Objectives of this chapter

Mantle convection involves many processes and physical effects that are seldom found in other convective systems. We evaluate the most important ones and discuss the impact of each one on the heat loss properties and thermal structure of the mantle. We developscalings for the heat flux and typical velocity and demonstrate that they can all be reduced to simple statements on the dynamics of a thermal boundary layer. We aim at an understanding of each process and its control variables, and do not attempt to build an all-encompassing physical model of mantle convection.

Introduction

Compared to Rayleigh–Benard convection that has been studied in Chapter 5, convection in the Earth's mantle involves a series of processes that all act to enhance the impact of the upper thermal boundary layer on heat transport. The surface heat flux evacuates heat released by radioactive decay within the mantle as well as sensible heat due to secular cooling. The presence of continents over part of the Earth's surface restricts the efficiency of heat loss to the atmosphere and hydrosphere, and enhances heat flux through oceanic areas. Separation of the oceanic and continental domains with different heat transport characteristics at the Earth's surface generates large-scale horizontal temperature variations in the shallow mantle. Mantle rheology is highly sensitive to temperature, implying that cold material in the upper thermal boundary layer deforms less readily than the interior.