Details of seismic velocity structure

• Velocity dependence on density and rock strength
• Rock strength is usually measured by how brittle or ductile it is (elasticity)
• more brittle - more strong and higher elasticity
• more ductile - less strong and lower elasticity
• Elasticity of rocks is strongly affected by temperature, and affects the velocity of seismic waves

Temperatures inside the Earth and heat sources

• Sources of heat in the earth:
  • Heat generation due to natural radioactivity
  • Primordial heat remaining from accretion
• Temperature gradients (geothermal gradients) near surface 15-20° C/Km.
• Temperature distribution in the interior must be in accord to keep the mantle in a solid state, i.e., local temperature in depth cannot exceed the melting point of material at that depth.
• Close approach to near the melting point at about 100 km depth, but far away from the melting point of materials at the surface and in the layer below about 350 km.
• Laboratory data of the temperature at the core-mantle boundary: ~4500° C

Rheological structure of the crust and mantle:

• To understand the dynamic behavior of the earth we need to look at the variation in strength of rocks with depth in the earth.
• Lithosphere: The outer most layer: cool, rigid and brittle, ~ 100 km thick, includes both oceanic and continental crust and the uppermost parts of the mantle.
• Asthenosphere: hot, weak, plastic, 100 km to ~350 km, able to flow.
• Mesosphere:: hot, but strong due to the effect of pressure, local temperature although high, is well below the melting temperature,from about 350 km to the core-mantle boundary at 2900 km depth

Modes of heat flow

• Conduction: no material transport, energy flows from the hotter body to the colder.
• Convection: rising of hot material upward and sinking of cold material downward.
• Radiation: energy emitted at very high temperatures, not important in the solid earth.

Conduction and convection are the only important mechanisms of heat transport in the earth. Of these convection is important for the dynamics of the surface regions of the body of the earth.

Convection currents in the mantle tend to create horizontal motions of the earth's outermost rigid layer, the lithosphere.