Abstract Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea where they form long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean (see Fig. 1 ). The concept of plate tectonics explains the locations of volcanoes and their relationship to other large-scale geologic features. The Earth's surface is made up of a patchwork of about a dozen large plates and a number of smaller ones that move relative to one another at <1 cm to ~10 cm/yr (about the speed at which fingernails grow). These rigid plates, with average thickness of ~80 km, are separating, sliding past each other, or colliding on top of the Earth's hot, viscous interior. Volcanoes tend to form where plates collide or spread apart ( Fig. 2 ) but can also grow in the middle of a plate, like the Hawaiian volcanoes ( Fig. 3 ). Of the more than 1,500 volcanoes worldwide believed to have been active in the past 10,000 years, 169 are in the United States and its territories ( Ewert et al., 2005 ) (see Fig. 4 ). As of spring 2007, two of these volcanoes, Kilauea and Mount St. Helens, are erupting, while several others, including Mauna Loa, Fourpeaked, Korovin, Veniaminof, and Anatahan, exhibit one or more signs of restlessness, such as anomalous earthquakes, deformation of the volcano's surface, or changes in volume and composition

Abstract In this paper, we consider the primary controls on gas and liquid geochemistry at The Geysers geothermal field (California) prior to reservoir exploitation and reinjection programs. Well discharges vary considerably in steam/gas ratio, gas composition, and dD and δ 18 O of steam. Many of the variations can be linked to the degree of liquid saturation or steam fraction (Y) within the reservoir. Discharged fluids from the central Northwest Geysers have low molar steam/gas (<200) and are produced from reservoir vapor because little condensed liquid water appears to exist in that part of the system (i.e., they are high Y fluids). The gas is relatively uniform in composition, typically with ~60 mol percent CO 2 and around 10 mol percent NH 3 + CH 4 on an H 2 O-free basis. N 2 /Ar ranges to values >500. Discharges from the central Northwest Geysers are interpreted to contain a mixture of connate and metamorphic gases derived from high-temperature breakdown of carbon- and nitrogen-bearing metasediments, either within or below the geothermal reservoir. Input of volcanic gas from underlying intrusions appears to be present but minor. The gas-rich end member is less evident in the Southeast and Central Geysers where discharged fluids consist primarily of steam boiled from condensed reservoir liquid (i.e., they are low Y fluids). Molar steam/gas in these parts of the field commonly exceeds 3,000; N 2 /Ar approaches that of airsaturated meteoric H 2 O (~38). Isotopes within reservoir steam (dD and δ 18 O) are only slightly shifted from local meteoric waters. Reservoir gases in the Southeast and Central Geysers are thus diluted by the dominant input of meteoric water, which disguises the connate and/or metamorphic signature of the gas. The resulting small proportion of gas is highly variable in composition.

Comb-layered quartz is a type of unidirectional solidification texture found at the roofs of shallow silicic intrusions that are often associated spatially with Mo and W mineralisation. The texture consists of multiple layers of euhedral, prismatic quartz crystals (Type I) that have grown on subplanar aplite substrates. The layers are separated by porphyritic aplite containing equant phenocrysts of quartz (Type II), which resemble quartz typical of volcanic rocks and porphyry intrusions. At Logtung, Type I quartz within comb layers is zoned with respect to a number of trace elements, including Al and K. Concentrations of these elements as well as Mn, Ti, Ge, Rb and H are anomalous and much higher than found in Type II quartz from Logtung or in igneous quartz reported elsewhere. The two populations appear to have formed under different conditions. The Type II quartz phenocrysts almost certainly grew from a high-silica melt between 600 and 800°C (as β-quartz); in contrast, the morphology of Type I quartz is consistent with precipitation from a hydrothermal solution, possibly as α-quartz grown below 600°C. The bulk compositions of comb-layered rocks, as well as the aplite interlayers, are consistent with the hypothesis that these textures did not precipitate solely from a crystallising silicate melt. Instead, Type I quartz may have grown from pockets of exsolved magmatic fluid located between the magma and its crystallised border. The Type II quartz represents pre-existing phenocrysts in the underlying magma; this magma was quenched to aplite during fracturing/degassing events. Renewed and repeated formation and disruption of the pockets of exsolved aqueous fluid accounts for the rhythmic banding of the rocks.