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27-655: The Brawley Seismic Zone represents the northernmost extension of the spreading center axis associated with the East Pacific Rise which runs up the axis of the Gulf of California and is in the process of rifting the Baja California peninsula away from the mainland of Mexico , with significant subsidence taking place at southern California's Salton Sea and at Laguna Salada in Baja California. Other major locations along

54-764: A mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), at a divergent tectonic plate boundary , located along the floor of the Pacific Ocean . It separates the Pacific plate to the west from (north to south) the North American plate , the Rivera plate , the Cocos plate , the Nazca plate , and the Antarctic plate . It runs south from

81-455: 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 thrust onto and 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

108-407: A density of about 2.7 grams per cubic centimeter. The crust uppermost is the result of the cooling of magma derived from mantle material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas. As

135-604: A pronounced portion of the activity occurring within the Brawley Seismic Zone that separates the San Andreas fault to the north and the Imperial fault to the south. The Imperial fault was the source of the 1940 El Centro earthquake and the 1979 Imperial Valley earthquake . There was more than 30 km (19 mi) of surface rupture associated with the 1979 event along the northwest trending Imperial fault from just north of

162-699: A swarm included one quake which was measured at 5.8. The swarm activity is not understood completely, but the new restlessness could generate data that will help scientists to gain a better understanding of the region. According to a geologist at the United States Geological Survey (USGS), such swarms are typical for the Brawley zone: "The area sees lots of events at once, with many close to the largest magnitude, rather than one main shock with several much smaller aftershocks." The Imperial Valley of southern California experiences high rates of seismicity with

189-410: Is continuously being created at mid-ocean ridges. As continental plates diverge at these ridges, magma rises into the upper mantle and crust. As the continental plates move away from the ridge, the newly formed rocks cool and start to erode with sediment gradually building up on top of them. The youngest oceanic rocks are at the oceanic ridges, and they get progressively older away from the ridges. As

216-449: Is in the west Pacific and north-west Atlantic  — both are about up to 180-200 million years old. However, parts of the eastern Mediterranean Sea could be remnants of the much older Tethys Ocean , at about 270 and up to 340 million years old. The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the basalt . A symmetrical pattern of positive and negative magnetic lines emanates from

243-544: The Azores and Iceland . Prior to the Neoproterozoic Era 1000 Ma ago the world's oceanic crust was more mafic than present-days'. The more mafic nature of the crust meant that higher amounts of water molecules ( OH ) could be stored the altered parts of the crust. At subduction zones this mafic crust was prone to metamorphose into greenschist instead of blueschist at ordinary blueschist facies . Oceanic crust

270-699: The Gulf of California in the Salton Sea basin in Southern California to a point near 55°S 130°W  /  55°S 130°W  / -55; -130 , where it joins the Pacific-Antarctic Ridge (PAR) trending west-south-west towards Antarctica , near New Zealand (though in some uses the PAR is regarded as the southern section of the EPR). Much of the rise lies about 3,200 km (2,000 mi) off

297-783: The Gulf of California Rift Zone . In this area, newly formed oceanic crust is intermingled with rifted continental crust originating from the North American plate. Near Easter Island , the East Pacific Rise meets the Chile Rise at the Easter Island and Juan Fernandez microplates, trending off to the east where it subducts under the South American plate at the Peru–Chile Trench along the coast of southern Chile . This portion of

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324-517: The Mexico–United States border to an area south of Brawley and the BSZ was found to have ruptured, with surface cracks, for a length of 13 km (8.1 mi). The 1940 and 1979 mainshocks were of similar size and both were followed by damaging aftershocks near Brawley. Sources East Pacific Rise The East Pacific Rise ( EPR ) is a mid-ocean rise (usually termed an oceanic rise and not

351-455: The Salton Trough . On August 26, 2012, near Brawley , a swarm of more than 300 small to moderate earthquakes occurred with the largest two reaching a maximum of 5.3 and 5.5. Some light cosmetic damage was found on some older buildings in downtown Brawley. Earthquake swarms are not unusual for the area. In 2005, also near Brawley, there was a swarm with a peak magnitude of 5.1, while in 1981

378-656: The Southeast Indian Ridge at the Macquarie triple junction south of New Zealand . The southern stretch of the East Pacific Rise is also one of the fastest-spreading divergent boundaries on Earth, peaking at 79.3 mm (3.12 in)/year. Along the East Pacific Rise the hydrothermal vents called black smokers were first discovered by the RISE project in 1979, and have since been extensively studied. These vents are forming volcanogenic massive sulfide ore deposits on

405-661: The Rise has been referred to as the Cape Adare-Easter Island Ridge, Albatross Cordillera, Easter Island Cordillera, Easter Island Rise, and Easter Island Swell. Parts of the East Pacific Rise have oblique spreading, such as the Nazca–Pacific plate boundary between 29°S and 32°S. This is seafloor spreading that is not orthogonal to the nearest ridge segment. The southern extension of the East Pacific Rise (the PAR) merges with

432-489: The South American coast and reaches a height about 1,800–2,700 m (5,900–8,900 ft) above the surrounding seafloor. The oceanic crust is moving away from the East Pacific Rise to either side. Near Easter Island the rate is over 150 mm (6 in) per year which is the fastest in the world. However, on the northern end, it is much slower at only roughly 60 mm ( 2 + 1 ⁄ 2  in) per year. On

459-544: The axis include the Cerro Prieto spreading center located south of Mexicali, and Wagner Basin (a submarine depression in the Gulf of California). The Salton Buttes on the south shore of the Salton Sea are on the north margin of the Brawley Seismic Zone and are linked to volcanic and geothermal activity within the zone. The Brawley Seismic Zone has been interpreted as an "onshore spreading center" which runs diagonally across

486-469: The eastern side of the rise, the eastward-moving Cocos and Nazca plates meet the westward moving South American plate and the North American plate and are being subducted under them. The belt of volcanos along the Andes and the arc of volcanoes through Central America and Mexico are the direct results of this collision. Due east of the Baja California peninsula , the Rise is sometimes referred to as

513-412: The lavas cool they are, in most instances, modified chemically by seawater. These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. But most magma crystallises at depth, within the lower oceanic crust . There, newly intruded magma can mix and react with pre-existing crystal mush and rocks. Although

540-406: 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 half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses

567-438: 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 ,

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594-413: 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. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle. The most voluminous volcanic rocks of the ocean floor are

621-480: The ocean floor. Many unique deep-water creatures have been found with vents, that subsist in a chemosynthetic ecosystem rather than one using photosynthesis . Oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates . It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust , composed of troctolite , gabbro and ultramafic cumulates . The crust overlies

648-508: The oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it. 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

675-453: The rigid uppermost layer of the mantle . The crust and the rigid upper mantle layer together constitute oceanic lithosphere . 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 3.0 grams per cubic centimeter as opposed to continental crust which has

702-479: 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. The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson Cycle . The oldest large-scale oceanic crust

729-526: The solidus and melts at lesser 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 greater depth, creating more melt and a thicker crust. An example of this is Iceland which has crust of thickness ~20 km. The age of

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