Chapter 27 The thermal history of the Earth

Starting with Kelvin there have been many controversies and paradoxes associated with the evolution of the Earth. These are not faith-based controversies, in the ordinary sense; they are based on calculations and assumptions which are a form of faith . Present-day heat flow is determined by the amount and distribution of radioactive elements, their secular decline with time, the delay between heat production and its appearance at the surface, secular cooling of the interior, and a variety of minor sources of heating which are usually overlooked. Chemical stratification of the mantle slows down the cooling of the Earth; the upward concentration of radioactive elements reduces the time between heat generation and surface heat flow. The initial conditions of the Earth cannot be ignored; they have not been forgotten. Continents divert mantle heat to the ocean basins and, in addition, tend to move toward cold downwellings , thereby protecting their cold keels . Surface conditions including the thickness of plates -and mantle viscosity control convective vigor and cooling history of the mantle. A global accounting of the heat lost fi·om the interior by hydrothermal processes at the surface is a lingering issue. A chemically layered mantle with upward concentrations of the radioactive elements, shallow return flow, and a low-viscosity asthenosphere is the most plausible model of mantle dynamics . The migration of ridges and trenches is an important aspect of thermal history and geodynamics. The heat budget of the Earth cannot be treated as an instantaneous one-dimensional heat-flow problem, or one that involves a homogenous mantle with uniform and static boundary conditions. Both the radial and lateral structure of the Earth must be considered. Viscosity of the mantle is temperature, pressure, composition, location and time-dependent. Magma oceans, plate tectonics, heat pipes and stagnant-lid convection have transferred heat out of the interior at various times. The current Earth approximates stagnant-lid conditions in one hemisphere and a lid that fully participates in internal convection in the other. On early Earth, the surface may have been covered with thick accumulations of unsubductable basalt, penetrated by pipes of magma, as appears to be the case on Io. In such a scenario the rate of heat loss is regulated by the stress and density of the basalt pile as much as by the viscosity of the interior. The interior cools by depression of the cold surface layer and delamination, which displace hot material upwards. Even on today's planet, eruption and mantle cooling depend on the stress state of the overlying plate.