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Sources of Geothermal Heat: Earth as a Heat Engine 29

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FUrTher InFormaTIon
Rudnick, R. L., ed. 2005. The Crust. New York: Elsevier, 683 pp.
This reference book is for the advanced student with a strong geological background. It
contains numerous articles contributed by recognized scientists that provide an up-to-date
scientific description of many of the processes described in this and other chapters.
United States Geological Survey
The USGS Web site (http://www.usgs.gov/) has numerous excellent pages describing the
basic elements of plate tectonics and how they relate to volcanism and seismicity.
University of California Museum of Paleontology
The Web site for the museum (http://www.ucmp.berkeley.edu/geology/tectonics.html) has
an excellent series of animations relating to motion of the tectonic plates.

sIdebar 2.1 power, energy, and Units
Throughout this book, the conventions of the International System of Units, or SI units, will be used. The SI system
is employed by the scientific community worldwide to assure consistency and avoid confusion when measurements
of any type are being done. The beginnings of a standardized decimal metric system began with the adoption by
the French Assembly of a standard meter and kilogram in the late eighteenth century. Since that time, the interna-
tional scientific community has developed an internally consistent and well-defined set of units and constants for
key fundamental physical properties. The latest internationally accepted version of these standards can be accessed
through the National Institute of Standards and Technology (NIST) Web site, http://physics.nist.gov/cuu/Units/ (see
also Thompson and Naylor 2008). The following discussion is based on information available through NIST.
The SI system is based on seven base units that are independent and from which all other units are derived. The
base units and the quantities they relate to are:
• meter (m): length
• kilogram (kg): mass
• second (s): time
• ampere (a): electric current
• kelvin (K): temperature
• mole (mol): amount of substance
• candela (cd): luminous intensity
For our considerations of geothermal energy, some of the key derived units we must use relate to heat energy,
power, and work. Of these units, the joule, which is a unit of energy or work, is fundamentally important. A joule (J)
is defined as the work done (or energy expended) when 1 newton (N) of force is applied to an object that is displaced
1 meter. The newton is also a derived unit, defined as the force required to give a 1 kg mass an acceleration of 1 m/
2
s . Thus, a joule can be defined as:
J ≡ N × m = (m × kg/s ) × m = m × kg/s .
2
2
2
Power is the rate at which energy is used to accomplish work. In SI units, the rate at which energy is used is
measured in watts (W), which are defined as
W ≡ J/s.
Throughout this book, power generation or consumption will be described as the amount of time over which
energy is used at a specified rate. For example, if 50 J/s are consumed by a laptop computer and it is used for one