Geological hazards take many shapes
and forms, from earthquakes and volcanic eruptions to the
slow downhill creep of material on a hillside and the expansion
of clay minerals in wet seasons. Natural geologic processes
are in constant operation on the planet. These processes
are considered hazardous when they go to extremes and
interfere with the normal activities of society. For instance,
the surface of the Earth is constantly moving through plate
tectonics, yet we do not notice this process until sections of
the surface move suddenly and cause an earthquake.
The Earth is a naturally dynamic and hazardous world,
with volcanic eruptions spewing lava and ash, earthquakes
pushing up mountains and shaking Earth’s surface, and
tsunami that sweep across ocean basins at hundred of miles
per hour, rising in huge waves on distant shores. Mountains
may suddenly collapse, burying entire villages, and slopes are
gradually creeping downhill moving everything built on
them. Storms sweep coastlines and remove millions of tons of
sand from one place and deposit it in another in single days.
Large parts of the globe are turning into desert, and glaciers
that once advanced are rapidly retreating. Sea level is beginning
to rise faster than previously imagined. All of these natural
phenomena are expected consequences of the way the
planet works, and as scientists better understand these geological
processes, they are able to better predict when and
where natural geologic hazards could become disasters and
take preventive measures.
The slow but steady movement of tectonic plates on the
surface of the Earth is the cause of many geologic hazards,
either directly or indirectly. Plate tectonics controls the distribution
of earthquakes and the location of volcanoes and causes
mountains to be uplifted. Other hazards are related to
Earth’s surface processes, including floods of rivers, coastal
erosion, and changing climate zones. Many of Earth’s surface
processes are parts of natural cycles on the Earth, but they are
considered hazardous to humans because we have not adequately
understood the cycles before building on exposed
coastlines and in areas prone to shifting climate zones. A third
group of geologic hazards is related to materials, such as clay
minerals that dramatically expand when wetted, and sinkholes
that develop in limestones. Still other hazards are extraterrestrial
in origin, such as the occasional impact of meteorites and
asteroids with Earth. The exponentially growing human population
on Earth worsens the effect of most of these hazards.
Species on the planet are now experiencing a mass extinction
event, the severity of which has not been seen since the extinction
event 66 million years ago that killed the dinosaurs and
many of the other species alive on Earth at the time.
Many geologic hazards are the direct consequence of
plate tectonics, associated with the motion of individual
blocks of the rigid outer shell of the Earth. With so much
energy loss accommodated by plate tectonics, we can expect
that plate tectonics is one of the major energy sources for natural
disasters and hazards. Most of the earthquakes on the
planet are directly associated with plate boundaries, and
these sometimes devastating earthquakes account for much of
the motion between the plates. Single earthquakes have killed
tens and even hundreds of thousands of people, such as the
1976 Tangshan earthquake in China that killed a quarter million
people. Earthquakes also cause enormous financial and
insurance losses; for instance, the 1994 Northridge earthquake
in California caused more than $14 billion in losses.
Most of the world’s volcanoes are also associated with plate
boundaries. Thousands of volcanic vents are located along
the mid-ocean ridge system, and most of the volume of
magma produced on the Earth is erupted through these volcanoes.
However, volcanism associated with the mid-ocean
ridge system is rarely explosive, hazardous, or even noticed
by humans. In contrast, volcanoes situated above subduction
zones at convergent boundaries are capable of producing
tremendous explosive eruptions, with great devastation of
local regions. Volcanic eruptions and associated phenomena
have killed tens of thousands of people in this century,
including the massive mudslides at Nevada del Ruiz in
Colombia that killed 23,000 in 1985. Some of the larger volcanic
eruptions cover huge parts of the globe with volcanic
ash and are capable of changing the global climate. Plate tectonics
is also responsible for uplifting the world’s mountain
belts, which are associated with their own sets of hazards,
particularly landslides and other mass wasting phenomena.
Some geologic hazards are associated with steep slopes,
and the effects of gravity moving material down these steep
slopes to places where people live. Landslides and the slow
downhill movement of earth material occasionally kill thousands
of people in large disasters, such as when parts of a
mountain collapsed in 1970 in the Peruvian Andes and buried
a village several tens of miles away, killing 60,000 people.
More typically, downhill movements are more localized and
destroy individual homes, neighborhoods, roads, or bridges.
Some downslope processes are very slow and involve the
gradual, inch-by-inch creeping of soil and other earth material
downhill, taking everything with it during its slide. This process
of creep is one of the most costly of natural hazards, costing
U.S. taxpayers billions of dollars per year.
Many other geological hazards are driven by energy
from the Sun and reflect the interaction of the hydrosphere,
lithosphere, atmosphere, and biosphere. Heavy or prolonged
rains can cause river systems to overflow, flooding low-lying
areas and destroying towns, farmlands, and even changing
the courses of major rivers. There are several types of floods,
including flash floods in mountainous areas or regional
floods in large river valleys such as the great floods of the
Mississippi and Missouri Rivers in 1993. Coastal regions
may also experience floods, sometimes the result of typhoons,
hurricanes, or coastal storms that bring high tides, storm
surges, heavy rains, and deadly winds. Coastal storms may
cause large amounts of coastal erosion, including cliff retreat,
beach and dune migration, and the opening of new tidal
inlets and closing of old inlets. These are all normal beach
processes but have become hazardous since so many people
have migrated into beachfront homes. Hurricane Andrew
caused more than $19 billion of damage to the southern
United States in 1992.
Deserts and dry regions are associated with their own set
of natural geologic hazards. Blowing winds and shifting
sands make agriculture difficult, and deserts have a very limited
capacity to support large populations. Some of the greatest
disasters in human history have been caused by droughts,
some associated with the expansion of desert regions into
areas that previously received significant rainfall and supported
large populations dependent on agriculture. In this century,
the sub-Saharan Sahel region of Africa has been hit with
drought disaster several times, affecting millions of people
and animals.
Desertification is but one possible manifestation of global
climate change. The Earth has fluctuated in climate extremes,
from hot and dry to cold and dry or cold and wet, and has
experienced several periods when much of the land’s surface
was covered by glaciers. Glaciers have their own set of localscale
hazards that affect those living or traveling on or near
their ice—crevasses can be deadly if fallen into, glacial meltwater
streams can change in discharge so quickly that
encampments on their banks can be washed away without a
trace, and icebergs present hazards to shipping lanes. Glaciers
may significantly reflect subtle changes in global climate—
when glaciers are retreating, climate may be warming and
becoming drier. When glaciers advance, the global climate
may be getting colder and wetter. Glaciers have advanced and
retreated over northern North America several times in the
past 100,000 years. We are currently in an interglacial
episode, and we may see the start of the return of the continental
glaciers over the next few hundred or thousand years.
Geologic materials themselves can be hazardous. Asbestos,
a common mineral, is being removed from thousands of buildings
in the nation because of the perceived threat that certain
types of airborne asbestos fibers present to human health. In
some cases (for certain types of asbestos fibers), this threat is
real. In other cases, the asbestos would be safer if it were left
where it is rather than disturbing it and making the particles
airborne. Natural radioactive decay is releasing harmful gases
including radon that creep into our homes, schools, and
offices, and causing cancer in numerous cases every year. This
hazard is easily mitigated, and simple monitoring and ventilation
can prevent many health problems. Other materials can
be hazardous even though they seem inert. For instance, some
clay minerals expand by hundreds of percent when wetted.
These expansive clays rest under many foundations, bridges,
and highways and cause billions of dollars of damage every
year in the United States.
Sinkholes have swallowed homes and businesses in Florida
and other locations in recent years. Sinkhole collapse and
other subsidence hazards are more important than many people
realize. Some large parts of southern California near Los
Angeles have sunk tens of feet in response to pumping of
groundwater and oil out of underground reservoirs. Other
developments above former mining areas have begun sinking
into collapsed mine tunnels. Coastline areas that are experiencing
subsidence have the added risk of having the ocean
rise into former living space. Coastal subsidence coupled with
gradual sea-level rise is rapidly becoming one of the major
global hazards that the human race is going to have to deal
with in the next century, since most of the world’s population
lives near the coast in the reach of the rising waters. Cities
may become submerged and farmlands covered by shallow
salty seas. An enormous amount of planning is needed, as
soon as possible, to begin to deal with this growing threat.
Occasionally in the Earth’s history the planet has been
hit with asteroids and meteorites from outer space, and these
have completely devastated the biosphere and climate system.
Many of the mass extinctions in the geologic record are now
thought to have been triggered, at least in part, by large
impacts from outer space. For instance, the extinction of the
dinosaurs and a huge percent of other species on Earth 66
million years ago is thought to have been caused by a combination
of massive volcanism from a flood basalt province
preserved in India, coupled with an impact with a six-mile
(10-km) wide meteorite that hit the Yucatán Peninsula of
Mexico. When the impact occurred, a 1,000-mile (1,610-km)
wide fireball erupted into the upper atmosphere, tsunami
hundreds or thousands of feet high washed across the
Caribbean, southern North America, and across much of the
Atlantic, and huge earthquakes accompanied the explosion.
The dust blown into the atmosphere immediately initiated a
dark global winter, and as the dust settled months or years
later, the extra carbon dioxide in the atmosphere warmed the
Earth for many years, forming a greenhouse condition. Many
forms of life could not tolerate these rapid changes and perished.
Similar impacts have occurred at several times in the
Earth’s history and have had a profound influence on the
extinction and development of life on Earth.
The human population is growing at an alarming rate,
with the population of the planet currently doubling every 50
years. At this rate, there will only be a 3-foot by 1-foot space
for every person on Earth in 800 years. Our unprecedented
population growth has put such a stress on other species that
we are driving a new mass extinction on the planet. We do
not know the details of the relationships between different
species, and many fear that destroying so many other lifeforms
may contribute to our own demise. In response to the
population explosion, people are moving into hazardous locations
including shorelines, riverbanks, along steep-sloped
mountains, and along the flanks of volcanoes. Populations
that grow too large to be supported by the environment usually
suffer some catastrophe, disease, famine, or other mechanism
that limits growth, and we as a species need to find ways
to limit our growth to sustainable rates. Our very survival on
the planet depends on our ability to maintain these limits.
Advances in science and engineering in recent decades
have dramatically changed the way we view natural hazards.
In the past, we viewed destructive natural phenomena (including
earthquakes, volcanic eruptions, floods, landslides and
tsunamis) as unavoidable and unpredictable. Our society’s
attention to basic scientific research has changed that view
dramatically, and we are now able to make general predictions
of when, where, and how severe such destructive natural
events may be, reducing their consequences significantly. We
are therefore able to plan evacuations, strengthen buildings,
and make detailed plans of what needs to be done in natural
disasters to such a degree that the costs of these natural geological
hazards have been greatly reduced. This greater understanding
has come with increased governmental responsibility.
In the past, society placed little blame on government for the
consequences of natural disasters. For instance, nearly 10,000
people perished in a hurricane that hit Galveston, Texas, on
September 8, 1900, yet since there were no warning systems
in place, no one was to blame. In 2001, 2 feet (0.6 m) of rain
with consequent severe flooding hit the same area, and
nobody perished. However, billions of dollars worth of insurance
claims were filed. Now things are different, and few disasters
go without blame being placed on public officials,
engineers, or planners. Our extensive warning systems, building
codes, and understanding have certainly prevented the loss
of thousands of lives, yet they also have given us a false sense
of security. When an earthquake or other disaster strikes, we
expect our homes to be safe, yet they are only built to be safe
to a certain level of shaking. When a natural geological hazard
exceeds the expected level, a natural disaster with great
destruction may result, and we blame the government for not
anticipating the event. However, our planning and construction
efforts are only designed to meet certain levels of force
for earthquakes and other hazards, and planning for the rare
stronger events would be exorbitantly expensive.
Geologic hazards can be extremely costly, in terms of
price and human casualties. With growing population and
wealth, the cost of natural disasters has grown as well. The
amount of property damage measured in dollars has doubled
or tripled every decade, with individual disasters sometimes
costing tens of billions of dollars. A recent report (2000) to the
Congressional Natural Hazards Caucus estimated the costs of
some recent disasters; Hurricane Andrew in 1992 cost $23 billion,
the 1993 Midwest floods cost $21 billion, and the 1994
Northridge earthquake cost $45 billion. In contrast, the entire
first Persian Gulf war cost the United States and its allies $65
billion. That the costs of natural geologic hazards are now similar
to the costs of warfare demonstrates the importance of
understanding their causes and potential effects.
See also ASBESTOS; HURRICANE; MASS WASTING; PLATE
TECTONICS; RADON.
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