Page 40 - Geothermal Energy Renewable Energy and The Environment

Sources of Geothermal Heat: Earth as a Heat Engine 23

By analogy with classic models of convection cells, one would also assume that, a priori, subduc-
tion zones must be regions of very low heat flow, since they mark those places where the cooler,
down-welling portion of a convection cell returns to the mantle. And yet, subduction zones are
marked by the globes highest concentration of volcanoes, which bring tremendous volumes of mol-
ten rock (and heat) to the surface.
This apparent contradiction results from the fact that subduction zones transport water into the
mantle. Water is primarily contained in certain hydrous minerals that form during the alteration
and metamorphism of the oceanic crust as it migrates away from spreading centers. These hydrated
minerals are stable at relatively low temperatures, but recrystallize to new, less hydrated minerals at
elevated temperatures. When the oceanic crust descends into the mantle at subduction zones it heats
up, eventually reaching temperatures at which the hydrated minerals begin to recrystallize to new
mineral phases that do not accommodate water in their structure. As a result, the water molecules
that are released form a separate fluid phase. This process of dehydration of the original mineral
phase to form an anhydrous mineral and a coexisting water phase can be represented by the follow-
ing reaction that represents the dehydration of serpentine and brucite (common hydrated minerals
in the oceanic crust) to form olivine:

Mg Si O (OH) + Mg(OH) < = > 2Mg SiO + 3H O
2
4
5
2
4
2
3
2
Serpentine Brucite Olivine Water.
Dry rock, when sufficiently heated, begins to melt. Wet rock, when sufficiently heated, also
melts, but does so at much lower temperatures than dry rock. The water released during subduction
causes melting to occur in the hot mantle immediately above the descending oceanic crust. The
resulting melt is less dense than the solid rock from which it formed and migrates upward. This
process is, in essence, a secondary convecting system that brings molten rock and heat to the surface
in the vicinity of subduction zones (Figure 2.4).
Heat is also brought to the surface in the regions behind the volcanic front that forms at sub-
duction zones. It is believed that the subduction process gives rise to small scale convection cells
above the descending slab (Hart, Glassley, and Karig 1972). The upwelling part of these convection
cells often cause the overlying crust to spread apart, forming rift zones and rift basins that can be
places where shallow-level magma chambers develop. The northern part of the North Island of New
Zealand is such a place, and also happens to be the site where the first large-scale geothermal power
development was pioneered. The Basin and Range province of the western United States is also, at
least in part, likely to reflect similar processes.
An additional tectonic setting that is commonly geothermally significant are locations that are
known as hotspots. Hotspots are places that persist for tens of millions of years at which magma rises
from the deep mantle to the Earth’s surface in a nearly continuous fashion. The geological reasons
why hotspots form, and what maintains their persistent eruption histories, is a matter of considerable
debate. Regardless of what powers them, they are prodigious sources of heat. Hawaii and Iceland are
two prominent examples of hotspots, and both are well-known sites for geothermal energy.
Shown in Figure 2.5 are the locations of geothermal power plants around the world. Their cor-
respondence with spreading centers, hotspots, and subduction zones is obvious. Shown in Figure 2.6
are the high heat flow regions of the Earth. Their relationship to key plate tectonic elements is clear.
This correspondence has been an influence in guiding exploration for geothermal resources suitable
for electricity generation, and continues to be important. In Table 2.4, the power generation facilities
around the world are linked to the plate tectonic settings that power them.

aVaIlabIlITy and UTIlIzaTIon oF GeoThermal enerGy
Geological controls on the near-surface (meaning less than 3 km depth to the resource) availability
of geothermal energy has resulted in its concentration in specific regions. As Figure 2.5 makes