Mantle convection

[4] The Earth's lithosphere rides atop the asthenosphere, and the two form the components of the upper mantle.

Accretion occurs as mantle is added to the growing edges of a plate, associated with seafloor spreading.

At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of subduction usually at an oceanic trench.

Secondary convection may cause surface volcanism as a consequence of intraplate extension[8] and mantle plumes.

[12][13] Although elements of this debate still continue, results from seismic tomography, numerical simulations of mantle convection and examination of Earth's gravitational field are all beginning to suggest the existence of whole mantle convection, at least at the present time.

[8] Many geochemistry studies have argued that the lavas erupted in intraplate areas are different in composition from shallow-derived mid-ocean ridge basalts.

Others, however, have pointed out that geochemical differences could indicate the inclusion of a small component of near-surface material from the lithosphere.

On a global scale, surface expression of this convection is the tectonic plate motions and therefore has speeds of a few cm per year.

[18] Currently, whole mantle convection is thought to include broad-scale downwelling beneath the Americas and the western Pacific, both regions with a long history of subduction, and upwelling flow beneath the central Pacific and Africa, both of which exhibit dynamic topography consistent with upwelling.

[20] The persistence of net tectonic divergence away from Africa and the Pacific for the past 250 myr indicates the long-term stability of this general mantle flow pattern[20] and is consistent with other studies[21][22][23] that suggest long-term stability of the large low-shear-velocity provinces of the lowermost mantle that form the base of these upwellings.

The strength of olivine is proportional to its melting temperature, and is also very sensitive to water and silica content.

Below 400 km, the olivine undergoes a pressure-induced phase transformation, which can cause more deformation due to the increased ductility.

[25] Further evidence for the dominance of power law creep comes from preferred lattice orientations as a result of deformation.