plumes - both deep and shallow
plumes are persistent upwellings of hot mantle material that are typically
associated with mafic volcanism at hot spots such as Hawaii.
A clear understanding of the nature and depth of mantle plumes has been
limited by our general lack of knowledge of the characteristics of the deep
mantle, and by the difficulties in resolving seismic signals that pass through
the lower mantle.
from Princeton University have developed a new mathematical approach for
processing seismic data, enabling them to model the lower mantle more precisely
than before (Montelli et al., 2004). They
have processed data from over 1,500,000 seismic records to create tomographic
images of temperature anomalies in the mantle, with the specific objective of
determining the diameter and depth of mantle plumes.
Locations of some of the relatively deep and relatively shallow mantle plumes are shown on the map below. The ones depicted here as being deep are described by Montelli et al. (2004) as having velocity anomalies down to the 2800 km depth level, while those labelled as "shallow" have maximum anomaly depths of 650 to 1000 km.
Velocity anomaly representations of both deep and shallow mantle plumes are shown below. Areas with relatively low wave velocities (ie. warm mantle) are shown in yellows and reds.
|Velocity anomalies for two "deep" mantle plumes||Velocity anomalies for two "shallow" mantle plumes|
Montelli et al. (2004) conclude that the deep plumes originate near to the CMB, and that at least some of the shallow plumes originate near to the 670 km phase-change boundary. Although there is little evidence for a thermal boundary at 670 km, the authors suggest that this boundary could affect mantle flow dynamics such that plumes are generated in this region. They also state that their results are consistent with mantle plume velocities of less than 10 cm/y.
Montelli, R, Nolet G, Dahlen, F, Masters G, Engdahl, E and Hung, S., 2004, Finite-frequency tomography reveals a variety of mantle plumes, Science, Vol. 303, p. 338-343. (January 2004)
Earle, 2004. Malaspina University-College, Geology Department, Return to Earth