THE COMPLEXITY OF CAVE ORIGIN AND KARST PROCESS IN THE PACIFIC NORTHWEST

Cave science in the Pacific Northwest (PNW) of the US and Canada is often overlooked given the paucity of carbonate but are a continuum of time scale, lithology, and critical zone process. State and federally managed parks and forests in the PNW host extensive karst resources. This presentation summarizes the types of PNW caves, the explorers who document them, and the connection with indigenous peoples.

In contrast to the vast expanses of low-lying Paleozoic carbonate east of the Rockies, metamorphosed and highly tectonized carbonates in the PNW represent portions of the Mesozoic margin of the North American craton or accreted terranes. Well-known caves in the Marble Mountains, at Oregon Caves National Monument, and near Horne Lake on Vancouver Island have a particularly long history of exploration and study. New exploration east in the Rockies has uncovered deep caves, such as Bisaro Anima (683 m) and Tears of the Turtle (626 m).

Layered on this backdrop are Miocene-present igneous provinces from Cascade volcanism and flood basalts. Lava tubes are frequent in the basalt flows of some Cascade volcanoes. The complex associations of terrane and volcanism in a precipitation-rich landscape of the Cascades leads to speleogenesis that includes tree molds and soil piping in ash and lahars beneath and between individual flows. In Washington alone, cave surveys document more than 130 km of these caves. The organization and structure of this volcanogenic karst convey important information on paleo-landscape and flow dynamics.

As a figurative “icing” on this complex geology are glaciers that mantle some Cascade volcanoes. The interplay between ice accumulation and ablation result in conduits in these glaciers that transmit water and heat. In valley glaciers, sub-glacial conduits move melt and thermal groundwater downslope along the ice-substrate boundary. Inside volcanic craters, volcanic fumaroles melt pockets in glacial ice that convey steam and volcanic gas by advection to crevasses and the ice margin. Glaciovolcanic karst is ephemeral—as glaciers have retreated, sub-glacial conduits on Mounts Rainer (Paradise Ice Cave, >22 km) and Hood (Snow Dragon System. 2.2 km) have disappeared. In contrast, fumarole ice caves in the craters of Mounts Rainer (3.9 km) and Saint Helens (2.3 km) have remained stable or grown, respectively.