Global Heat Flow
- All energy on Earth ultimately comes from either:
- 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
- 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
- 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 t1/2, of
1.4x1010 years (14 billion years).
- 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 (t1/2)
of 4.5x109 (4.5 billion years).
- 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 (t1/2) of 1.3x109
years (1.3 billions years).
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
- 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
This is a similar map but the heat flow values are color coded according to
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