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Understanding the stress evolution of extinct volcanoes can improve efforts to forecast flank eruptions on active systems. Field, petrographic, and seismic data are combined with numerical modeling to investigate the paleo-stress field of New Zealand's Akaroa Volcano, or Akaroa Volcanic Complex. Field mapping identifies 86 radially oriented dikes and seven lava domes found only within a narrow elevation range along Akaroa's erosional crater rim. These observations suggest that crater rim dike emplacement resulted from lateral deflection of vertically ascending intrusions from a centralized magma source, which in turn may have facilitated formation of the lava domes, as well as two scoria cones. We postulate that dike deflection occurred along a stress barrier, as neither a compositional change nor structural boundary are present. We use a finite element model (FEM) simulating Akaroa to test how different factors may have influenced the system's stress state and dike geometry. Elastic, non-flexural ("roller") model configurations containing a large, oblate, and shallow magma chamber produce stress barriers most conducive to radial dike emplacement along Akaroa's crater rim. These configurations also simulate rapid edifice construction above a preexisting lithospheric "bulge." Conversely, simulating flexural stresses exerted on the lithosphere by Akaroa's large mass hinder rather than promote radial dike emplacement. Temperature-dependent viscoelastic relaxation promotes gradual increases in stress barrier elevation, though this effect is strongly dependent on magma chamber parameters. These results suggest that Akaroa was constructed rapidly (within ∼100 kyr) prior to crater rim dike emplacement, which occurred throughout the volcano's remaining active lifespan.

Recent analytical investigations have suggested that magma buoyancy is critical for triggering catastrophic caldera forming eruptions. Through detailed assessment of these approaches, we illustrate how analytical models have been misapplied for investigating buoyancy and are, therefore, incorrect and inconclusive. Nevertheless, the hypothesis that buoyancy is the critical trigger for larger eruptions warrants further investigation. As such, we utilize viscoelastic finite element models that incorporate buoyancy to test overpressure evolution and mechanical failure in the roof due to the coalescence of large buoyant magma bodies for two model cases. In the first case, we mimic empirical approaches and include buoyancy as an explicit boundary condition. In the second set of models, buoyancy is calculated implicitly due to the density contrast between the magma in the reservoir and the host rock. Results from these numerical experiments indicate that buoyancy promotes only minimal overpressurization of large silicic magma reservoirs ( 100 km3) they surpass a rheological threshold where their subsequent evolution is controlled by host rock mechanics. Consequently, this results in a thermomechanical division between small systems that are triggered “internally” by magmatic processes and large systems that are triggered “externally” by faulting related to roof uplift or tectonism. Finally, critical assessment of recent analytical approaches illustrates that care must be used when applying previously derived analytical solutions to ensure that assumptions used in the original formulation are not violated during application to new geologic problems.

Understanding the connection between shallow subsurface magmatism and related surface expressions provides first-order insight into the volcanic and tectonic processes that shape a planet's evolution. When assessing the role of flexure, previous investigations assumed homogeneous host rock, but planetary lithospheres typically include crust and mantle material, and the mechanical response of a layered lithosphere subjected to flexure may influence both shallow magma reservoir failure and intrusion propagation. To assess the formation of giant radial dike systems, such as those observed on Venus, we create axisymmetric elastic finite element models of a spherical reservoir centered at the contact between stiff, dense mantle overlain by softer, lighter crust. We analyze magma chamber stability, overpressure at rupture, and resulting intrusion types for three distinct environments: lithostatic, upward flexure, and downward flexure. In the lithostatic case, reservoir failure at the crust-mantle contact favors lateral sill injection. In the flexure cases, we observe that failure location depends upon the crust/lithosphere thickness ratio and, at times, will favor radial dike intrusion. Specifically, upward flexure can promote the formation of giant radiating dike swarms, a scenario consistent with a plume-derived origin. Our results present a mechanical explanation for giant radial dike swarm formation, showing that both the stability of magma chambers on Venus and the type of intrusions that form are influenced by lithospheric layering. Furthermore, where dike swarms occur, our approach provides a powerful new way to constrain local crust/mantle layering characteristics within the lithosphere at the time the swarm was forming.

The transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER—the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear- or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

In this study, we utilize thermomechanical models to investigate how magma chambers overpressurize as the result of either magmatic recharge or volatile exsolution. By implementing an adaptive reservoir boundary condition we are able to track how overpressure dissipates as the magma chamber expands to accommodate internal volume changes. We find that the size of the reservoir greatly impacts the resultant magma chamber overpressure. In particular, overpressure estimates for small to moderate-sized reservoirs (1–10 km 3 ) are up to 70% lower than previous analytical predictions. We apply our models to Santorini volcano in Greece where recent seismic activity and ground deformation observations suggested the potential for eruption. The incorporation of an adaptive boundary condition reproduces Mogi flux estimates and suggests that the magma reservoir present at Santorini may be quite large. Furthermore, model results suggest that if the magma chamber is >100 km 3 , overpressures generated due to the high magma flux may not exceed the strength of the host rock, thus requiring an additional triggering mechanism for eruption. Although the adaptive boundary condition approach does not calculate the internal evolution of the magma reservoir, it represents a fundamental step forward from elastic Mogi models and fixed boundary solutions on which future investigations of the evolution of the magma can be built.

[1] Using parameterized finite element models, we investigate the emplacement of both radial and circumferential intrusions in the configuration observed at Fernandina volcano in the Galapagos Islands. When situated within the edifice at depths consistent with petrological and surface displacement data, inflation of a mildly oblate magma reservoir to the point of rupture can initiate either radial or circumferential intrusions in response to minor, volcanologically plausible variations in reservoir geometry (i.e., aspect ratio). In addition, more oblate reservoirs inject lateral sills into an inflation-derived stress field consistent with rotation about their propagation axis to form gently dipping radial dikes, a mode of behavior recently inferred from InSAR data at Fernandina. All three styles of intrusion occur in near-surface configurations consistent with field observations.

Approximately every 100,000 years the Earth experiences catastrophic caldera-forming “supereruptions” that are considered to be one of the most hazardous natural events on Earth. Utilizing new temperature-dependent, viscoelastic numerical models that incorporate a Mohr-Coulomb failure criterion, we find that eruptive failure of the largest magma chambers is a function of the geometry of the overlying roof and the location of the brittle-ductile transition. In particular, the ductile halo created around the hot magma chamber buffers increasing overpressures and prevents pressure relief via magmatic injection from the magma chamber. The numerical results indicate that as chamber volume increases, the higher temperatures in the host rock and the decrease in the roof aspect ratio cause a shift from reservoir-triggered eruption to an external roof-triggered mechanism. Specifically, as overpressure increases within the largest magma chambers, extensive uplift in the overlying roof promotes the development of through-going faults that may trigger eruption and caldera collapse from above. We find that for magma chamber volumes > 103 km3, and roof aspect ratios (depth/width)

Eruptions fed from subsurface reservoirs commonly construct volcanic edifices at the surface, and the growth of an edifice will in turn modify the subsurface stress state that dictates the conditions under which subsequent rupture of the inflating reservoir can occur. We re-examine this problem using axisymmetric finite element models of ellipsoidal reservoirs beneath conical edifices, explicitly incorporating factors (e.g., full gravitational loading conditions, an elastic edifice instead of a surface load, reservoir pressures sufficient to induce tensile rupture) that compromise previous solutions to illustrate why variations in rupture behavior can occur. Relative to half-space model results, the presence of an edifice generally rotates rupture toward the crest of a spherical reservoir, with increasing flank slope (for an edifice of constant volume) and larger edifices (or greater reservoir scaled depths) normally serving to enhance this trend. When non-spherical reservoirs are considered, the presence of an edifice amplifies previously identified half-space failure characteristics, shifting rupture to the crest more rapidly for prolate reservoirs while forcing rupture closer to the midpoint of oblate reservoirs. Rupture is always observed to occur in the σ t orientation, and depending on where initial failure occurs rupture favors the initial emplacement of either lateral sills, circumferential intrusions or vertically ascending dikes. Ultimately, integration of our numerical model results with other information, for instance the sequence of intrusion/eruption events observed at a given volcano, can provide useful new insight into how a volcano's subsurface magma plumbing system evolved. We demonstrate this process through application of our model to Summer Coon, a well-studied stratocone on Earth, and Ilithyia Mons, a large conical shield volcano on Venus.

Elastic, gravitationally loaded finite element models of an ellipsoidal magma reservoir in mechanically layered host rock yield new insight into the effect of layer stiffness, thickness, and spatial configuration on the resulting surface displacement curve and tensile rupture of the reservoir wall. Decreasing layer stiffness enhances surface and subsurface deformation, focusing pressure across a narrow area, and generally decreases the likelihood of vertical magma intrusion and surface eruption. In contrast, increasing layer stiffness impedes uplift and distributes strain across a broad area, enhancing the likelihood of vertical magma intrusion. Application of the model to deformation that occurred between 1985 and 1999 at the Long Valley caldera, California shows that the predicted reservoir radius and depth change by 16% and 12% respectively when mechanical layers inferred to be present are included in the model, highlighting the importance of explicitly incorporating mechanical layering. Analysis of rupture conditions for the reservoirs modeled in the present paper indicates that vertical intrusions are likely. In addition, analysis of rupture conditions for Long Valley magma reservoirs recently inferred from surface displacement inversions indicates that they will fail before achieving sufficient pressure to reproduce measured surface displacement.

Results from a finite element model characterizing tensile rupture of an internally pressurized ellipsoidal magma reservoir within an axisymmetric elastic half space illustrate that gravity plays a critical role in this process. Failure to incorporate gravitational loading correctly, which is the case for most published models, affects for example: (a) application of corrections designed to account for the presence of the free surface in analytical models; (b) inferences about the internal pressure that a reservoir can sustain prior to rupture; (c) conclusions about the importance of neutral buoyancy, i.e. the relative host rock and magma density structures; and, (d) predictions about the location at which rupture of the reservoir wall will occur and the style of intrusion which will be favored. Analyses that reduce magma reservoirs to a cavity within an unloaded elastic medium, inflated by only an excess pressure component, sacrifice important information and should not be used to interpret reservoir activity or to calibrate more advanced models of volcanic regions and phenomena; an exception to this rule occurs, however, when constraining the pressure that can be inferred from surface displacements for a reservoir of known geometry. In a gravitationally loaded model, the characteristics of the failure process are insensitive to geologically plausible variations in the tensile strength, shear modulus, density structure and gravitational acceleration. As a result the half-space analysis presented here, which will benefit from future expansion to include topography and other factors, can yield insight into not only magma reservoirs on Earth but those thought to have formed within the crusts of Mars, Venus and other solar system bodies as well.

▪ Abstract Earth, Venus, and Mars all exhibit populations of giant (radiating, linear, and arcuate) mafic dike swarms hundreds to >2000 km in length. On Earth the dikes are exposed by erosion, while on Venus and Mars their presence is mainly inferred from associated volcanic morphology and surface deformation. The apparent absence of plate tectonics in the geologic record of Venus and Mars means that the observed population of swarms remains geometrically intact, while on Earth plate tectonics has fragmented swarms. About 30 giant radiating swarms have so far been identified on Earth, but with further study the number is expected to rise and may eventually coincide with the hundreds of mantle plume head events now being proposed. On Venus, at least 118 radiating swarms are distributed across the planet, and new high resolution mapping is revealing additional swarms. On Mars, up to 16 giant dike swarms are observed, most associated with the Tharsis region.

Stratigraphic study of a distributed population of 118 giant radiating dike swarms on Venus reveals that within each dike-intruded region, emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera, regional plains, and most wrinkle ridges. The density of impact craters superimposed on the swarm population (1.80±0.57 craters/10 6 km 2 ), when compared with the average global density (2.01±0.14 craters/10 6 km 2 ) and the densities reported for other geologic units, is consistent with the observed stratigraphy. On the basis of these data, we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing. The stresses recorded by the dike swarm population, when combined with its age and compared with the predictions of several proposed resurfacing models, best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer. Formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long-wavelength gravity and topography. In addition, the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long-wavelength characteristics. We interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long-wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago.

Theoretical calculations of extrusive volcanic degassing on Venus yield atmospheric pressure-related rock density profiles consistent with the formation of magma neutral buoyancy zones and magma reservoirs at different depths as a function of altitude (Head and Wilson, J. geophys. Res. 97, 3877, 1992). Global analysis of radiating dike swarms interpreted to originate at magma reservoirs show that their distribution matches these predictions across approximately 90% of the planet's surface; only those highland regions whose elevations exceed 6053 km appear anomalous. The distribution of the large volcano population (extrusive reservoir products) (Keddie and Head, Planet. Space Sci. 42, 455, 1994) has yielded similar results. Comparison between the dike swarm (intrusive) and large volcano (extrusive) populations suggests that neutral buoyancy plays an important role in governing volcanic processes near the venusian surface and that the depth to the level of neutral buoyancy increases systematically at altitudes above 6051 km.

Magellan radar data of Venus reveal 163 large radial lineament systems composed of graben, fissure, and fracture elements. On the basis of their structure, plan view geometry, and volcanic associations, at least 72% are interpreted to have formed primarily through subsurface dike swarm emplacement, the remainder through uplift or a combination of these two mechanisms. The population of swarms is used to determine regional and global stress orientation. The stress configuration recorded from 330-210 deg E (Aphrodite Terra) is best explained by isostatic compensation of existing long wavelength topography or coupling between mantle flow and the lithosphere. The rest are correlated with concentrations of rifting and volcanism in the Beta-Atla-Themis region. The global stress field on Venus is different than that of Earth, where plate boundary forces dominate.

[1] The growth and evolution of large volcanic edifices on Venus should reflect interactions between local magma reservoir-induced stresses and broader-scale stresses resulting from flexure of the lithosphere beneath the edifice load. Here, we explore the relationship between magma movement in the lithosphere and the flexural stress state via static, gravitationally loaded, axisymmetric finite element models. We find that reservoirs situated in the lower (extensional) lithosphere fail at the bottom and are therefore not viable long-term conduits for upward magma transport. Furthermore, for high-stress conditions (e.g., large edifices or thin lithospheres), chambers in the lowermost lithosphere exceed the failure criterion even before pressurization and are therefore unstable. In contrast, magma chambers located in the upper (compressional) lithosphere fail at or somewhat above the reservoir midsection, promoting lateral sill injection; continued failure in this mode would tend to produce oblate magma chambers with zones of intrusion at their margins. Reservoirs near the flexural neutral plane require the greatest overpressure to reach failure, emplacing cone sheets that transition to sills further from the chamber. The out-of-plane orientation of principal extensional stresses in the flexed lower lithosphere predicts the presence of radial dikes that are likely the main conduits for any subsequent magma ascent from the mantle melt source region. Our results also explain how the evolving stress state in the lithosphere tends to redirect magma passage over time: magma ascending into the lithosphere beneath the edifice is diverted to lateral sills in the upper lithosphere, inhibiting summit eruptions and possibly shifting eruption locations to the lower flanks at and beyond the distal margins of an oblate chamber or sill complex. We apply these results to interpret the observed structure and tectonism of Sapas Mons, Venus, in terms of flexurally controlled intrusive processes.

[1] The total volume of extrusive volcanism on the Moon provides a basic thermal and geologic constraint, and accurate volume assessments are contingent upon constraining lava flow depths. Here, employing UV-VIS data from the Clementine mission, we estimate mare thickness values in the Imbrium Basin by analyzing ejecta from large (>10 km diameter) impact craters that penetrate (or failed to penetrate) through the mare. Mare thickness values are found to range from at least ∼2 km at the basin center to 1.5–2.0 km in the buried ring shelf annulus. This corresponds to a basalt volume of ∼1.3 × 106 km3, almost a factor of two greater than the volume estimated from partially filled craters alone. Our results indicate that thickness measurements from penetrating craters, combined with minimum estimates from partially filled and non-penetrating craters, provide a more complete picture of the spatial variation of basalt thickness values than could previously be obtained.

[1] Radar bright lava flows in the Radunitsa Labyrinthus region of the Carson quadrangle on Venus traveled up to 200 km within a network of orthogonal grabens. Using a cooling model for basalt to evaluate the graben-channelized lava flow emplacement process, we demonstrate that the lava flows would need to either be very thin (