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Geology of the Pacific Ocean

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The East Pacific Rise near Easter Island is the fastest spreading mid-ocean ridge, with a spreading rate of over 15 cm/yr. 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.

Most subduction zones around the rim of the Pacific are directed away from a large area in the southern Pacific. At the core–mantle boundary below this area there is a large low-shear velocity province (LLSVP). Most of Pacific hotspots are located above the LLSVP while the longest Pacific hotspot tracks are located at or near its boundaries pointing at the positions of large igneous provinces.

Charles Darwin proposed a theory that explained the existence of reefs by means of slow subsidence of the ocean floor. His theories have been verified and expanded in the development of plate tectonics.

History

In the Early Jurassic, the supercontinent Pangaea was surrounded by the superocean Panthalassa, the ocean floor of which was composed of the Izanagi, Farallon, and Phoenix plates. These three plates were joined at a migrating, or unstable, ridge-ridge-ridge (RRR) triple junction from which the Pacific plate began to grow 190 million years ago in an area east of the Mariana Trench; this area, known as the Pacific Triangle, is the oldest part of the Pacific plate and therefore the oldest ocean floor of the Pacific. Spreading laterally from this triangle are the Hawaiian, Japanese, and Phoenix magnetic lineations, the earliest traces of how the Pacific plate began to grow as a RRR triple junction fell apart into three triple junctions. Virtually all of the three Panthalassic plates have now been subducted beneath surrounding continents but their spreading rates have been preserved in the magnetic lineations around the Pacific Triangle.

In the North Pacific, magnetic anomalies south of the Aleutian Islands indicate the presence of a now almost completely subducted tectonic plate, known as the Kula plate, which probably existed from the Late Cretaceous to the Eocene (c. 83–40 Ma). This plate probably broke off the Farallon plate and, when subduction in the North Pacific shifted from Siberia to the Aleutian Trench c. 50 Ma, spreading ceased between the Kula plate and the Pacific plate.

The Pacific plate kept growing and lineations south of the Pacific Triangle indicate the Pacific–Phoenix ridge remained a simple N–S trending spreading system 156–120 Ma. Following the formation and break-up of the Ontong Java–Hikurangi–Manihiki large igneous province 120 Ma, however, the Phoenix plate broke into several smaller tectonic plates. The complexity of the Mid-Pacific and Magellan lineations indicate the presence of a series of microplates around the Pacific–Phoenix–Farallon triple junction.

The Farallon plate subducted under North America from the late Mesozoic, while spreading between the Pacific and Farallon plates was initiated 190 Ma and lasted until the break-up of the Farallon plate 23 Ma. During the Cenozoic the Farallon plate broke up along the eastern Pacific margin into the Kula, Vancouver/Juan de Fuca, and Cocos plates.

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