Pacific Northwest Geologic Mapping: Northern Pacific Border, Cascades and Columbia

Science Center Objects

The Pacific Northwest is an area created by active and complex geological processes. On its path to the Pacific Ocean, the Columbia River slices through a chain of active volcanoes located along the western margin of the U.S. in Washington, Oregon, and northern California. These volcanoes rest above the active Cascadia subduction zone, which is the boundary where the oceanic tectonic plate dives beneath the continental plate. Consequently, this area with urban centers and transportation networks is subject to earthquakes, volcanic activity, landslides, and floods. Geologic mapping and research supports resource assessments, the understanding of natural hazards, the delineation of ecosystems, and defines the framework geology of this unique region. 

The primary focus of this project is geologic mapping. Most mapping is conducted at the 7.5' quadrangle scale but in many instances these maps are compiled into regional compilations such as the Portland basin map. The focus areas for mapping are strategically chosen based on the overall project objectives but also in consultation with other USGS researchers, universities, State and local agencies, and the private sector.

Project tasks:

Cascadia Linkages 

Physiographic Cascadia

Cascadia encompasses the volcanoes of the Cascade Range and the subduction zone that feeds them. The region is home to nearly 10 million people. Where and how these people live, work, and play are influenced by the shape, or physiography, of Cascadia. This map is a tool for visualizing and interpreting the physiography of Cascadia.

(Public domain.)

The term “Cascadia” encompasses the volcanoes of the Cascade Range and the subduction zone that feeds them. This area in the Pacific Northwest is a region of significant seismic hazards, and much is unknown about the potential size and magnitude of earthquake ruptures and the effects of earthquake shaking. This translates into major uncertainties in earthquake hazard assessments of the U.S. Pacific Northwest that can lead to ineffective preparedness measures. 

The primary objectives of this task are to evaluate upper-plate deformation in the Cascadia forearc (i.e., the area between the Cascade Range and the oceanic subduction zone), determine its linkages to the active Cascadia subduction zone, and quantify the associated seismic, tsunami, and landslide hazards. 

A photograph of the Columbia River within the Columbia River Gorge National Scenic Area, Oregon

A view west of the western Columbia River Gorge National Scenic Area from near Cascade Locks, Oregon. The toe of the Bonneville landslide is on the right, which blocked the Columbia River about 500 years ago.

(Credit: Jim E. O'Connor, USGS. Public domain.)

Columbia River Corridor Mapping 

The lower Columbia River corridor hosts most of Oregon’s population and is an internationally vital transportation link connecting the interior United States with Trans-Pacific trading partners. The Columbia River is unique; it is by far the largest river in the world to bisect an active arc (i.e., volcanoes of the Cascade Range) which forms above a subduction zone. Vital infrastructure, including interstate highways, hydropower dams, rail lines, natural gas and petroleum pipelines, electrical-power transmission lines, and fiber-optic communications cables, are all routed through the Columbia River Gorge, where the river bisects the Cascade Range. In this narrow gorge, infrastructure is vulnerable to a variety of hazards: earthquakes, volcanoes, landslides, and floods. As urbanization expands into these areas, increasing conflicts arise with resource extraction (water, forestry, fisheries, aggregate resources, hydroelectric power), recreational activities, and natural hazards. 

The primary objective of this task is to understand the Columbia River’s recent history and basin evolution by providing a synoptic geologic framework based on 7.5-minute scale mapping of this region.

Columbia Basin Landscape Evolution 

The Columbia River and the tributaries that feed it have evolved dramatically over recent geologic time, beginning with the large Miocene flows of the Columbia River Basalt Group (CRBG) that filled a pre-existing topographic basin east of the Cascade Range. Deformed sedimentary strata of the Ringold, Palouse, and other formations blanket the basalt. River incision has carved through these units. Recent glaciations have left a legacy of temporary lakes, massive outburst floods, and deposits of glacial sediments and landforms. 

Photograph of the Columbia River flowing through the Pasco Basin, Washington

The Columbia River flowing through the Hanford reach of the arid Columbia River basin, eastern Washington. This reach is one of the few undammed segments of the Columbia River within the United States. Photograph taken from the White Bluffs, composed of sediment deposited in late Miocene and Pliocene Lake Ringold which occupied the Pasco Basin.

(Credit: Jim E. O'Connor, USGS. Public domain.)

The primary objective of this task is to understand the overall geologic evolution of structures (faults and folds) and landforms resulting from glacial retreat and floods from ice-dammed lakes the Columbia Basin, including the forces and events driving basin integration and river pattern development; the topographic, geologic, and ecologic effects of Quaternary ice sheets and associated megafloods; and the relations of these driving forces to regional hazards, resources, and ecosystems.  

Evolution of the Cascade Range 

The Cascade Range in Washington, Oregon, and northern California is comprised of dozens of iconic and active stratovolcanoes such as Mount St. Helens and Mount Hood, all forming above an active subduction zone. Yet the volcanoes are only one element of this continental scale mountain range which has had a 40-million-year history of crustal deformation, vertical uplift, volcanism, and erosion. The modern Cascade Range plays a critical role in regional climate and weather patterns, distribution of mineral and water resources, ecosystems, and the types and magnitudes of natural hazards.   

This primary objective of this task is to understand the past and ongoing drivers for the growth and evolution of the Cascade Range and how these factors relate to resources, hazards, and ecosystems. 

Map showing tectonic plate boundaries in the Pacific northwest

Subduction in Cascadia has been underway for about 35 million years. It has produced a crustal architecture known from subduction zones around the world: an accretionary prism, extending from the deformation front into the continental shelf; a fore-arc region; a volcanic arc; and a back-arc region. These features are developed within a pre-existing continental framework, are modified by motion along the continental margin, and are overprinted by other plate boundary regimes as the extent of the subduction zone changes through time.

(Public domain.)

 

Columbia River Basalt Group Stratigraphy and Deformation 

The Columbia River Basalt Group (CRBG) is the youngest large igneous flood basalt province on Earth and covers an area of ~160,000 km2, mostly in eastern Washington and Oregon, and western Idaho. Within this area, known as the Columbia Plateau or Columbia Basin, the CRBG is up to 2 km thick. Individual lava flows extend throughout this region and across the Willamette Valley and Coast Ranges to the west, where some flows reach the Pacific Ocean. The Grande Ronde Basalt (GRB), which makes up ~85% by volume of the CRBG, hosts the primary aquifer systems in the region, particularly in the arid Columbia Plateau province. This region is seismically active and includes numerous dams along the Columbia River as well as the Hanford nuclear production complex (mostly decommissioned) that houses large quantities of high-level radioactive waste. 

The primary objective of this task is to use geochemical and paleomagnetic techniques to map the stratigraphy of the basalt flows of the CRBG and GRB. An improved map will support hydrogeological models and studies to assess the potential for carbon dioxide sequestration in porous volcanic rocks.

Photograph of scientists standing on rocks above a steep canyon

An enigmatic outcrop of 5 million year old basalt sits 300 meters above the Deschutes and Columbia rivers. USGS scientists are carefully studying this basalt flow to determine if it came all the way from central Oregon, 160 kilometers away.

(Credit: Anthony Pivarunas, USGS. Public domain.)