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The Structure and Origin of the Ocean Basins

The water Planet Shouldn’t our planet be called “Water” rather than “Earth”?

If you look at a world map in an atlas, you will see that there is more water than land. • The oceans cover about 71% of the earth's surface. They are not distributed

equally with respect to the Equator. • Southern Hemisphere: ~ 80% is ocean. • Northern Hemisphere: ~ 61% is ocean. • Average depth is 3636 m, Greatest (maximum) depth: ~ 11,022 m (Marianas

Trench) (Land: Mt Everest 8,848 m), and 84% > 2000m depth. Formation of the Oceans How did water (and eventually oceans) form on the Earth?

• Gases, including water vapor, were released by the process of outgassing1, where gases from the mantle were vented out of volcanoes to the earth’s surface.

• The outgassed water vapor remains in the atmosphere for millions of years. This is because the earth was too hot to allow the vapor to condense to form liquid water.

• Eventually, the earth cooled enough for water vapor to condense into clouds in the upper cooler atmosphere.

• Hot rain fell, however, since the earth's surface was so hot, the hot rains will boil back into the clouds.

• As this process continued, the surface cooled enough for water to collect in basins, forming the earliest oceans.

The world Oceans

The ocean surrounds landmasses known as continents, which constitute the remaining 29% of Earth’s surface. Today's oceans are traditionally divided by the continents into four major parts or basins: the Pacific, the Atlantic, the Indian and the Arctic. The continuous body of water that surrounds Antarctica is called the Southern Ocean. ________________________________________________________________________________________________________________________________________________________________________________________________________________________

1 Outgassing is the slow release of a gas that was trapped in some material

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Table Areas, volumes, mean depths and maximum depths of the four major ocean basins.

Ocean Area (106 km2)

Volume (106 km3)

Mean Depth (m)

MaximumDepth (m)

Pacific 181.3 714.4 3940 11022 Atlantic 94.3 337.2 3575 8605 Indian 74.1 284.6 3840 7450 Arctic 12.3 13.7 1117 4600

The oceans and seas of the world

Marginal Seas Often, where two continents lie close together, a smaller part of an ocean called a sea is formed. Mediterranean Sea, Red Sea, Arabian Gulf, the Gulf of Mexico, the Baltic Sea, North Sea, and South China Sea are examples of these seas. Owing to their restricted connections with the open ocean, most marginal seas have unique oceanographic characteristics. For example, the presence of a shallow-water barrier, or sill (Gibraltar Sill), restricts exchange between the Mediterranean and Atlantic Ocean. Additionally, the local excess of evaporation relative to precipitation increases the salinity within the Mediterranean. These differences in the physical characters (temperature, salinity and consequently density) between Mediterranean and Atlantic Ocean are resulted in dividing the two seas into two separate entities by means of barrier. Allah Q has informed us about this barrier in the Nobel Qur’an that:

الى ال اهللا تع ان{:ق رين يلتقي رج البح ان * م رزخ ال يبغي ا ب ذبان * بينهم ا تك أي آالء ربكم ؤ * فب ا اللؤل رج منهم يخ ].22-19: الرحمن [ }والمرجان

The interpretation of this verse is that; when the water from one sea enters the other sea, it looses its distinctive characteristics and becomes homogenized with the other water. In a way, this barrier serves as a transitional homogenizing area for the two waters. This is an excellent example of the Scientific Inimitability of the Noble Qur'an.

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Ocean Floor Features Bathymetry: Measuring the Ocean Depths and mapping of ocean floor features. How did early scientists recognize the ocean floor features? Early bathymetric studies were often performed using a weighted line dropped to measure the depth. Advances in Bathymetry Echo sounding Echo sounding is a method of measuring depth using powerful sound pulses. After the sound wave hits the bottom, the returning signal, called an echo, is received by a depth recorder in the ship. The time it takes for the sound pulse to travel to the sea bed and bounce back is a measure of the depth. Two pieces of information are needed to calculate the depth—the speed of sound in water (1454 meters per second) and the time it takes for the signal to reach the bottom. If for example a signal takes one second (after being sent) to return to the ship, then it takes one-half second to travel to the bottom. Since sound travels 1454 meters per second underwater, the depth of the water = 1454 (1/2 × 1) = 727 meters. Sonar (sound navigation ranging) is a technique or equipment that used to locate objects underwater by the detection of echoes. Sonar is also very useful to help ships navigate in shallow waters. For example, a reef may be located only several meters below the surface -close enough to make ships cautious when they pass by. Modern fishing boats also use sonar to locate schools of fish.

Echo sounding Multibeam System

Multibeam Systems Multibeam systems collect data from as many as 121 beams to measure the contours of the ocean floor.

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Satellite Altimetry (قياس اإلرتفاعات) Satellite altimetry is an indirect way of measuring depth and detecting sea floor features. Satellites measure the sea surface height from their orbits (i.e. the distance between the satellite and the sea surface) by bouncing rapid pulses of radar energy off the ocean surface. These measurements show that the sea surface is not smooth but undulates by as much as 100 m above and below a smooth reference sea surface. Sea floor features like submerged mountains -see figure below- have more mass than sea water. This extra mass pulls the sea surface into gentle “hills” above the features. Thus we can locate the features below the surface by measuring the variations in sea surface height.

The sea floor The sea floor to be subdivided into three distinct regions: continental margins, deep ocean basins and midoceanic ridges.

Continental Margins: The continental margins represent the submerged ends of the continents which composed mostly of granite. It represents 7-8% of the total ocean area

Components of Continental margins: 1. Continental shelf ) اري )الرفرف الق : a shallow, gently sloping (~1 degree), submerged edge of the continent. It extends from roughly 10 km to over 300 km from the strand line. Though they make up only about 8% of the ocean’s surface area, continental shelves are the biologically richest part of the ocean, with the most life and the best fishing. The continental shelf ends at the shelf break, where the slope abruptly gets steeper. The shelf break usually occurs at depths of 100-200 m. 2. Continental slope ( اري ( المنحدر الق : is the area where the seafloor drops steeply at the outer edge of the continental shelf.

3. Submarine canyons الوديان البحرية( ) The continental shelf and continental slope are usually dissected by a V-shaped depression called submarine canyons that act as channels for down-slope transport of sediment. Underwater landslides called turbidity currents commonly flow down submarine canyons. The debris settles out to build up a fan-shaped sediment called a submarine fan at the base of the canyon and on deep sea floor.

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4. Continental rise ) اري ع الق )المرتف : A slightly elevated region formed as a result of accumulations of sediment at the base of the continental slope. Deep-Sea Floor (قرار البحر العميق) The deep sea floor beyond the continental margin; made up of oceanic crust, which is composed mostly of volcanic basalt. Most of the deep-sea floor lies at a depth of 3,000 to 5,000 m, averaging about 4,000 m

Deep Ocean Province is between the continental margins and the midoceanic ridge and includes a variety of features such as; Abyssal plain, Abyssal hills, Seamounts, and Deep sea trenches Abyssal plain ) ة )السهول المحيطي : are broad flat areas of ocean floor found between the continental margins and the mid-ocean ridges, average 4000 m depth and coated by sediments. Abyssal hills are small, extinct volcanoes or rock intrusions that poke up through the sediments coating the abyssal plains. Seamounts are steep-sided volcanoes that rise up from the sea floor but which do not stick up above sea level. Mid-oceanic islands rise up from the deep sea floor to the surface due to hot spots in the crust (e.g.., Hawaiian Islands)

How did these huge canyons form? Submarine canyons developed during the last ice age, when the sea level was much lower. At these times most of the continental shelves were exposed and rivers and glaciers (A glacier is a mass of moving ice formed on mountains from compacted snow) flowed across the continental shelves and eroded deep river valleys. When the sea level rose at the end of the ice age, these valleys were submerged, creating the submarine canyons.

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Guyots are flat-topped seamounts. Today they occur far below sea level, but at one time they were islands that poked up above the surface. They were eroded flat by wave action, and then gradually sank downward. Trenches ) ادق )الخوانق أو الخن : are the deepest parts of the sea floor and may be over 10000 m deep. They are found at the margins of the crustal plates and formed by subduction, where one plate descends into the mantle below the other plate. The deepest ocean trench is the Mariana Trench (11,022 m), located in the western Pacific Ocean.

Two examples of continental margins, showing various topographic features.

Midoceanic ridge )الحواجز المحيطية( systems: A continuous chain of volcanic Submarine Mountain that encircle the globe, marking the boundaries of several crustal plates and extends for about 60,000 km around the Earth. These are areas of sea floor spreading and the formation of new oceanic crust (see the plate tectonic theory). They rise 2000-4000 meters from the ocean floor, and parts sometimes reach the sea surface, forming emergent islands (e.g., Iceland). At the center of the ridge there is a great gap or depression known as the central rift valley.

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Hydrothermal Vents Occur near the axes of oceanic ridges. Cold, dense seawater seeps down through fractures in the ocean floor and gets heated to very high temperatures by the hot mantle material. The heated water then forces its way back up through the crust and emerges in hydrothermal vents, or deep-sea hot springs. As the hot water seeps through cracks in the earth’s crust, it dissolves a variety of minerals. When the mineral-laden hot water emerges at the vent, it mixes with the surrounding cold water and is rapidly cooled. This causes many of the minerals to solidify, forming mineral deposits around the vents in chimney-like structures called smokers. The “smoke” is actually a dense cloud of mineral particles. Types of Smokers

o White smokers are warm water vents (10-20°C) containing white particles of barium sulfate

o Black smokers are hot water vents (350°C - 400°C) containing black metal sulfides

Deep-sea hot springs are of great interest to biologists. One of the most exciting developments ever in marine biology has been the discovery of unexpectedly rich marine life around hydrothermal vents.

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The Mediterranean Sea The Mediterranean Sea is a semi-enclosed basin that extends for 3800 km in an East to West direction and for 800 km North to South in the widest sections. It covers an area of 2.5 million km2 and contains a volume of 3.7 million km3 of water. The average depth of the basin is thus 1500 m. Depths over 4000 m are common and the maximum depth is 5121 m off the Southern coast of Greece. The Mediterranean Sea harbours most of the topographical structures; such as submarine canyons, seamounts, or deep trenches. The continental shelves are relatively narrow, cover only 30% of the Mediterranean sea surface. Continental shelves are only extensive near the major river mouths. The continental slope is steep dropping rapidly into submarine canyons that lead into deep basins. This is particularly true in the eastern basin where the continental shelf is not continuous but dissected by its complex topography. The Mediterranean Sea is comprised of two major basins, western and eastern, that are divided by the relatively shallow strait of Sicily which acts as a geographical and hydrological frontier between them. These two basins are in turn divided into a series of interacting parts and adjacent seas. The Western Mediterranean covers about 0.85 km2 and the Eastern Mediterranean about 1.65 km2.

Fig. The Mediterranean Sea is formed of various interconnecting but nearly autonomous basins. In the Western Mediterranean, the Alboran, Balearic, Tyrrhenian and Ligurian Seas can be identified though only bottom topography, hydrographic or even historical boundaries separate one from the other. In the Eastern Mediterranean, the Aegean and the Adriatic Seas are clearly separated from the rest of the basin while no real separations exist between the Ionian and Levantine Seas. The Mediterranean Sea is connected with (and separated from) the Atlantic by the Strait of Gibraltar (15 kilometers wide, with an average depth of 290 meters and a maximum of 950 meters), with the Sea of Marmara* by the Dardanelles (between 450 meters and 7.4 kilometers wide and 55 meters deep), and with the Red Sea by the Suez Canal (1200 meters wide and 12 meters deep). * Marmara inland sea in north-west Turkey connected to the Black Sea by the Bosporus and connected to the Aegean by the Dardanelles

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Plate Tectonics Theory نظرية تكتونية الصفائح

Is the comprehensive theory that tied the ideas of continental drift and seafloor spreading theories together to explain the horizontal crustal movements. The concept of the theory is that, the outer part of the earth is split up into a set of rigid, moving plates. These plates move because of slow convecting currents of hot rock inside the earth.

The continental drift زحزحة القارات( ) This theory is proposed by Wegener, a German meteorologist, astronomer, and Arctic explorer. He proposed the existence of a single supercontinent, he called Pangaea. Wegener thought that forces arising from the rotation of the earth began Pangaea's breakup. First, the northern portion composed of North America and Eurasia, which he called Laurasia, separated from the southern portion formed from Africa, South America, India, Australia, and Antarctica, for which he retained the earlier name Gondwanaland. The continents as we know them today then gradually separated and moved to their present positions. Wegener based his ideas on the following: 1. The geographic fit of the continents, and the way in which some of their older mountain ranges and rock formations appeared to relate to each other when the landmasses were assembled to form Pangaea. 2. He also noted that fossils more than 150 million years old collected on different continents were remarkably similar, implying the ability of land organisms to move freely from one landmass to another. Fossils from different places dated after this period showed quite different forms, suggesting that the continents and their evolving populations had separated from one another.

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Wegener's theory however was rejected; because most geologists agreed that it was not possible to move the continental rock masses through the rigid basaltic crust of the ocean basins. There was no mechanism to cause the drift. Eventually new technology provided evidence to support his idea. Seafloor spreading ( أرضية المحيطاتساع( Sea-floor spreading is only part of the story of plate tectonics. The earth’s surface is covered by a fairly rigid layer composed of the crust and the uppermost part of the mantle. This layer, approximately 100 km thick, is called the lithosphere( الغالف الصـخريــألرض ــلب لـ The .(الصـlithosphere is broken into a number of plates called lithospheric plates. A plate can contain continental crust, oceanic crust, or both. The lithospheric plates float on the rest of the upper mantle, called the asthenosphere ( ــالف الغ which is denser ,(الــواهنand more plastic. In the early 1960s H.H. Hess and Robert Dietz proposed that within the earth's mantle there are currents of low-density molten material heated by the earth's natural radioactivity. When these upward-moving mantle currents reach the lithosphere, they move along under it, cooling as they do so until they become cool enough and dense enough to sink down toward the

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core again. These patterns of moving mantle material are called convection ( حمـل حـراري) cells or convection currents.

These currents drive the plates apart. As a result, the rocks of the oceanic crust break and form a crack between the plates. Magma rises through the cracks and seeps out onto the ocean floor, which then cooled and solidified as it meets the water forming new oceanic crust. The process continues today. Molten mantle materials continually rise to fill the cracks formed as the plates move slowly apart from each other. The mid-ocean ridges form the edges of many of the plates. It is at the ridges that the lithospheric plates move apart and new sea floor—that is, new oceanic lithosphere—is created by sea-floor spreading. If the plate includes a block of continental crust, the continent is carried along with the plate as it moves away from the ridge. Because new crust is being formed and added to the ocean floor during sea-floor spreading process at mid ocean ridges, the processes are Constructive بناء. The seafloor spreading at the Mid-Atlantic Ridge causes the North America and South America plates move westward, while the Eurasian and African plates move eastward.

The major lithospheric plates and their direction of relative movement are shown here. The boundaries between plates correspond to

most of the earth’s earthquakes and volcanoes.

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Trench formation If new crust is being produced in the manner described above, a mechanism is needed to remove old crust since there is no measurable change in the size of the earth. In fact, as the lithospheric plates diverged at the mid-oceanic ridge it will be converged with another plate on the other side. In some places, the movement of the plates causes them to collide with one another. When this occurs, earthquakes are formed, and the heavier and denser oceanic plates sink into the hot mantle beneath the lighter continental plates in a process called subduction The subduction process depresses the sea floor forming a trench. As an oceanic .االندساس plate is subducted into the Earth, it is subjected to increased pressure and temperature. These conditions cause the plate materials to melt. This molten material moves upward through crustal fractures adjacent to the subduction zone and forms belts of volcanoes along the trench and above subducted plates. Because old plates are being destroyed during trench formation, the processes are destructive هدام. When volcanoes produced in this way are close to the continent they often causes the development of continental volcanoes which may form coastal mountain ranges (e.g.. Andes Mtns. along Pacific coast of South America) When volcanoes are separated from a continent they often form island arc systems األقواس Volcanic action of this type form the island arc chains of Japan, Philippines, and .الجزريةMalaysia. Plate tectonics in our region Palestine is located on the Sinai sub-plate (part of the African plate).The Dead Sea Rift which is part of long fissure in the earth's surface called the Great Rift Valley represents a key tectonic feature in the eastern Mediterranean region. The Dead Sea Rift is considered to be a plate boundary between the Arabian plate in the east and the Sinai subplate in the west. The tectonic forces on the Sinai subplate and the Arabian plate are a transform fault, where the adjacent plates grind past each other. The rate of displacement across the fault has been estimated as 1–10 mm/yr. From the Dead Sea the fault extends almost due south to the Red Sea, and almost due north along the Jordan River and up into Lebanon, eventually wending its way into southern Turkey. Earthquakes in the Eastern Mediterranean Region, are associated mainly with the northward movement of the Arabian plate and its collision with the Eurasian plate.

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Evidence supporting the theory of plate tectonics:

a. Radiometric dating of rocks revealed that the oceanic crust is not more than about 200 million years old anywhere, while the rocks from the land are much older.

b. Echo sounders revealed the shape of the Mid-Atlantic Ridge and trenches. c. Seismographs revealed that volcanoes and earthquakes occur mostly in narrow belts

or zones on the earth correspond to the edges of tectonic plates (plate boundaries) and are associated with the mid-ocean ridge.

d. Samples of the sediments that cover the ocean bottom indicated that, sediment age and thickness were shown to increase with distance from the ocean ridge system.

e. The discovery of magnetic anomalies (الحيود المغناطيسية)

• Scientists discovered that the magnetic properties of rocks on both sides of the ridge were symmetrical.

• Marine magnetic anomalies always run parallel to mid ocean ridges. • Magnetic stripes reflect times when Earth's magnetic field is alternatively normal

and reversed polarity.