Age of oceanic crust. The red is most recent, and blue is the oldest.

Oceanic crust has a crust made up of basalt and gabbro and the upper mantle consits of peridoite. 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. It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is denser, having a mean density of about 2.9 grams per cubic centimeter.^[1]

Composition[link]

Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging (especially from ridge crests and fracture zones) and drilling. Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers.

• Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge. Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents.^[2]
• Layer 2 could be divided into two parts: layer 2A – 0.5 km thick uppermost volcanic layer of glassy to finely crystalline basalt usually in the form of pillow basalt, and layer 2B – 1.5 km thick layer composed of diabase dikes.
• Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and cumulate ultramafic rocks. It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.

Geochemistry[link]

The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low-potassium tholeiitic magmas. These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements. There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands, the Azores and Iceland.^[3]

Life cycle[link]

Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges.

As the mantle rises it cools and melts, as the pressure decreases and it crosses the solidus. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr^-1 half-rate) produce thinner crust (4-5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at a lower depth, thereby producing less melt and thinner crust. An example of this is the Gakkel Ridge under the Arctic Ocean. Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a lower depth, creating more melt and a thicker crust. An example of this is Iceland which has crust of thickness ~20 km.^[4]

The oceanic lithosphere subducts at what are known as convergent boundaries. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old.^[5] The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson cycle.

Magnetic lines[link]

The oceanic crust displays an interesting pattern of parallel magnetic lines, parallel to the ocean ridges, frozen in the basalt. In the 1950’s, scientists mapped the magnetic field generated by rocks on the ocean floor. They noticed a symmetrical pattern of positive and negative magnetic lines as they moved along the ocean floor, and the line of symmetry was at the mid ocean ridge. The fact that the anomalies were symmetrical at the mid-ocean ridge was explained by the hypothesis that new rock was being formed by magma at the mid-ocean ridges, and the ocean floor was spreading out from this point. When the magma cooled to form rock, it aligned itself with the current position of the north magnetic pole of the Earth (which has reversed many times in its past) at the time of its cooling. New magma forced the older cooled magma away from the ridge. Approximately half of the new rock was formed on one side of the ridge and half on the other.

See also[link]

• Continental crust
• Mid-Ocean Ridge
• Mohorovičić discontinuity

Notes[link]

1. ^ Rogers, N.; Blake, S.; Burton, K. (2008). An introduction to our dynamic planet. Cambridge University Press. p. 19. ISBN 978-0-521-49424-3. http://books.google.co.uk/books?id=WA9ST5S_2v0C&pg=PA44&dq=density+%22oceanic+crust%22&hl=en&sa=X&ei=P4IUT6fiPJH5sgb87rAY&ved=0CG8Q6AEwCTgU#v=onepage&q=density%20%22oceanic%20crust%22&f=false. Retrieved 16 January. 
2. ^ D. R. Bowes (1989) The Encyclopedia of Igneous and Metamorphic Petrology, Van Nostrand Reinhold ISBN 0-442-20623-2
3. ^ Clare P. Marshall, Rhodes W. Fairbridge (1999) Encyclopedia of Geochemistry, Kluwer Academic Publishers ISBN 0-412-75500-9
4. ^ C.M.R. Fowler (2005) The Solid Earth (2nd Ed.), Cambridge University Press ISBN:0-521-89307-0
5. ^ Condie, K.C. 1997. Plate Tectonics and Crustal Evolution (4th Edition). 288 page, Butterworth-Heinemann Ltd.

References[link]

• Marshak, Stephen (2005) Earth: Portrait of a Planet (41-87)
• Russell E. McDuff, G. Ross Heath. "Ocean 540: Oceanic Lithosphere; Plate Tectonics; Seafloor Topography". www2.ocean.washington.edu. http://www2.ocean.washington.edu/oc540/lec01-1/. Retrieved 2009-08-09. 

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Ioánnis Fokás (Greek: Ιωάννης Φωκάς), better known by the Spanish transcription of his name, Juan de Fuca (born 1536 on the Ionian island of Cefalonia; died there 1602^[1][2]), was a Greek-born maritime pilot in the service of the king of Spain, Philip II. He is best known for his claim to have explored the Strait of Anián, now known as the Strait of Juan de Fuca, between Vancouver Island, now part of British Columbia, Canada, and northwestern Washington State, United States.

Family and early life[link]

Fokás's grandfather, Emmanouíl Fokás (Gk: Εμμανουήλ Φωκάς), fled Constantinople at its fall in 1453, accompanied by his brother Andrónikos (Gk: Ανδρόνικος). The two settled first in the Peloponnese, where Andrónikos remained, but in 1470 Emmanouíl moved to the island of Cefalonia. Iákovos (Gk: Ιάκωβος), Ioánnis's father established himself in the village of Valerianos (Gk: Βαλεριάνος) on the island and came to be known as "the Valeriáno Fokás" (Gk: ο Φωκάς ο Βαλεριάνος) to distinguish him from his brothers.^[2]

It was in this village of Valeriáno that Fokás was born in 1536. Little to nothing is known about his life before he entered the service of Spain, some time around 1555.^[3]

Name[link]

The name of the man known to history as Juan de Fuca is the source of some confusion. While Juan de Fuca is clearly a Spanish rendering of Ioánnis Fokás (Gk: Ιωάννης Φωκάς), some sources cite Apóstolos Valeriános (Gk: Απόστολος Βαλεριάνος) as his "real" name. It is possible that Fokás was baptized Apóstolos and later adopted the name Ioánnis/Juan (i.e., John) because Apóstol is not much used as a name in Spanish. Given that Fokás/Fuca was the family name borne by the seafarer's father and grandfather, Valeriános is likely to be a nickname used on the island which would have been quite meaningless in the Spanish Empire.

Early career[link]

Fuca's early voyages were to the Far East, and he claimed to have arrived in New Spain in 1587 when, off Cabo San Lucas in Baja California, the English privateer Thomas Cavendish seized his galleon Santa Ana and deposited him ashore. He was a well-traveled seaman, perfecting his skill as a pilot in the Spanish fleet. The king of Spain, he also claimed, recognized him for his excellence and made him pilot of the Spanish navy in the West Indies (a title he held for forty years), but there is no record in Spanish Archives of his name or position or of his visit to the royal court.^[4] Before he made his famous trip up the northwest coast of the North American continent, he sailed to China, the Philippines and Mexico. The Strait of Juan de Fuca between the United States of America and Canada was named for him by British Captain Charles Barkley because it was at the same latitude that Juan de Fuca described as the location of the Strait of Anian.^[5]

Voyages to the north[link]

Fuca Pillar at Cape Flattery, Washington beside the Strait of Juan de Fuca.

According to Fuca's account, he undertook two voyages of exploration on the orders of the Viceroy of New Spain, Luis de Velasco, marqués de Salinas, both intended to find the fabled Strait of Anián, believed to be a Northwest Passage, a sea route linking the Atlantic and Pacific Oceans. The first voyage saw 200 soldiers and three small ships under the overall command of a Spanish captain (with Fuca as pilot and master) assigned the task of finding the Strait of Anián and fortifying it against the English. This expedition failed when, allegedly due to the captain's malfeasance, the soldiers mutinied and returned home to California.^[2] (Note that in this period, Spanish doctrine divided control of ships and fleets between the military commander, who was an army officer, and the sailing and navigation commander, who was a mariner.)

In 1592, on his second voyage, Fuca enjoyed success. Having sailed north with a caravel and a pinnace and a few armed marines, he returned to Acapulco and claimed to have found the strait, with a large island at its mouth, at around 47° north latitude. The Strait of Juan de Fuca is in fact at around 48° N, although Fuca's account of sailing into it departs from reality, describing a region far different from what actually existed there.^[6] The Strait follows directly into Puget Sound, Washington, beginning at around 47° 59' N and continuing as far south as 47° 01' N at Tumwater, Washington. During the voyage, Fuca also noted a "high pinnacle or spired rock", which may have been Fuca Pillar, a tall, almost rectangular, rock on the western shore of Cape Flattery on the northwestern tip of Washington beside the Strait of Juan de Fuca - although Fuca noted it being on the other side of the strait.^[6]

Despite Velasco's repeated promises, however, Fuca never received the great rewards he claimed as his due. After two years, and on the viceroy's urging, Fuca travelled to Spain to make his case to the court in person. Disappointed again and disgusted with the Spanish, the aging Greek determined to retire to his home in Kefallonia but was in 1596 convinced by an Englishman, Michael Lok (also spelled as Locke in English and French documents from the period), to offer his services to Spain's archenemy, Queen Elizabeth. Nothing came of Lok and Fokás' proposals, but it is through Lok's account that the story of Juan de Fuca entered English letters.^[2]

Controversy[link]

Because the only written evidence for Fokás's voyages lay in Lok's account — researchers being unable to find records of the expedition in Spanish colonial archives — there was long much controversy over his discovery and, indeed, whether he had ever even existed as a real person; several scholars have dismissed Juan de Fuca as entirely fictitious, and the 18th-century British explorer Captain Cook strongly doubted that the strait Fokás claimed to have discovered even existed^[7] (although Cook actually sailed past the Strait of Juan de Fuca without entering it and did stop at Nootka Sound on the west coast of Vancouver Island). With later English exploration and settlement of the area, however, Fokás's claims seemed much more credible.

Finally, in 1859, an American researcher, with the help of the U.S. Consul in the Ionian Islands, was able to demonstrate not only that Fokás had lived but also that his family and history were well known on the islands.^[2] While we may never know the exact truths that lay behind the account published by Lok, it must be considered unlikely that the man himself was a fiction.

Legacy[link]

When the English captain Charles William Barkley, sailing the Imperial Eagle in 1787, (re)discovered the strait Fokás had described, he renamed it the Strait of Juan de Fuca.

The Juan de Fuca Plate, a tectonic plate underlying much of the coastline he explored, is also named for Fokás.

Notes[link]

Further reading[link]

• British Columbia: From the Earliest Times to the Present, Chapter II, The Apocryphal Voyages, pp. 19-31, Ethelbert Olaf Stuart Scholefield, publ. S.J. Clarke, Vancouver, 1914

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