Cascadia Subduction Zone Volcanism in British Columbia

By: Doug Geller

Emporia State University

April 2008


ES 767 Global Tectonics

Table of Contents

Introduction

Plate Movement and Subduction

Cascade Volcanic Arc

Volcanism in British Columbia

Subduction Earthquakes

Summary

References


INTRODUCTION

This paper examines volcanism and tectonic processes associated with the Cascadia subduction zone located off the western coast of North America, roughly from northern California to coastal British Columbia. The region includes the spreading center of the Juan de Fuca plate, transforms, including the Mendocino system, a subduction zone, and continental arc volanism as exemplified in the system of stratovolcanos extending from Lassen Peak in California to peaks such as Mt. Garibaldi north of Vancouver, B.C. Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism that has included such notable eruptions as Mazama (Crater Lake) several thousand years ago and Mt. St. Helens in 1980. (see map, below). The Cascadia Subduction Zone extends from mid-Vancouver Island to northern California. It separates the Juan de Fuca and North America plates. New ocean floor is being created offshore of Washington and Oregon at the Juan de Fuca ridge or spreading center. As new oceanic crust is produced along the ocean ridge, the Juan de Fuca plate east of the ridge is pushed toward and under the North American plate. The Cascadia Subduction Zone is where the two plates meet. This is a seismically active zone, with information recorded here Pacific Northwest Seismograph Network

Figure source: PNSN

PLATE MOVEMENT AND SUBDUCTION

The system of plates, spreading centers, transforms and subduction zones off the northwestern coast of the U.S. and western Canada is referred to as the Cascadia Subduction Zone or sometimes called the Juan de Fuca plate system. There are actually three distinct and relatively small oceanic plates subducting under North America (from north to south): The Explorer Plate, near the northern tip of Vancouver Island, the Juan de Fuca plate, off the Washington and Oregon coasts, and the Gorda plate off the northern coast of California, north of the San Andreas-Mendocine fault and fracture zone system. Several offsetting transform faults mark the boundaries between these three plates. The oceanic crust of the Juan de Fuca plate moves away from the speading center at an average rate of approximatley 3 cm per year. There is a transition from north to south in the subduction zone geometry: to the south, the fault zone associated with subduction widens and the dip of the subducting slab steepens. The relative motion of subduction is oblique, with the motion of oceanic crust relative to continental plate having a northeasterly component.

CASCADE VOLCANIC ARC

The Cascade Volcanic Arc is a continental island arc that extends from northern California to the coastal mountains of British Columbia. The arc consists of a series of Quaternary (Pleistocene to Holocene) age stratovolcanoes that grew on top of pre-existing geologic materials that ranged from Miocene volcanics to glacial ice. The volcanos lie approximately 100 km inland from the coast, and form a north-to-south oriented chain of peaks that average over 3,000 m (10,000 ft) in elevation. The major peaks from south to north include:

The most active volcanoes in the chain include Mt. St. Helens, Mt. Baker, Lassen Peak, and Mt. Hood. St. Helens captured worldwide attention when it erupted catastrophically in 1980. St. Helens continues to rumble albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity. Most of the volcanoes have a main, central vent from which the most recent eruptions have occurred. The peaks are composed of layers of solidified andesitic to dacitic magma, and the more siliceous (and explosive) rhyolite. Descriptions of Cascade volcanoes can be found here USGS Juan de Fuca subduction volcanoes

VOLCANISM IN BRITISH COLUMBIA

The Garibaldi Volcanic Belt forms the northern (Canadian) continuation of the Cascades Volcanic Belt of the U.S. Pacific Northwest, and contains the most significant stratovolcanoes in Canada. The coastal volcanoes are also close to British Columbia's densely populated lower mainland, not unlike Mt. Hood in Oregon and Mt. Rainier in Washington. The peaks of the Garibaldi Volcanic Belt are stratovolcanoes typical of subduction zone volcanoes, and include Mt. Garibaldi, Mt. Price, the Black Tusk, Mt. Cayley, Mt. Fee, Mt. Meager and Mt. Silverthrone. Mt. Meager erupted approximately 2350 years ago and volcanologists consider this the most recent explosive type volcanic eruption in Canada. The Meager eruption was of a similar magnitude to the 1980 eruption of Mt. St. Helens in Washington. The mountainous areas around the coastal B.C. volcanoes are generally more glaciated than the neighboring peaks south of the border, and some of the volcanoes formed within or on top of glaciers and ice sheets.

Subglacial volcanic features distinguish the Garibaldi Volcanic Belt from the Cascade volcanoes south of the 49th parallel. Such glaciovolcanic features include tuyas (stacks of flat-lying lava flows erupted under glacial ice), subglacial volcanic domes, and ice marginal lava flows (Kelman et al 2002). The bulk of the Mt. Garibaldi volcanic edifice is believed to have been built on an existing ice sheet. Although not part of the Garibaldi Belt, the northern coastal volcano Mt. Hoodoo is perhaps the best example in B.C. of a flat-topped subglacial tuya volcano. Hoodoo currently has a large (3 km diameter) icecap that reaches 150 meters thick. Although the volcano is in a relatively remote area, scientists are concerned that a future eruption could catastrophically melt the icecap and cause large mudflows to surge down the pristine Iskut River valley.

View of Mt. Hoodoo and its icecap


View of Mt. Garibaldi (image source: Natural Resources Canada)

East of the Coast Mountains the Chilcotin basalts occupy a relatively broad plateau area in the dry interior of B.C. These volcanics consist of basaltic lava flow eruptions about 150 km inland from and aligned parallel to the Garibaldi Volcanic Belt. Volcanic activity in the Chilcotin is likely due to crustal thinning and extension, which is known as back-arc extension volcanism (Wood and Kienle 1990). The Chilcotin volcanic eruptions comprised two main phases, the first from 6-10 million years ago and the more recent phase occuring 2-3 million years ago, during the early part of Garibaldi belt activity. Limited lava flows from the Pleistocene have also been found in the Chilcotin. The latter eruptions may have formed in contact with glacial ice.

Chilcotin lavas eventually accumulated to a thickness of more than 2 km (1.2 mi). As the molten rock came to the surface, the earth's crust gradually sank into the space left by the rising lava. The subsided basin containing the lavas is known as the Chilcotin Plateau. The tectonic setting and the internal flow structure of the Chilcotin basalts resemble those of the more extensive and voluminous lavas of the Columbia River Basalt Group of Washington, Oregon and Idaho.

Map showing location of Chilcotin volcanic field (dark area), immediately west is the Garibaldi volcanic belt of B.C. Map

Besides a well-developed volcanic arc complex that includes stratovolcanoes and plateau basalts, B.C. also has a volcanic hot spot. The Anahim Volcanic Belt runs approximately east to west from near Bella Bella B.C. It is shown as the elongate zone on the above map, immediately north of the coastal (Garibaldi) belt and the Chilcotin lava plateau. The volcanics have erupted through continental crust as the North American Plate moved west over the hot spot. Due to this movement, the youngest volcanics are found in the eastern part of the belt at Nasko Cone (see map below) while the oldest remnants are found near the coast.

SUBDUCTION EARTHQUAKES

Subduction zone earthquakes include deep (Benioff Zone) events, with focal depths of 30-70 km and shallower crustal quakes with focal depths less than 25 km. Major deep earthquakes of magnitude 9.0 are considered possible, with the last such event occuring in January 1700, as evidenced by tsunami-deposited sands along coastal areas, Japanese records of tsunamis, native traditional (spoken) knowledge, and devegetation of former freshwater environments near the coast due to invasion of saltwater. The probable return period on the major (magnitude 9.0) deep earthquakes is on the order of 400 to 600 years. A joint Japanese-U.S.-Canada research team made the link between tsunami evidence in Japan and the 1700 Cascadia earthquake just a few years ago link to 2003 Science Daily article

Recent deep earthquakes occurred in 1949 (magnitude 7.1), 1965 (magnitude 6.5) and 2001 (magnitude 6.8). These events were felt throughout northwest Washington and southwest B.C. The most significant shallow earthquake occured on Vancouver Island in 1974 (magnitude 7.4).

Seismologists typically group subduction earthquakes into two types: the so-called megathrust earthquakes, which involve failure of a "locked" portion of the subduction zone fault across a large area and crustal quakes, which invol more focused or localized slip within the descending oceanic slab or the overlying continental crust (Geological Survey Canada 2008). Large and relatively rapid megathrust failures occur offshore and are believed to be responsible for the large tsunamis that have inundated coastal areas in prehistoric times. Also, research shows that as elastic strain builds so does the likelihood of future major rupture occuring within the subduction zone (Hyndman et al 2008).


Dead cedar trees, possibly killed by saltwater inundation following 1700 Tsunami

Cross section showing dynamics of Cascadia megathrust earthquakes (image sources: Geological Survey Canada)

SUMMARY

The Cascadia Subduction Zone lies off the west coast of North America between northern California and the northern tip of Vancouver Island in Canada. The subduction of the Juan de Fuca and related plates under the North American continent represents a highly active tectonic region adjacent to major population centers such as Vancouver, B.C. and Seattle, Washington. Inland from the subduction zone are the major volcanoes of the Cascade Volcanic Arc, including glaciated high peaks such as Mt. Rainier in Washington and Garibaldi in British Columbia. Within British Columbia, the Garibaldi Belt is an important chain of volcanoes in the coastal mountains north of Vancouver. The B.C. volcanoes are generally less active than their neighbors to the south, and characterized by glaciovolcanic features such as flat-topped tuyas. Inland from the andesitic stratovolcanoes are plateau lava basalts of the Chilcotin Plateau, which form in an extensional back-arc tectonic setting.

Subducting plate movements have produced major earthquakes in historic and pre-historic times. The so-called "megathrust" events occur offshore within the subduction fault zone and can be catastrophic (magnitude 9). The last such event occurred approximately 300 years ago. While relatively infrequent, the megathrust earthquakes have a well documented return interval of 400 to 600 years. More common earthquakes occur within subducting oceanic crust, or within overlying continental rocks, and the 2001 Nisqually earthquake in Washington State provides a recent example. Researchers believe that strain within the locked zone of the subduction fault is accumulating, and as it does so, the chances of a major subduction earthquake event increase.

REFERENCES

Weblinks and Related Presentations

Geological Survey of Canada Catalogue of Canadian Volcanoes (GSC 2008)

USGS Cascade Range Summaries

ES 767 Special Presentation on Cascade Volcanism (J.S. Aber 2008)

ES 767 Student Presentation on Northern B.C. Tectonics ( C. Siegel 2005)

Publications

Hyndman et al 2008. Giant Earthquakes Beneath Canada's West Coast. Geological Survey of Canada link to online article.

Kelman, M, J. Russell and C. Hickson, 2002. Effusive Intermediate Glaciovolcanism in the Garibaldi Volcanic Belt in S.W. British Columbia in J.Smedlie and M Chapman (eds) Volcano-Ice Interaction on Earth and Mars. Geological Survey of London Spec Pub V202, p 195-211.

Wood, C. and J. Kienle, 1990. Volcanoes of North America: U.S. and Canada. Cambridge University Press, New York, 354 p.