ABSTRACT
The source of all the energy that heats the air, the ground and the seas, and drives the winds and ocean currents, is the sun. By comparison, the flow of heat from the Earth's interior (geothermal heat) is quite negligible, and is generally supposed to have been so for at least the last 500 million years. Averaged over the Earth, and over the year, about 720 g cal of radiation for each square centimetre of surface (corresponding to 2 cal/cm2/min falling on any surface normal to the solar beam) are received daily from the sun at the outer limit of the atmosphere; and between 300 and 350 cal reach and are absorbed at the Earth's surface. The geothermal heat flux from the Earth's interior averages 0·1-0·2 cal/cm2/day (see footnote 1, p. 17); it is locally rather higher, 0·4-0·6 cal/cm2/day, in the extensive Tertiaryera volcanic areas in Iceland, and only very locally in contemporary volcanic areas are average values ten times or more than this attained (though in a volcanic vent enormous heat fluxes occur during eruptions). Even so, it is useful to remember that the temperature in all parts of the world normally increases with depth in the ground: the world average rate is about 3 °CI 1 00 m depth in the uppermost layers of the crust -lower down the rate eases off, so that at 35 km depth the average temperature is thought to be about 600°C. In much of Iceland - the youngest volcanic land area of its size in the world - the rate of temperature increase with depth in the uppermost layers is as great as 10-15 °CI 1 00 m, rising in the immediate vicinity of hot springs and steam holes to over 10° per metre.
