Plate tectonics and associated hazards

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Structure of the Earth
Lithosphere= top 100km of Earth.Asthenosphere = upper most mantle and crust (down to 700km).The crust itself is mad up of continental and oceanic plates:Continental Thicker (30-40km on average) over 1500 million years old lighter (density of 2.6) mainly made up of granite Oceanic: Thinner under 200million years old heavier (3.0 density)

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Plate tectonic theory
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Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen "plates" that move around across the earth's surface relative to each other. There have been many factors that have provided evidence to this theory: Idea that Europe, Africa and the Americas fit together like a jigsaw. The Wallace line, dividing Indonesia in half with the Western part having animal life similar to continental Asia and the eastern part resembling those found in Australia. Alfred Wegener's Pangea Similar fossil records found across Africa and S.America Paleomagnetism. Discovery of the Mid-Atlantic Ridge, providing evidence towards Hass' theory that this was where the plates were splitting apart, allowing magma to rise in-between the cracks. 

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Constructive plate margins
Occur when two plates diverge. As the plates diverge, the magma rises to fill any possible gaps, creating new oceanic crust. initially forms submarine volcanoes, that over time build up above sea level. They also produce oceanic ridges. Where there is lateral movement, transform faults are produced at right angles to the plate boundary. Oceanic ridges separation of 10-15mm per year: wide ridge axis with deep central rift valley. 50-90mm per year: less well marked rift. >90mm per year: produces smooth crest and no rift. 

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Rift valleys
Tensional forces as the crust pulls the rocks away from each other normal faults/ cracks appear. crusts on either sides of the faults pull apart, causing the rock layers in between to sink. East African Rift systemThe African plate carries most of the continent, while the smaller Somali plate carries the Horn of Africa. The Arabian plate, Nubian plane and Somali plate meet at a triple junction, where they are pulling away from each other, creating a rift system. The 60km crack that opened up in the region of Afar, Ethiopia in 2005 will eventually form into an ocean, splitting Africa into two.

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Hot spots
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Hot Spots are areas of crust with unusually high heat flow found away from plate boundries. Eruptions are usually of low-viscosity basaltic magmas.HawaiiHawaii currently sits on top of a hot spot in the Pacific Ocean, which has led to the formation of the Hawaiian Island chain.  The hot spot has led to a series of volcanic eruptions which have created  the island chain which have slowly risen out of the sea. While the hot spot remains stationary, the Pacific Plate moves in a North-Westerly direction above it. As you move in a North-westerly direction from Hawaii, the age of the islands increase, with Kauai being 5.1million years old compared to Hawaii which is only 400,000 years old.

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Destructive plate margins
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Causes: The denser oceanic crust is subducted beneath the lighter continental plate. The crust is destroyed into magma due to the heat generated by friction between the two plates. The newly formed magma is less dense than the mantle, so rises to the Earth's surface to form volcanoes. These margins can also occur between two oceanic plates, with the plate on top forming a continental shelf. Collision margins are former destructive plate boundaries between two continental plates. Earthquakes at destructive marginsBenioff Zone: An inclined zone along the subduction zone along which earthquakes are recorded.Shallow earthquakes: In the descending slabs due to outer part of cold oceanic lithosphere fracturing. In the overlying continental due to block uplift and subsidence. Intermediate/deepdue to compression and extension within slab.

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Landforms- Fold mountains
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Material is deposited in geosynclines. As the two continental plates move towards each other, the sediment becomes compressed and is turned into rock, which begins to fold, becoming fold mountains.

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The nature of volcanic eruptions
Magma contains large volumes of dissolved gases confined under high pressure. Once pressure drops, gases exolve with rapid expansion of volume, forcing magma to the surface. Lava with low viscosity enables gas to escape freely through vesicles, while more viscous lava traps the gas until sufficient pressure to blow lava apart. Lava blown out of the vents in fragments is called pyroclastic material. this material settles through the air and is called tephra. Words such as bomb, lapilli ad ash describe tephra of decreasing size. Sometimes the highly viscous lava is pushed down the sides of volcano. The pyroclastic material and gas are mixed in a hot cloud called a nuee ardente.Lava types:basaltic: Free flowing and fast moving (low viscosity). Eruptions produce large volumes of lava with little pyroclastic material. Andesite: more viscous, producing less lava and more pyroclastic material. Generally found at destructive margins.Rhyolite: extremely viscous that it does not move away from the vent, and is forced into dome like mounds called tholoids.The extremely viscous eruptions produce virtually no lava, with all products being in pyroclastic form. 

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Shield volcanoes- Mauna Loa
A very wide volcano, of 3 or 4 miles and heights of 1,500 to 2,000m.  Mauna Loa has heights of 28,000 feet. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic  lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone.  The magma has very low gas content and is low in silica, allowing it to flow large distances.
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A strato volcano is a tall, conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, strato volcanoes are characterized by a steep profile and periodic, explosive eruptions and quiet eruptions. The lava that flows from strato volcanoes typically cools and hardens before spreading far due to high viscosity. The increase in temperature causes the dome to expand while its outer lava cools. This growth causes the newly hardened surface to splinter, causing loose debris to fall from its sides, the reason for its appearance.
Strato volcano
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Other types of volcano
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