Chapter (Thompson, 1991). Chapter 2. Convergent plate boundaries

Chapter1.

                                                     Introduction1.1 General statement: Plate tectonics is a logical hypothesis that recognizes theextensive scale movements of Earth’s lithosphere. Where plate’s boundariescombine, the movement of these plates decides that what type of boundary formhere, Example convergent plate boundary, divergent plate boundary, transformplate boundary. Along these plate boundaries, Earthquakes, volcanic activity, mountain-building,and Oceanic trench arrangement happen along these plate limits. The horizontalrelative development of the plates normally shifts from zero to 100 mm everyyear.

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The Earth’s crust is divided into 12 plates which changes thereposition in various directions with the passage of time. The motion of theseplates causes, collision (collide each other), Pull apart or slide past withone another. With plate interaction different type of geologic structures ortectonic feature are developed (Figure 1).

Certaintheories of plate tectonic are present which explain that at ocean ridges theplates are generated and along Subduction zones the plate are consumed, andareas where transform fault present the plates are slide past each other andwhen two plates collision occur, then suturing of plates occur (Condie, 1997).  1.2 Types of plate boundaries:Plate boundaries are classified into three types, on thebasis of motion relative to one another (Kious et al, 1996).a.      Convergent plate boundary: Oneplate dives beneath another (Subduction) or two plates collide without eithersubductingb.     Divergent plate boundary: New lithosphere forms as platespull apart. c.

      Transform plate boundary: Plates grind past each other, nochange in Lithosphere.Thesethree plate boundaries have different type of stresses so because of thisproduce different structure. At divergent plate boundary extensional stressesare generated which make normal faults and graben which is uncommon. Attransform plate boundary shear stresses produced which causes bending andfractures in the rock, fault, folds and uplift of rock occur. At convergenceplate boundary compressive stresses are produced, because of which folds,reverse faults and thrust fault (Thompson, 1991).   Chapter 2.                              Convergentplate boundaries2.

1 Convergent plate boundaries:At convergent plate boundaries collision of two plate occurbecause of when two plates are moving towards each other. In these boundariesthe heavier (Denser) plate drive undergo the less heavy (lighter) plate. Theprocess in which one plate will go below the other is called Subduction, theplate which is going below the other are reach into the mantle and here theymelts because temperature is very high in the mantle.At convergence plate boundaries continuously new Lithosphereis produced occur along oceanic ridges, but there is no changing occur in theearth total volume. The volume of earth remains same as previous one (earthvolume remain constant). There is a cyclic process continue, the older oceaniclithosphere will go into the mantle as same rate as the rate of production ofsea floor.

2.2 Subduction zones:Subduction zones which is also known as Convergentboundaries. Subduction zones are the place where one plate is undergo beneaththe other (subducted) and will reach to the mantle. Subduction of two platesoccur when there is density contrast between the two plates for example the oceaniclithosphere is subduct below the continental plates because oceanic plates aredenser than continental plate. The continental lithosphere is less dense sothere is no subduction occur and the oceanic plate will have high density andwill subduct up to greater depth(Zhao et al., 1992).

 2.3 Deep-oceantrenches:Whenthe lithosphere having oceanic origin descend down and will go into mantle asurface like feature are produced called deep ocean trenches, these trenchesare in the form of long and deep depressions. Example Pero Chile trench whichis 4499 km in length and 7.9 km below sea level present along the west coast ofS. America. Other example of western Pacific are Mariana trench and Tongatrench which are deeper than those which are present in the Eastern Pacific.Theoceanic plate slabs are goes down into the mantle with small angle or at 90degree.

The angle downward movement is depend upon the density of the plate.For example when the spreading zone is around the subduction zone along Peru Chiletrench, the subducting material are younger and are hot and floating outward.Here small angle of descent are form. Because of this the area around the PeruChile trench have large no of earthquakes i.

e., 2010 Chile earthquake which isalso included in top 10 largest recorded earthquake. Properties of all theconvergent plate boundaries are same and they have very complex structures.Each of these feature is form from different material and different tectonicenvironment (DeLong and Fox, 1977). 2.4 Typesof convergent plate boundaries:Whentwo ocean plate, or one continental and ocean plate or either two continentalplate meet or collide convergent boundaries are generated On the basis of thisthere are three type of convergent plate boundaries (Frederick, et al 2012).1.     Oceanic-continentalconvergence2.

     Oceanic-Oceanicconvergence3.     Continental-continentalconvergence 2.4.1Oceanic-continental convergenceThisis the type of convergence in which one continental plate and one oceaniccollide with one another. In this convergence the plate having higher density(oceanic plate) goes down and reach into the mantle and the lesser densityplate (continental plate) floating. When these plate slabs reaches into themantle here they started melting of the block of asthenosphere, these processesoccur at a depth of 90-100 km (Figure, 3.A).Inthis type of convergence partial melting of rock occur, which produced about 11percent molten material and these molten material are then mixed with mantlerock those which is not melted.

When the oceanic plate goes downward, theycontain sediment which have high amount of water, these are derived fromoceanic crust, and the high pressure and temperature condition remove waterfrom the pore spaces. This process causes melting of rock which is calledpartial melting. These melting rock are less dense than the surrounding rocki.e. mantle rocks so these material rises upward in the form of volcanoes, butall of these material not reaches to the surface and solidifies within the earthcrust which causes the increase in the crust volume.Exampleof the above process is Andes Mountain, which is form from the subduction ofNazca plate below the South American plate, these are form from subduction ofoceanic lithosphere and known as continental volcanic arcs.

Other examplesinclude the mountains of California contain Mount Rainer, Mount Shasta, andMount of St. Helens which is reach to the Canada and here it include MountGaribaldi, Mount Silverthorne and others (Frederick, et al 2012)  (Figure 3.B).2.4.

2Oceanic-Oceanic convergence:Whentwo ocean plate are meet or collide each other, they form as oceanic-oceanicconvergence boundaries. Many feature which are produced along this type ofconvergence are commonly present in continental plate boundaries. At thisboundary also subduction of one plate occur which reaches into the mantle andtriggers volcanic activity, as same process which is occur alongcontinental-oceanic convergence boundaries (FIGURE 4.A).Thetwo ocean plate convergence setting, the volcanic activity generated from theocean floor. In this a chain of volcanic mountains are produced which is occurin large amount and form as Islands. These are called as volcanic Island arc orIsland arc (FIGURE 4.

B)ExampleMariana and Tonga Islands (young island arc). The Island arc produced 99-301 kmfrom deep ocean trench. The western pacific ocean contain most island arcs, theAtlantic Ocean contain only two island arcs. Antilles Island are form becauseof the subduction of the Atlantic beneath the Caribbean plate, present inUnited States, British virgin island and also present in Martinique islandexample of this is Mount Pelee which is erupted in 1902 and kill 28,000 people.Theisland arc which is form newly have simple structure and underlain by oceaniccrust, commonly less than 21 km thick. And the older island arc have complexstructures and are underlain by highly deformed crust which reaches to 36 km inthickness.

Examples Japan, Indonesia, and Philippines. These island arc aregenerated form early form island arc system (Frederick, et al 2012). 2.4.3Continental-continental convergence:Thistype of convergent boundary is form from the collision of two continentalplates. In this type of boundary no subduction occur, because of this boundarymountain ranges are form. (FIGURE 5.

A). When the collision of two continentalplate occur it will causes the formation of folds and deform the sediment andsedimentary rock which is accumulate here along continental plate boundaries.This collision causes the formation of high mountain ranges which is highlydeformed and contain sedimentary and metamorphic rocks (FIGURE 5.C).Exampleinclude the collision of indian plate and the Eurasisan plate, starts before51-60 million years ago, produced Himalayas, which is the very important andhave the world most highest peaks. In this collision the crustal shorteningoccur and increase in thickness takesplace.

Otherexample include, Alps, Appalachians, and Urals which are also form as a resultof collision of two plates (Frederick, et al 2012) (Figure 5.C).Chapter 3.                                                      SummaryThis report is about the convergent plate boundaries, inwhich explain the formation of convergent plate boundary, how subduction occurand how oceanic trenches are formed then explain the different types ofconvergent plate boundaries, Threedistinctive types of convergence are recognized: (1) the convergence of twooceanic plates, (2) the convergence of a continental plate and an oceanicplate, and (3) the convergence of two continental plates.Convergentplate boundaries (continental-continental) is a place where two tectonic platesmeet each other. All tectonic plates are constantly being moved because of theconvection currents that occur in the mantle. Continental plates are creatingmountains when they pushed into each other. A few examples of this feature arethe Himalaya Mountain Range and the Appalachian Mountain Range.

Asubduction zone also develops where oceanic and continental plates converge.The less-dense continental crust always resists subduction into the densemantle and over rides the oceanic plate. Consequently, the volcanic arc formson the continent, and compression may deform the continental margin into afolded mountain belt.Thesimplest type of convergent plate boundary is ocean-ocean convergence which consistsof two oceanic plates. As the plates collide, one is thrust under the otherforming a subduction zone.

The subducting plate descends into the mantle, whereit is heated, triggering the generation of magma. The magma, being less densethan the surrounding rock, rises and erupts on the seafloor, ultimatelybuilding an arc of volcanic islands. Andesite is the volcanic rock thatcharacteristically forms at such sites.