Global Heat Flow

• All energy on Earth ultimately comes from either:
  • nuclear fusion (sun, H-bombs), or
  • nuclear fission (within Earth)
  • For example, where does Oklahoma get its electricity?
• Although nearly all the energy we use (food, gasoline, electricity, etc.) comes from the Sun, it turns out that fusion and the Sun's energy has almost nothing to do with earthquakes, volcanoes, or plate tectonics!
• It is nuclear fission (radioactive decay) within the Earth which produces heat, drives mantle convection (more on this in the next unit), drives plate tectonics, volcanism, mountain building and earthquakes
• Radioactive decay of the unstable isotopes of the following three elements produce most of Earth's heat (atom/isotope refresher):
  1. Thorium: Thorium has many isotopes (atoms with the same number of protons - namely 90 for thorium - but different numbers of neutrons. ²³²Th (said "thorium 232") has 90 protons (or else it wouldn't be thorium) and 142 neutrons, for a total "mass number" of 232 (90 + 142). It is the isotope of thorium that produces the most heat in the Earth. ²³²Th has a half-life (time for half the thorium to decay), or t_1/2, of 1.4x10¹⁰ years (14 billion years).
  2. Uranium: Uranium has 92 protons. Again, uranium has many different isotopes, but one that produces the most heat in the Earth is ²³⁸U, which has 146 neutrons and therefore a mass number of 238 (92 + 146).  ²³⁸U has a half-life (t_1/2) of 4.5x10⁹ (4.5 billion years). 
  3. Potassium has 19 protons. The isotope of potassium which produces the most heat in the Earth is ⁴⁰K, which has 21 neutrons, and a half-life (t_1/2) of 1.3x10⁹ years (1.3 billions years).

• When we compute the total amount of energy generated by ²³²Th, ²³⁸U, and ⁴⁰K, we find that the total, global, energy production is 3.8x10¹³ Watts, or 38,000,000,000,000 Watts, or 38 trillion Watts!
• For comparison purposes, in the United States, our energy consumption averages about  3.0x10¹¹ (300 billion or 0.3 trillion) Watts.
• Most of the energy by radioactive decay in the Earth escapes as heat and eventually radiates into outer space.
• However, a tiny bit of this energy is released in earthquakes: less than 0.001%! So, as an "earthquake-producing machine," the Earth is terribly inefficient!

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: