Earthquakes & Volcanoes

• Plate tectonics - a theory developed in the 1960s that helps explain the formation of some of the important features on the Earth’s surface and how the continents move

  • helps explain global distribution
  • explains landforms like volcanoes, earthquakes and mountain ranges called fold mountains
• Fold mountains - mountains created where two or more tectonic plate are pushed together, compressing the rocks and folding them upwards
• Two types of crusts:

  • Oceanic crust (sima):
  • mainly made of a rock called basalt
  • thinner, average depth is 6km
  • denser (heavier), 3 g cm^-3
  • younger
  • can sink and is continually being renewed and destroyed
  • Continental crust (sial):
  • mainly made of a rock called granite
  • thicker, average depth is 35km but it can be over 100km under mountain ranges
  • lighter, 2.6 g cm ^-3
  • older
  • it cannot sink and is neither destroyed or renewed
• It is important to remember the differences between oceanic and continental crust
• The earth’s surface is not a continuous layer

  • it is fractures and the sections are called tectonic plates
• Asthenosphere - the layer of the Earth below the lithosphere, it is hotter and weaker than the lithosphere above and is capable of plastic flow (deformation of material that remains rigid)
• Sima - another name for the oceanic crust, which is rich in silicate and aluminium minerals
• Sial - another name for the continental crust, which is rich in silicate and aluminium minerals
• Basalt - a fined-grained extrusive igneous rock formef by the cooling of lava at constructive plate margins
• Granite - a coarse-grained intrusive igneous rock comprising the minerals quartz, feldspar and mica, it is formed at destructive plate margins
• Tectonic plate - a piece of lithosphere that moves slowly on the asthenosphere, seven major, eight minor and numerous micro plates have been identified

  • made up of crust and upper mantle
• Lithosphere - the outer and rigid layer of the Earth, comprising the crust and the upper part of the mantle
• The surface of the earth is divided into 7 major and 8 minor plates

  • largest plates - antartica, eurasian and north american
• Plates can be continental, oceanic, or a mixture of both

  • continental plates are on average 100km thick and lighter than oceanic plates
• Plates float like rafts on the mantle

  • heat from the core creates convection currents in the magma of the mantle and these causes the plates to move
  • where the convection currents rises to the surface, the plates move away from each other
  • where the convection current sinks, plates move towards each other
• Convection currents - transfer heat from place to place, dense colder fluid sinks into warmer areas, heat from the Earth’s core causes convection currents in the mantle
• Plate boundary - where two or more plates meet, tthe three main types of plate boundary are constructive, destructive and conservative

  • can be called plate margin
• Most earthquakes and volcanic eruptions occur along or close to the plate boundaries

  • however, not all tectonic hazards are found at plate boundaries
  • some volcanoes are found above hot spots and earthquakes can occur wherever the crust moves
• Three types of plate boundaries:

  • constructive (divergent)
  • destructive (divergent)
  • conservative
• Constructive (divergent) - two oceanic plates are pulling away from each other, leaving a gap or weakness

  • magma from the mantle rises to the surface because of convection currents
  • magma solidifes when contact is made with cold ocean water
  • magma turns to lava and forms new basaltic ocean crust
• Sea-floor spreading - the process by which oceans are formed at constructive plate boundaries, new oceanic crust is formed as two oceanic plates move apart

  • small earthquakes are triggered
  • the new ocean crust builds up to form mid-ocean ridges and can also form submarine volcanies which may grow and appear above sea level as volcanic islands
• Ridge push - a gravitational force that causes and oceanic plate to move away from the crest of a mid-ocean ridge and into a subduction zone, it works together with slab pull
• Shield & basic volcanoe - a broad volcano built up from the repeated eruption of basalt

  • usually have non-explosive eruptions because there is little pressure build up
• Radiometric dating and fossil evidence shows that ocean floors becomes progressively older in both directions away from mid-ocean ridges

  • e.g. Eurasian plates moves away from the North American plate along the Mid-Atlantic Ridge
• If two continental plates move away from eachother, a rift valley may form as the central block of land drops down between the faults
• Rift valley - an area where a continent is being stretches and the central block moves downwards
• Destructive (converging) (1)

  • boundary can be found between a continental plate and an oceanic plate, between two oceanic plates and between two continental plates
  • at a destructive plate boundary, n oceanic plate and a continental plate moves towards eachother becuase of convection currents in the mantle
  • the denser, oceanic plate is forced down or subducted under the lighter, continental plate
  • this slab pull process takes place in the subduction zone and forms an ocean trench where the oceanic plate is subducted
• Destructive (converging) (2)

  • as the oceanic plate moves downwards, pressure and friction trigger svere earthquakes in the Benioff zone
  • heat produced by friction turns the descending plate into magma and by 700 km down the plate has completely disintegrated
  • magma starts to rise and becuase of the release of pressure, it can erupt through a weakness in the crust as an explosive composite (strato) or acidic volcano
• Destructive (converging) (3)

  • fold mountains are also formed on the continental plate as the sediments get pushed upwards and folded
  • when two oceanic plates move towards each other, the older and denser plate is subducted - the magma that rises upwards and erupts at the surface forms a chain of volcanic islands
• Slab pull - the force at a destructive plate boundary, where the oceanic plate sinks beneath the adjacent plate, as a result of its own weight, the descending plate is pulled by gravity through the asthenosphere

  • takes place in the subduction zone
• Subduction zone - a zone where the oceanic plate is deflected (subducted) down into the mantle, at the surface the subduction zone coincides with ocean trenches
• Ocean trench - a depression of ocean floor that runs parallel to a destructive plate boundary

  • these trenches are the deepst part of oceans
• Benioff zone - a zone of earthquake foci in the upper part of a subducting oceanic plate at a destructive plate boundary
• Composite volcano - a volcano built up by alternating layers of lava and ash, conical in shape

  • also known as a strato volcano
• Acidic volcano - a volcano that is made up of just lava, it is steeped sided as the lava does not flow easily, and it builds up into a convex cone shape
• Island arc - a chain of volcanoes, generall with an arc shape, that run parallel to an oceanic trench at a destructive (oceanic-oceanic) plate boundary
• Collision zone - a destructive plate boundary between 2 continental plates, resulting in fold mountains

  • sediments between two plates are squeezed together and pushed upwards to form fold montains
  • earthquakes occur but no volcanic activitiy because there is no subduction of an oceanic plate
• Conservative

  • no plate is being destroyed or creates
  • the plates are sliding past eachother
  • plates get locked together and pressure builds up until it is eventually released as an earthquake
• Different plate boundaries

  A) mid-ocean ridge

  B) volcanic island

  C) continent

  D) oceanic plate

  E) oceanic plate

  F) mantle

  G) magma

  H) magma

  I) convection currengts

  J) plate

  K) ocean trench

  L) mountains range (fold mountains)

  M) volcano

  N) Benioff zone

  O) continental plate

  P) magma

  Q) subduction zone

  R) mantle

  S) oceanic plate

  T) fold mountains

  U) continental plate

  V) continental plate

  W) seismic waves

  X) epicentre

  Y) focus

  Z) constructive plate boundaruy

  [) destructive plate boundary

  \) collision zone

  ]) conservative

  ^) plate boudnary

  
• An earthquake is when the ground shakes or moves in sudden jerks

  • earthquakes result from a build up and sudden release of tension, usually along a fault line
  • sends out shock waves (seismic waves) that travel outwards from the focus