Global Heat Flow

Let's get back to the total heat for a moment. Although 38 trillion Watts is a lot of energy, when we spread it out over the entire surface of the Earth, the average global heat flow is only about 0.075 Watts/meter2. Suppose you owned a square plot of land that was 32 meters (about 100 feet) on each side. The area of your land would be 1000 square meters (32 times 32). If you could somehow capture all of the heat escaping your property, it would only be 75 Watts, or enough to light a single 75 Watt light bulb! (Take 1000 square meters times 0.075 Watts/square meter.) This means that in most places, geothermal energy is not a practical energy source.

However, the heat flowing out of the Earth is not uniform. Most of it is released by volcanism associated with mid-ocean ridges, active mountain belts, rifts, hot spots, and so on. So there are a few places, like Iceland, or The Geysers, California, where there is enough geothermal heat to be a practical source of energy.

Heat flow on a global scale is based on individual measurements distributed like this:

This is a similar map but the heat flow values are color coded according to value:

 

Data above  from Pollack et al., Heat flow from the Earth's interior: analysis of the global data set, Rev. Geophys., 31, 267-280, 1991

One can then "contour" the data, except this is done by spherical harmonic analysis. The higher the degree (and order), the more detail.

Global Heat Flow Map (Degree 12 Spherical Harmonic)

This map shows color-coded contours of the global distribution of heat flow at the surface of the Earth's crust. Major plate boundaries and continent outlines are also shown. The fundamental data embodied in this map are the more than 24,000 field measurements in both continental and oceanic terrains, supplemented by estimates of the heat flow in the unsurveyed regions. The estimates are based on empirically determined charactersitic values for the heat flux in various geological and tectonic settings. Observations of the oceanic heat flux have been corrected for heat loss by hydrothermal circulation through the oceanic crust. The global data set so assembled was then subjected to a spherical harmonic analysis. The map is a representation of the heat flow to spherical harmonic degree and order 12.

Here's another representation, in three different views, to degree and order 8: