Abyssal plain

Abyssal plain

kacimo samy   1:28 م

Abyssal Plain

Abyssal plain is a submerged plain on the profound sea depths, generally found at profundities somewhere around 3000 and 6000 m. Lying for the most part between the foot of a mainland rise and a mid-sea edge, deep fields cover more than half of the Earth's surface. They are among the flattest, smoothest and slightest investigated areas on Earth. Deep fields are key geologic components of maritime bowls (alternate components being a lifted mid-sea edge and flanking deep slopes). Notwithstanding these components, dynamic maritime bowls (those that are connected with a moving plate tectonic limit) likewise commonly incorporate a maritime trench and a subduction zone. 

Deep fields were not perceived as unmistakable physiographic components of the ocean depths until the late 1940s and, until as of late, none had been concentrated on an efficient premise. They are inadequately safeguarded in the sedimentary record, on the grounds that they have a tendency to be devoured by the subduction process. The making of the deep plain is the deciding consequence of spreading of the ocean bottom (plate tectonics) and softening of the lower maritime outside layer. Magma ascends from over the asthenosphere (an upper's layer mantle) and as this basaltic material achieves the surface at mid-sea edges it frames new maritime outside layer. This is always pulled sideways by spreading of the ocean bottom. Deep fields result from the covering of an initially uneven surface of maritime outside by fine-grained residue, for the most part earth and sediment. A lot of this silt is stored by turbidity streams that have been diverted from the mainland edges along submarine gorge down into more profound water. The rest of the dregs is made essentially out of pelagic residue. Metallic knobs are regular in a few regions of the fields, with shifting groupings of metals, including manganese, iron, nickel, cobalt, and copper. These knobs may give a critical asset to future mining endeavors. 

Owing to some extent to their incomprehensible size, deep fields are as of now accepted to be a noteworthy supply of biodiversity. The void additionally applies noteworthy impact upon sea carbon cycling, disintegration of calcium carbonate, and environmental CO2 fixations over timescales of 100–1000 years. The structure and capacity of deep biological systems are emphatically impacted by the rate of flux of nourishment to the ocean bottom and the material's creation that settles. Components, for example, environmental change, angling practices, and sea treatment are relied upon to have a generous impact on examples of essential creation in the euphotic zone. This will without a doubt affect the flux of natural material to the void in a comparative way and along these lines have a significant impact on the structure, capacity and assorted qualities of deep biological system.

Oceanic zones

The sea can be conceptualized as being partitioned into different zones, contingent upon profundity, and vicinity or nonattendance of daylight. About all life frames in the sea rely on upon the photosynthetic exercises of phytoplankton and other marine plants to change over carbon dioxide into natural carbon, which is the fundamental building piece of natural matter. Photosynthesis thusly obliges vitality from daylight to drive the substance responses that deliver natural carbon. 

The water's stratum segment closest the sea's surface (ocean level) is alluded to as the photic zone. The photic zone can be subdivided into two diverse vertical areas. The highest part of the photic zone, where there is satisfactory light to bolster photosynthesis by phytoplankton and plants, is alluded to as the euphotic zone (likewise alluded to as the epipelagic zone, or surface zone). The lower segment of the photic zone, where the light force is inadequate for photosynthesis, is known as the dysphotic zone (dysphotic signifies "dim" in Greek). The dysphotic zone is likewise alluded to as the mesopelagic zone, or a twilight zone. Its lowermost limit is at a thermocline of 12 °C (54 °F), which, in the tropics by and large lies somewhere around 200 and 1000 meters. 

The euphotic zone is fairly self-assertively characterized as stretching out from the surface to the profundity where the light force is more or less 0.1–1% of surface daylight irradiance, contingent upon season, scope and level of water turbidity. In the clearest sea water, the euphotic zone may stretch out to a profundity of around 150 meters, or once in a while, up to 200 meters. Broken up substances and strong particles ingest and scramble light, and in beach front locales the high convergence of these substances makes light be lessened quickly with profundity. In such zones the euphotic zone may be just a couple of many meters profound or less. The dysphotic zone, where light power is extensively under 1% of surface irradiance, reaches out from the base of the euphotic zone to around 1000 meters. Stretching out from the base of the photic zone down to the seabed is the aphotic zone, a locale of interminable dimness. 

Since the normal profundity of the sea speaks the truth 4300 meters, the photic zone speaks to just a modest portion of the sea's aggregate volume. Then again, because of its ability for photosynthesis, the photic zone has the best biodiversity and biomass of every single maritime zone. Almost all essential creation in the sea happens here. Life shapes which occupy the aphotic zone are frequently fit for development upwards through the water section into the photic zone for sustaining. Else, they must depend on material sinking from above, or discover another wellspring of vitality and nourishment, for example, happens in chemosynthetic archaea found close aqueous vents and frosty leaks. 

The aphotic zone can be subdivided into three distinctive vertical areas, in light of profundity and temperature. In the first place is the bathyal zone, reaching out from a profundity of 1000 meters down to 3000 meters, with water temperature diminishing from 12 °C (54 °F) to 4 °C (39 °F) as profundity increments. Next is the deep zone, reaching out from a profundity of 3000 meters down to 6000 meters. The last zone incorporates the profound maritime trenches, and is known as the hadal zone. This, the most profound maritime zone, reaches out from a profundity of 6000 meters down to give or take 11000 meters. Deep fields are normally situated in the deep zone, at profundities running from 3000 to 6000.

Formation

Maritime outside, which frames the bedrock of deep fields, is consistently being made at mid-sea edges (a sort of dissimilar limit) by a procedure known as decompression softening. Tuft related decompression liquefying of strong mantle is in charge of making sea islands like the Hawaiian islands, and in addition the sea outside at mid-sea edges. This marvel is likewise the most widely recognized clarification for surge basalts and maritime levels (two sorts of vast volcanic areas). Decompression liquefying happens when the upper mantle is somewhat softened into magma as it moves upwards under mid-sea edges. This up welling magma then cools and hardens by conduction and convection of warmth to frame new maritime outside. Growth happens as mantle is added to the developing edges of a tectonic plate, normally connected with ocean bottom spreading. The period of maritime outside layer is in this way an element of separation from the mid-sea edge. The most youthful maritime covering is at the mid-sea edges, and it turns out to be continuously more established, cooler and denser as it moves outwards from the mid-sea edges as a major aspect of the procedure called mantle convection. 

The lithosphere, which rides on the asthenosphere, is partitioned into various tectonic plates that are ceaselessly being made and devoured at their inverse plate limits. Maritime covering and tectonic plates are shaped and move separated at mid-sea edges. Deep slopes are shaped by extending of the maritime lithosphere. Utilization or demolition of the maritime lithosphere happens at maritime trenches (a kind of joined limit, otherwise called a dangerous plate limit) by a procedure known as subduction. Maritime trenches are found at spots where the maritime lithospheric pieces of two unique plates meet, and the denser (more established) section starts to dive once more into the mantle. At the utilization edge of the plate (the maritime trench), the maritime lithosphere has thermally contracted to end up entirely thick, and it sinks under its own particular weight during the time spent subduction. The subduction procedure devours more established maritime lithosphere, so maritime hull is from time to time more than 200 million years of age. The general procedure of rehashed cycles of creation and decimation of maritime hull is known as the Supercontinent cycle, initially proposed by Canadian geophysicist and geologist John Tuzo Wilson. 

New maritime outside layer, nearest to the mid-maritime edges, is for the most part basalt at shallow levels and has a tough geology. The unpleasantness of this geology is a rate's component at which the mid-sea edge is spreading (the spreading rate). Sizes of spreading rates differ fundamentally. Commonplace qualities for quick spreading edges are more prominent than 100 mm/yr, while moderate spreading edges are ordinarily under 20 mm/yr. Studies have demonstrated that the slower the spreading rate, the rougher the new maritime hull will be, and the other way around. It is thought this marvel is because of blaming at the mid-sea edge when the new maritime covering was framed. These issues plaguing the maritime covering, alongside their bouncing deep slopes, are the most widely recognized tectonic and topographic components on the Earth's surface. The procedure of ocean bottom spreading serves to clarify the idea of mainland float in the hypothesis of plate tectonics. 

The level appearance of adult deep fields results from the covering of this initially uneven surface of maritime outside by fine-grained residue, basically dirt and sediment. Quite a bit of this residue is stored from turbidity streams that have been diverted from the mainland edges along submarine gorge down into more profound water. The rest of the silt involves mainly tidy (mud particles) extinguished to ocean from area, and the remaining parts of little marine plants and creatures which sink from the upper layer of the sea, known as pelagic residue. The aggregate residue affidavit rate in remote regions is assessed at a few centimetres for every thousand years. Silt secured deep fields are less regular in the Pacific Ocean than in other significant sea bowls on the grounds that residue from turbidity streams are caught in maritime trenches that fringe the Pacific Ocean. 

Deep fields are typically secured by remote ocean, yet amid parts of the Messinian saltiness emergency a great part of the Mediterranean Sea's deep plain was presented to air as an unfilled hot dry salt-stunned sink.

Earth's interior and plate tectonics

Earth's interior and plate tectonics

kacimo samy   9:41 ص

Plate Tectonics Concepts

Plate Tectonics Concepts

kacimo samy   8:11 م

                     PLATE TECTONICS CONCEPTS

Introduction

Plate tectonics- The idea that Earth’s surface is divided into large plates that move slowly and change in size over time.

This idea provides a model for understanding many geologic features: faults, folds, volcanoes, earthquakes, and mountain belts.

Plate Tectonics is the culmination of two pre-existing ideas:

1) Continental Drift- The idea that continents move freely over Earth’s surface, changing their position relative to each other.  This concept was proposed by German meteorologist Alfred Wegener in the early 1900’s.

2) Seafloor Spreading- The concept that new seafloor forms at mid-ocean ridges, and then moves horizontally away from the ridge toward an ocean trench/subduction zone (the seafloor is a conveyor belt).  This idea was proposed by Princeton geologist Harry Hess in 1962.

The Early Evidence for Continental Drift

1) The continents look like they could fit together (like a puzzle).

2) Rock types are correlated from continent to continent (across the oceans).

3) The extinct plant fossil, Glossopteris, which grew in temperate climates, is found in South America, Africa, India, Antarctica, and Australia.

4) The extinct reptile, Mesosaurus, is found only in Brazil and South Africa.  The mesosaurus lived only in freshwater, and could not swim across oceans.

5) Ancient glacial deposits in South America, Africa, India, Antarctica, and Australia suggest these continents were all located near a polar region
.Wegener's Concept of Continental Drift: Polar Wandering

Alfred Wegener proposed that the continents were once assembled into a supercontinent called Pangaea.

Wikipedia Commons Image.

He also proposed that Pangaea split into two parts:

    1) Northern Pangaea (which includes present day North America and Eurasia) became Laurasia.

    2) Southern Pangaea (including South America, Africa, India, Antarctica, and Australia) became Gondwanaland.

After Pangaea fragmented, Laurasia drifted northward and Gondwanaland drifted southward.

Wegener knew that coral reefs form near the equator, deserts form about 30° north and south of the equator, and glaciation occurs near the poles.  Based upon this information, he determined the North and South Pole positions over geologic time.  He called the apparent movement polar wandering.

There were two possible explanations:

    1) The continents remained motionless and the poles actually moved (literal polar wandering).

    2) The poles stood approximately still and the continents moved (continental drift).  Wegener preferred this explanation.

Opposition to Continental Drift

-Some scientists argued that some fossils (especially fossil plants) could have been spread from one continent to another by wind or ocean currents.

-Land dwelling reptiles could have spread from continent to continent by land bridges that rose up from the seafloor (this idea was pure speculation, as no appropriate mechanism was known).

-Wegener lacked a plausible mechanism by which continents could actually drift.

-Wegener's Polar wandering might reflect true wandering of poles rather than drifting of continents.

 New Evidence for Continental Drift

  

1) Paleomagnetism- The study of Earth’s magnetic field through time; geologists look at the way magnetic minerals in rocks preserve the magnetic field.

When a magnetic mineral crystallizes and cools below its Curie point, its magnetic alignment is locked in.

The direction of the mineral alignment gives the direction of magnetic North.  The dip of the mineral alignment gives the latitudinal distance from magnetic North.

  

For example, Permian age rocks in North America point to an apparent magnetic North pole in Asia whereas Permian age rocks in Europe point to an apparent magnetic North Pole in Japan.

Note: Today, when geologists use the term polar wandering they are referring to an “apparent” wandering of the poles.  We know that the poles themselves did not wander (although, as we shall see, the poles flipped or reversed many times throughout geologic history).

2) The continental slope- If the continental slope is taken into account, the plates fit together extremely well.

3) GPS technology- Global positioning satellites allow us to actually watch and measure the drift of the continents (1-16 cm/year).

Seafloor Spreading

Wegener thought that the seafloor remained stationary whereas the continents moved.

In contrast, in the 1960's Harry Hess proposed that the seafloor was also moving.

According to Hess, oceanic crust is produced at mid-ocean ridges and subducted at trenches.  The driving force for seafloor spreading is convection (hot mantle rises near the mid-ocean ridges and cold mantle sinks near trenches). 

The best evidence for sea floor spreading was provided by magnetometer surveys perpendicular to mid-ocean ridges.  A zebra pattern of magnetic reversals reflects episodic flips in Earth's North and South poles.

Image from the USGS.

The zebra pattern of magnetic anomalies is symmetrical about the ridge crest.

The concept of seafloor spreading explains the age of the sea floor.  Near mid-ocean ridges: sea floor is young and lacks sediment.  Away from ridges: seafloor gets older and acquires a thick blanket of pelagic sediment.

 The Big Picture: Plate Tectonics

By the late 1960’s, the hypotheses of continental drift and seafloor spreading had been combined into a single, unified theory of plate tectonics.

Recall that a plate is a thick, mobile slab of the Earth’s surface made of lithosphere (crust + upper mantle).  Plates glide on the ductile asthenosphere.

There are 3 types of tectonic plate boundaries:

1) Divergent Plate Boundaries

Divergent plate boundaries can occur in the middle of oceans or in the middle of a continent.

The result of a divergent plate boundary is to create a new ocean basin.

When a supercontinent like Pangaea breaks up, the divergent boundary is found in the middle of a continent, marked by a continental rift.

During the rifting event, the continental crust is stretched and thinned, producing a normal fault.  Topographically this results in a rift valley with a central graben.

The fault provides a pathway for basaltic magma, which rises up from the mantle to form basalt flows and cinder cones.

As divergence continues, sea water will eventually fill the split.

True oceanic crust is eventually produced at a mid ocean rift between the two diverging continents.

The trailing edge of the continent on each side of the rift becomes a passive margin.

2) Transform Boundaries

Transform boundaries involve strike-slip motion of plates (a conservative boundary).

Transform boundaries are characterized by shallow earthquakes.

The San Andreas fault is a transform boundary.  A previous plate was subducted, and after subduction was complete a subsequent plate arrived with a strike-slip orientation to the mainland.

Transform boundaries also occur along the fracture zones of mid-ocean ridges.

3) Convergent Plate Boundaries

At convergent plate boundaries, two plates move towards each other.

The character of the convergent boundary depends on the types of plates that are converging:

Ocean-ocean convergence-  One of the oceanic plates will subduct.  The subduction results in an island chain of volcanoes called an island arc (for example, the Phillipines).

Ocean-continent convergence- The dense oceanic crust is subducted below the continental crust.  A chain of volcanoes forms on the continental crust as a magmatic arc.

Regional metamorphism will occur due to the rising hot magmas and also due to the convergent forces.  On the landward side of the arc, a fold and thrust belt will form.

Continent-continent convergence-  In the case of continental-continental convergence, neither plate will subduct.  First, ocean floor in-between them the continents is subducted.  Eventually, when all the oceanic crust is subducted, the continents will collide.  The two continents will weld together along a suture zone.  Fold and thrust belts will form from the convergence and regional metamorphism will occur (Himalayan Mountains).

A closer look at subduction (for ocean-ocean or ocean-continent subduction).

A Benioff zone defined by shallow, intermediate, and deep earthquakes will define the top of the down-going plate.

At a depth of ~100 km, magmas will be generated in the asthenosphere overlying the down-going plate.  The magmas will rise upward, creating a chain of volcanoes on the overlying plate that parallel the subduction zone.

What Causes Plate Motions?

Rock deep within the Earth’s interior is heated and rises whereas shallow, colder denser rock sinks.  This sets up a convection current.

Huge convection cells may extend from the heat source at the core-mantle boundary all the way to the base of the lithosphere.

Several mechanisms assist in the movement of plates:

1) Ridge-push- Plates move apart at the midocean ridge due to down slopes. 

2) Slab-pull- Subducting slabs pull the surface part of the plate away from the ridge. 

3) Trench-suction- The subducting plate falls into the mantle.  As a result, the overlying trench and plate are pulled horizontally, seaward, toward the subducting plate.

Mantle Plumes and Hot Spots

A modification to the convection model suggests that the mantle transfers heat in the form of narrow columns of hot rock called mantle plumes.

The mantle plumes may rise from the core-mantle boundary and stay stationary.

The mantle plumes have a wide, mushroom-shaped head and a long, narrow tail.

Mantle plumes are proposed to produce “hot-spot” volcanic activity on the earth’s surface, sometimes far away from any plate margins.

Hawaii volcanism is “hot-spot” volcanism.  Hawaii is located in the middle of the Pacific plate, on oceanic crust.

Yellowstone volcanism is another example of “hot-spot” volcanism.  Yellowstone is located on the North American plate in the middle of continental crust.

Hot spot volcanism fed by a mantle plume is proposed to be the reason for the breakup of Pangaea.