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land, Kamchatka, and parts of Japan, Indonesia and the Philippines. Volcanic
areas are also found above "hot spots" or mantle plumes outside the subduction
zones. Examples are the Yellowstone volcanic fields or the Clear Lake Volcanic
Field with the geothermal field of the Geysers. Continental rifts are also zones
with recent and young volcanism. Rifting (spreading) of continental crust occurs
in regions where the magma flow in the upper mantle is in direction of the earth
surface causing an uplift of the mantle crust boundary and a thinning of the crust.
A prominent example of a continental rift system with a high geothermal potential
is the East African Rift or Graben. This Graben is extending from the Red Sea to
Mozambique in the southern part of Africa. The largest mass of hot magma is
ascending along the mid-ocean ridges. Most of the spreading centres are under
water. There are, however, a few regions astride these ridges that are above water
and accessible for extraction of geothermal heat. The best-known example is Ice-
land.
Fumarole
Fumarole
Caprock
Caprock
Reservoir
Reservoir
Bedrock
Bedrock
Magma
Magma
Fig. 10.13
Scheme of a geothermal system /10-11/
The potential for geothermal power production from geothermal fields accom-
panied with volcanism is tremendous. Recent estimates reach up to 22,000 TWh/a
/10-11/. For comparison, the geothermal power production in the year 2004
amounted to about 57 TWh/a. This shows that geothermal power production from
geothermal areas can be increased by at least two or three orders of magnitude
compared to the current state. Many developing countries in South and Central
America, in Asia, and Africa could and some do supply a major portion of their
electricity consumption by geothermal power production from such geothermal
fields associated with volcanism.
Such geothermal fields are generally characterised on the one hand by tempera-
tures above 150 °C at relatively shallow depths (frequently under 1,000 m), and
on the other hand by a high degree of hydraulic permeability. Though their poten-
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