THE HYDROGEOLOGY OF BASALTIC TERRANE

Basalt flows are the most permeable rocks widely exposed at the Earth’s surface. Permeability of young, unaltered basalt flows is on the order of 10^-11 to 10^-9 m² in geologic settings as diverse as the flanks of the mid-ocean ridge (Stein and Fisher, G-Cubed, 2003), ocean islands (e.g. Hawaii: Ingebritsen and Scholl, Geothermics, 1993), and continental volcanic arcs (e.g. the Oregon Cascade Range: Manga, WRR, 1996, 1997). Consequences of this high near-surface permeability include great depth to water table (100s of m on Kilauea volcano or the High Lava Plains of Oregon), temperatures and heat flow much lower than would be predicted from conduction-only models, and dynamic groundwater-surface water interaction, with high recharge rates, large springs, and abrupt losses and gains in streamflow. Recharge rates are as large as several m/yr in areas of high precipitation such as the windward sides of the Hawaiian Islands. Vertical downflow fluxes of only a few cm per year are sufficient to entirely suppress near-surface heat flow, so that large areas of the mid-ocean ridge, Kilauea caldera (Hawaii), and the Quaternary volcanic fields of the continents are characterized by near-zero near-surface conductive heat flow, despite being underlain by magmatic heat sources. Much of the heat is advected laterally and eventually discharged by large springs km to tens of km distant – half of the large (≥100 cfs) springs of the conterminous United States (as inventoried by O. E. Meinzer, USGS WSP 557,1927) occur in volcanic terrane of Oregon, northern California, and southern Idaho. All of these characteristics testify to the generally high permeability of basalts, but basalt permeability is also heterogeneous at a range of scales, varying by 10⁴ or more between massive flow centers and interflow breccias, between lava flow and cross-cutting dikes, and between the surface and ~1 km depth in (for instance) the rift zones of Kilauea. Thus basaltic terrane is also often characterized by perched and “stair-stepped” water-table configurations and complex patterns of solute transport. Further, basalts exposed to high temperatures (~>50^oC) and consequent hydrothermal alteration lose most of their initial permeability (e.g. Blackwell, DOGAMI OFR O-94-07, 1994); the permeability of such basalts is low-to-moderate, and depends mainly on secondary characteristics such as the degree of fracturing.