Seafloor spreadingAge of oceanic cr

Seafloor spreading

Age of oceanic crust; youngest (red) is along spreading centers.
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to form new sea floor. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone.

Earlier theories (e.g. by Alfred Wegener and Alexander du Toit) of continental drift were that continents "ploughed" through the sea. The idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s.[1] The theory is well accepted now, and the phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere.[2]

Incipient spreading

Plates in the crust of the earth, according to the plate tectonics theory
In the general case, sea floor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today. The process starts with heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see isostasy). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120 degree angles. These areas are named triple junctions and can be found in several places across the world today. The separated margins of the continents evolve to form passive margins. Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge.

If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift'. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea. The East African rift was thought to be a "failed" arm that was opening somewhat more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment reported that in the Afar region last September, a 60 km fissure opened as wide as eight meters. During this period of initial flooding the new sea is sensitive to changes in climate and eustasy. As a result, the new sea will evaporate (partially or completely) several times before the elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists.

Sea floor spreading can stop during the process, but if it continues to the point that the continent is completely severed, then a new ocean basin is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the Niger Delta. The Niger River has formed in the failed rift arm of the triple junction.

Continued spreading and subduction

Spreading at a mid-ocean ridge
The new oceanic crust is quite hot relative to old oceanic crust, so the new oceanic basin is shallower than older oceanic basins. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of the Pacific Ocean. The Pacific is also home to one of the world's most active spreading centres (the East Pacific Rise (EPR)) with spreading rates of up to 13 cm/yr. The Mid-Atlantic Ridge is a "textbook" slow-spreading centre, while the EPR is used as an example of fast spreading. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.[3]

Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean.

Debate and search for mechanism
At the Mid-Atlantic Ridge (and in other areas), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift. When Alfred Wegener first presented a hypothesis of continental drift in 1912, he suggested that continents ploughed through the ocean crust. This was impossible: oceanic crust is both more dense and more rigid than continental crust. Accordingly, Wegener's theory wasn't taken very seriously, especially in the United States.

Since then, it has been shown that the motion of the continents is linked to seafloor spreading. In the 1960s, the past record of geomagnetic reversals was noticed by observing the magnetic stripe "anomalies" on the ocean floor. This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. The seafloor must have originated on the Earth's great fiery welts, like the Mid-Atlantic Ridge and the East Pacific Rise.

The driver for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along. The magmatism at the ridge is considered to be "passive upswelling", which is caused by the plates being pulled apart under the weight of their own slabs.[4] This can be thought of as analogous to a rug on a table with little friction: when part of the rug is off of the table, its weight pulls the rest of the rug down with it.

Hasil (Bahasa Indonesia) 1:

Dasar laut yang menyebarkanUsia kerak Samudera; bungsu (red) adalah sepanjang menyebarkan pusat.Penyebaran dasar laut adalah proses yang terjadi di pegunungan mid-ocean, mana kerak Samudera yang baru dibentuk melalui aktivitas gunung berapi dan kemudian secara bertahap bergerak dari punggungan. Dasar laut menyebarkan membantu menjelaskan pergeseran benua dalam teori lempeng tektonik. Ketika piring oceanic menyimpang, tensional stres menyebabkan patah tulang terjadi di litosfer. Magma basaltik naik patah tulang dan dingin di dasar laut untuk membentuk dasar laut baru. Batuan akan ditemukan jauh dari zona menyebarkan sementara batu-batu yang lebih muda akan ditemukan dekat zona menyebar.Teori-teori sebelumnya (misalnya oleh Alfred Wegener dan Alexander du Toit) pergeseran benua adalah bahwa benua "dibajak" melalui laut. Gagasan bahwa dasar laut itu sendiri bergerak (dan membawa benua dengan itu) sebagai mengembang dari poros tengah diusulkan oleh Harry Hess dari Princeton University pada tahun 1960. [1 teori] adalah diterima dengan baik sekarang, dan fenomena ini dikenal disebabkan oleh konveksi arus dalam plastik, sangat lemah atas mantel, atau astenosfer. [2]Baru jadi menyebarkanPiring di kerak bumi, menurut teori lempeng tektonikDalam kasus umum, dasar laut menyebarkan mulai sebagai pertikaian dalam massa tanah kontinental, mirip dengan Timur Laut Merah sistem retakan Afrika hari ini. Proses dimulai dengan pemanas di dasar kerak benua yang menyebabkan hal itu menjadi lebih plastik dan kurang padat. Karena benda-benda padat kurang naik dalam kaitannya dengan benda-benda padat, daerah yang dipanaskan menjadi luas dome (Lihat isostasy). Sebagai kerak busur ke atas, patah tulang terjadi itu berangsur-angsur tumbuh ke dalam perpecahan. Sistem retakan khas ini terdiri dari tiga lengan keretakan pada kira-kira 120 derajat sudut. Wilayah ini diberi nama tiga persimpangan dan dapat ditemukan di beberapa tempat di seluruh dunia hari ini. Margin terpisah dari benua berevolusi untuk bentuk margin pasif. Hess teori adalah dasar laut yang baru terbentuk ketika magma dipaksa ke atas ke arah permukaan pada punggung.Jika penyebaran berlanjut melewati tahap baru jadi dijelaskan di atas, dua lengan keretakan akan terbuka sementara lengan ketiga berhenti pembukaan dan menjadi 'gagal keretakan'. Sebagai perpecahan aktif dua terus terbuka, akhirnya kerak benua adalah dilemahkan sejauh itu akan meregangkan. Pada titik ini basaltik kerak Samudera mulai bentuk antara fragmen kontinental pisah. Ketika salah satu perpecahan terbuka ke laut yang ada, sistem retakan ini dibanjiri dengan air laut dan menjadi laut baru. Laut Merah adalah contoh dari lengan yang baru dari laut. Retakan Afrika Timur diperkirakan sebagai lengan "gagal" yang membuka agak lebih lambat dari dua lengan lain, tetapi pada tahun 2005 Ethiopia jauh Geofisika Lithospheric percobaan melaporkan bahwa di Afar region September lalu, 60 km celah membuka lebar delapan meter. Selama periode awal banjir laut baru adalah sensitif terhadap perubahan iklim dan eustasy. Sebagai akibatnya, laut baru akan menguap (sebagian atau seluruhnya) beberapa kali sebelum ketinggian rift valley telah diturunkan ke titik bahwa laut menjadi stabil. Selama periode penguapan deposito besar evaporite akan dibuat dalam rift valley. Kemudian endapan ini memiliki potensi untuk menjadi hidrokarbon segel dan kepentingan tertentu untuk ahli geologi minyak bumi.Penyebaran dasar laut dapat berhenti selama proses, tetapi jika itu berlanjut ke titik bahwa benua benar-benar dipotong, kemudian baskom laut baru akan dibuat. Laut merah telah tidak belum benar-benar memisahkan Arab dari Afrika, namun fitur serupa dapat ditemukan di sisi lain dari Afrika yang telah rusak benar-benar gratis. Amerika Selatan pernah masuk ke dalam kawasan Niger Delta. Sungai Niger telah terbentuk di lengan gagal retakan persimpangan triple.Terus menyebarkan dan subduksiMenyebar pada punggungThe new oceanic crust is quite hot relative to old oceanic crust, so the new oceanic basin is shallower than older oceanic basins. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of the Pacific Ocean. The Pacific is also home to one of the world's most active spreading centres (the East Pacific Rise (EPR)) with spreading rates of up to 13 cm/yr. The Mid-Atlantic Ridge is a "textbook" slow-spreading centre, while the EPR is used as an example of fast spreading. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.[3]Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean.Debate and search for mechanismAt the Mid-Atlantic Ridge (and in other areas), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift. When Alfred Wegener first presented a hypothesis of continental drift in 1912, he suggested that continents ploughed through the ocean crust. This was impossible: oceanic crust is both more dense and more rigid than continental crust. Accordingly, Wegener's theory wasn't taken very seriously, especially in the United States.Since then, it has been shown that the motion of the continents is linked to seafloor spreading. In the 1960s, the past record of geomagnetic reversals was noticed by observing the magnetic stripe "anomalies" on the ocean floor. This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. The seafloor must have originated on the Earth's great fiery welts, like the Mid-Atlantic Ridge and the East Pacific Rise.The driver for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along. The magmatism at the ridge is considered to be "passive upswelling", which is caused by the plates being pulled apart under the weight of their own slabs.[4] This can be thought of as analogous to a rug on a table with little friction: when part of the rug is off of the table, its weight pulls the rest of the rug down with it.