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.