Cascadia Subduction Zone

Subduction zones produce some of Earth’s most devastating geological events. Recent eruptions of Mount St. Helens and great earthquakes and tsunamis in Japan and Sumatra provide stark examples of the destructive power of subduction-related hazards. In the Cascadia subduction zone, large earthquakes, tsunamis, and volcanic eruptions have occurred in the past and geologic records imply that these events will occur in the future. As the population and infrastructure increase in the region, resilience to these natural hazards requires a detailed scientific understanding of the geologic forces and processes involved, combined with a society motivated to mitigate risks.

The Cascadia subduction zone continuously shapes the landscape of the Pacific Northwest of North America and the cultures of its inhabitants. The impacts of subduction processes on Pacific Northwest societies and cultures are varied, but Native Americans and European settler cultures alike have described geological processes through oral histories and have relied on resources provided by the subduction zone. Indigenous peoples focus many aspects of their religious practices and art around the geohazards of the Cascadia region, and our melded modern cultures continue to take part in storytelling related to subduction zone hazards through movies and other forms of narration.

Volcanic activity is dictated by crustal pathways and reservoirs through which magma ascends and collects. The Cascades are a natural laboratory to study the influence of the crust on erupted magmas and subterranean reservoirs. The interaction of ascending magmas with the variable subduction geometry and tectonics of the overriding North American plate has given rise to a diversity of magmatic storage conditions. The confluence of geochemical and geophysical investigations emphasizes that most magmatic systems in the Cascades have been built at multiple levels in the crust, as determined by tectonics, pre-existing structure, and magmatic flux from the mantle.

The Cascade arc has produced a remarkable diversity of volcanic rocks over the Quaternary period. The major stratovolcanoes that define the arc front are dominated by eruptions of andesitic and dacitic intermediate magmas, produced largely by fractionation, melting, assimilation, and mixing within the crust. In addition, relative to many other subduction zones, the arc has produced significant mafic volcanism. These more primitive magmas reveal complexity in mantle wedge dynamics, sources, and magma production processes, and suggest that there are significant differences along the arc in the amount of magma that enters the lower Cascade crust from the underlying mantle.

The Cascadia subduction zone, where the young and thin oceanic Juan de Fuca plate sinks beneath western North America, represents a thermally hot endmember of global subduction systems. Cascadia exhibits complex and three-dimensional heterogeneities including variable coupling between the overriding and downgoing plates, the amount of water carried within and released by the oceanic plate, flow patterns within the mantle wedge and backarc, and the continuity and depth extent of the subducting slab. While recent research has benefitted from extensive onshore and offshore deployments of geophysical instrumentation, a consensus on many important aspects of Cascadia’s magmatic, tectonic, and geodynamic setting remains elusive.

The well-studied Cascadia subduction zone has enriched our general understanding of global subduction zones. This Elements issue explores the interconnected set of processes that link geodynamics, tectonics, and magmatism at depth and the surface expressions of these processes, which shape the landscape and give rise to natural hazards in the Cascadia region. This issue also addresses the impact of subduction zone processes on human populations using cultural records, and reviews the state of knowledge of Cascadia while highlighting some key outstanding research questions.