Tectonics and heat

Tectonics and the transport of heat through the mantle are intimately related. In the picture developed in Part 3, plate tectonics and plumes are forms of mantle convection, each is therefore a form of heat transport, and each is also a tectonic mechanism. This is the most direct connection. Plumes are the mechanism by which heat from the core is transported into the mantle and plates are the mechanism by which heat is removed from the mantle. Plate tectonics is the dominant tectonic mechanism of the earth and plumes are an important secondary mechanism.

If we understand the mechanisms by which the mantle transports heat, then it is possible to calculate the rate at which the mantle's heat will change, under given assumptions. That is, we can calculate the temperature as a function of time, or the thermal history of the mantle. A different aspect of the relationship between heat and tectonics then comes into play, because there are reasons to suspect that the present tectonic mechanisms might not have been able to operate in the past when the mantle was probably hotter. We must then ask what tectonic mechanisms might have operated instead, or in other words, how might the mantle have transported its heat. Any proposed tectonic mechanism must then satisfy two fundamental requirements: it must be dynamically viable, that is there must be appropriate forces available to drive it, and it must be capable of transporting heat at a sufficient rate to cool the mantle. The latter requirement arises from the geological evidence that the mantle was hotter in the past, and it is sufficiently stringent to throw some dynamically attractive possibilities into doubt.