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Tectonic Processes Plate tectonic theory The location of continents today of far removed from what it was millions of years ago, when it is believed that all continents were joined to one land mass. Alfred Wegener put forward a theory regards their movement in 1912 with his theory of Continental Drift. His theory was based on observations such as: Biological: Coal is found in UK but needs warm, wet, humid conditions to form. Observational: The shapes of countries appear to 'fit'one another, for example, S.America And Africa. In the 1940's-1960's the theory was revised and Plate Tectonic theory emerged. It is based on the premise that the lithosphere (crust and rigid upper mantle) is divided into plates, which are moved by convection currents coming from the earth's core. Plates and Plate Movement Characteristics of the crust There are 2 main types of crust that lie on plates and each has its own characteristics: Oceanic Crust (Sima) Young (under 200 million years), thin, dense and heavy, will sink. Always being created and destroyed. For example, Basalt. Continental Crust (Sial) Old, light, thick (up to 150km under mountains) permanent, does not sink. For example, Granite. Plate margins Movement of plates is in one of 3 ways: Towards each other: Convergent (destructive or collision). Away from each other: Divergent/ Constructive Alongside each other: Transform or transcurrent. There is a further subdivision for Convergent plates, which is dependent on the properties of the plates moving towards each other (oceanic /continental crust).
This is usually associated with plate boundaries. Volcanoes vary greatly in their shape, as does the type of material emitted during an eruption. Volcanoes range from steep to gently sided. Types of volcano Two contrasting types exist; those dominated by lava, usually found at diverging boundaries, where basalt can rise freely to the surface (volcanoes of Iceland) and those dominated by Ash, usually found along subduction zones where large amounts of pyroclastic material is ejected (Japan -Fuji Yama) The cone shapes of volcanoes are summarised in the table below: Cone Shape: Characteristics: Fissure Very gentle slope, found at diverging ocean plates, basaltic lava, can flow over large distances. Basic / Shield Have gentle slopes, steeper than fissure due to repeated explosions and subsequent build up of basalt based lava (Mauna Loa Hawaii). Cone Symmetrical in shape, A) acid where thick viscous lava, rapidly cools, B) Ash / Cinder. Composite Very large old volcanoes. Both ash and lava are deposited (Mt. St. Helens). Crater / Caldera Form when a very violent eruption occurs after a build up of gas beneath the volcano. Can destroy the magma chamber leaving a large crater. Volcanic Hazards Types of lava flow This ranges from being extremely thick and viscous, to highly fluid. The amount of pyroclastic material also varies. The two types are: Aa flow: This is a few metres thick, a mix of uneven shaped, sharp edged ash and cinder blocks.
Hazard prevention exists in a number of ways: Training and Education: This is often cited as the best and most effective way of reducing the impact of hazards. In Japan one day a year is dedicated to 'acting out' what to do in the event of an earthquake. Citizens of The USA and New Zealand know what steps to take in an earthquake as public awareness has been raised via brochures and such like. In the Philippines the first week in July is now dedicated as 'National Disaster Consciousness Week' Building Restrictions and Land Use planning: This is far more commonplace in MEDC's where there is a lower density of population and the money available to enforce such codes. Buildings can be restricted in height, have to conform to a certain design to ensure they have the greatest chance of remaing intact in the event of a hazard. For example on the North Shore of Hawaii residents on the coast are not allowed to live on the ground floor of their homes to reduce the impact of tsunami. Problems exist in LEDC's such as the Philippines where housing is vulnerable, due to material being salvaged from homes destroyed in a typhoon in order to rebuild others. There is currently a move towards educating the public on low-cost typhoon resistant housing. Aid: Often the most contentious issues regards preventing hazards. LEDC's are eager to limit short-term handouts, such as clothes, money, food, and focus on long-term measures that they believe can help a country become more self-dependant and learn to cope with the impacts of hazards for themselves.
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