Tectonic hazards

Tectonic hazards

1. The Earth's structure

   • Core - extremely hot and under a lot of pressure
     • Inner core - solid
     • Outer core - liquid
   • Mantle - surrounds the core, made of solid material that can flow very slowly
     • The upper portion of the mantle is a weak layer called the asthenosphere
       • The outermost layer is the crust which is vey thin in comparison to the mantle and the core
         • Oceanic crust
           • Thinner and denser
           • Formed of basaltic rock
           • Recycled at destructive margins - usually less than 200 million years old
         • Continental crist
           • Thicker and lighter
           • Composed mainly of granite rock
           • Is not destroyed - up to 3.8 million years old
         • The crust forms a rigid shell at the surface of the earth called the lithosphere
           • The lithosphere is broken into several major fragments known as tectonic plates

2. How do tectonic plates move?

   • Convection
     • The hot core (around 6000*C) causes magma to rise in the mantle and sink towards the core when it cools
       • The currents flow beneath the lithosphere building up lateral pressure and carrying the plates with them
   • Slab pull and ridge push
     • At constructive margins ocean ridges form high above the ocean floor
       • Beneath ocean ridges the mantle melts
         • The molten magma rises as the plates move apart and cools to form new plate material
           • As the lithosphere cools it becomes denser and starts to slide down away form the ridge
             • This causes plates to move away from each other
     • At destructive margins the denser plate sinks back in the mantle due to gravity which pulls the rest of the plate along behind it (slab pull)

3. Where are earthquakes and volcanoes located?

   • Pattern matches where plate margins are located
     • Earthquakes are found at all types of plate margins
     • Volcanoes are found at constructive and destructive plate margins
   • Earthquakes and volcanoes are found both on land and in the sea
     • Chain of volcanoes and earthquakes which runs along the west coast of North and South America
     • The large band of volcanoes which circles the Pacific Ocean is know as the ring of fire
   • Not every earthquake and volcanoes lies along a plate margin - some occur in the middle of plates
     • These are known as hot spots where the crust is though to be particularly thin
       • E.g. Hawaii has formed due to volcanic eruptions at a hot spot that is located far from the edge of the Pacific plate

4. Plate boundaries

   • Constructive plate margins
     • Tectonic plates moving apart
       • Most move a few centimetres a year
         • E.g. Eurasian plate and North American plate move away from each other at a rate of 5cm a year
     • As the plates are moved apart by slab pull, ridge push and convection currents, magma rises to fill the gap
       • The magma cools to become part of the oceanic plate
         • The new solid plates sometimes fractures as it moves causing earthquakes
           • Shallow and usually small and not violent
       • The magma erupts in some places producing lava that is runny and spreads out before solidifying
         • Over many eruptions a shield volcano is formed
           • Wide base and gentle slopes
       • Lots of the magma never reaches the surface but pushes up the crust
         • This creates mid-ocean ridges
           • E.g. Mid-Atlantic ridge where the ridge is above sea level, forming Iceland where volcanoes are also formed
       • These two plates can start as one plate which is stretched
         • Forms a steep-sided Rift Valley - this part has become thinner
           • Fissures may be created as the plates are pulled apart allowing magma to rise and causing shallow earthquakes
   • Destructive plate boundaries
     • Where an oceanic and continental plate collide
       • The oceanic plate is subducted under the continental plate
         • The front of the continental plate is pushed up creating fold mountains creating a deeper part
           • Creates a deep ocean trench
             • At the destructive plate margin of the Pacific plate and the Philippine plate is the Mariana trench
               • Over 10km deep - the deepest known part of the ocean
         • This causes friction and the heat form the mantle melt the plate
           • Some of this material can work its way up through fissures and cracks in the crust to collect in magma chambers
             • Some can erupt out of the surface creating a volcanic arc
               • The magma becomes increasingly viscous (sticky) as it rises to the surface producing composite volcanoes (steep-sided, violent eruptions)
               • Japan has 118 active volcanoes
                 • Form part of the ring of fire
                 • Japan lies of the margin of four plates
                   • The Pacific plate subducts beneath the North American plate and the Philippe plate subducts beneath the Eurasian plate and
         • Causes earthquakes due to the increasing pressure built up in the plates as they move
           • The front edge of the continental plate is dragged down and then springs back up again
           • Many parts of Japan have experienced earthquakes due to the pressure built up in the plates
   • Collisional plate boundary
     • Two of the same plates moving together
       • Similar densities means subduction doesn't occur and therefore volcanoes are very rare here
       • As the plates are pushed together compression forces the plates to collie and fold upwards to form fold mountains
         • In the ocean, the sediment from the oceanic plate forms fold mountains
         • The Himalayas have formed as a result of the collision between the Indian plate and the Eurasian plate which began 50 million years ago and continues today
           • The Himalayas are still rising by more than 1cm per year
             • India continues to move northwards into Asia, causing shallow focus earthquakes
   • Conservative plate boundary
     • When tectonic plates move parallel to each other
       • In the same direction at different speeds or in different directions
       • Volcanoes not found at conservative plate boundaries because no gap is created as the plates move alongside each other and therefore, no magma can rise to the surface
       • Small but damaging earthquakes
         • Have a shallow focus as there is no subduction
           • When subduction is involved the focus is deeper down
       • San Andreas Fault
         • Found along the margin of the North American plate and the Pacific plate
           • Same direction but the Pacific plate is moving faster

5. Volcanic hazards

   • Types of volcanism
     • Volcanic explosivity index (VEI)
       • Scale of 1-8 describing how explosive an eruption is
       • Based on the volume of material ejected during an eruption
     • Effusive
       • Thin runny lava
         • Fast flowing
         • Found a large distance from crater
         • Silica poor
       • Constructive plate boundaries (shield volcanoes)
       • Much more frequent than explosive eruptions
     • Explosive
       • Thick viscous magma
         • Only flows a short distance
         • Silica rich
       • Subduction areas at destructive plate boundaries
       • Much less frequent than effusive eruptions
   • Lava flow
     • Molten rock that flows out of a volcano or volcanic vent
     • Can be easily avoided by a person onfoot
     • Cause severe damage to buildings and vegetation (extremely hot)
   • Volcanic bombs
     • A mass of molten rock (tephra) larger than 64mm in diameter
     • Formed when a volcano ejects viscous fragments of lava during an eruption
     • Very hazardous to people around the active vents
     • Cool into solid fragments before they reach the ground
   • Volcanic ash
     • Made up of tiny fragments of jagged rock, minerals and volcanic glass
       • These rock fragments can fix with the solidified lava fragments in the air and create an ash cloud
     • Wind can carry small volcanic ash particles great distances
       • Dangerous to moving aircraft
         • Iceland eruption 2011 cost aviation industry $2.6 billion dollars
     • Can cause a person to suffocate
     • When mixed with rain, volcanic ash turns into a heavy sludge that is able to collapse roofs
   • Volcanic gases
     • Magma contains dissolved gases and is released as it rises (pressure decreases)
     • Large eruptions can release enormous amounts of gases in a short time
     • Gases potentially hazardous to people, animals, agriculture and property
   • Pyroclastic flows
     • Contain a high density mix of hot lava blocks, pumice, ash and volcanic gas
     • Move at high speed down volcanic slopes
     • Form in different ways:
       • Collapse of eruption column
         • During a highly explosive eruption the column ejected upwards can become too cool and dense to maintain momentum
       • "Boiling over" from eruptive vent
         • Material is erupted without forming a high plume and rapidly moves down the slope
       • Collapse of lava domes or flows
         • The fronts can become so steep that they collapse due to gravitational force
   • Lahars
     • Mixtures of water and tephra
     • Have the consistency of wet concrete and can grow down the slopes of volcanoes or down river valleys at rapid speeds
     • Can carry debris
   • Supervolcanoes
     • Erupts at least 1000km3 of material
     • Forms a depression (caldera)
     • Ridge of higher land around it
     • Eruptions thousands of years apart