The Differences between Convergent Boundary and Divergent Boundary 地质学系专业英语论文.doc

The Differences between Convergent Boundary and Divergent Boundary Abstract: Plate boundaries are generally divided into three types: convergent boundary, divergent boundary and transform boundary. In this paper, the author mainly introduced the first two types of the boundaries and compared them with each other. This problem were explained in tectonics and material. Key Words: convergent boundary, divergent boundary, tectonic, materialIntroductionDivergent boundaries(Figure 1-1) are places where the crust is being extended, thinned, and fractured as magma, derived from the partial melting of the mantle, rises to the surface, intrudes into vertical fractures, and flows out onto th sea floor forming pillow lavas. Whats more, divergent boundaries also occur under continents during the early stages of continental breakup. When the separating continues, the new oceanic crust is formed. At that time, the continental rift ridge will become a mid-ocean ridge. Its happening to Red Ocean today. We believe that it can become a large ocean plate just like Atlantic Ocean today. Figure 1-1. Convergent boundaries occur where one plate subducts into The model of divergent boundary the other plate. When it comes to convergent boundaries, we often devide them into three types as following: oceanic-oceanic boundaries, oceanic-continental boundaries and continental-continental boundaries. Now lets intruduct them one by one.(Figure1-2)Oceanic-Oceanic Boundaries: When two oceanic plates converge, one of them is subducted under the other along an oceanic-oceanic boundary. In this case, the subducting plate will remelt at a certain depth which leads to form magma. When the magma rise up to the nonsubducted plate, it will be cooled on the surface of the plate and form a set of islands called volcanic island arc. Figure 1-2The model of convergent boundaryOceanic-Continental Boundaries: An oceanic-continental boundary occurs when oceanic crust is subducted under continental crust just like what is happening to Pacific Ocean today. In this case, when the ocean plate subducts at a certain depth, it will causes melting. One day, the magma will rise up to the continental plate and form a set of mountains called volcanic arc. If the subducting is faster than the separating of mid-ocean bridge, the oceanic crust will dissappear in the end.Continental-Continental Boundaries: Two continent approaching each other will initially be separated by an ocean floor that is being subducted under one continent. The edge of that continent will display the features characteristic of oceanie-continental convergence. As the ocean floor continues to be subducted, the two continents will be closer and closer until they eventually collide.During this progress, it will form new mountains like Himalayas of China. DescriptionAccording the above-mentioned, we can easily know that there are many differences between divergent boundaries and convergent boundaries. Now, lets discuss the topic respectively from the two aspects as tectonic and material. In tectonics, those two types of the boundaries suffer from completely different stress which causes different models of plates movement. Divergent boundary is under tensional stress, so it makes two plates move against with each other. When it happens to continental crust, the divergent boundary will creat a continental rift. If the process is going on, the continental crust will turn to be oceanic crust. That is to say that a new ocean is created by the tension stress. And the rift will become the new oceans middle ridge.On the contrary, convergent boundary is under compressive stress. It makes two plates move towards each other and lead one of them to subduct into the other one. If it happens between oceanic plate and oceanic plate, the remelting magma will rise up, be cooled, and form volcanic island arc. If it happens between oceanic plate and continental plate, the oceanic crust will subduct into mantle. When it rise up to the surface of continental plate, it will creat a set of volcanic arc which is a part of continental crust. When the subducting continues and is faster than plates separating, the ocean will be closed in the end. If there is a convergent boundary between continental plate and continental plate, a new mountain will form nearby the boundary. In materials, there are still a lot of great diffrences between divergent boundary and convergent boundary. But the cause is same to what is in tectonic: tensional stress and compressive stress. That is to say, different tectonic environment has different typical materials. In divergent boundary which is under tensional stress, with the separatist of the continental plates, magmas alkalinity gradually decline until the formation of the oceanic rift. It makes fractional crystallization become an important role in the evolution of the rift magma, which determines the rift within the presence of a variety of igneous rocks and fractionation series. Thats the main reason of why the continental crust can become oceanic crust.In convergent boundary which is under compressive stress, basalt, K-feldspar rocks and amphibolite in the subducting oceanic plate turn to be quartz eclogite because of dehydrating. The water they release leads to upper mantle rocks partial melting. When partial melting of magma upwell, olivine will crystallize out and generate TH magma which can erupt near the island arc. If the subducting continues, the talc, rhyolite , brucite in oceanic crust will dehydrate, which makes the oceanic crust become melt and genarate the lava with large ion lithophile element. It will crystallize during the rising process, precipitating garnet and pyroxene. At last, it will form the calc-alkaline magma which is mainly maked up with andesite. ConclusionAccording to what we have discussed, we can summarize the differeces between divergent boundary and convergent boundary as following:In tectonics, divergent boundary under tensional stress tends to generate continental rift on the continental crust and mid-ocean ridge on the oceanic crust. And convergent boundary under compressive stress tends to generate volcanic island arc over the surface of oceanic crust, volcanic arc over the boundary of oceanic plate and continental plate and collision mountains between continental plate and continental plate.In materials, divergent boundary makes the rift within the presence of a variety of igneous rocks and fractionation series. And convergent boundary can turn basalt, K-feldspar rocks and amphibolite into quartz eclogite first, and with the subducting process continuing, the calc-alkaline magma will be formed at last. References1 Wang T, Zheng YD, Liu SW, Li TB and Ma MB. 2002. Mylonitic potassic granitoids from the Yagan metamorphic core complex on Sino-Mongolian border: a mark of transition from contranctile to extensional tectonic regime2 Reed Wicander and James S. Monroe. 1995. Essentials of Geology3 Han YW and Ma ZD. 2003. Geochemistry4 Zen ZX and Fan GM. 2008. Tectonic Geology5 Hong Kong geology webpage about convergent boundary and divergent boundary