Intellectual merit. Volcanoes that form in intraplate fields, although small and (mainly) monogenetic, result from eruptive phenomena ranging from quiet effusion of lava to relatively explosive Strombolian activity and, if external water interacts with ascending magma, hydrovolcanic activity. The generation and eruption of magmas in relatively small batches over dispersed areas, often with evidence for direct ascent from the upper mantle to the surface, suggests a driving process that is hybrid between strong hotspot-style mantle upwelling and scattered pockets of incipient melt that are passively mobilized by tectonic deformation (high- and low-magma flux end members of volcanic fields, respectively).
Integrated approaches involving physical volcanology, petrology, and geochemistry can provide important insights into these processes and the links between magma dynamics at depth and eruption processes on the surface. The lack of long-lived crustal magma reservoirs) allows investigation of the influences of deep magma source(s) and shallow plumbing systems on eruption styles. It is hypothesized that eruption style is influenced by physical and chemical characteristics of magma sources. To test this hypothesis comprehensive data will be integrated from the well-exposed Lunar Crater Volcanic Field (central Nevada). Direct studies of eruptive facies, of exposed shallow conduits (at older, eroded volcanoes), and of abundant mantle xenoliths will provide insights into the magma sources and ascent processes. Existing data on broad geochemicsl trends and age relationships will be integrated with new EarthScope seismic tomography data to provide a framework for understanding: (1) the interplay between pre-existing structure, topography, and vent location; (2) shallow plumbing geometries; (3) shallow controls on magmatic eruption styles, including relationships between eruption style, clast texture and shape, volatile content, and mineral chemistry; (4) spatial and temporal variations in magma sources and magma differentiation processes at individual volcanoes and across the field as a whole; (5) depth of melting and volatile contents of parent magmas; and (6) correspondence between volcano location, melting depths, and upper mantle seismic structure.
Broader impacts. This work will support improvements in volcanic risk assessment, both in terms of the probability (volcano timing and location) of future events and in terms of their consequences (related to eruption processes). The project will also support the training of three Ph.D. students (two of whom are female minorities) and will have a component of international collaboration with the volcanology and geochemistry group at the Universidad Autónoma de México. The proposing team is in communication both with the Shoshone Tribe (Duckwater Reservation) and the Bureau of Land Management so that we can share our results with them and (through BLM) with the public.
The purpose of this project was to improve our understanding of a type of volcanism – referred to as volcanic field volcanism - that produces large areas with scattered small volcanoes, compared to single large volcanic cones or shields. Most large volcanoes have many eruptive episodes over lifetimes of hundreds of thousands of years. Volcanic fields have similar lifetimes, but the individual volcanoes within them typically only each have a single eruptive episode that can last for a month to a few decades. Also, volcanic fields occur in all types of geologic settings, and are often not linked to the best-understood causes of volcanism, namely subduction zones, hot spots, or mid-ocean ridges. The Lunar Crater Volcanic Field lies in central Nevada, and contains more than 100 small volcanoes scattered over an area of about 1000 square kilometers. We analysed the composition of nearby volcanoes. The compositions constrain the type of rocks in the mantle that melted to produce the volcanoes. Our data indicate that the source varied in composition over very small scales of hundreds of meters. It is likely that this variability, which would cause variability in the amount of magma present at depth, plays a role in how the magmas (molten rock) are collected at depth, prior to their ascent through the crust. Compositional data from individual minerals within the erupted magmas show that older volcanoes were fed by magma that rose relatively quickly through the Earth’s crust, while the magmas that fed younger volcanoes stalled in the middle of the crust at depths of about 15 km. In a volcanic field where magmas are rising quickly without stalling, monitoring may not give much warning time, reducing our ability to mitigate hazards. Volcanoes within the Lunar Crater field tend to form in clusters. This is consistent with the geochemical data that suggest there are variations in the amount of magma present at depth. Understanding the origin of these clusters can help with forecasts of likely future eruption locations. Clusters form over areas where there is a larger fraction of magma that is sustained for tens or hundreds of thousands of years. The magmas rise through fractures, which they generate themselves, until they reach the Earth’s surface. Initial eruptions may occur along fissures, but eruptions tend to focus into points and cones build up around these points. Our field studies suggest that the focusing of magma occurs mainly in the upper few hundred meters of the crust, and especially in the upper tens of meters, to form conduit-like structures. As a volcanic cone builds up over one of these points, it sometimes may "leak" through fractures that radiate from the cone’s center, which we described in detail in an eroded volcano in the southern part of the study area. These "leaks" typically feed lava flows, while explosions mainly happen at the cone’s summit. In a small percentage of cases, rising magma encounters groundwater. Rapid heating of the groundwater can cause violent explosions, which our studies indicate are repeated tens or hundreds of times as magma continues to rise into the wet environment. The resulting eruptions produce craters, rather than cone, and could be a major hazard in populated volcanic fields. Our studies show that such explosive activity can occur even after a cone has already formed, rather than only before cone-forming activity as many researchers have assumed. This adds to the unpredictability, and hazard, of volcanic fields.
