THE GEODETIC CHARACTER OF THE CASCADE ARC (THERE’S MORE HERE THAN YOU MIGHT THINK)

Based on the two most recent eruptive episodes of Mount St. Helens (1980–86 and 2004–08), it is tempting to speculate that Cascade volcanoes do not show signs of deep-seated deformation prior to eruption, and the only pre-eruptive deformation is localized to the vent area immediately before or during eruption. Geodetic data collected throughout the arc over several decades, however, tell a different story. Of the 13 major volcanic centers in the arc, 5 are or have recently been deforming (Mount Baker, Mount St. Helens, South Sister, Medicine Lake, and Lassen Volcanic Center), 5 have shown no evidence of deformation since the 1980s (Mount Rainier, Mount Hood, Newberry Volcano, Crater Lake, and Mount Shasta), and 3 do not have the ground-based observations needed to assess their deformation state (Glacier Peak, Mount Adams, and Mount Jefferson). In addition, gravity changes have been detected at 2 of the 3 volcanoes where measurements have been repeated (Mount St. Helens and Mount Baker show changes, South Sister does not). With the exception of localized pre- and co-eruptive displacements at Mount St. Helens, deforming Cascade volcanoes are characterized by low displacement rates, in the range of mm to a few cm per year, that are overprinted by tectonic motions of several cm/yr. Tracking such deformation is best done via continuous GPS networks, which provide a means of recognizing tectonic signals so that they may be isolated from volcanic sources. Repeated deployments of semipermanent or campaign GPS stations can be used to densify continuous networks and improve signal to noise, and spatial resolution can be improved with InSAR observations. Mechanisms of geodetic change at Cascade volcanoes include magma accumulation/withdrawal, cooling of recent deposits, magmatic-tectonic interactions, post-eruptive groundwater accumulation, and loss of volatiles plus densification of magma. The Cascade Range thus offers an outstanding opportunity for investigating a wide range of volcanic processes, including how volcanoes work before, during, and after eruptions, and how tectonic and magmatic processes are co-manifested. There may also be areas of geodetic change that have yet to be discovered, perhaps in areas of distributed mafic volcanism between the major volcanic centers that have been the traditional focus of study.