Gorringe Ridge

Gorringe Ridge
Image Map of the ridge
Summit depth	25 m (82 ft)
Summit area	60 km × 180 km (37 mi × 112 mi)
Location
Location	Atlantic Ocean
Coordinates
Geology
Type	Underwater ridge
History
Discovery date	6 November 1875
Discovered by	USS Gettysburg (1858), United States Coast Survey

The Gorringe Ridge is a seamount in the Atlantic Ocean. It is located about 130 miles (210 km) west of Portugal, between the Azores and the Strait of Gibraltar along the Azores–Gibraltar fault zone. It is about 60 km wide and 180 km long in the northeast direction.

Discovery

In the nineteenth century the United States Coast Survey embarked on an ambitious program to map the seafloors of the world's main oceanways. This produced extensive maps of the more shallow areas, but deep-ocean work was hampered by lack of robust equipment. In 1872, English scientist Sir William Thomson invented a wire-based depth-sounding mechanism which was a significant improvement over rope-type equipment used previously. This Thomson Sounding Machine made its first discovery in 1874, of several seamounts west of the Hawaiian Islands. Its second use was on the USS Gettysburg (1858), an ocean-going vessel used in 1875 to extensively map the Eastern Atlantic seafloor. The ship was commanded by Captain Henry Honychurch Gorringe. On 6 November 1875 this expedition discovered the raised area (which was referred to as Gorringe Bank in reference to the ship's captain), and spent time mapping it. They determined that it contained two significant peaks, which they named Gettysburg (the highest, at 20 meters depth) and Ormonde (the second highest, at 33 meters depth).

Subsequent explorations

In the early twentieth century, Albert I, Prince of Monaco spent considerable time exploring and mapping the Gorringe Bank, using a total of three ships: Princess Alice, Princess Alice II, and Hirondelle II. The ships' names were given to several mounds and large banks between Madeira and the Azores.

In June 2005, the Oceana Organization mounted an extensive exploration of the biota on Gorringe Ridge's two largest peaks. It aims to categorize and determine relative abundance of the diverse lifeforms there.

Geology

The Gorringe Bank was eventually renamed Gorringe Ridge owing to its extensive length and the determination that it is the result of two tectonic plates which are sliding into and past each other. The plate boundaries here are converging at 4 mm/y, as well as sliding past each other. Upper mantle and oceanic crust are exposed along this ridge. Ferrogabbro dated at 77 Mya has been intruded, Also at 66 Mya the Canary hotspot mantle plume passed by and caused alkaline magma to intrude. Where there is crust, it is very thin, so that the Moho comes up to the sea floor. Sediment overlies the mantle, so this could be considered as crust. Since the Miocene Era there has been shortening of the ocean crust absorbed by folding, and thrusting.

A 2003 study of the ridge's gravity and magnegic anomalies concluded that the Moho is relatively flat across the ridge, and that the ridge's upper part corresponds to a northwestwards vergent fold. The thrusting activity probably started some 20 million years ago, and has covered about 20 km. The seamount is composed of gabbros of the oceanic crust, serpentized rocks and alkaline basalts.

1755 Lisbon earthquake

Modern seismologists who studied the cause of the 1755 Lisbon earthquake and the resulting tsunami initially suspected a displacement in the Gorringe Ridge, but later concluded that there was a simultaneous event involving two separate faults along the African Plate boundary, both faults displacing by around 20m.

See also

• Geology of the Iberian Peninsula