Shallow plumbing systems for small-volume basaltic volcanoes

• Published in: Bulletin of volcanology Vol. 70; no. 5; pp. 563 - 582
• Main Authors: Keating, Gordon N., Valentine, Greg A., Krier, Donathon J., Perry, Frank V.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.03.2008; Springer; Springer Nature B.V
• Subjects: Basalt; Conduits; Crystalline rocks; Dikes; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Exact sciences and technology; Geology; Geophysics; Geophysics/Geodesy; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Magma; Mineralogy; Research Article; Rocks; Sedimentology; Volcanic eruptions; Volcanoes; Volcanology; United States; New Mexico; Nevada; Magmatic; Conduit modeling; Fissure eruption; Eruptive conduit; Basalt; Cinder cone; Dike; dikes; eruption dynamics; flow mechanism; North America; fissure eruptions; country rocks; basalts; sills; craters; uncertainties; volatile elements; models; volcanic rocks; field studies; host rocks; volcanoes; igneous rocks; subsurface; heterogeneity; cinder cones; vents; pyroclastics; lithostatic pressure; depth; rhyolite tuff; Dike, Magmatic, Fissure eruption; brecciation; tuff; erosion; geometry
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/s00445-007-0154-1

Eruptive dynamics in basaltic volcanoes are controlled, in part, by the conduit geometry. However, uncertainties in conduit shape and dike-to-conduit transition geometry have limited our predictive capability for hazards assessments. We characterize the subvolcanic geometry of small-volume basaltic volcanoes (magmatic volatile-driven eruptions, 0.1 to 0.5 km 3 ) based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder dikes, conduits, and vent areas ≤250 m depth. Study areas include East Grants Ridge (New Mexico, USA), Basalt Ridge, East Basalt Ridge, Paiute Ridge, and Southeast Crater Flat (Nevada, USA). Basaltic feeder dikes 250 to 100 m deep have typical widths of 4–12 m, with smooth host-rock contacts (rhyolite tuff). At depths less than 100 m, heterogeneities in the host rock form preferential pathways for small dike splays and sills, resulting in a 30-m effective width at 50 m depth. The development of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country rock. The overall zone of effect at depths less than 50 m is ≤110 m wide (220 m elongated along the feeder dike). Based on comparisons with theoretical conduit flow models, the width of the feeder dike at depths from 250 to 500 m is expected to range from 1 to 10 m and is expected to decrease to about 1–2 m at depths greater than 500 m. The flaring shape of the observed feeder systems is similar to results of theoretical modeling using lithostatic pressure-balanced flow conditions. Sizes of observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface (<50 m depth), but at depths greater than 100 m the difference is a factor of 2 to 4. This difference is primarily due to the fact that observed eroded conduits record the superimposed effects of multiple eruptive events, while theoretical model results define dimensions necessary for a single, steady eruption phase. The complex details of magma-host rock interactions observed at the study areas (contact welding, brecciation, bifurcating dikes and sills, and stoping) represent the mechanisms by which the lithostatic pressure-balanced geometry is attained. The similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness of the pressure-balanced modeling approach, consistent with the conclusions of Wilson and Head (J Geophys Res 86:2971–3001, 1981) for this type of volcano.