Plate Tectonics- Plate Movement

Plate Margins

• Towards each other: Convergent (destructive or collision)
• Away from each other: Divergent
• Alongside each other: Transform

Constructive Margins

Plates move away from each other, for example, N. American and Eurasian plates, creating mid-ocean ridges such as the Mid Atlantic Ridge.

Features of Constructive margins:

• Mid-Ocean Ridges- long high and often complex structures of rifts and scarps.
• Volcanoes- occur along mid-Atlantic ridges, sometimes rising above the sea to form islands such as Surtsey, Iceland. These volcanoes have basaltic lava which has low viscosity and can flow over great distances with gentle sides. Volcanoes are also formed at rift valleys, Africa; these volcanoes are different to ones associated with mid -ocean ridges.
• Rift Valleys- on continental areas due to the fracturing of the brittle crust. Area of the crust drops down between parallel faults to form the feature. The African rift is thought to be an emerging plate boundary as east Africa splits from the rest of the continent. 

Destructive Margins

Oceanic and Continental

• Off western South America where the denser oceanic Nazca Plate is subducting under the less dense continental South American Plate

Oceanic and Oceanic

• In the western Pacific Ocean where the Pacific Plate is subducting under the smaller Philippine Plate forming island arcs. One example would be the Solomon Islands.

Continental and Continental

• The Indo-Australian Plate meeting the Eurasian Plate in southern Asia. Here the two plates have lower density than the underlying layers, so there is little subducting and the plate is forced upwards to form fold mountains, Himalayas.

Earthquakes are associated with all these types. Shallow, intermediate and deep earthquakes are associated with oceanic/continental and oceanic/oceanic convergences, but only shallow earthquakes are found in continental/continental collisions.

Features of Destructive Margins:

• Ocean trenches- as the denser plate subducts, the ocean floor is pulled down to form a trench. Peru-Chile trench off western South America and the Mariana trench in the western Pacific.
• Fold mountains- sediments accumulating on the continental shelf are focused upward and are deformed by folding and faulting e.g. Andes. When continental plate meet, the edges are forced up e.g. Himalayas
• Volcanoes- heat generated by friction, cause the plate to melt in an area known as the Benioff zone. This is lighter than the surrounding asthenosphere and rises towards the surface as magma. This is viscous and forms composite and explosive volcanoes.
• Island arcs- magma comes to the surface under water to form a line of volcanoes e.g. the Mariana Islands formed in association with the Mariana trench.

Conservative margins

Plates slide past each other and there is no creation or destruction of crust. There is also no volcanic activity. However there are shallow focused earthquakes. 

The best known example is the where the Pacific Plate and North American Plate are sliding past each other at different rates, forming the San Andreas Fault, California. 

Hot Spots

These are examples where volcanic activity is not linked to plate margins as in the case of the Hawaiian Islands. 

This is believed to be due to the presence of 'hot spots' - places of localised heat under the earth's crust that then find their way to the surface, similar to solar flares on the sun, volcanoes are formed and as the plate moves over the hot spot it creates a chain of islands and over time they are eroded by the sea and weathering.