Abundant sulfates appear to exist on the surface of Mars and have commonly been attributed to a planet-wide volcanogenic ‘acid fog’—where clouds of acid mist react directly with surface rocks or acidify surface waters —from which the sulfates precipitated [e.g., Clark and Baird, 1979]. In particular, Meridiani Planum, a plain located just south of the Martian equator, hosts many sulfate minerals. Squyres et al. [2004, 2006] and Squyres and Knoll [2005] hypothesized that the sulfate minerals there, particularly acid sulfates such as jarosite, precipitated from high concentrations of sulfuric acid in flowing and standing water and groundwater. However, such explanations are problematic, because magnesia and alkali-bearing minerals in the basaltic regolith should have been able to neutralize sulfuric acid rapidly, making precipitation of acid sulfates such as jarosite impossible. Alternatively, the sulfate-rich sediments at Meridiani can be explained in terms of impact surge deposition [e.g., Knauth et al., 2005]. The only water involved in making jarosite, in this explanation, is moisture (i.e., frost or water vapor), none of it initially acidic.

A notable feature and, in many cases, a distinguishing aspect of welded tuff is the occurrence of numerous fractures. These fractures, generally oriented in a nearly vertical fashion, are rather equally spaced, and show little or no displacement. Produced by cooling and contraction perpendicular to isotherms in tuff after its emplacement and during its welding, these fractures are often referred to as cooling joints, resembling columnar joints developed in basaltic lava flows. Notable exceptions are where joints form plumose patterns in fumarolic zones. These fractures play a dominant role in the structural integrity of welded tuffs, their hydrology, and, by secondary processes, their mineralogy. 

The Bandelier Tuff of New Mexico provides field data that illustrate fracture characteristics. Erupted 1.13 million years ago, the upper member of the Bandelier Tuff (Tshirege Member) is well exposed in numerous canyons that cut the Pajarito Plateau in northern New Mexico. It displays prominent fractures, which likely play an important role in the vadose-zone hydrology of the tuff and the surface manifestations of the tectonic fabric. Detailed canyon wall maps, discussed here, record the location and morphology of over 5000 fractures, represented in nearly 8 km of canyon-wall exposure. Being best developed in more strongly welded zones, fractures extend from the surface of the tuff to the pumice fallout underlying it. The average fracture spacing is 1.5 m in studied locations giving a linear density of 65 fractures per hundred meters. Notable increases in fracture density up to 230 per hundred meters occur over tectonic lineaments associated with the Pajarito fault system. Fracture strikes are widely dispersed, but do show a crude bimodal distribution that defines a conjugate system of northwesterly- and northeasterly-oriented fracture sets. Since fracture maps represent vertical cross sections along generally west-to-east canyon walls, a fracture set generally paralleling these canyons is not well documented, but its presence is indicated by simple trigonometric approximations.

 Rare surface exposures of fracture sets show a polygonal, often rhombohedral, pattern, though in places the pattern is nearly orthogonal or hexahedral. Most fractures are steeply dipping (~80°), but nearly horizontal fractures are also evident. They display both planar (constant aperture) and sinuous (variable aperture) surfaces, averaging 0.7–1.0 cm aperture in all studied areas; fractures in tectonic zones, however, show average apertures up to 5 cm. Although most fractures are not filled at depth, they are packed with detritus and secondary minerals within about 15 m of the surface. Combining linear density and fracture aperture data shows that an average of 0.7 m of total aperture exists over any 100-m interval, but it can rise to over 5 m of total aperture per hundred meters over tectonic zones. 

An interesting result of this characterization is a demonstration of how a fractured-welded tuff can conceal faults by accommodating strain incrementally in each fracture over a wide area. Calculations based on the fracture data indicate that the Bandelier Tuff conceals fault displacement of up to several meters. The occurrence and distribution of fractures in welded tuff is an important consideration for slope stability and infiltration of surface water and contaminants. The development of new dual-porosity/dual-permeability numerical procedures for predicting contaminant dispersal relies on accurate input of fracture characteristics. Providing obvious pathways for dispersal, the Bandelier Tuff shows that fracture fillings of detrital materials and secondary minerals actually block migration along fractures near the tuff’s surface. At deeper levels, fractures make the tuff very permeable and capable of containing large quantities of water.

Explosive volcanic eruptions are the result of intensive magma and rock fragmentation, and they produce volcanic ash, which consists of fragments 62 mm in average diameter. The problem with volcanic ash is that its formation is poorly understood from the standpoint of eruption energetics. Because the source of explosive eruption energy should be the thermal energy of magma, and because an explosion requires rapid conversion of energy into a mechanical form, and because of the physical properties of magma thermal energy is dominantly released by conduction, the energy release on a short time scale (explosion) in volcanic processes has to be the result of a special mechanism, probably a positive feedback mechanism of fragmentation and heat exchange. In fact, the most explosive volcanic explosions are characterized by the most intensive fragmentation. In any fragmentation mechanism the generated particle sizes reflect the kinetic energy available (i.e. the fragmentation energy density). Consequently, fine ash (<62 mm) provides information on fragmentation mechanisms that are the most energetic and related to the highest explosive energy release. In this letter we discuss mechanisms of formation of fine volcanic ash, using experimental results, theoretical considerations, and field observations. We focus on the potency of these mechanisms to explain fine ash produced by explosive volcanism. We conclude that quantitative analysis of fine ash particles is necessary to estimate the mechanical energy of volcanic explosions.

Volcanology and Geothermal Energy is a book devoted to portraying volcanoes and volcanic regions in a manner that illuminates the origin and location of their associated potential geothermal reservoirs. Through years of experience, the authors have realized that their is a traditional gap between the geologists and drilling engineers that has in many cases resulted in exploration failures. This gap can be bridged by geologists using recent volcanological theory and models to focus on field observations and measurements that explicitly guide drilling programs. In review of recent advances in volcanology, the book gives theory and numerous practical examples of how geological field data give unique evidence of the location, nature, and magnitude of a geothermal resource. Important topics discussed include the characterization of petrogenesis, tephra deposits, and volcanic structure, all focussed on determining the location, size, and longevity of the magmatic heat source, the presence and abundance of water necessary for development of a hydrothermal system, and how the two have combined to form a geothermal reservoir.

This book is a one-of-a-kind compilation that uniquely satisfies both requirements for planning and operating a successful exploration project. With abundant background information and examples of volcanological study, geologists with minimal training in volcanology or engineers with none may begin to fully appreciate the methods developed to optimize exploration and minimize costs.

Volcanic ash deposits are found thorughout the entire geologic record. Pyroclastic rocks are present as layers in metamorphic rocks over a billion years old, interbedded with sedimentary rocks deposited throughout Paleozoic and Mesozoic times, and in recent deposits only a few years old. Many of these deposits have served as the source or host of a variety of mineral resources such as uranium, lithium, copper, and diamonds. Volcanic ash is also an important soil constituent in many areas of the world. This useful atlas provides descriptions of volcanic ash from different eruption types, sequences of ash layers, and weathered and metamorphosed ash. Divisions of eruption, and therefore asht ypes are based upon the mechanism of ash formation and the chemical composition of the ash. The numerous samples that are presented typify each known type of explosive eruption and its ash deposits.

Volcanic Ash is a reference work for those interested in the identification of volcanic ash and its physical properties. For the geologist and volcanologist, the atlas provides examples of ash from most eruption types and from a variety of eruption sequences. These examples can be used to interpret explosive eruption phenomena and physical characteristics of magmas. Examples of volcanic ash collected in eruption plumes will also be of interest to those in the atmospheric, engineering, and physical sciences.

Explosive volcanism: Inception, evolution, and hazards. Studies in geophysics

Experiments have determined that the optimal mass mixing ratio of water to basaltic melt for efficient conversion of thermal energy into mechanical energy is in the range of 0.1 to 0.3. For experiments near this optimum mixture, the grain-size of explosion products is always fine (less than 50 mm). The particles generated are much larger (greater than 1-10 mm) for explosions at relatively low or high ratios. Both natural and experimental pyroclasts produced by hydroexplosions have characteristic morphologies and surface textures. SEM micrographs show that blocky, equant grain shapes dominate. Glassy clasts formed from fluid magma have low vesicularity, thick bubble walls, and drop-like form. Microcystalline essential clasts result from chilling of magma during or shortly following explosive mixing. Crystals commonly exhibit perfect faces with patches of adhering glass or large cleavage surfaces. Edge modification and rounding of pyroclasts is slight to moderate. Grain surface alteration (pitting and secondary mineral overgrowths) are a function of the initial water to melt ratio as well as age. Deposits are typically fine-grained and moderately sorted, having distinctive size distributions compared with those of fall and flow origin.

Hydrovolcanic processes occur at volcanoes of all sizes ranging from small phreatic craters to huge calderas. The most common hydrovolcanic edifice is either a tuff ring or a tuff cone, depending on whether the surges were dry (superheated steam media) or wet (condensing steam media). Hydrovolcanic products are also a characteristic component of eruption cycles at polygenetic volcanoes. A repeated pattern of dry to wet products (Vesuvius) or wet to dry products (Vulcano) may typify eruption cycles at many other volcanoes. Reconstructions of eruption cycles in terms of water-melt mixing is extremely useful. In modeling processes and evaluating risk at active volcanoes.

Three stages of ash development are considered: (1) formation by the eruptive process; (2) modification by transport abrasion; (3) alteration by post-emplacement processes. Ash collected from pyroclastic surge deposits shows a higher percentage of broken or planar surfaces than does ash from air-fall deposits which are more vesicular. Surge ash from tuff ring deposits shows a higher percentage of rounding by transport and is generally more altered than associated air-fall ash.

The principal bed forms are massive beds, planar beds showing inverse grading, and sandwave beds with dunes, ripple and cross laminations, and antidunes. Markov analysis shows that sections are characterized by a dominance of either (1) sandwave and massive beds (sandwave facies); (2) planar, massive, and sandwave beds (massive facies); or (3) planar and massive beds (planar facies). Facies distribution maps demonstrate a systematic lateral variation away from the vent. Sandwave facies predominate in sections nearest the vent, massive facies dominated in sections at an intermediate distance from the vent, and planar facies occur in sections farthest from the vent.

The spatial distribution of surge facies is compatible with a fluidization-deflation model of pyroclastic-surge transport and deposition. A pyroclastic-surge cloud that is intially fluidized at the vent deflates (defluidizes) as it moves laterally. During transport the cloud passes from a proximal viscous mode of flow characterized by deposition of the sandwave facies to a distal inertial mode of flow represented by deposition of the planar facies. The gradual transition from viscous to inertial flow is coincident with deposition of the massive facies.