ABSTRACT The current tectonic framework for understanding Cenozoic magmatism in western North America was laid out in a series of influential papers in the early days of the plate-tectonics revolution. These ideas, largely developed through deductive analysis, were so revolutionary yet seemingly self-evident that they quickly passed from hypothesis to axiom. These include the following. (1) Inboard and outboard sweeps of magmatism resulted from shallowing and then rapid steepening of a subducted slab. (2) The Oligocene–Miocene ignimbrite flareup resulted from sinking and rollback of a shallow slab. (3) Late Cenozoic basaltic magmatism resulted from opening of a slab window. (4) The current Cascade arc is the remnant of a much more continuous ancestral arc that ran the length of western North America. When tested against current databases of igneous rock ages and chemical analyses, these conjectures largely fail; some are clearly contradicted, whereas others are possible but ad hoc and unfalsifiable. Ironically, the plate-tectonics revolution nicely explains plate-boundary magmatism in much of the world but is less successful in western North America, where many of these links were first developed. It is time for a second revolution.

ABSTRACT The Columbia River Gorge is the Columbia River’s long-held yet evolving passage through the volcanic arc of the Cascade Range. The globally unique setting of a continental-scale river bisecting an active volcanic arc at the leading edge of a major plate boundary creates a remarkable setting where dynamic volcanic and tectonic processes interact with diverse and energetic fluvial processes. This three-day field trip explores several elements of the gorge and its remarkable geologic history—cast here as a contest between regional tectonic and volcanic processes building and displacing landscapes, and the relentless power of the Columbia River striving to maintain a smooth passage to the sea. DEDICATION Dedicated to Russell C. Evarts (7 April 1947–11 July 2017) and his contributions to Pacific Northwest geology. Russ Evarts devoted most of his 30-year career with the U.S. Geological Survey to geologic mapping of Oregon and Washington. His thorough geologic mapping of the near-vertical terrain of the western Columbia River Gorge underpins much of what is reported in this guide and continues to inspire our studies of the geology of the Pacific Northwest.