DESCRIPTION:
Juan de Fuca Volcanics

... In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features. ...

According to this theory, the Earth's surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth's hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes.

The boundary between the Pacific and Juan de Fuca Plates is marked by a broad submarine mountain chain about 500 kilometers long, known as the Juan de Fuca Ridge. Young volcanoes, lava flows, and hot springs were discovered in a broad valley less than 8 kilometers wide along the crest of the ridge in the 1970's. The ocean floor is spreading apart and forming new ocean crust along this valley or "rift" as hot magma from the Earth's interior is injected into the ridge and erupted at its top.

In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust is forced deep into the Earth's interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone.

The remaining part of the Pacific Plate currently converging with the American Northwest is the Juan de Fuca Plate, with small platelets at its northern (Explorer Plate) and southern Gorda Plate) terminations. The Explorer Plate separated from the Juan de Fuca approximately 4 million years ago and is apparently no longer being subducted (Hyndman, et.al., 1979); the Gorda split away between 18 and 5 million years ago (Riddihough, 1984). The present slow rate of convergence (3-4 centimeters per year) of the Juan de Fuca Plate is only about half its value at 7 million years (Riddihough, 1984), which probably explains the reduced seismicity, lack of a trench, and debatable decline in volcanic activity. ...

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[Map,11K,InlineGIF]
Juan de Fuca Ridge - Gorda Ridge - Axial Seamount -- showing features of Pacific/Juan de Fuca/North American subduction system relative to Western United States. Open blue arrows, ridge-spreading directions; solid blue arrow, convergence direction.
-- Modified from: Swanson, et.al., 1989, IGC Field Trip T106: Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon, p.2

Axial Seamount rises 700 meters (m) above the mean level of the central Juan de Fuca Ridge crest about 480 kilometers (km) west of Cannon Beach, Oregon to within about 1400 m of the sea surface. The volcano is the most magmatically robust and seismically active site on the Juan de Fuca Ridge between the Blanco Fracture Zone and the Cobb offset. The summit is marked by an unusual rectangular-shaped caldera (3 x 8 km) that lies between two rift zones. The caldera is breached to the SE and is defined on three sides by boundary faults of up to 150 m relief. Hydrothermal vents colonized with biological communities are located near the caldera fault or along the rift zones. Following the discovery of hydrothermal venting north of the caldera in 1983, a concentrated mapping and sampling effort was made in the mid-late 1980s. An eruption was detected seismically in January 1998; later mapping revealed a lava flow erupted from a 9-km-long fissure at the southern end of Axial caldera. The new lava flows were seen to partially bury a seismometer/ pressure gauge and mooring line deployed in 1997.

[Map,20K,InlineGIF]
Juan de Fuca Subduction - Juan de Fuca Ridge - Cascade Range
-- Modified from: Brantley, 1994

The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. ...

From tomography, Rasmussen and Humphreys (1988) interpret the subducted Juan de Fuca plate as a quasi-planar feature dipping about 65 degrees to about 300 kilometers under the southern Washington Cascades. The plate is poorly defined seismically, however, owing to a lack of earthquakes within it. Guffanti and Weaver (1988) show that the present volcanic front of the Washington Cascades, defined by the westernmost young vents, parallels the curved trend of the subducting plate reflected by the 60 kilometer-depth contour. The front trends northwest in northern Washington -- where Glacier Peak, Mount Baker, and the volcanoes of southern British Columbia occur along a virtually straight line -- and northeast in southern Washington. A 90-kilometer gap free of young volcanoes between Mount Rainier and Glacier Peak is landward of that part of the subducting plate with the least average dip to a depth of 60 kilometers. South of Portland (Oregon), the volcanic front is offset 50 kilometers eastward and extends southward into California, probably still parallel to the trend of the convergent margin.

Earth scientists believe that most earthquakes are caused by slow movements inside the Earth that push against the Earth's brittle, relatively thin outer layer, causing the rocks to break suddenly. This outer layer is fragmented into a number of pieces, called plates. Most earthquakes occur at the boundaries of these plates. In Washington State, the small Juan de Fuca plate off the coast of Washington, Oregon, and northern California is slowly moving eastward beneath a much larger plate that includes both the North American continent the land beneath part of the Atlantic Ocean. Plate motions in the Pacific Northwest result in shallow earthquakes widely distributed over Washington and deep earthquakes in the western parts of Washington and Oregon. The movement of the Juan de Fuca plate beneath the North America plate is in many respects similar to the movements of plates in South America, Mexico, Japan, and Alaska, where the world's largest earthquakes occur. ...

The Cascadia subduction zone off the coasts of Washington, Oregon, and northern California is a convergent boundary between the large North America plate and the small Juan de Fuca plate to the west. The Juan de Fuca plate moves northeastward and then plunges (subducts) obliquely beneath the North America plate at a rate of 3 to 4 centimeters per year.

Washington has features typical of convergent boundaries in other parts of the world:

1. A zone of deep earthquakes near the probably boundary between the Juan de Fuca plate and the North America plate. The 1949 magnitude 7.1 Olympic earthquake and the 1965 magnitude 6.5 Seattle-Tacoma earthquake occurred within this deep zone.

2. The active or recently active volcanoes of the Cascade range created by the upward migration of magma (molten rock) above the Juan de Fuca plate. Rock in the subducting plate may melt at depths of 100 kilometers or more in the Earth. Because melted rock is lighter, it can sometimes rise to the surface through weakened areas in the overlying materials.

3. Young, highly deformed mountains composed of formerly oceanic rocks scraped off the Juan de Fuca plate during subduction and piled up on the Olympic peninsula.

4. Deformed young sediments offshore in the Pacific Ocean where the converging plates meet.

In sum, the subduction of the Juan de Fuca plate beneath the North America plate is believed to directly or indirectly cause most of the earthquakes and young geologic features in Washington and Oregon.

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