The earliest human exploration of the oceans is poorly known, but pictures of boats on early cave drawings in Norway illustrate Viking-style ocean vessels known to be used by the Vikings centuries later. Other rock drawings around the world show dugout canoes, boats made of reeds, bark, and animal hides. Early migrations of humans must have utilized boats to move from place to place. For instance, analysis of languages and of genetics shows that the Polynesians moved south from China into southeast Asia and Polynesia, then somehow made it, by sea, all the way to Madagascar off the east coast of Africa. other oceanic migrations include the colonization of Europe by Africans about 10,000 years ago, explorations and trade around and out of the Mediterranean by the Phoenicians about 3,000 years ago, and the colonization of North America by the siberians and Vikings. Ming Dynasty ocean explorations in the early 1400s were massive, involving tens of thousands of sailors on 317 ships. The Chinese ships were huge, including as many as nine masts more than 444 feet (135 m) in length and 180 feet (55 m) in width. The Chinese mounted these expeditions to promote Chinese culture, society, and technology but did not contribute significantly to understanding the oceans.
The first European to reach North America was probably Leif Eriksson, who, in the year 1,000, landed at L'Anse-aux Meadows in the Long Range Peninsula of Newfoundland, after becoming lost on his way from Greenland to Norway. The Vikings established a temporary settlement in Newfoundland, and there are some speculations of further explorations by the Vikings to places as far south as New England and Narragansett Bay in Rhode Island. Their colonies disappeared during the Dark
Ages, probably as a result of a global climate cooling trend that turned previously arable lands into arctic tundra.
Ptolemy (in the year 140) published maps of Europe's coastline that were largely inaccurate and took many years of ocean exploration to correct. The Greeks and Islamic explorers had made great strides in understanding the geography of the world centered on the Mediterranean Sea and Arabian Peninsula, and the records of these explorations eventually made it into European hands, where this knowledge was used for further explorations. The Portuguese, most notably Prince Henry the Navigator (1392-1460), were the most avid explorers of the Atlantic, exploring northwest Africa and the Azores in the early 1400s. In the late 1400s, Vasco da Gama (1460-1524) made it to southern Africa and eventually around the Cape of Good Hope, past Madagascar, and all the way to India in 1498. These efforts initialized economically important trade routes between Portugal and India, building the powerful Portuguese Empire. The timing was perfect for establishment of ocean trade routes, as the long-used overland Silk Roads had become untenable and dangerous with the collapse of the Mongol Empire and the Turk conquest of Constantinople (Istanbul) in 1453.
In the late 15 th and early 16th centuries, many ocean exploration expeditions were mounted as a precursor to more widespread use of the oceans for transportation. In 1492, Christopher Columbus sailed from Spain to the east coast of North America, and from the late 1400s to 1521 Ferdinand Magellan sailed around the world, including a crossing of the Pacific Ocean, followed by Sir Francis Drake of England. Later, Henry Hudson explored North American waters, including attempts to find a northwest passage between the Atlantic and Pacific. During the 1700s, Captain James Cook made several voyages in the Pacific and coastal waters of western North America, improving maps of coastal and island regions.
The early explorations of the oceans were largely concerned with navigation and determining the positions of trade routes, coastlines, and islands. Later, sea-going expeditions aimed at understanding the physical, chemical, biological, and geological conditions in the ocean were mounted. In the late 1800s the British Royal Society sponsored the world's most ambitious scientific exploration of the oceans ever, the voyage of the H.M.S. Challenger. The voyage of the Challenger in 1872-76 established for the first time many of the basic properties of the oceans and set the standard for the many later expeditions.
ocean exploration today is led by American teams based at several universities, Scripps Institute of oceanography, and Woods Hole Oceanographic Institute, where the deep submersible Alvin is based and from where many oceanographic cruises are coordinated. The Ocean Drilling Program (formerly the Deep-Sea Drilling Project) has amassed huge quantities of data on the sediments and volcanic rocks deposited on the ocean floor, as well as information about biology, climate, chemistry, and ocean circulation. Many other nations, including Japan, China, France, and Russia have mounted ocean exploration campaigns, with a trend toward international cooperation in understanding the evolution of the ocean basins.
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ophiolites A distinctive group of rocks that includes basalt, diabase, gabbro, and peridotite, the ophiolites may also be associated with chert, metallif erous sediments (umbers), trondhjemite, diorite, and serpentinite. Many ophiolites are altered to serpenti-nite, chlorite, albite, and epidote-rich rocks, possibly by hydrothermal seafloor metamorphism. German geologist G. Steinman introduced the term in 1905 for a tripartite assemblage of rocks, including basalt, chert, and serpentinite, that he recognized as a common rock association in the Alps. Most ophiolites form in an ocean-floor environment, including at mid-ocean ridges, in back-arc basins, in extensional forearcs, or within arcs. Ophiolites are detached from the oceanic mantle and have been thrust upon continental margins during the closure of ocean basins. Lines of ophiolites decorate many sutures around the world, marking places where oceans have closed. In the 1960s and 1970s much research focused on defining a type of ophiolite succession that became known as the Penrose-type of ophiolite. More recent research has revealed that the variations between individual ophiolites are as significant as any broad similarities between them.
A classic Penrose-type of ophiolite is typically three to nine miles (5-15 km) thick and if complete, consists of the following sequence from base to top, with a fault marking the base of the ophiolite. The lowest unit in some ophiolites is an ultramafic rock called lherzolite, consisting of olivine + clinopyrox-ene + orthopyroxene, generally interpreted to be fertile, undepleted mantle. The base of most ophiolites
Mesozoic ophiolites Paleozoic ophiolites Proterozoic ophiolites Tethyan suture zones
Mesozoic ophiolites Paleozoic ophiolites Proterozoic ophiolites Tethyan suture zones
Map showing distribution of ophiolites in the Tethyan orogenic belt, showing the location of Proterozoic, Paleozoic, and Mesozoic ophiolites
Shallow-water or terrestrial sedimentary rocks Pelagic, hemipelagic, or volcanic sediments Volcanic breccia, volcaniclastic rocks Silicic intrusion
Mafic extrusion—pillow lava and massive flows
Mafic sheeted-dike complex
Massive gabbro, diorite, or plagiogranite
C Intra-arc— Smartville
D Hot spot-Oceanic Plateau
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Cross sections through typical ophiolites, including different types of ophiolites produced at slow, intra-arc, and hotspot types of tectonic settings consists of an ultramafic rock known as harzburgite, consisting of olivine + orthopyroxene (± chromite), often forming strongly deformed or transposed compositional layering, forming a distinctive rock known as harzburgite tectonite. In some ophiolites, harzburgite overlies lherzolite. The harzburgite is generally interpreted to be the depleted mantle from which overlying mafic rocks were derived, and the deformation is related to the overlying lithospheric sequence flowing away from the ridge along a shear zone within the harzburgite. The harzburgite sequence may be six miles (10 km) or more thick in some ophiolites, such as the Semail ophiolite in Oman, and the Bay of Islands ophiolite in Newfoundland.
Resting above the harzburgite is a group of rocks that were crystallized from a magma derived by partial melting of the harzburgite. The lowest unit of these crustal rocks includes crystal cumulates of pyroxene and olivine, forming distinctive layers of pyroxenite, dunite, and other olivine + clinopyroxene + orthopyroxene peridotites, including wehrlite, web-sterite, and pods of chromite + olivine. The boundary between these rocks (derived by partial melting and crystal fractionation) and those below from which melts were extracted is one of the most fundamental boundaries in the crust, known as the Moho, or base of the crust, named after the Yugoslavian seismologist Andrija Mohorovicic, who noted a fundamental seismic boundary beneath the continental crust. In this case, the Moho is a chemical boundary, without a sharp seismic discontinuity. A seismic discontinuity occurs about 1,500-1,600 feet (half a kilometer) higher than the chemical Moho in ophiolites.
The layered ultramafic cumulates grade upward into a transition zone of interlayered pyroxenite and plagioclase-rich cumulates, then into an approximately half-mile (1 km) thick unit of strongly layered gabbro. Individual layers within this thin unit may include gabbro, pyroxenite, and anorthosite. The layered gabbro is succeeded upward by one to three miles (2-5 km) of isotropic gabbro, which is generally structureless but may have a faint layering. The layers within the isotropic gabbro in some ophiol-ites define a curving trajectory, interpreted to represent crystallization along the walls of a paleomagma chamber. The upper part of the gabbro may contain many xenoliths of diabase and pods of trondhjemite (plagioclase plus quartz) and may be cut by diabase dikes.
The next highest unit in a complete, Penrose-style ophiolite is typically a sheeted dike complex, consisting of a 0.3-1.25-mile- (0.5-2 km-) thick complex of diabasic, gabbroic, to silicic dikes that show mutually intrusive relationships with the underlying gab-bro. In ideal cases, each diabase dike intrudes into the center of the previously intruded dike, forming a sequence of dikes that have chilled margins developed only on one side. These dikes are said to exhibit one-way chilling. In most real ophiolites, examples of one-way chilling may be found, but statistically the one-way chilling may only show directional preference in 50-60 percent of cases.
The sheeted dikes represent magma conduits that fed basaltic flows at the surface. These flows are typically pillowed, with lobes and tubes of basalt forming bulbous shapes distinctive of underwater basaltic volcanism. The pillow basalt section is typically 0.30.6 mile (0.5-1 km) thick. Chert and sulfide minerals commonly fill the interstices between pillows.
Deep-sea sediments, including chert, red clay, in some cases carbonates, or sulfide layers, overlie many ophiolites. Many variations are possible, depending on tectonic setting (e.g., conglomerates may form in some settings) and age (e.g., siliceous biogenic oozes and limestones would not form in Archean ophiol-ites, before the life-forms that contribute their bodies developed).
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