The Cocos Ridge is an aseismic ridge within the Cocos plate that runs northeastwards from just north of the

Galápagos islands The Galápagos Islands () are an archipelago of volcanic islands in the Eastern Pacific, located around the equator, west of the mainland of South America. They form the Galápagos Province of the Republic of Ecuador, with a population of sli ...

to the

Middle America Trench The Middle America Trench is a major subduction zone, an oceanic trench in the eastern Pacific Ocean off the southwestern coast of Middle America, stretching from central Mexico to Costa Rica. The trench is 1,700 miles (2,750 km) long an ...

offshore

Panama Panama, officially the Republic of Panama, is a country in Latin America at the southern end of Central America, bordering South America. It is bordered by Costa Rica to the west, Colombia to the southeast, the Caribbean Sea to the north, and ...

. It records the effects of the

Galápagos hotspot The Galápagos hotspot is a volcanic hotspot in the East Pacific Ocean responsible for the creation of the Galápagos Islands as well as three major aseismic ridge systems, Carnegie, Cocos and Malpelo which are on two tectonic plates. The hots ...

on the Cocos plate since the establishment of the

Cocos–Nazca spreading centre The Cocos–Nazca spreading centre (CNSC, Cocos–Nazca spreading system) is the divergent boundary between the Cocos and Nazca plates. It extends from close to the East Pacific Rise northwest of the Galápagos Islands, to the southeastern end o ...

during the break-up of the Farallon plate towards the end of the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

. It is the counterpart to the

Carnegie Ridge The Carnegie Ridge is an aseismic ridge on the Nazca plate that is being Subduction, subducted beneath the South American plate. The ridge is thought to be a result of the passage of the Nazca Plate over the Galápagos hotspot. It is named for th ...

, which developed on the

Nazca plate The Nazca plate or Nasca plate, named after the Nazca region of southern Peru, is an oceanic list of tectonic plates, tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru– ...

Cocos Island Cocos Island () is a volcanic island in the Pacific Ocean administered by Costa Rica, approximately southwest of the Costa Rican mainland. It constitutes the 11th of the 15 districts of Puntarenas Canton of the Puntarenas Province, Province of ...

is the only emergent part of the ridge. The volcanic activity that formed the island is much younger than the part of the ridge where it is located.

Extent

The Cocos Ridge is a ~1000 km long bathymetric high, varying in width up to 200 km. It lies entirely within the Cocos plate. The presence of a shallow area between the Galápagos islands and Cocos Island was first identified by

Alexander Agassiz Alexander Emmanuel Rodolphe Agassiz (December 17, 1835March 27, 1910), son of Louis Agassiz and stepson of Elizabeth Cabot Agassiz, was an American scientist and engineer. Biography Agassiz was born in Neuchâtel, Switzerland, and immigrated t ...

in 1892, naming it the Galapagos Plateau. After the acquisition of more depth soundings it became clear that the shallow area continued almost up to the coast and it was given the name Cocos Ridge in 1950, for Cocos Island. The ridge is typically about 2–3000 m shallower than the nearby areas of the Cocos plate. On the northwestern flank of the ridge there is a broad area in which many seamounts are developed. Far fewer seamounts are found on the crest or the southeastern flank giving the ridge an overall asymmetric geometry. This asymmetry is also expressed in the chemistry of the lavas found, with the northwestern seamounts having an

ocean island basalt Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified ...

composition, while lavas on the southeastern flank have compositions more typical of

mid-ocean ridge A mid-ocean ridge (MOR) is a undersea mountain range, 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 ...

basalts.

Development

The ridge began to form immediately after the establishment of the Cocos–Nazca spreading centre in the late Oligocene. Initially (at about 23 Ma) the spreading centre is interpreted to have had a relatively simple SW–NE trend, with the hotspot lying just on its northwestern side, starting the formation of the Cocos Ridge. By 19.5 Ma, the spreading centre was now north of the hotspot and, during the next 5 million years, the spreading centre reorganised, with the spreading elements rotating clockwise, trending west-east. During this period the hotspot mainly lay to the south of the spreading centre on the Nazca plate, forming the broad part of the Carnegie Ridge that is now subducting beneath the South America plate. From 14.5 Ma to 12 Ma the spreading centre moved south so that the Nazca plate was less affected, causing narrowing of the Carnegie Ridge at this time. This was in contrast a major growth period for the Cocos Ridge. At this time the Malpelo Ridge began to rift away from the Carnegie Ridge. After 12 Ma the spreading centre began to move slowly northwards. At about 9.5 Ma, spreading ceased between the Malpelo and Carnegie ridges. At about 3.5 Ma a new spreading centre developed briefly on the northwestern edge of the Cocos Ridge. This rift failed by about 2.6 Ma and alkalic vulcanism associated with this formed Cocos Island and the Cocos seamount province. The spreading centre continued its overall northward motion, but to the east of the hotspot the spreading centre repeatedly jumped southwards towards the hotspot, leading to the formation of the Galápagos transform. Following the formation of the transform the Northern Galápagos Volcanic Province developed between the Galápagos islands and the spreading centre.

Structure

The Cocos Ridge consists of thickened

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

. Geophysical investigations using

seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...

and wide-angle techniques have been used to determine the internal structure of the ridge. Observations on the oldest part of the ridge offshore

Costa Rica Costa Rica, officially the Republic of Costa Rica, is a country in Central America. It borders Nicaragua to the north, the Caribbean Sea to the northeast, Panama to the southeast, and the Pacific Ocean to the southwest, as well as Maritime bo ...

show variations in thickness of the various layers both parallel to the ridge axis and parallel to the Middle America Trench. The ridge's crustal thickness is at least twice as large as the surrounding crust of the Cocos plate. Typical oceanic crust has three main layers: layer 1, which consists of sediments; layer 2, typically 2 km thick, which consists of extrusive and intrusive

basaltic 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% ...

material; layer 3, typically about 6 km thick, which is thought to have a

gabbroic Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemi ...

composition. Over most of the area surveyed, the ridge is characterised by an additional 2 km of extrusive basalts above the normal 2 km thick layer 2. Locally, beneath the Quepos plateau, this additional layer reaches 3 km in thickness. On the flanks of the ridge the lower crustal layer 3 is about 6 km thick, similar to average oceanic crust. However, in the central part of the ridge this more than doubles, reaching a maximum of 17 km in some parts. The additional thickness is also thought to be gabbroic in type from the observed seismic velocities. All the extra thickness observed in layers 2 and 3 is thought to be a result of past proximity to the Galápagos hotspot.

References

{{Reflist Underwater ridges of the Pacific Ocean