1 Summary Geography Class 13 ## Last class revision(5.13 PM). ## Ocean-Ocean convergence(5.20 PM): - When 2 oceanic plate margins converge oceanic plates of higher density descend into the asthenosphere, this process is called a subduction zone. - The process of subduction leads to the formation of trenches which are the deepest regions on the surface of the earth. - For example subduction of the pacific plate below the Eurasian plate has led to the formation of the Mariana Trench, and Japan Trench. - The subducting plate undergoes deformation, intense compression, metamorphism, and melting as it reaches the deeper parts. - Some of the molten material from the subducting plate rises upwards and accumulate continually on the other oceanic plates, this results in the formation of Volcanic mountains on the seafloor. - When these Volcanic mountains rise above the sea level it results in the formation of volcanic islands. - The continued volcanic activity gradually increases the size and elevation of volcanic islands. - These islands are arranged parallel to the trenches in an arc shape and are called island arcs, for example, japan, Aleutian, and the Caribbean islands. - An Archipelago is a group of scattered islands in the ocean formed due to ocean-ocean convergence plate boundary with intense volcanic activity, for example, Indonesia, Philippines, etc. - A subduction zone is the site of widespread volcanic activity and earthquakes. - In ocean-ocean convergent, all 3 kinds of earthquakes shallow, intermediate, and deep-seated are found. - The majority of the earthquake appears to be confined to a dipping zone along the subduction plate called Benioff Zone. ## Ocean-Continent convergent plate boundary(5.35 PM): - When a plate with an oceanic margin collides with a plate of the continental margin oceanic crust is denser and subducts beneath the continental plate which is more buoyant, for example, the Nasca plate subducting below the South American plate. - During the subduction, the thick sequence of rocks along the continental margin is compressed and deformed to form a chain of fold mountains, for example, Andes and rocky mountains. - As the oceanic plate subducts it creates trenches and earthquakes are generated at Benioff zones. - As the oceanic plate subducts deeper it gets melted and the magma starts to rise. - The rising magma accumulates within the adjacent continental crust which eventually rises to the surface resulting in the formation of volcanoes. - For example, the subduction of the Nasca plate below the  South American plate has resulted in a series of volcanos in the Andes, such as Ozes del Salado and Cotopaxi. - St. Helens volcano in the Rockies is also similar. - Ocean-continent convergence results in all the 3 types of earthquakes. - The Pacific Ocean is surrounded by trenches on all sides, which are zones of intense volcanic activities and earthquakes, therefore it is called as Pacific ring of fire. ## Continental-continental convergence(6.09 PM): - Before the collision of the continental margin, the land masses are usually separated by the oceanic crust. - As the two plates converge, the oceanic crust located in between starts to subduct beneath one of the plates. - The oceanic plate completely breaks from the continental block and assimilated into the mantle, at this point, Volcanic activity stops. - The continued convergence causes the two continual masses to get stitched together along a zone called a suture zone. - When continental margins continue to collide the low density of continental material doesn't permit its subduction. - The continued convergence forces the continental crust partially under the other one creating an unusually thick layer of intense folding. - The sediments deposited in the basis between the continental margins undergo continued compression resulting in the formation of very high-fold mountains, for example, the Himalayas. - Shallow and intermediate earthquakes are absorbed along this margin. ## Parallel plate/Conservative plate margin(6.39 PM): - At the conservative plate margins, the plate slightly passes each other without any formation of new crust. - The transform fault roughly moves parallel to the direction of plate movement, for example, the San Andreas fault of Califonia. - A conservative plate margin involves no volcanic activity. - The earthquakes observed are shallow in depth but they are more frequent and intense. - Such a series of transform faults are also found along the oceanic plate margins where they offset the oceanic ridges throughout their length. ## Table summarizing plate boundaries(7.16 PM): - | | | | | | | | | --- | --- | --- | --- | --- | --- | --- | | | Divergent(O-O) plates | Divergent(C-C) plates | Convergent(O-O) plates | Convergent(O-C) plates | Convergent(C-C) plates | Conservative/parallel plates | | Features. | Mid-oceanic ridges, transform faults. | Rift valley, shallow sea, MOR. | Trenches, subduction zones, archipelago,  island arcs, Benioff zone. | Trenches, subduction zones, fold mountains, volcanoes Benioff zone. | suture zone, partial subduction, geo-syncline, fold mountains, nappe. | Series of transform faults | | Earthquakes. | shallow. | shallow. | shallow, intermediate, and deep. | shallow, intermediate, and deep. | shallow and intermediate | shallow. | | volcanism | Yes | Yes | Yes | Yes | No | No | | Examples | Mid Atlantic Ridge, Carlsberg Ridge. | East African rift valley, Red sea, Mid Atlantic ridges | Japan, Aleutian, Indonesia, etc. | Andes, Rockies mountains. | The Himalayas. | San Andreas. | ## Causes of Plates movements(7.32 PM): - ==Diagramatic representation of various causes:== - ==== - ==The convection currents:== - They are generated due to the intense heat released from the interior of the earth. - The theory of convention current was proposed by Arthur homes. - As the currents ascend from below they diverge and spread laterally. - ==Mantle plumes:== - It is a type of mantle convention that involves jet-like plumes of low-density magma material from the core-mantle boundary. - Mantle plumes are also the cause of the formation of volcanic hot spots, for example, reunion, Hawai, etc. - ==Ridge push:== - The magma rushes along the oceanic ridges from wedges of the new lithosphere on either side trail edge of the plate and causes the plates to be pushed apart. - ==Gravity sliding:== - The spreading centers along the mid-ocean ridges stand high on the oceanic floor this results in the gravitation side of the lithospheric slab being away from the oceanic ridges. - ==Slab pull:== - It is considered to be along the subduction zone where the subducting plate pulls the rest of the slab along. ## The topic for the next class: Dictation for causes of plate movement and volcanism.