Geology Reference
In-Depth Information
IV. Introduction to Sustainability: Resource Planning and Global Change
253
Renewable
often is interpreted to mean derived
from living matter and replenished with the growing
season or at least over a short time period. Some nonliv-
ing geological resources, such as water, are renewable.
In fact, oil is produced in sediments on a scale of hun-
dreds of thousands of years; gold and other metals are
slowly produced in volcanic settings such as the rift at
the bottom of the Red Sea, and gravel is renewed
annually at the base of an alluvial fan. Most of these
rates of "renewal" of geologic resources are slow com-
pared to the rates of human use, and the resources are
therefore considered nonrenewable.
Nonrenewable
is a "one-crop" resource. Nonre-
newable is the usual category for mineral resources.
When a mining district or an oil field is exhausted,
there is no more of that resource to be had. Geothermal
energy, when extracted at a high rate, may be a nonre-
newable resource. The same geothermal resource, if
extracted at a rate lower than the heat and water are
recharged, is a renewable resource.
Natural resources are also characterized as
exhaustible or inexhaustible.
Exhaustible
resources are
(1) of limited quantity compared to rate of use and,
(2) those renewable resources whose productivity may
be destroyed (whale oil).
Inexhaustible
resources include
those available in exceptionally great quantities (magne-
sium in seawater), those easily renewed, and those with
a low rate of use. With unlimited energy, we would be
able to mine low-grade deposits for specific mineral
resources and to avoid or repair environmental damage.
Energy might be the ultimate geologic resource.
If a natural resource is exhaustible, we would
like to know when it will be used up. If we are using
the resource at a certain rate per year and there is a
known quantity, then we should be able to estimate its
lifetime, by:
FIGURE IV.1
Classification of mineral resources by feasibility
of economic recovery and certainty of geologic existence
(Brobst and Pratt, 1973).
Survey and the U.S. Bureau of Mines have classified
mineral resources using a scheme developed by Brobst
and Pratt (1973). One of the key components of this
scheme (Figure IV.l) is the difference between mineral
reserves
and other mineral resources. A
reserve
is a min-
eral resource whose extent and grade are known and
for which extraction is now economically, technologi-
cally, and politically feasible. It is a resource that can
readily be withdrawn from the "bank."
The processes of formation of mineral resources
are basically the geologic processes of the Earth sys-
tem. Mineral deposits form by igneous, metamorphic,
and sedimentary processes. The superficial process of
weathering also concentrates elements and materials
that are useful to humans. Biological processes are
largely responsible for phosphorus deposits and sedi-
mentological, biological, and chemical processes play
a role in formation of fossil fuels.
Estimated lifetime (yrs) = Original amount
(tons)/rate of use (tons/year)
However, it is not as simple as that. The rate of use may
increase or decrease, usually increasing with popula-
tion and decreasing with substitution and improved
technology (we are much more efficient now at gener-
ating electric energy per pound of coal than we were in
the early 1900s). Also we do not know the full extent of
the resources that might eventually become reserves
and be used. For this reason, a classification of mineral
resources is particularly useful in looking at the avail-
ability of all resources in the future.
IMPORTANCE OF GEOLOGIC RESOURCES
Mineral resources are an essential part of society and
improvement of the quality of life. Use of mineral
and energy resources has been positively correlated
with gross domestic product, although with conser-
vation measures, renewable energy use, technologi-
cal advances, and the growing concern about
environmental degradation, these relationships could
change. In addition to energy resources, both metallic
and industrial (nonmetallic) rocks and minerals are
essential to a quality life. According to V. McKelvey
(1973), a former director of the U.S. Geological Sur-
vey, the level of living (L) depends on the following
relationship:
MINERAL RESOURCES: CLASSIFICATION
AND ORIGINS
Mineral resources
are concentrations of naturally occur-
ring material (solid, liquid, gas) in or on the crust of
the Earth in a form such that economic extraction is
currently or potentially feasible. The U.S. Geological
L = (R
x
E
x
I)/P
