Terrestrial Heat Flow and Geothermics
The Earth steadily loses heat through its surface, and mapping this heat flow, together with the temperature gradient it implies, reveals the planet's thermal structure and the energy that drives its internal engine.
Definition
Terrestrial heat flow is the rate at which heat is conducted out of the Earth per unit area of surface, determined from the geothermal gradient and thermal conductivity; geothermics is the broader study of the Earth's temperature field and heat transport.
Scope
This topic covers the measurement and interpretation of the Earth's surface heat flow: how heat flow is determined from temperature gradients and thermal conductivity in boreholes and on the seafloor, the global heat-flow distribution and its relation to crustal age and tectonic setting, and the geotherm describing temperature with depth. It treats conductive and convective heat transport, the cooling of oceanic lithosphere as recorded in heat flow, and the contribution of crustal radiogenic heat. The emphasis is on the observed heat budget at the surface and the thermal regime it constrains.
Core questions
- How is surface heat flow measured from temperature gradients and conductivity?
- How does heat flow vary with crustal age and tectonic setting?
- What does the geotherm reveal about temperature with depth?
- How do conduction and convection share the transport of the Earth's heat?
Key concepts
- Geothermal gradient and thermal conductivity
- Surface heat flow and its global distribution
- Continental versus oceanic heat flow
- The geotherm and depth-temperature profile
- Crustal radiogenic heat production
Key theories
- Conductive heat flow and Fourier's law
- Heat flow through the crust is governed by Fourier's law, the product of the geothermal gradient and thermal conductivity, so measuring temperature with depth and rock conductivity yields the local surface heat flow.
- Heat flow and lithospheric cooling
- Oceanic heat flow decreases with the age of the seafloor as the lithosphere cools by conduction away from spreading ridges, in agreement with cooling-plate models, while continental heat flow reflects crustal radiogenic heat and mantle contributions.
Mechanisms
Heat is conducted up the geothermal gradient toward the cooler surface at a rate set by Fourier's law; in young oceanic crust hydrothermal circulation also carries heat convectively, while in continents the upper crust generates additional heat through radioactive decay, so the measured surface heat flow combines deep mantle heat, lithospheric cooling, and shallow radiogenic production.
Clinical relevance
Heat-flow measurements constrain the Earth's thermal structure and energy budget, identify regions favorable for geothermal energy, and inform basin maturation models used in petroleum exploration and the assessment of crustal thermal regimes.
History
Bullard pioneered both continental and seafloor heat-flow measurements in the mid-twentieth century, the systematic decrease of oceanic heat flow with age confirmed lithospheric cooling models, and global compilations have refined the estimate of the Earth's total heat loss to about 47 terawatts.
Key figures
- Edward Bullard
- Claude Jaupart
- Jean-Claude Mareschal
Related topics
Seminal works
- turcotte2014
- jaupart2011
- davies2010
Frequently asked questions
- How is heat flow from the Earth measured?
- Scientists measure how temperature increases with depth in a borehole or seafloor probe and multiply this geothermal gradient by the rock's thermal conductivity; the product gives the heat flowing out per unit area at that location.
- Why is heat flow higher near mid-ocean ridges?
- At ridges hot mantle rises and fresh, young lithosphere forms, so the crust is hottest there; as the lithosphere ages and moves away it cools, and the surface heat flow correspondingly declines with the age of the seafloor.