Gorringe Ridge
Encyclopedia
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.
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
, 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 Ormond (the second highest, at 33 meters depth).
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.
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
basalt
s.
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.
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
in the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. It is located about 130 miles (210 km) west of Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
, between the Azores
Azores
The Archipelago of the Azores is composed of nine volcanic islands situated in the middle of the North Atlantic Ocean, and is located about west from Lisbon and about east from the east coast of North America. The islands, and their economic exclusion zone, form the Autonomous Region of the...
and the Strait of Gibraltar
Strait of Gibraltar
The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Spain in Europe from Morocco in Africa. The name comes from Gibraltar, which in turn originates from the Arabic Jebel Tariq , albeit the Arab name for the Strait is Bab el-Zakat or...
along the Azores-Gibraltar fault zone
Azores-Gibraltar Transform Fault
The Azores-Gibraltar Transform Fault, also called the Azores-Gibraltar fault zone , is a major geologic fault which runs eastward from the eastern end of the Terceira Rift in the Azores, extending through the Strait of Gibraltar and into the Mediterranean Sea. It forms part of the tectonic boundary...
. It is about 60 km wide and 180 km long in the northeast direction.
Discovery
In the nineteenth century the United States Coast SurveyU.S. National Geodetic Survey
National Geodetic Survey, formerly called the U.S. Coast and Geodetic Survey , is a United States federal agency that defines and manages a national coordinate system, providing the foundation for transportation and communication; mapping and charting; and a large number of applications of science...
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
William Thomson, 1st Baron Kelvin
William Thomson, 1st Baron Kelvin OM, GCVO, PC, PRS, PRSE, was a mathematical physicist and engineer. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging...
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
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
. Its second use was on the USS Gettysburg
USS Gettysburg
Three ships of the United States Navy have been named USS Gettysburg for the Battle of Gettysburg. was built in 1858, captured in 1863, commissioned in 1864 and decommissioned in 1879...
, an ocean-going vessel used in 1875 to extensively map the Eastern Atlantic seafloor. The ship was commanded by Captain Henry Honychurch Gorringe
Henry Honychurch Gorringe
Henry Honychurch Gorringe was a United States naval officer who attained national acclaim for successfully completing the removal of Cleopatra's needle from Alexandria, Egypt to Central Park, New York City.-Early life:...
. 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 Ormond (the second highest, at 33 meters depth).
Subsequent explorations
In the early twentieth century, Albert I, Prince of MonacoAlbert I, Prince of Monaco
Albert I was Prince of Monaco and Duke of Valentinois from 10 September 1889 until his death.-Early life:...
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
Madeira
Madeira is a Portuguese archipelago that lies between and , just under 400 km north of Tenerife, Canary Islands, in the north Atlantic Ocean and an outermost region of the European Union...
and the Azores.
In June 2005, the Oceana Organization
Oceana (non-profit group)
Oceana is the largest international ocean conservation and advocacy organization. Oceana works to protect and restore the world’s oceans through targeted policy campaigns....
mounted an extensive exploration of the biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...
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 mantleMantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
and oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
are exposed along this ridge. Ferrogabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
dated at 77 Mya has been intruded, Also at 66 Mya the Canary hotspot
Canary hotspot
The Canary hotspot, also called the Canarian hotspot, is a volcanic hotspot believed to be located at the Canary Islands off the north-western coast of Africa, although alternative theories to explain the volcanism there exist. The Canary hotspot is believed to be underlain by a mantle plume that...
mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
passed by and caused alkaline magma to intrude. Where there is crust, it is very thin, so that the Moho
Mohorovičić discontinuity
The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...
comes up to the sea floor. Sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
overlies the mantle, so this could be considered as crust. Since the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
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
Vergence (geology)
In structural geology, the vergence of a fold is the direction in which an antiform is inclined or overturned. The term vergence comes from the German vergenz, which means "overturn"....
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
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s.
1755 Lisbon earthquake
Modern seismologists who studied the cause of the 1755 Lisbon earthquake1755 Lisbon earthquake
The 1755 Lisbon earthquake, also known as the Great Lisbon Earthquake, was a megathrust earthquake that took place on Saturday 1 November 1755, at around 9:40 in the morning. The earthquake was followed by fires and a tsunami, which almost totally destroyed Lisbon in the Kingdom of Portugal, and...
and the resulting tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...
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
African Plate
The African Plate is a tectonic plate which includes the continent of Africa, as well as oceanic crust which lies between the continent and various surrounding ocean ridges.-Boundaries:...
boundary, both faults displacing by around 20m.