Geoscience Reference
In-Depth Information
Subduction zone
Hot spot
Continent
Mid-ocean
ridge
Lower
mantle
Upper
mantle
Core
Subduction zone
Hot spot
Continent
Mid-ocean
ridge
Lower
mantle
Core
Upper
mantle
Figure 11.14
Two models of convection within the mantle. Top: whole-mantle convection. Subduction of
lithospheric plates and rise of plumes form the convective system. Bottom: two-layer convection
separated by the 660 km transition zone (notice the option of thermal coupling in the transition
zone: upper mantle plumes are localized above the hot ascending zones of the lower mantle).
number of layers existing within the mantle and the way they contribute to its overall
motion took decades to emerge.
The idea that signs of recycling of ancient subducted tectonic plates could be found at
the surface in the source of MORB or OIB arose in the late 1970s (
Fig. 11.14
,
top) under
the pen of Al Hofmann and Bill White. It is now envisaged that ancient oceanic crust,
transformed into pyroxenite by pressure, supplies a large part of the fusible material that
gives rise to OIB. New OIB can be made out of old MORB! In the model of whole-mantle
convection, the plates sink to the base of the mantle. The intense flow of heat at the core-
mantle interface causes gravitational instabilities and after one or two billion years the
deep material rises rapidly in the form of plumes (blobs). Isotope and trace element data
were persuasive evidence for this concept in the early 1980s, but four factors soon came to
oppose the idea of whole-mantle convection based on simple export of lithospheric plates
to the lower mantle:
•
The rate at which material is exchanged within the mantle at the present-day rate of
plate tectonics would eradicate any differences between the lower and upper mantle in a
