SIMULTANEOUS MAGMATIC/HYDROMAGMATIC ERUPTIONS REVEAL MAGMA INTERACTION WITH HETEROGENEOUS AQUIFERS

Simultaneous magmatic and hydromagmatic eruptions from adjacent vents in continental settings are very rare in the historical record. Three Neogene examples of interbedded magmatic and hydromagmatic eruptive products illustrate the impact of aquifer heterogeneity on eruption style. (1) The 5.3 Ma Steamboat Rock fissure eruption (Deschutes basin, Oregon) records a 12-km-long dike that fed simultaneous Hawaiian and hydromagmatic eruptions, each focused along adjacent en-echelon segments. Hydromagmatic-explosion breccia is dominated by basalt blocks from a modern, spatially restricted, high-discharge perched aquifer that apparently was not intersected by the dike segment that fed the magmatic eruptions. (2) Simultaneous basaltic eruptions from three, 2.7 Ma, fault-aligned vents at Cochiti Dam (Santo Domingo basin, Rio Grande rift, New Mexico) produced two tuff rings and a shield volcano over a distance of 4 km. The dike penetrated a facies boundary in the rift-basin fill with hydromagmatic eruptions occurring where magma moved through high-hydraulic-conductivity (K) axial-river gravel, and Hawaiian eruptions occurred where magma ascended through low-K piedmont sand and silt. (3) A sub-plinian eruption at 6.96 Ma from the Bearhead Peak rhyolitic center (Jemez Mountains, New Mexico) generated ignimbrites with >680^oC emplacement temperatures (from thermal demagnetization data). The upper part of the ignimbrite sequence is interbedded with cold-emplaced hydromagmatic tuffs that formed an 8-km diameter tuff ring, centered 7 km from Bearhead Peak. Contrasting accessory ejecta show that the magma feeding the hydromagmatic eruption explosively excavated a gravelly sedimentary aquifer whereas the magmatic eruption occurred through older volcanic rocks. The hydromagmatic eruptions at Steamboat Rock and near Bearhead Peak culminated in magmatic extrusion of lava shields and a lava dome, respectively. Simultaneous hydromagmatic/magmatic activity reflects lateral variations in substrate porosity. Hydromagmatism is driven by magma interaction with, and steam ejection of, water in storage; groundwater-flow rates are insufficient to maintain persistent hydromagmatic activity after this initial interaction.