1.B Tectonic hazards

• The inner core of the earth is formed of solid iron and nickel
• The outer core of the earth is formed of liquid iron and nickel
• The mantle of the earth is formed of semi molten rock, convection currents occur here
• the asthenosphere is the top 100km of mantle where convection currents converge
• the lithosphere is the crust and upper most solid mantle
• oceanic crust is 6-10km thick and made of silica and magnesium
• continental crust is up to 70km thick and made of silica and aluminium
• Internal heat from the core is transferred towards the surface of the Earth by convection currents. The hot materials in the mantle rise and move laterally in the asthenosphere causing the plates above to

  move. The convection currents then cool and then

  descend and the cycle continues.
• Ridge Push Mechanism - molten magma rises at a mid

  ocean ridge boundary. Rocks in the lithosphere are heated

  and pushed upwards forming a mid ocean ridge. As the

  rock cools it becomes dense and gravity causes it to force

  the plates apart.
• Slab Pull Mechanism – At convergent destructive boundaries one

  plate is denser and heavier than the other. The dense

  plate subducts beneath the less dense plate and as it sinks

  it pulls more of the plate with it.
• Plates move laterally alongside each other at

  Transform/Conservative margins. E.g. Strike/slip San

  Andreas Fault, N American and Pacific plates. There is

  no subduction or volcanic activity.
• Conservative margin: The plates move laterally side by side, and a stick-slip motion results in the creation of significant

  earthquakes. Pressures builds up due to friction

  between the plates and when the plates break apart the

  energy is sent through the earth as seismic waves in the

  form of an earthquake. High pressure faults release

  pressure easily and may cause many minor earthquakes

  a day. If pressure builds up due to restricted movement

  major infrequent events can occur, such as the Haiti EQ

  of Jan 12 th 2010.
• Plates move towards each other at converging/

  destructive boundaries.
• Continental Arcs - Lateral limb of two convection currents

  converge in the asthenosphere. Older, denser oceanic

  plate is subducted beneath the lighter, larger continental

  plate. The dense oceanic crust sinks into the

  asthenosphere creating a subduction zone due to a slab-

  pull mechanism and this pulls new crust behind it, forming

  a deep ocean trench. Seismic activity can occur due to

  friction as the plate subducts in the Benioff Zone.
• The Ring of Fire is the name associated with

  volcanoes encircling the Pacific Ocean, created by

  convergent plates.
• Oceanic Nazca and Continental South

  American plates create the Andes Fold Mountains,
• If both diverging plates are oceanic (sima) they form ocean

  ridges and volcanic islands.
• Plates moves away from each other, at diverging/

  constructive boundaries.
• Ocean ridge - The rising limb of convection

  current spreads in the asthenosphere. Tension in crust

  forces the lithospheric plates to lift, dome and diverge by a

  ridge-push mechanism, creating an ocean ridge. The new

  ocean floor slides away from either side of the ridge under

  the influence of gravity (sea floor spreading). Basaltic

  magma accumulates and rises through the fractures

  (cracks). Over time the lava will cool and solidify to form a

  new basaltic crustal material.
• Where rising magma

  continues to build up above the ocean surface, a gently

  sloping shield volcanic island may be formed. Volcanoes are

  gentle and frequent; magma is usually runny and basaltic.

  Example: N American and

  Eurasian plates diverge at 2.5cm/yr, forming Surtsey volcanic island

  1963.
• If both diverging plates are continental (sial), then Rift

  Valleys are formed. The crust widens and thins and

  subsidence between the faults leads to the formation of rift

  valleys. Streams and rivers may flow into the low valley and

  long lakes can be created.
• Hotspots – located away from plate boundaries. These are plumes of magma with a high heat flow that rise through the

  asthenosphere. The magma will push though fractures in the weak crust, forming volcanoes at the surface.
• An oceanic hotspot is formed when the

  plate moves over the hotspot it can create

  a chain of volcanic islands which move with

  the plate. As the oceanic volcanic islands

  move away from the hotspot they cool and

  subside, remaining as islands e.g Hawaiian islands in Pacific ocean
• A continental hotspot example is the Yellowstone Caldera in Yellowstone National Park in the United States.
• Earthquakes occur at:

  Divergent boundaries – along ocean

  ridges. EQs are shallow and generated by

  volcanic activity along oceanic ridges.
• Earthquakes occur at:

  Convergent boundaries – due to

  subduction and friction earthquakes are

  frequent and can be high magnitude.

  Tsunamis are commonly generated, such

  as the 2004 Boxing Day tsunami.
• Earthquakes occur at: Collision boundaries - continental crust

  collides, creating fold mountain belts. E.g.

  Himalayan Mountain chain. Shallow

  earthquakes create a high hazard risk to a

  broad area, such as the 1990 Bam

  earthquake, Iran.
• Earthquakes occur at: Transform boundaries - Lateral crust

  movement in continental areas produce

  shallow earthquakes of high magnitude,

  such as the conservative margin of the San

  Andreas fault system.
• Earthquakes occur at: Intra-plate earthquakes - Away from plate

  boundaries. Stresses in crustal rocks are

  located along ancient fault lines causing

  unpredictable and dangerous events. E.g.

  Tangshen, China 1976
• Earthquakes occur at: Quasi-natural earthquakes - generated by

  human activity, such as fracking. 1993

  Killari, India earthquake triggered by the

  newly built reservoir lubricating the fault.
• Volcanoes occur at:

  Divergent plate boundaries - 75% of magma that reaches the earth’s

  surface comes from this boundary. This occurs at mid-ocean ridges where

  upper mantle melting produces basaltic magma. These are non-violent and

  mainly occur on the sea floor. On the land fissure eruptions produce lava

  plateaus, such as Iceland.
• Volcanoes occur at:

  Convergent plate boundaries - create subduction volcanoes (80% of world’s

  volcanoes). Oceanic plate is subducted and melts under pressure; magma

  rises upwards and mixes with the continental crust to produce andesitic

  magma, resulting in violent volcanic eruptions. E.g. Vesuvius, Italy.
• Volcanoes occur at: Hotspot eruptions are Effusive with huge quantities of low viscosity basaltic

  magma that are less hazardous for people. E.g. Hawaii, 2018.
• Shield volcano examples: Kilauea and

  Mauna Loa,

  Hawaii
• Strato volcano: Explosive eruptions that are infrequent. Andesitic

  viscous lava is slow flowing and does not travel far from

  vent. Steep sided layers of solidified lava and ash, cone

  shaped. Pyroclastic flow as gas pressure builds up inside

  vent – released violently as superheated 800 O C+ steam,

  ash and rock. Occur at convergent margins. VEI of 4+.
• Strato volcano examples: Mt Fuji, Japan

  Cotopaxi,

  Ecuador
• Cinder/ash volcano: Small, steep bowl shaped crater volcanoes. Gas forces

  basaltic lava high into the air where it breaks into small

  glassy fragments called cinders. Often formed on the

  side of shield volcanoes.
• Cinder/ash volcano example: Paricutin,

  Mexico
• Acid/dome volcano: Viscous rhyolitic acidic lava erupts effusively and cools

  and solidifies quickly creating a steep sided cone.
• Acid volcano example: Mount Pelée,

  Martinique