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The ascent of magma to the Earth's surface is commonly modeled by assuming a fixed dike or flow geometry from a deep subsurface reservoir to the surface. In practice, however, this flow geometry is produced by deformation of the crust by ascending overpressured magma. Here, we explore how this elastic deformation is coupled to magma ascent using a model of a planar dike whose width is allowed to evolve with depth in the crust. The model predicts that, for points well below the surface, the dike gradually narrows with height above the source reservoir as the overpressure of the magma relative to the minimum horizontal stress in the crust decreases. For a bubbly compressible magma, the flow accelerates to the surface and reaches the speed of sound at the vent, as for rigid conduit flow. However, it is now able to decompress to atmospheric pressure at the vent by contraction of the conduit. Our calculations predict eruption rates on the order of 0.1–10 $m^2/s$ per unit length of a dike, for magma supplied from a reservoir with overpressure in the range − 10 MPa to + 20 MPa.

A low-energy (Volcanic Explosivity Index [VEI] 1), small-volume (0.001 km3 Dense Rock Equivalent [DRE]) eruption of highly crystalline basalt occurred at Cerro Negro volcano, Nicaragua, August 5–7, 1999. This eruption followed three earthquakes (each ∼Mw 5.2) with strike-slip and oblique-slip focal mechanisms, the first of which occurred approximately 11 h before eruptive activity and within 1 km of Cerro Negro. Surface ruptures formed during these events extend up to 4 km from Cerro Negro, but concentrate ∼1 km south of Cerro Negro. Surface ruptures did not occur within 300 m of the cone, however, three new vents formed on the south flank and base of Cerro Negro and on trend with the Cerro La Mula–Cerro Negro volcanic alignment. Earthquake swarms were located northwest and southeast of Cerro Negro and seismicity was elevated for up to 11 days after the initial event. The temporal and spatial patterns of earthquake swarms, surface ruptures, and the eruption location can be explained using the Hill [J. Geophys. Res. 82 (1977) 1347] model for earthquake swarms in volcanic regions, where an eruption is triggered by tectonically induced changes in the regional stress field. In this model, tectonic strain, rather than magmatic overpressure causes dilation of the conduit for magma ascent. Numerical simulations for the 1999 eruption illustrate that the observed velocities (up to 75 m s−1) and fountain heights (50–300 m) can be achieved by eruption of magma with little excess magmatic pressure, in response to changes in Coulomb stress along the Cerro La Mula–Cerro Negro alignment. These observations and models show that 1999 Cerro Negro activity was a tectonically induced small-volume eruption in an arc setting, with the accommodation of extensional strain by dike injection.

The goal of probabilistic volcanic hazard assessment is to translate complex volcanological data and numerical models into practical hazard estimates for communities potentially affected by volcanic eruptions. Probabilistic volcanic hazard assessment quantifies volcanic hazards and illustrates uncertainties about the magnitude and consequences of volcanic activity. Planning based on probabilistic volcanic hazard assessment has the potential of mitigating the effects of volcanic eruptions when they occur. This paper presents an approach developed to estimate volcanic hazards related to tephra fallout and illustrates this approach with a tephra fallout hazard assessment for the city of Leon, Nicaragua, and the surrounding area. Tephra fallout from eruptions of Cerro Negro volcano has caused damage to property and adverse health effects and has disrupted life in this area. By summarizing the geologic and historical records of past eruptions of Cerro Negro on a probability tree, it is shown that the inhabitan...

Xenoliths in pyroclastic fall deposits from the 1975 Tolbachik eruption constrain the timing and development of subsurface conduits associated with basaltic cinder cone eruptions. The two largest Tolbachik vents contain xenoliths derived from magmatic and hydromagmatic processes, which can be correlated with observed styles of eruption activity. Although many basaltic eruptions progress from early hydromagmatic activity to late magmatic activity, transient hydromagmatic events occurred relatively late in the 1975 eruption sequence. Magmatic fall deposits contain 0.01–0.3 vol.% xenoliths from 500 m depth into the dry-out zone around the conduit, disrupting and ejecting 10 5 –10 6 m 3 of wall-rock through hydromagmatic processes with conduits widening to 8–48 m. Hydromagmatic falls contain 60–75 vol.% of highly fragmented xenoliths, with juvenile clasts displaying obvious magma-water interaction features. During the largest hydromagmatic event, unusual breccia-bombs formed containing a wide range of fresh and pyrometamorphic xenoliths suspended in a quenched basaltic matrix. Hydromagmatic activity during the 1975 Tolbachik eruption occurred below likely fragmentation depths for a basalt containing 2.2 wt.% magmatic water. This activity is more likely related to conduit-wall collapse rather than variations in conduit-flow pressure. In contrast, larger volume silicic eruptions may have transient hydromagmatic events in response to conduit flow dynamics above the magma fragmentation depth. The 1975 Tolbachik volcanoes are reasonably analogous to Quaternary basaltic volcanoes in the Yucca Mountain region and can guide interpretations of their poorly preserved deposits. The youngest basaltic volcanoes near Yucca Mountain have cone deposits characterized by elevated xenolith abundances and distinctive xenolith breccia-bombs, remarkably similar to 1975 Tolbachik deposits. Extrapolation of 1975 Tolbachik data suggests conduits for some Yucca Mountain basaltic volcanoes may have widened locally on the order of 50 m in response to late-stage hydromagmatic events.

Cerro Negro volcano, Nicaragua, continued a 147-yr-long duration of cinder-cone activity with a major eruption in 1995. Small, phreatically driven eruptions began in May 1995 and continued for 79 days. Following a 95 day repose, the main eruption produced 8 × 106 m3 of basalt from Cerro Negro over 13 days of activity and deposited 5 mm of ash in the city of Leon. Although the damage from the 1995 eruptions was fortunately minor, previous tephra falls from Cerro Negro have produced significant crop damage and multiple deaths through building collapse. In spite of its apparent longevity for a historically active cinder cone, Cerro Negro has mass-flow rates typical of arc-related basaltic cinder cone volcanoes. Volcanic hazards beyond 3 km from Cerro Negro consist of tephra falls. Few models are available to calculate tephra-fall risks from basaltic volcanoes such as Cerro Negro, and none have been applied to dispersive cinder cone eruptions. A convective-dispersive model of Suzuki is modified and evaluated using detailed data from the 1995 Cerro Negro eruption and is found to reasonably calculate tephra-fall thickness between 8 and 30 km from the vent. This model is used with detailed data from previous Cerro Negro eruptions in a tephra-fall hazard assessment. Cerro Negro also appears to have had a steady-state eruption rate since about A.D. 1900, which is used to estimate the timing of the next eruption as before A.D. 2006. The potential tephra fall from Cerro Negro in Leon, Nicaragua, is calculated as 2.2 mm/yr until 2006, with 95% confidence that deposits will be

The June–August 1995 eruption of Cerro Negro, a small-volume basaltic cinder cone in Nicaragua, provided a unique opportunity to quantify 222Rn degassing from soils in response to explosive volcanic activity. 222Rn was monitored at 29 stations using electrostatically charged teflon 222Rn detectors distributed north and southeast of the volcano. A pulse of elevated 222Rn degassing occurred early in the eruption along a > 1 km long zone, extending at least 750 m beyond the base of the cinder cone. Observation of this 222Rn pulse shows that large changes in soil 222Rn concentration can occur simultaneously at widely separated stations in response to even comparatively small-volume intrusions and volcanic eruptions. Frequent 222Rn sampling at multiple stations provides a sense of the magnitude, time scales and area affected by convective gas transport during volcanic eruptions.

The distribution and timing of areal basaltic volcanism are modeled using three nonhomogeneous methods: spatio-temporal nearest neighbor, kernel, and nearest-neighbor kernel. These models give nonparametric estimates of spatial or spatio-temporal recurrence rate based on the positions and ages of cinder cones and related vent structures and can account for migration and shifts in locus, volcano clustering, and development of regional vent alignments. The three methods are advantageous because (1) recurrence rate and probability maps can be made, facilitating comparison with other geological information; (2) the need to define areas or zones of volcanic activity, required in homogeneous approaches, is eliminated; and (3) the impact of uncertainty in the timing and distribution of individual events is particularly easy to assess. The models are applied to the Yucca Mountain region (YMR), Nevada, the site of a proposed high-level radioactive waste repository. Application of the Hopkins F test, Clark-Evans test, and K function indicates volcanoes cluster in the YMR at the >95% confidence level. Weighted-centroid cluster analysis indicates that Plio-Quaternary volcanoes are distributed in four clusters: three of these clusters include cinder cones formed

We conducted low-frequency (16 to 100 MHz) ground-penetrating radar surveys on the eroded lava flows at Craters of the Moon (Idaho, USA) volcanic field to evaluate the potential of future radar-sounding investigations on Mars to map shallow subsurface features. Radar-sounding profiles were obtained from three locations: above a lava tube, across a volcanic rift, and over a scoria cone. Results were combined with laboratory permittivity and magnetic permeability measurements of field-collected samples to deconvolve the electromagnetic attenuation and scattering losses from the total losses and therefore separately quantify both effects on the radar penetration depth. Our results demonstrate a constrained performance for low-frequency sounding radars to characterize mafic, arid volcanic terrains that contain a significant amount of ferro-oxides (∼14%), mainly in the form of olivine and magnetite. Penetration depths of 35 m were achieved at a frequency of 100 MHz, and depths of 80 m were achieved at 16 MHz, with an effective dynamic range of 60 dB. Results indicate that for frequencies below 100 MHz, the electromagnetic attenuation dominated the signal losses while above this frequency threshold the volume scattering dominated the losses. Over our frequency range, the observed electromagnetic attenuation and penetration depths were strongly dependent on the magnetic losses, ground porosities, and degree of heterogeneity rather than the sounding frequency. In light of these results, we suggest average attenuation and scattering losses measured in terms of dB/m and discuss the expected penetration depth for the Mars orbital radar-sounding instruments SHARAD and MARSIS in mafic volcanic terrains.

An orthogonal system of tube-bearing joints constitutes the oldest fractures in the Tiva Canyon Tuff at Yucca Mountain, Nevada. The joints formed within a month of ignimbrite deposition, prior to major degassing. The system consists of (1) narrow, persistent, northeast-striking joint swarms with trace lengths typically greater than 5 m and between-joint spacings of less than 20 cm and (2) northwest-striking swarms that have a more en echelon geometry and greater between-joint spacings compared to the northeast-striking swarms. Between-swarm spacing for both trends is ∼50 m. Questions concerning the joints include the following: (1) What was the origin of driving stress(es) for formation of the joints, particularly as their orientations were not consistent with the regional stress geometry at the time of their formation? (2) What mechanism caused the horizontal principal stresses to be reoriented so as to yield an orthogonal geometry? (3) What insights can be developed for predicting joint geometry in unexposed rock volumes by understanding joint origin? These questions are important because the joints and other fractures may affect the performance of a proposed high-level nuclear waste repository within Yucca Mountain. To address these questions, we use new and existing field data about joint geometry and the relationships of joints to degassing structures, numerical modeling of fault behavior, and work by previous authors. Our interpretation begins with the initial ignimbrite eruption and deposition during caldera collapse. The ignimbrites were deposited over a preexisting topography that possibly included a shallow northwest-trending basin in the Yucca Mountain area. During initial ignimbrite cooling, joint swarms formed as elements of orthogonal fumarolic ridge systems where degassing was associated with vertical dilation. Both joint sets have unusual tubes that are interpreted to have formed during dilation and segmentation of joint faces resulting from lithophysae inflation in the cooling ignimbrite deposit. Modeling supports the interpretation that a combination of regional stresses and stress related to slip on local normal faults controlled the orientation of the joint swarms and favored the formation of the northeast-striking joints first. The faults might have moved in a stress field already perturbed by caldera collapse. Formation of the northwest-striking joints occurred after a local 90° switch of horizontal principal stress directions due to the presence of the northeast-striking swarms, possibly aided by differential compaction across the northwest-trending basin. Tube- bearing joints occur in all lithophysae-bearing lithostratigraphic units of the Topopah Spring Tuff, which is the stratigraphic interval for the proposed nuclear waste repository within Yucca Mountain. We conclude that the tube-bearing joints formed in the same manner and share similar geometric characteristics, adding a persistent joint population to the overall fracture system that influences hydrological and mechanical properties.

We examine the possible ascent of alkali basalt magma containing 2 wt percent water through a dike and into a horizontal subsurface drift as part of a risk assessment for the proposed high-level radioactive waste repository beneath Yucca Mountain, Nevada, USA. On intersection of the dike with the low pressure, horizontal drift, the ascending magma will be diverted into the drift. The fragmenting mixture expands down the drift to reach speeds of order 100–300 m/s. After this initial disruptive activity, parts of the repository can be filled with magma within a matter of hours until a pathway is found to allow the magma to vent. Magma flow through the drift network will cause intense heating of any waste canisters located along the pathway between the dike and the surface conduit. The assessments suggest a greater number of waste packages may be adversely affected than previously recognized.

A new compilation of 23 K-Ar and 40 Ar- 39 Ar radiometric ages, stratigraphic and paleomagnetic correlations, and cone morphology from the SP cluster, San Francisco volcanic field, Arizona, is used to illustrate a quantitative approach to long-term volcanic hazards assessment for basaltic volcanic fields. SP cluster is a group of 67 basaltic cinder cones, tuff rings, spatter cones, and lava flows, all predominantly younger than 1700 ka. Relatively steady-state volcanic activity (1 volcano per 15 k.y.) has characterized the SP cluster since 780 ka. This activity has been concentrated in an ∼250 km 2 area since about 300 ka. We estimate that the chance of an eruption in the SP cluster during the next 1000 yr has an upper bound (95% confidence level) of 13%. Spatial and temporal probability models further indicate with 90% confidence that an eruption will take place within this 250 km 2 area of the SP cluster within the next 22–26 k.y., a rate of activity significantly greater than predicted by field-wide averages. Thus, spatial variations in recurrence rate of basaltic volcanism, such as those recognized in the SP cluster, should be considered in the formulation of hazard assessments.

The 1975 Tolbachik, Kamchatka, Russia, eruption resulted in the formation of three basaltic cinder cones. During this eruption, shallow dike injection into a nearby cinder cone, Cone 1004, caused deformation and slumping of the cone. In 1995, temperatures at 2 m depth, electromagnetic anomalies, and geologic units were mapped on the slump block. These data reveal details about the cooling rates of a shallow dike in the slump block, including (1) measured temperatures as much as 475 °C, (2) temperatures greater than 200 °C along a 160-m-long and 30-m-wide zone that trends oblique to bedding, (3) a low resistivity zone at 80 m depth that may delimit a larger intrusion beneath the shallow dike, (4) a dry zone extending 15–20 m from the axis of the thermal anomaly, and (5) a condensation zone beyond this dry-out zone, characterized by steaming ground and high convective heat flux. Analytic models indicate that the surprisingly high temperatures persist because of the low thermal diffusivity of the scoria into which the dike is injected, a direct result of the scoria9s high porosity. Two-phase nonisothermal flow models suggest that the observed patterns of dry-out and condensation are expected for small volume fractions of water in the scoria (≤10%). Thus, field observations from Tolbachik provide a graphic example of the roles of rock thermophysical properties and water in the unsaturated zone on rates of dike cooling.

Natural disasters like volcanic eruptions occur infrequently, but if they occur near nuclear power plants or high-level radioactive waste repositories, local and global communities can be threatened. Ideally, such facilities should be constructed only where geologic risk is very low. Estimating the probabilities of such events requires a comprehensive understanding of site geology and the geologic processes operating in the site region on timescales of 104 to 107 years. In light of these requirements, geologists and geophysicists must continually improve techniques for site characterization.