geography - restless earth

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  • geography - restless earth
    • the structure of the earth
      • the earth is made up of layers
      • the solid crust has relatively low density, allowing it to rest on the fluid mantal
      • the upper mantle is 200-400*C
      • the earths crust is very thin compared to the diameter of the planet
      • the crust is made up of plates ranging from small to the size of a continent
      • continental crust is much thicker than oceanic crust
      • continental crust is made of less dense material than oceanic
      • the earths surface and its interior are constantly moving but you will never notice unless you are in an eathquake
      • beneth the earths crust lies the mantal, the upper part of which is solid
      • the crust and upper mantle are known as the lithosphere
      • below the lithosphere the mantle moves in convectional currents
      • the upper part of the mantle is called the asthenosphere
      • in the asthenosphere the flows affect the lithosphere above
      • because of the heat and pressure which builds up beneth the surface the crust is constantly being stressed which breaks it up
      • large scale processes within the earths crust are known as plate techtonics
      • when 2 plates are being pulled away from eachother deep cracks are opened in the crust
        • this allows magma to rise to the surface and then when it cools it forms new crust in the shape of a ridge
      • plate size can vary greatly from a few hundred to thousands of km across
      • the pasific and antartic plates are 2 of the largest
      • continental crust is composed of gannet rock which is low in density
      • oceanic crust is composed of basaltic rocks which are more dense
      • when oceanic crust meets continental crust it is the oceanic plate that is forced downwards
      • the difference in thickness of the two crusts are natures way of balancing the density of the 2 crusts
      • the continental crust can be as thick as 100km thick whereas the oceanic crust is usually only 7km thick
      • the convection currents in the mantle are driven by the heat of the core
      • heat is created by the pressure of the overlying materials and the radioactiveness of the core material
      • temperatures of the core are probably similar to those of the surface of the sun
      • the core is mostly made up of iron and nickle
      • the core is just over half the diameter of the earth
      • the core is 1 6th of the volume of the earth
      • the core is 1 3rd of the earths mass
      • the outer core is liquid
      • the inner core is solid
      • the currents in the outer core generate the earths magnetic feild
    • the 4 types of plate boundary between tectonic plates
      • there are 4 types of plate boundaries between tectonic plates - destructive, constructive, collision and conservative
      • destructive plate boundaries
        • distructive plate boundaries are found where two plates are moving together and oceanic plate is destroyed
        • they are associated with frequent earthquakes and volcanoes
        • the collision of 2 plates buckles the leading edge of the continental plate forming fold mountains and causing earthquakes
        • the oceanic plate is dragged downwards below the continental plate. this is known as subduction
        • magma rised through weaknesses in the continental crust forming volcanoes on the surface
      • constructive plate boundaries
        • these are found when new basaltic material rises to the surface, forcing plates appart
        • rising convection currents in the earths mantle cool and spread outwards as they near the surface this pulls the crust apart and creates fissures and faults through which molten magma can reach the surface
        • creation of new crust usually takes place  in the sea forming a ridge and chains of submarine volcanoes. but sometimes these volcanoes reach the surface to form islands
        • constructive plate boundries evolve from rising magma splitting apart continental crust and creating new oceans
      • collision plate boundaries
        • these are found where 2 continental plates move towards each other. neither is destroyed but buckling takes place
        • earthquakes are common
        • because there is no suduction, no volcanoes are formed
        • the buckling has lead to the formation of the world biggest mountain range
      • conservative plate boundaries
        • these occur when plates are sliding past eachother
        • there is no plate being created or destroyed because there is no magma or subduction
        • the sliding is not smoth -  there is friction between the 2 plates and extreme stresses build up in the crustal rocks when this is eventually relised it results in an earthquake
        • the size of the earthquake relates to the frequency of movement
        • on this boundry the plates are moving in the same direction but at different rates
        • some faults are visible on the surface but many are not
    • earthquakes and volcanoes
      • there is a close relationship between plate margins and tectonic activity
      • the hot spot theory is the theory that there are fixed spots in the mantle where magma rises to the surface. as the crust moves over these fixed spots volcanoes are created eventually forming chains of volcanoes.
      • there are many hot spots on the earth some of which have been extremely destructive in the past and pose threats for the planet in the future
      • there are many different types of volcano
      • experts have recognized 539 volcanoes that have erupted in recorded history these are classified as active
      • there are 529 volcanoes that have not erupted in historic times but exhibit clear evidence to do so again these are dormant
      • volcanoes that have not errupted in recorded history and show no signs of ever doing so are called extinct
      • in general the shape and structure of a volcano and explosive threat which they pose is related to the type of magma which created them
        • basaltic magma is usually found on constructive plate margins. it is high in temperature, very low in silica, with low gas content. this type of magma produces lava flow with relatively little explosive activity when it reaches the surface.
        • andesitic magma is formed at destructive margins where continental rocks are melted by rising magma. it is lower in temperature, has more silica and a lot more of dissolved gases. As a result this magma is much less fluid than basaltic magma and is more likely to explode when it reaches the surface.
      • the shape of volcanoes is a result of the type of magma that creates them and the frequency of their eruptions
      • volcanoes formed by magma that is basaltic tend to form very large gently sloping shapes known as shield volcanoes
      • volcanoes made up of anddesitic lavas tend to form composite volcanoes which are steep sided and make up of layers of lava and ash.
      • the ash is formed out of the material destroyed in the explosive eruptions which often blow the top of the mountain to pieces
      • pieces of as and rock are known as pyroclastics
    • the effects and impacts of volcanic hazards
      • earthquakes and volcanoes are good examples of tectonic hazards
      • not all hazards are equally devastating. the size and type of event is crucial
      • not all of earths inhabitants are at equal risk from natural hazards
      • unless people live close to a plate boundary it is very rare for them or their propety to be damaged by earthquakes - they are not vulnerable to the earthquake
      • if you do not live close to a volcano then you are not likely to be threatened by lava flows
      • even if you dont live near a volcano you can be affected my the clouds of volcanic ash which can significantly alter the climate of places miles, even continents, away from their point of origin.
      • capasity refers to the ability of a community to absorb and recover from the effects of a natural hazard
      • it is important to distinguish between the primary impacts and the secondary impacts of disasters
      • primary impacts are those that take place at the time of the event its self and are directly caused by it
      • secondary impacts are those that follow the event and are indirectly caused by it
      • the social and economic impacts of volcanic erruptions can be very considerable, and can include evacuation
    • management of tectonic hazards
      • the management of volcanic and earthquake hazards is expensive
      • the pattern of volcanoes and earthquakes are fairly well known and communities in the tectonically active areas can develop management strategies to cope
      • preparedness - being ready
        • preparation means that the government communities and individuals are ready to respond when disaster strikes and cope with the situation effectively
        • measures include the formulation of emergency plans, the development of warning systems and the training of personnel.
        • the measures may include evacuation plans for areas that may be at risk from a disaster and training for search and rescue terms
        • preparedness therefore encompasses those measures taken before a disaster event which are aimed at minimizing loss of life, disruption of critical services, and damage when the disaster occurs
      • mitigation - reducing the impact
        • mitigation measures are taken to reduce both the effect of the hazard and the vulnerability to it in order to reduce the scale of the disaster
        • they can be forced on the hazard itself or on the elements exposed to the threat
        • hazard specific measures include relocating people away from hazard prone areas and strengthening structures and using hazard resistant design to reduce damage when a hazard occurs
      • building design
        • one of the main ways of mitigating the impact of an earthquake is to improve the building design
        • engineers have developed ways to build earthquake resistant structures - houses office blocks and bridges.
        • for small to medium sized buildings the simpler reinforcement techniques include bolting buildings to their foundations and priding support walls called shear walls
        • shear walls are made from concrete that has steel rods inside to help strengthen the structure
        • the center of the building can be constructed from shear core
        • walls may be enforced with cross bracing
        • medium sized buildings are constructed using base insulators between the building and foundations which are made of steal and rubber
        • base insulators absorb some of the sideways motion that would otherwise damage a building
        • sky scrapers need special construction to make them earthquake resistant the foundations need to be very deep
        • sky scrapers need a reinforced frame work with stronger joints than those used in an ordinary building
        • sky scraper framework makes the makes the sky scraper strong enough but flexible enough to absorb the energy of an earthquake -  flexibility is the key
        • heavy things like furniture and other structures should be fastened down to prevent them from falling
        • gas and water lines must be specially reinforced with flexible joints to prevent breaking
        • fire fueled my broken gas pipes is a risk after earthquakes
        • in the developed world all of these methods are used especially for important buildings and in richer areas
        • in the developing world money is rarely available to spend on these methods so simpler ways of strengthening buildings are used

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