Objectives of this chapter

Heat is transported by convection in magmas, in hydrothermal systems, and in the Earth's mantle. In this section, we investigate thermal convection in the simplest geometrical configurations and boundary conditions, and hence do not deal with specific geological examples. We illustrate important aspects of convective flows and developthe tools that will be used later to analyze a number of natural convective systems. We do not provide a comprehensive account of flow patterns but focus on the characteristics of heat transport. We define dimensionless numbers and use them to characterize the different regimes of convection. We show that, even in highly non-linear convective systems, relationships between physical quantities can be reduced to simple scaling laws. We review experimental data and numerical calculations that support these scaling laws.

Isolated heat sources: Plumes and thermals

When heat is released from a small area at the base of a fluid layer, such as a heating coil at the bottom of a container, convection develops in an isolated rising element. This is relevant to the atmosphere or the ocean above a lava flow, and to a magma chamber which gets replenished by primitive melt. Over a heated horizontal plate of large width, plumes can be generated by instabilities in a thermal boundary layer. One distinguishes between a plume, which occurs when heat is continuously supplied and which remains connected to the source at all times, and a thermal, which is generated by the release of a finite amount of energy and which detaches from the source (Figure 5.1).