Augustine is an active stratovolcano occupying almost entirely a small island in the Cook Inlet in Alaska. In this study, we use the arrival time data from local seismicity recorded by 15 permanent seismic stations deployed on the flanks of Augustine Volcano by the Alaska Volcano Observatory and invert them by the body wave local earthquake tomography. The resulting model includes the 3D distributions of the P and S wave velocities (Vp and Vs), Vp/Vs ratio and the relocated seismic events. The resolved area extends down to the depth of 2 km below sea level (b.s.l). At shallow depths, within the edifice, we observe a contrasted anomaly with very high Vp, low Vs and very high Vp/Vs, which represents the rigid volcano core composed of igneous rocks (high Vp), but strongly fractured and saturated with fluids and melts (low Vs). At larger depths, below sea level, we retrieve a columnar anomaly of high Vp/Vs ratio representing the magma conduit. On the top of this anomaly, there is a zone with low Vp/Vs coinciding with the strong seismicity cluster, which is associated with the degassing of deep fluids.

Cover.Photograph from the southwest Aniakchak caldera rim looking northeast at the steep, ~300-m-high exposure through Half Cone.Light tan tephra and dark gray vent-filling lava flows (middle cliffs) from pre-400 yr BP Half Cone eruptions are overlain by reddish orange and black pyroclastic deposits from the ~400 yr B.P. eruption.The blocky surface of the Cobweb lava flow, mantled by 1931 tephra, lies to the right of Half Cone

Tumuli are small, dome-like features that form when magmatic pressures build within a subsurface lava pathway, causing the overlying crust to bulge upwards. As the appearance of these features has been linked to lava flow structure (e.g., underlying lava flow tubes) and conditions, there is interest in identifying such features in satellite images so they can be used to expand our understanding of lava flows within regions difficult to access (such as on other planets). Here, we define a methodology for identifying (and measuring) tumuli within satellite imagery, and validate it by comparing our results with fieldwork results of terrestrial tumuli reported in the literature and with independent measurements we made within Amboy Field, CA. In addition, we present aggregated results from the application of our methodology to satellite images of six terrestrial fields and seven martian fields (with >2100 tumuli identified, per planet). Comparisons of tumuli morphometrics on Earth and Mars yield similarities in size and overall shape, which were surprising given the many differences in the environmental and planetary conditions within which these features have formed. Given our measurements, we identify constraints for tumulus formation models and drivers that would yield similar shapes and sizes on two different planets. Furthermore, we test a published hypothesis regarding the number of tumuli that form per a square kilometer, and find it unlikely that a diagnostic “tumuli density” value exists.

The Golden Trout Volcanic Field (GTVF) produced the only Quaternary eruptions of mafic magma within the southern Sierra Nevada block. Approximately 38×106m3 of basalt, trachy-basalt, basaltic trachy-andesite, and basaltic andesite (50.1–56.1% SiO2, 1.1–1.9% K2O, and 5.4–9.1% MgO) was erupted from four vents within a ~10km2 portion of the GTVF, which also includes rhyolite domes that are not considered in this study. The vents include, from oldest to youngest: Little Whitney Cone, South Fork Cone, Tunnel Cone, and unglaciated Groundhog Cone. Little Whitney Cone is a 120m-high pile of olivine-CPX-phyric scoria produced during a Strombolian-style eruption overlying two columnar jointed lava flows. Tunnel Cone formed through a Hawaiian-style eruption along a 400m-long north-south trending fissure that excavated at least three 25–65m-wide craters. Crater walls up to 12m high are composed of plagioclase-olivine-phyric spatter-fed flows that dip radially away from the crater center and crumble to form Tunnel Cone's steep unconsolidated flanks. South Fork Cone is a 170m-high pile of plagioclase-olivine-phyric scoria that formed during Strombolian to violent Strombolian eruptions. South Fork Cone overlies the South Fork Cone lava, a 9.5km-long flow (~12×106km3) that reached the Kern River Canyon to the west. Scoria and airfall deposits originating from South Fork Cone are located up to 2km from the vent. Groundhog Cone is a 140m-tall cinder and spatter cone breached on the north flank by a 13×106m3 lava flow that partially buried the South Fork Cone lava and extends 7.5km west to Kern River Canyon. Incompatible trace element concentrations and ratios show vent-specific trends but are unsystematic when plotted in terms of all mafic GTVF vents, implying that GTVF basalts were derived from a lithospheric mantle source and ascended through thick granitic Sierra Nevada crust as discrete batches that underwent different degrees of crustal contamination, differentiation, and magma mixing. Clinopyroxene-liquid thermobarometry calculations of the oldest and most primitive GTVF sample indicate that most clinopyroxene crystals record nucleation conditions of 1172–1196°C and 896–1115MPa. These pressures correspond to depths equivalent to, or up to 10km shallower than, the Moho in this region. Deposits from the oldest GTVF vents are more primitive and homogeneous in terms of major and trace element concentrations, phenocryst textures and compositions, and their general absence of crustal xenoliths and xenocrysts compared to younger eruptive products. The eruption rate of the GTVF (~0.05km3/Myr) is two orders of magnitude less than neighboring and contemporaneous Big Pine and Coso volcanic fields to the north and east, respectively. We interpret these differences to result from a relative lack of strain in the GTVF region of the southern Sierra Nevada block, which limits magma accumulation and ascent.

The manner in which a volcano erupts is largely controlled by the rate of magma ascent and the geometry of the volcanic conduit connecting the volcanic vent at the surface to the magma reservoir region in the shallow crust. Factors influencing the rate in which magma ascends to the surface during eruptions include both physical and chemical properties of the magma, such as its temperature, composition, volatile budget, and crystallinity, all of which affect its viscosity and density, as well as the permeability of the conduit walls. This chapter offers a summary of different techniques used to examine and constrain the rates of magmatic processes that occur in shallow crustal reservoirs as well as in conduits while magma ascends to the surface during eruption. Whereas compositional zoning patterns in phenocrysts and diffusion modeling indicate that relatively short-lived magmatic processes like magma mixing occur over timescales of 100–102 days, processes like magma assimilation and crystal-melt fractionation persist for 103–106 years based on studies that utilized radioactive isotopes from whole-rock or mineral separates. Thus, such timescales must also be required for the accumulation and storage of magma in the shallow crust, particularly in the case of silicic magmas emplaced as a result of voluminous caldera-forming eruptions. Techniques aimed at understanding syneruptive magma ascent involve magma extrusion rate studies as well as experimentally replicating a variety of decompression-induced mineral-melt reactions, such as bubbles, microlites, and reaction rims. Results from these studies suggest a general relationship between magma ascent rates in excess of ∼0.2 m/s and explosive eruptions. Moreover, magma ascent rates appear to be relatively consistent over a wide range of magma compositions.

The data on the geochemistry of the rocks of Kizimen Volcano and results of microprobe studies of major and trace elements in plagioclase grains from acid lavas and basalt inclusions are presented. The characteristics of the Kizimen Volcano are the following: (1) basalt inclusions are abundant in acid lavas; (2) banded, mixed lavas occur; (3) the distribution curves of rare-earth elements of acidic lavas and basalt inclusions intersect; (4) Sr-Nd isotope systematics of the rocks and inclusions do not indicate mixture with crustal material; (5) plagioclase phenocrysts are of direct and reverse zonation; (6) olivine and hornblende, as well as acid and mafic plagioclases, coexist in the rocks. The studies revealed that the rocks are of a hybrid nature and originated in the course of repeated mixture of acid and mafic melts either with chemical and thermal interaction of melts or exclusively thermal ones. Study of the major- and trace-element distribution in zonal minerals provides an informative tool for understanding the history of the generation and evolution of melts in a magma chamber.

This digital publication contains the geologic map information used to publish the U.S. Geological Survey Scientific Investigations Map 3145 (Coombs and others, 2012), which depicts the geology of Mount Gareloi, Gareloi Island, Alaska.

This article describes results from morphological, textural, mineralogical, and compositional analyses of lava clasts and quenched mafic inclusions from 10 Holocene debris-avalanche deposits (2200–125 years B.P.) that form the lower flanks of Augustine Volcano, Alaska. Mafic inclusions collected from the Rocky Point pyroclastic-flow deposit emplaced during the 2006 eruption are included for comparison. All deposits contain evidence for mixing between basalt and high-silica andesite prior to and during eruption, including compositional banding, disequilibria phenocryst assemblages, linear variation in major- and trace-element concentrations, and quenched mafic inclusions (51.3–57.3 wt.% SiO2) hosted by andesite lavas (59.1–62.6 wt.% SiO2). Generally, inclusions from all deposits share many morphologic and petrologic characteristics. Inclusions range in diameter from <1 to >36 cm, although 87% of the 959 inclusions analysed are less than 5 cm in diameter. Mafic inclusions account for an average of 8 vol.% of host andesitic lava clasts included in the oldest debris-avalanche deposits compared with subsequently emplaced deposits that contain an average of only 1–3 vol.%. Inclusions contain phenocrysts of plagioclase, amphibole, clinopyroxene, olivine, and rare orthopyroxene as well as microphenocrysts of plagioclase, amphibole, clinopyroxene, olivine, magnetite, ilmenite, and apatite in a glassy, vesicular, and acicular groundmass. Plagioclase phenocrysts in inclusions typically have a 30–150 μm-thick, fine-grained ‘dusty sieved’ rim superimposed over oscillatory zoned texture, suggesting that they existed in the host andesite magma prior to basaltic intrusion, but were engulfed during the intrusion and inclusion formation process. Mafic inclusions are calc-alkaline, low-K (0.45–0.82 wt.% K2O) basalts to basaltic andesites that are altogether different in terms of mineralogy and composition from olivine basalt contained in late Pleistocene-age fragmental deposits described by Plank et al. (2006, The Augustine basalt: Eos (American Geophysical Union Transactions), v. 87, Abstract V42B-06) and Larsen et al. (2010, Petrology and geochemistry of the 2006 eruption of Augustine Volcano, in Power, J.A., Coombs, M.L., and Freymueller, J.T., eds., The 2006 eruption of Augustine Volcano, Alaska: US Geological Survey Professional Paper 1769). Thus, the most reliable approximation of the basaltic endmember composition involved in magma mixing processes at Augustine Volcano over the past ∼2200 years are the most mafic inclusions, which consistently possess the (1) highest abundance of vesicles, (2) fewest number of phenocryst-sized plagioclase, and (3) largest volume of microlites compared with more contaminated inclusions.