Geology Reference
In-Depth Information
PART D. EARTH SYSTEMS AND GEOLOGIC
PROCESSES
Internal geologic processes
are driven by geother-
mal heat, produced primarily by the radioactive decay
of elements within the Earth. As indicated in Figure
1.3, the rotational energy of the solar system also acts
on the Earth, producing earth and ocean tides. The
result of all these processes is deformation of the crust,
primarily through the mechanism of plate tectonics, in
which large and small segments of the crust and upper
mantle known as plates collide, separate, deform,
grow, and disappear. Due to these plate tectonic
processes and to phase changes in minerals in the
Earth, the surface of the Earth rises and falls. Although
much of this deformation is very slow (on the order of
a few centimeters or less per year), some is very rapid.
These seismic events or earthquakes not only rapidly
displace portions of the crust, but cause shaking and
secondary processes that can seriously impact human
structures designed for a generally stable surface. In
addition to these tectonic processes, there are volcanic
processes that affect the surface and near surface areas
of the Earth. The rates at which many of the geologic
processes that shape the Earth occur are summarized
in Figure 1.4.
The interaction of the internal and external
processes working on earth materials comprises the
"geosystem" and produces our landscapes and the
substrate for human structures. As the surface
changes on short- and long-term bases, humans must
adjust to it. The climate component of the geosystem,
which is responsible for the relative importance of
different external geomorphic processes, is a major
factor in the rate and scale of changes on the Earth.
With the widespread recognition of global warming,
both natural and human-induced (IPCC, 2001, 2007),
our need for improved understanding of the geosys-
tem has grown. The impacts of global warming will
not all be negative; some regions may benefit
(Ausubel, 1991). Global change is now a focus of
much educational, research, and political activity.
Given the expected growth in the human component
of this system, we can expect humans to become a
more important geomorphic agent and to have a
more difficult time adjusting their activities to this
changing system. In partial recognition of this devel-
opment, the 1990s were designated the International
Decade of Natural Disaster Reduction (IDNDR). When
humans have the potential of being overwhelmed by
natural geologic processes, these processes are referred
to as geologic hazards. If the events actually produce
losses in property and life, they are then described as
geologic disasters.
Much of this topic explores the realm of geologic
hazards and disasters as they are a major component
of environmental geology. The last section explores
global climate change, possibly the most important
geologic hazard, in the context of sustainability.
The Earth's surface is not static; it changes in response
to processes that operate at the surface and within the
Earth. The results of these interactions are the land-
forms and landscapes on which the human colony
lives.
External geologic processes
are generated primarily
by solar energy and gravity. Precipitation and other
components of the hydrologic cycle work on the materi-
als of the Earth to shape the land. The processes include:
weathering of rocks and sediments; mass wasting (com-
monly described as landsliding); and erosion and sedi-
mentation by rivers, lakes, oceans, wind and glaciers.
Biologic processes also produce landforms, for exam-
ple, organic reefs. In fact, humans are the prime geo-
morphic agent. We are reshaping the Earth's surface
directly with anthropogenic (human-produced) land-
forms and indirectly through modification of the rates
and scales of natural geologic processes. The processes
working on the Earth are summarized in Figure 1.3. In
addition, humans are also modifying climate, which
controls many geologic processes. The Earth has been
both colder and warmer in the last 130,000 years, but
our current understanding of the Earth system incorpo-
rates a mechanism for increases in global temperatures
because of increased concentrations of greenhouse
gases. Water vapor accounts for 80 percent of the green-
house effect, which is an essential process for the main-
tenance of life-supporting temperatures on Earth
(Jacobsen and Price, 1990). Anthropogenic increases in
natural trace gases such as carbon dioxide (C0
2
),
methane (CH
4
), and tropospheric ozone (0
3
) and
manufactured gases chlorofluorocarbons (CFCs) and
halons are now causing significant changes in tempera-
tures, climate, and geologic processes. Excess quantities
of greenhouse gases are caused primarily by the burn-
ing of fossil fuels or "buried sunlight" (C0
2
, CH
4
and
N
2
0), but agricultural activity (livestock management,
rice paddies, biomass burning, nitrogenous fertilizers,
and deforestation) and industrialization (chlorofluoro-
carbons and halons) also contribute to these gases.
Industrial gases also cause stratospheric ozone deple-
tion, which leads to increases in ultraviolet (UV) radia-
tion and impacts human health, plant productivity, and
climate (Jacobson and Price, 1990).
Environmentalists commonly speak of "Space-
ship Earth" to describe the life-support characteristics
of the Earth. Note that the Earth is not a closed system.
For example, infall of meteoritic material is generally
a minor process; however, on very rare occasions (in
geologic time) large-scale meteorite impact occurs
and can cause significant changes in the Earth's land-
scape, climate, and life. Although not always obvious
on Earth, cratering is the most important geomorphic
process in the solar system.
