumarolic cassiterite may produce the colloform “wood tin” common in this deposit type. Deposits of this type occur in Nevada and New Mexico, as well as in many areas of northern Mexico. Fluorine-rich intrusive domes (better known as plutons) may also host metal mineralization, generally of the porphyry type. Examples include the well-known Climax-type porphyry molybdenum deposits of Colorado, New Mexico, and Utah, and the porphyry tungsten deposit at Mount Pleasant, New Brunswick, Canada. These subvolcanic deposits, in common with those associated with extrusive silicic lava flows and domes, are believed to have been derived by the crystallization-devolatization of highly fractionated magma.

High-silica, topaz-bearing rhyolites of Cenozoic age are widely distributed across the western United States and Mexico. Topaz rhyolites are characteristically enriched in fluorine (>0.2 wt%) and contain topaz crystallize during post-magmatic vapor-phase alteration. In the United States, their ages span much of the Cenozoic Era (50 to 0.06 Ma). Their emplacement followed or was contemporaneous with calc-alkaline and basaltic magmatism in the Basin and Range province, along the Rio Grande rift, and in Montana, and coincided with episodes of extensional tectonism in these regions. Nearly all topaz rhyolites extruded as small, endogenous lava domes with or without lava flows; no topaz-bearing ash-flow tuffs have yet been identified with certainty in the western United States. Most domes are underlain by a precursory blanket of non-welded tephra. A few are small, shallowly emplaced intrusive plugs. Volumes of rock (<1 to 100 km 3 ) in individual complexes composed of 1 to many separate extrusions suggest that the lavas were erupted from small to medium sized magma bodies. In addition to topaz, these rhyolites also contain garnet, bixbyite, pseudobrookite, hematite, and fluorite in cavities or in their devitrified groundmasses. All of these phases may form during vapor-phase crystallization. Magmatic phenocrysts include sanidine (ca. Or 50 ), quartz, sodic plagioclase (usually oligoclase), and F- and Fe-rich biotite in order of usual abundance. Fe-rich hornblende or clinopyroxene occur in a few lavas. Common magmatic accessory minerals include magnetite, ilmenite, zircon, apatite, allanite, and fluorite. Titanite and REE-rich phosphates have been identified in a few lavas. The rhyolites crystallized over a wide temperature interval (850 to 600°C, with most at the lower end of this range) and at variable oxygen fugacities. Titanite-bearing lavas crystallized above the NNO buffer under oxidizing conditions. Most others appear to have crystallized near the QFM oxygen buffer. For individual complexes, temperatures correlate negatively with F-content. All topaz rhyolites are high-SiO 2 rhyolites with elevated F, Na, K, Fe/Mg and low Ti, Mg, Ca, and P. Samples with F concentrations of about 1% have notably lower Si and higher Al and Na than other topaz rhyolite glasses. Most glasses from topaz rhyolites are metaluminous, but many appear to be slightly peraluminous. Fluorine concentrations in glasses range from slightly less than 0.2 to more than 1.0 wt%, and F/Cl ratios are high (3 to 10) compared to F-rich peralkaline glasses (<3). Topaz rhyolites are characteristically enriched in incompatible lithophile elements (Rb, U, Th, Ta, Nb, Y, Be, Li, and Cs). Elements compatible in feldspars (Sr, Eu, Ba), ferromagnesian minerals (Ti, Co, Ni, Cr), and zircon (Zr, Hf) are depleted. The REE patterns of most topaz rhyolites are almost flat (La/Yb N = 1 to 3) and have pronounced negative Eu anomalies (Eu/Eu* = 0.01 to 0.02). Both of these parameters decrease with differentiation as indicated by increasing F, U, Cs, and other incompatible elements. Titanite-bearing rhyolites have prominent middle REE depletions. Initial Sr-isotope ratios range from 0.705 to over 0.710. Geochemical trends at individual complexes are interpreted as arising from fractional crystallization of an initially more “mafic” rhyolite with about 0.2% fluorine. Extensive fractionation of sanidine, quartz, plagioclase, biotite, and Fe-Ti oxides (in proportions consistent with their modes) produced much of the trace element patterns. Zircon, apatite, and a REE-rich phase (allanite, monazite, or titanite) were minor but important fractionating phases. No liquid-state fractionation is required to explain the geochemical trends. The high F content and F/Cl ratios of topaz rhyolite melts may have modified phase relations so as to produce Na and Al enrichments for highly evolved magmas. Topaz rhyolites are intimately related to economic deposits of lithophile elements (i.e. Be, U, F, Li, and Sn). The volcanic rocks were probably ore- and, in some cases, fluid-sources for these mineral deposits. In their age, tectonic setting, mineralogy, chemistry, and style of emplacement, topaz rhyolites bear resemblance to the rhyolitic stocks associated with Climax-type Mo deposits, and some may be surface manifestations of such deposits. In their chemical composition and mineralogy, topaz rhyolites are distinct from both peralkaline rhyolites and calc-alkaline rhyolites with which they may be spatially and temporally associated. Some of the compositional differences between topaz rhyolites and peralkaline rhyolites may be attributed to the relative effects of F and Cl, on melt structure and phase relationships in their parental magmas. The F/Cl ratios of the melt or its source may determine the alumina saturation of the magma series. Topaz rhyolites are distinguishable from calc-alkaline rhyolites by lower Sr, Ba, and Eu, and higher F, Rb, U, and Th. The usually low La/Yb ratios of topaz rhyolites distinguish them from both peralkaline and calc-alkaline rhyolite suites. Topaz rhyolites are similar to other aluminous rhyolites erupted in bimodal associations with basalt in the western United States. They may be the equivalent of the topaz-bearing ongonites of central Asia. Topaz rhyolites from the western United States are not the eruptive equivalents of S-type granites; we liken them to the highly evolved, non-peralkaline, and F-rich anorogenic granites. Topaz rhyolites appear to have evolved from partial melts of a residual felsic granulite source in the lower or middle crust of the Precambrian continent. According to the proposed model, the passage of contemporaneous mafic magmas through the crust produced necessarily small volumes of partial melts as a result of the decomposition of small amounts of F-rich biotite that persisted in a high-grade metamorphic protolith. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas. Subsequent fractionation led to their extreme trace element characteristics.

Abstract Several deposits of uranium and other lithophile elements are located within or adjacent to small, fluorine-rich rhyolitic dome complexes. Examples are studied and a model is offered.

Subaerial pyroclastic deposits are of three genetic types: (1) fall, (2) surge, and (3) flow. Although pyroclastic flows include a wide range of volume magnitudes, only small-scale eruptions have been observed. Flows are composed of a dense, basal avalanche and an overriding cloud of entrained particles that rise by convective buoyancy. Theoretical models explain the generation of pyroclastic flows by gravitational collapse of an eruption column. The high apparent mobility of pyroclastic flows is largely due to the dissipation of kinetic and potential energy imparted at the time of eruption. The regular stratigraphic succession of fall, surge, and flow deposits is compatible with a column-collapse model. The textures, morphology, and distribution of pyroclastic flow deposits suggest emplacement as thick, dense beds that may be quasi-fluidized near their source, but which flow mainly in a laminar mode toward their distal reaches. Smaller flows may move entirely as single laminar beds. Large primary deposits may show a facies change controlled by the gradation from an inflated proximal stage into a deflated distal stage. Some deposits may undergo secondary (or tertiary) flowage following welding to produce textures and structures similar to lava flows. The various degrees of welding and secondary crystallization are principally controlled by emplacement temperature, thickness, and composition.