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Geography Revision (Topic 1 (Restless Earth (Volcanic and earthquake…

- - - - Plates at our planet’s surface move because of the intense heat in the Earth’s core that causes molten rock in the mantle layer to move. It moves in a pattern called a convection current that forms when warm material rises, cools, and eventually sink down. As the cooled material sinks down, it is warmed and rises again.
    - - Plate boundaries
        Subduction zones, or convergent margins, are one of the three types of plate boundaries. The others are divergent and transform margins.
        At a divergent margin, two plates are spreading apart, as at seafloor-spreading ridges or continental rift zones such as the East Africa Rift
    - - Volcanic eruptions arise through three main mechanisms
        Gas release under decompression causing magmatic eruptions
        Thermal contraction from chilling on contact with water causing phreatomagmatic eruptions
        Ejection of entrained particles during steam eruptions causing phreatic eruptions
    - - The first main earthquake hazard (danger) is the effect of ground shaking. Buildings can be damaged by the shaking itself or by the ground beneath them settling to a different level than it was before the earthquake (subsidence).
      - Buildings can even sink into the ground if soil liquefaction occurs. Liquefaction is the mixing of sand or soil and groundwater (water underground) during the shaking of a moderate or strong earthquake. When the water and soil are mixed, the ground becomes very soft and acts similar to quicksand. If liquefaction occurs under a building, it may start to lean, tip over, or sink several feet. The ground firms up again after the earthquake has past and the water has settled back down to its usual place deeper in the ground. Liquefaction is a hazard in areas that have groundwater near the surface and sandy soil.