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Tectonic Processes and Hazards EQ1- Tectonic Processes and Hazards,…

- - - - the mantle
        -semi-molten body of rock between the earths crust and its core
        
        The asthenosphere
        -part of the mantle
        -semi-molten
        -below the lithosphere, which floats on top of it
        
        The lithosphere
        -is the crust and upper mantle which form the tectonic plates
        -together they are 80-90km thick
        the crust changes in thickness under oceans and continents:
        -under oceans, the crust is 6-10km thick
        -under continents, the crust is 30-40km thick
  - - - What are some plate tectonic discoveries?
        1
        Wegener’s Continental Drift hypothesis
        2
        Seafloor spreading creates new crust
        3
        Slab pull drags down high density ocean floor at convergent boundaries
        4
        Gravitational sliding at divergent boundaries
        5
        Internal radioactive heat drives convection currents
    - - sea floor spreading
        -in the 1960s, there was a discovery of magnetic stripes in the oceanic crust of the seabed
        -palaeomagnetic signals from the past reversals of the earths magnetic field prove that new crust is created by the process of seafloor spreading at mid-ocran ridges
        
        slab pull
        -at convergent boundaires, the high density ocean floor is being dragged down by a downward gravitational force (slabpull) beneath the adjoining continental crust
      - wegner's continental drift hypothesis
        1912
        -our now seperate continents had once been joined together as supercontinents eg Pangea
  - - - at convergent margins
        -deep sea trenches and fold mountain ranges often form at convergent plate margins
        -volcanic activity at convergent plate margins is often violent due to high viscosity lava
        -earthquakes at convergent plate margis are high magnitude, deep focus (up to 700km deep) and frequent
        -an eg of this type of plate margin is western south america
        
        at conservative margins
        -no volcanic activity at these plate margins
        -earthquakes at conservative plate margins are high magnitude, shallow focus less than 70km deep and a commen occurance
        -an example of this type of plate margin is the San Andreas fault in California
      - at Collision margins
        -collision plate margins are characterised by the meeting of two continental landmasses resulting in the formation of a fold mountain belt
        -eg the collision between the indo-austailian and eurasian plates has created the himalayan mountain range
        -there is no volcanic activity at collision plate margins
        -earthquakes at collision plate margins are rare-when they do happen, they are high mag and shallow focus (less than 70km deep)
- - - - caused by release of built up stress
        -the plates on the earths crust are always moveing and so tentions can build up between rocks
        -this release of stress can cause parts of the surface to experience an intense shaking motion that lasts for a few seconds
      - terminology
        -the focus is where the pressure is released underground and where the energy radiates out from. this is the place with the strongest waves that cause the most damage
        -the epicentre is the point directly above the centre of the earthquake on the earths surface
        -seismic waves are when energy is released from the focus in shock waves
        -the most damage will occur at the places where the shock waves are strongest (ie closest to the epicentre)
  - - - underwater earthquakes
        -underwater earthquaes move the seabed, which causes water to get displaced. if enough water is displaced then tsunamies (series of enormous waves) can happen
        -underwater earthquakes can be shallow or deep focus. the deeper the focus, the bigger the tsunami
        -waves travel in deep water, and so tsunamis can strike shores with little earning. this can catch people off guard and cause many deaths
  - - - s-waves
        some earthquakes produce secondary waves that make the ground shake violently
        
        p-waves
        -priamry waves occus when tectonic stress energy is released
        -p waves are the fastest waves but they cause the least damage
- - - - secondary hazards
        -in mountainous areas, large landslides are a common one
        Developing counties eg nepal struggle to recover from landslides because of their lack of infrastructure and resources
        -the process of liquefaction is when earthquakes compact loose sediment like silts, sand and gravel that are waterlogged
        The compact ground forces water to rise to the surface and damage the buildings and infrastructure is common
        -sub marine earthquakes result in tsunamis, which can have devastating effects eg boxing day tsunami 2004
- - - - lava flows
        -not usually a amajor threat when they are molten lava as they are slowly moving
        -they create extensive areas of solidified lawa
        examples of land masses that have been created by lava flows are Hawaii and Southwest Iceland
    - - pyroclastic flows
        -they are very hot (800C), have a high velocity (over 200km/h) and are a mixture of gasses and tephra
        -pyroclastic flows can destroy everything in their paths
        -examples of eruptions that have produced large pyroclastic flows include
        The AD79 eruption of Mount vesuvius in Pompeii, italy
        The 1980 eruption of Mouth St Helens in the USA
  - - - lahars
        -they are fast mudflows that happen when rain mobalises deposits of volcanic ash
        -in 1985 the armero tragedy took place when the nevado del ruiz volcano erupted in colombia. four lahars came towards the town of armero and around 20,000 people died
- - - - short amplitude at sea
        -tsunamis have very short amplitudes at sea- they are usually about 1m in height and are barely noticeable
        -but, when tsunamis reach land, their height can increase to over 25m in some cases
        
        velocity
        -tsunamis move very quickly and can reach speeds of up to 700kph in deep water
        -tsunamis slow down as they reach shallow water
        
        drawback
        -one of the identifying factors before a tsunami happens is a localised drop in sea level (drawback), as water is drawn back and up for use by the approaching tsunami
        
        wave train
        the way that a tsunami hits a coastline is like a series of waves that we call a wave train