The Gorda Ridge (41°36'19.6"N 127°22'03.1"W), aka ''Gorda Ridges''

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

spreading center A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...

, is located roughly off the northern coast of

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

and southern

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

. Running NE – SW it is roughly in length.Carey, Stein, & Rona. (1990). Benthos of the Gorda Ridge axial valley (NE Pacific Ocean): Taxonomic composition and trends in distribution. Progress in Oceanography, 24(1), 47–57. The ridge is broken into three segments;Sverdrup, K. (1986). Multiple-event relocation of earthquakes on and near the Gorda Ridge. Geophysical Research Letters., 13(7), 674–677. the northern ridge, central ridge, and the southern ridge, which contains the Escanaba Trough.

Regional setting

The Gorda Ridge runs in a north-easterly direction, bounded at both ends by

transform faults A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subductio ...

. At the southern end, the ridge meets the Mendocino transform fault, while the northern end butts against the Blanco transform fault. To its east is the

Gorda Plate The Gorda Plate, located beneath the Pacific Ocean off the coast of northern California, is one of the northern remnants of the Farallon Plate. It is sometimes referred to (by, for example, publications from the USGS Earthquake Hazards Program) as ...

, which together with the Juan de Fuca Plate to its north, is what remains of the once-vast

Farallon Plate The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west c ...

. These two oceanic plates are currently moving east, subducting underneath the

North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...

in what is known as the

Cascadia Subduction Zone The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, and ...

. To the west and south of the ridge is the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

, which is currently moving west diverging from the Gorda Plate. The divergence of the Pacific Plate and the Gorda Plate has led to the creation of the Gorda Ridge. On the Pacific Plate, roughly 50 km west of the northern portion of the ridge, sit eight

seamounts A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abru ...

(underwater volcanoes), known as the

President Jackson Seamounts The President Jackson Seamounts are a series of seamounts ( underwater volcanoes) located on the Pacific Plate, off of California. It consists of 8 seamounts, 4 independent and 4 morpohologically fused, just west of the northern Gorda Ridge. The ...

Geological history

Roughly 30 Mya the Farallon Plate subducted beneath the North American Plate, segmenting the Pacific Farallon Ridge. This subduction created new microplates and new ridges, including the Juan de Fuca Plate and Juan de Fuca Ridge. As the Juan de Fuca Plate continued to subduct underneath the North American Plate it also segmented, creating the Gorda Plate and Gorda Ridge.

Spreading rate

The

is moving in a northwest direction, creating a divergence with the Gorda Plate at a speed of 5 cm per year. The Juan de Fuca Plate (including the

) is moving east-northeast, subducting under the North America Plate at a much slower rate of 2.5–3 cm per year. Due to the ridge being segmented into three distinct parts, each section has its own spreading rate, caused by the slab-pull and ridge-push of the surrounding

tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

. The northern segment is the narrowest, with portions as narrow as 3 km across, and has the fastest spreading rate of 2.9 cm per year (half-rate). The central segment is roughly 10 km wide with a spreading rate of 2.4 cm per year (half-rate). The southern segment has sections as wide as 18 km, and has the slowest spreading rate of 1.2 cm per year (half-rate). Juan de fuca plate

Seismicity

Due to the Gorda Ridge's proximity to the

Mendocino Triple Junction The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the ...

, the area experiences a significant amount of seismic activity. The majority of activity is seen on the Gorda Plate, however some occurs on the ridge itself. Most events are generated by the divergence of the Pacific Plate and the Gorda Plate. Since 1983 there have been approximately 80 magnitude 3 earthquakes happening at this location every year.

1996 eruption

On 28 February 1996 the northern segment of the Gorda Ridge (42 40'N 126 48'W) experienced a burst of seismic activity, which lasted roughly three weeks.Chadwick, Embley, & Shank. (1998). The 1996 Gorda Ridge eruption: Geologic mapping, sidescan sonar, and SeaBeam comparison results. Deep-Sea Research Part II, 45(12), 2547–2569. Concurrent with seismicity were a series of slow volcanic eruptions (1–10 m³/sec), forming thick flows of

pillow basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

. These pillow basalts are thickest to the north, indicating this region's activity lasted longer than the other portions of the ridge. The estimated volume of erupted magma during this event is 18x10⁶ m³, forming a blanket of new oceanic crust, averaging 75 m thick.

Axial Valley

Unlike other intermediate spreading centers, the Gorda Ridge has a large rift valley, which is typically seen in areas of slow spreading centers. This is caused by the oceanic crust beneath the ridge being thinner and mantle temperatures being cooler than most intermediate spreading centers. The Gorda Ridge has an average depth of 3000 m, with a few locations reaching depths of 3500 m. The walls of this valley are steep, in most cases giving a vertical relief of over 1000 m. The floor of the southern ridge valley has been filled in with roughly 1000 m of sediment from the

continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...

, mostly delivered by turbidity currents. The central ridge valley contains exposed basalt, and the northern ridge valley has a light sediment covering.

References

{{Tectonic plates Plate tectonics Oregon Coast Underwater ridges of the Pacific Ocean Geologic provinces of California Geology of Oregon