Plate tectonics and assosciate hazards revision notes- Part 1


Plate tectonics& associated hazards

Earth structure; internal structure of the earth

  • The internal structure of the earth is divided into different layers based upon its composition and properties.

  • At the centre of the earth lies the inner core (the densest part at 12g/cm3) composed of solid iron and nickel due to extreme pressures.

  • The surrounding outer core is made up of liquid iron, nickel and sulphides. As pressure is much less here a liquid state is allowed. It is also the area whereby the earth’s magnetic field is generated.

  • Outside of the Guttenburg discontinuity and outer core, the mantle can be found; making up about 80% of the earth’s volume. The lower mantle is a solid state compromising of silicate minerals and a density around a third of the inner cores of just 3.3-5g/cm3. The upper mantle is more complicated as it is a region which contains the asthenosphere which is a 5-10% partially molten layer, and the lithosphere consisting of the very upper part of the mantle and the rigid crust.

  • It must also be noted that information about the internal structure of the earth comes from the monitoring of seismic waves and volcanic activity in the upper mantle.  

Differences between the asthenosphere and lithosphere;                                                 

 The asthenosphere is in the upper mantle, generally laying 75-250kms below the surface. It is the layer beneath the lithosphere with a temperature of 1300c, hot enough to cause 5-10% melting of crystals(molten). The zone shows plastic properties known as rheid, the material is able to flow causing the generation of convection currents. This upper limit is defined by the abrupt decrease in velocity of seismic waves as they enter the region. As it has lost some of its rigidity P&S waves slow down.                                                                                                                    The lithosphere consists of the very upper part of the mantle and either continental or oceanic crust. It lays above the asthenosphere closer to the surface, temperatures are lower here and there is no partial melting. This solid, brittle layer makes up the plates, which are carried by the convection currents in the underlying asthenosphere.

The Crust; oceanic and continental

  • Oceanic crust is comprised of iron and magnesium, rock type is Basaltic.                 Density;2.9g/cm3                 Age;younger,0-200millionyears                                                                                                              Thickness; an average of 7kms

  • Continental crust is comprised of aluminium and silica, rock type is granite.     Density;2.7g/cm3                                                                                                                          Age;older,upto4000millionyears                                                                                                                                                     Thickness; an average of 35kms (90kms under mountains)

Characteristics of plates

  • Solid brittle layers consisting of the oceanic and continental crust and the upper most part of the mantle to form the lithosphere.

  • May be described as the outer shell of the earth

  • The plates are moved by the convection currents in the underlying asthenosphere.

  • There are 7 major plates and several smaller ones, each made up of both oceanic and continental crust. (Although the pacific plate is mainly oceanic)

  • Away from the plate boundaries there is little activity, known as aseismic regions.

  • The edges of the plates (or boundaries) are areas of instability with both seismic


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