How Energy Is Produced - The Renaissance of Renewable Energy - page 146

Geologically speaking, the Earth consists of three layers: crust, mantle and core. The
crust is the uppermost and thinnest layer - between 5 and 70 kilometres deep (it is thinner
beneath the oceans and thicker beneath the continents). The mantle extends a further 2,900
kilometres down, while the core is a metal sphere, solid on the inside and liquid on the
outside, 7,000 kilometres in diameter (roughly the size of Mars).
Some of the heat at the Earth's core has been trapped there by the crust blanket since
the formation of the solar system, while the remainder derives from the radioactive decay
of elements within the Earth. 26 But the Earth's layers are not homogeneous. In many areas
of the world (Iceland, Hawaii, Japan, California, and East Africa to name a few), the
heat from the core reaches the upper layers of crust, giving rise to volcanic activity and
other geothermal phenomena. Geothermal occurrence involves hot volcanic rock heating
wet sedimentary rocks that lie above. These rocks, in turn, heat groundwater, giving birth
to phenomena such as hot springs and geysers (see Figure 4.52 ) . There is also a steady
supply of milder heat, sufficient to heat air or water to an ambient temperature, just about
anywhere on Earth, starting at depths of just a few metres.
The word 'geothermal' derives from the Greek geo (earth) and thermos (heat), but its
use by humankind predates Hellenic culture. Hot springs have been used for ritual or
routine bathing since the Stone Age (Cataldi 1999 ) and perhaps for as long as our species
has existed. In fact, we are not the only species to take advantage of hot springs. In the
Jigokudani Monkey Park in Japan the resident snow monkeys ( Macaca fuscata ) while
away many winter hours soaking in the warm waters of a hot spring, before returning to the
forests at night (see Figure 4.50 ).