Geology of the Pacific Ocean

Plate subduction in the North Pacific 70 million years ago

The Pacific Ocean developed from the Panthalassa, a vast global ocean, following the breakup of the supercontinent Pangaea in the Mesozoic Era, gradually forming Gondwana and Laurasia, which in turn divided.[2][3] There is no firm date for when the changeover occurred, as the replacement of the sea bed is a continuous process, though reconstruction maps often change the name from Panthalassic to Pacific around the time the Atlantic Ocean began to open.[4] The Panthalassic Ocean first opened around 750 million years ago at the breakup of Rodinia,[4] but the oldest Pacific Ocean floor is only around 180 Ma old. Any older Pacific floor crust has been subducted.[5]

During the Jurassic period, four tectonic plates developed in the Pacific Basin: the Kula and the Farallon in the north, the large Pacific Plate in the centre and south and the Phoenix Plate in the far south. The Kula Plate was subducted under the eastern and south-eastern Asian landmass. The northern part of the Farallon was subducted under North America while its southern section, together with the Phoenix Plate, was subducted under South America and the Antarctic. During the late Tertiary period, the Farallon plate broke up, leaving the Juan de Fuca Plate in the north, the Cocos Plate off today's Central America, and the Nazca and Phoenix Plates in the southern Pacific.[6][7]

Charles Darwin had proposed a theory of the slow subsidence of the ocean floor based on his studies of Pacific oceanic islands, seamounts (underwater volcanoes) and guyots (flat-topped seamounts). His observations have been verified and expanded in the development of plate tectonic theory.[8]

The tectonic plates continue to move today. The slowest ridge spreading rate is across the Gakkel Ridge in the Arctic Ocean which separates at less than 2.5 cm/yr while the fastest, across the East Pacific Rise near Easter Island, has a spreading rate of over 15 cm/yr.[9] The Pacific Plate moves generally towards the northwest at between 7 and 11 cm/yr while the Juan De Fuca Plate has an east-northeasterly movement of some 4 cm/yr.[10]

The theory of plate tectonics, which is supported by seismology, volcanology, gravimetry, and paleomagnetism (remanent magnetism), explains the origin of the physical features of the Pacific.[8] The islands of the Pacific have developed in a number of ways. Some have originated as chains of volcanic islands on the tectonic plates either as a result of mantle plumes or by fracture propagation. Atolls have developed in tropical waters when, after volcanos sink, coral growth results in reefs as evidenced by the Cook Islands. Coral reefs can develop into islands over a submerged extinct volcano following uplift as in Makatea and Rennell Island in the Solomon Archipelago which have steep coral cliffs over 100 metres high.[1]

The collision of two plates, resulting in subduction, have contributed to the emergence of the western Pacific arcs of volcanic islands. A new crust has emerged as the East Pacific Rise. In the northeastern Pacific margin there is a lateral shift, through the San Andreas Fault system, of the North American Plate and the Pacific Plate.[8]

Plate movements have also caused fragments of continental crust to be rotated away from landmasses so as to form islands. Zealandia which broke off from Gondwana 70 million years ago with the spreading of the Tasman Sea, has since resulted in island protrusions such as New Zealand and New Caledonia. Related causes of island formation include obduction and subduction at convergent plate boundaries. Malaita and Ulawa in the Solomon Islands are the result of obduction while the effects of subduction can be seen in the formation of volcanic island arcs such as the Aleutian Arc off Alaska and the Kermadec-Tonga Subduction Zone north of New Zealand.[1]

As a result of the collision of the Nazca Plate and the South American Plate, the Andes Mountains emerged in western South America, close to the Peru–Chile Trench.[8] Many large fracture zones aligned in an east-west direction, occur in the floor of the northeastern Pacific.[8]

The Pacific is a broad ocean basin (unlike the narrow Atlantic Ocean) and extends over a width of 10,000 miles (16,000 km) between New Guinea and Peru.[11] The andesite line, a zone of intense volcanic and seismic activity, is a major regional distinction in the Pacific. The petrologic boundary separates the deeper mafic igneous rock of the Central Pacific Basin from the partially submerged continental areas of felsic igneous rock on its margins.[12] The andesite line follows the western edge of the islands off California and passes south of the Aleutian arc, along the eastern edge of the Kamchatka Peninsula, the Kuril Islands, Japan, the Mariana Islands, the Solomon Islands, and New Zealand's North Island.[13][14]

The dissimilarity continues northeastward along the western edge of the Andes Cordillera along South America to Mexico, returning then to the islands off California. Indonesia, the Philippines, Japan, New Guinea, and New Zealand lie outside the andesite line.

Outside of the andesite line, volcanism is explosive; the Pacific Ring of Fire is the world's foremost belt of explosive volcanism. The Ring of Fire is named after the several hundred active volcanoes that sit above the various subduction zones. In 2009, the deepest undersea eruption ever recorded occurred at the West Mata submarine volcano, a mile beneath the ocean, close to the Tonga-Kermadec Trench, within the Ring of Fire;[15] it was filmed by the US Jason robotic submersible which descended over 1,100 metres (3,600 ft).[16] The Pacific Ocean is the only ocean which is almost totally bounded by subduction zones. Only the Antarctic and Australian coasts have no nearby subduction zones.

In March and April 2008, a series or swarm of moderate earthquakes occurred both near and within the Blanco Fracture Zone. The swarm began on 30 March when over 600 measurable tremors began occurring north of the zone within the Juan de Fuca Plate.[17] A decade earlier, in January 1998, another swarm was detected at Axial Seamount in the Juan de Fuca Ridge.[18] At the time of its occurrence, scientists were not aware that the series of faults in this plate even existed.[19] In a remote region of the central Pacific Ocean, at the southeastern section of the Gilbert Islands, a major swarm of intraplate earthquakes occurred between December 1981 and March 1983, with no prior seismicity having been reported in this region previously.[20] Another swarm was detected on the Queen Charlotte Islands fracture zone in August–September 1967.[21]