Key Terms

• Accretionary wedge


  The top layer of material on a tectonic plate, that accumulate and deform where oceanic and continental plates collide. These sediments are scraped off the down-going oceanic crustal plate and appended to the edge of the continental plate.
• Asthenosphere


  the ductile part of the earth just below the lithosphere, including the lower mantle. The asthenosphere is about 180 km thick.
• Basalt


  A hard, black volcanic rock with less than about 52 weight percent silica and high amounts of heavy elements like iron and magnesium and low sodium and potassium. This magma has a high temperature and low viscosity and tends to erupt at spreading ridges.
• Crust

  The outermost layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes
• Lithosphere


  The outer solid part of the earth, including the crust and uppermost mantle. The lithosphere is about 100 km thick, although its thickness is age dependent. The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.
• Locked fault


  A fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault. Such faults may store strains for extended periods that are eventually released in an earthquake when frictional resistance is overcome.
• Mantle


  The part of the earth’s interior between the metallic outer core and the crust.
• Mid ocean ridge 


  The fracture zone along the ocean bottom where molten mantle material comes to the surface, this creates new crust. This fracture can be seen beneath the ocean as a line of ridges that form as molten rock reaches the ocean bottom and solidifies.
• Thrust fault


  Plate material is subducted or consumed in the mantle
• Subduction


  the process of the oceanic lithosphere colliding with and ascending beneath the continental lithosphere.
• Subduction zone


  The place where two lithospheric plates come together, one riding over the other. Most volcanoes on land occur parallel to and inland from the boundary between two plates.
• Divergent Margins 


  Divergent boundaries occur where plates are moving apart. Hot mantle rock rises and partial melting occurs. New crust is created by the magma pushing up from the mantle.
• Fast-spreading ridges 


  Mountain chains form along the crest of the ridge. High heat and magma input makes the ridge buoyant. New crust is added both as dikes and as erupted lava.
• Slow-spreading ridges


  They form valleys on the ridge crests. The early onset of extension forms a basin and range province of parallel mountains and valleys dispersed across a broad uplifted area.
• Transform Margins


  Two plates rub against each other and this creates friction
• Oceanic-oceanic convergence

  When two plates converge, one subducts beneath the other and in the process a trench is formed. An example of this is the Marianas Trench which plunges 11,000 m into the Earth's interior
• Oceanic-Continental convergence


  When an oceanic plate and continental plate collide the denser oceanic plate subducts under the continental plate. In this type of convergence, trenches, strong, destructive earthquakes and the rapid uplift of the mountain rangers are common. Volcanoes can also be created when the subducted plate is melted by the outer core and then this creates magma which rises up through the mantle and crust to create a volcano.
• Continental-contenential convergence


  When two continental plates meet head-on, neither is subducted because the continental rocks are relatively light and resist downward motion. Instead then the crust tends to buckle and be pushed upward or sideways.
• Tephra


  Any material ejected by a volcano into the air. This can be anything from fine ash to large volcanic bombs. It is hazardous and it can cover agricultural land, destroying crops. It can also cause air space to be closed and cause breathing difficulties for people with respiratory problems.
• Pyroclastic flows (nuees ardentes)


  It is a flow of fast moving hot gas and rock, which reaches speeds of 700 km/h. The gas can also reach temperatures of around 1000oC. They normally hug the ground and travel downhill. However their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They are a common and devastating result of certain explosive volcanic eruptions. The inhalation of hot and poisonous gas and ash causes almost instant death.
• Lava Flows

  Different volcanoes produce different types of lava. There is basaltic lava which is the hottest lava with a lower silica content and low viscosity. It is also able to to travel long distances even over relief that is relatively gently sloping, before cooling and solidifying as basalt

  Then there are acidic lavas such as rhyolite which is relatively cool with a temperature of around 700oC with a high silica content which has a high viscosity and forms only slow-moving flows.
• Volcanic Gases


  Volcanic eruptions release a wide range of toxic gases that include Co, CO2 and SO2. These gases can threaten human life and when SO2 combines with water in the atmosphere acid rain is produced. This pollutes surface water, enhances weathering and can damage crops.
• What is a normal faulting?


  The extensional forces and crustal stretching that occur at these boundaries create the necessary tensional stress to cause rock layers to break and vertically displace along normal fault lines.(  A line of braking in the rock caused by pulling way of the rocks causing one side to slip below the fault line.)
• Sequence of a tropical storm

  1. Air is heated above warm tropical oceans
  2. Air rises (low pressure) and condenses and forms clouds. This releases energy that increases wind speed
  3. Strong winds forms as rising air draws in more air (winds blow high to low) and moisture causing torrential rain
  4. Air spins due to Coriolis effect and spirals round the eye in a eyewall
  5. Cold, denser air sinks in the eye to it is clear and dry
  6. Outflow of moisture-laden air at top causes wide cloud cover
  7. Landfall - loses source of heat and moisture (sea) so loses power
• Nature of Earthquakes 


  1. Caused by strain energy (tension) building up at margin
  2. Plates jerk past each other and send out seismic waves = shaking
  3. Shock waves spread out from focus (source of earthquake)
  4. Epicentre is point on Earth's surface where earthquake is felt first
  5. Ground shakes and can rupture (split apart) along fault
• Composite Volcanic eruptions 


  Magma created as subducted friction heats crust and becomes molten magma. Andesitic and rhyolitic lava - is cooler and more sso flows less easily. Can form blockages, causing pressure to build. Usually infrequent, short eruptions. Form strato/composite volcanoes. Violent eruptions at subduction zones.
• Magma Plumes 


  Not at plate margin. Magma plume column rises up from mantle. This can cause a chain of volcanic islands where a magma plume is stationary but crust moves so chain of volcanoes form. E.g. Hawaii
• Earthquakes at Constructive margins

  Pressure can build up as some plates move faster than others. Generally low magnitude.