Geology Reference
In-Depth Information
8
Perspective
A fuller picture of mantle convection. Active plates and active plumes.
The distinct roles of plates and plumes in heat transport. Plume tecton-
ics cannot replace plate tectonics. How plates and plumes affect each
other.
Plumes are not the return mode of plate flow. There is normally no
active upwelling under ridges. There is no significant 'decoupling' layer.
Return flow is not shallow, 'drag' is small. There is no seafloor 'flatten-
ing', though there is some anomalous seafloor elevation. 'Superswells'
and residual thermal variations. Layered convection?
Rifts and flood basalts. Superplumes? Small-scale modes? Possible, but
evidence is marginal. Edge modes. Drips. Mantle wetspots.
8.1 Separate but interacting
The picture of mantle convection developed so far is of two thermal boundary
layers, each driving a distinctive form of convection. Because the two modes of
convection are so different, it has been useful to consider the thermal boundary
layers separately. Of course the two modes do interact, but not as strongly as in low-
Rayleigh-number 'textbook' convection (in which the modes are tightly coupled;
Figure 6.2), as we will discuss after a brief assessment of the story so far.
The top thermal boundary layer is very directly implied by all the observations
that indicate a steep temperature gradient near the Earth's surface and a shallower
gradient further down. The near-surface temperature gradient is directly measured,
and the need for a shallower gradient deeper down is implied by the fact that
the mantle is not liquid (from seismology) and by temperatures inferred from
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