Geography Reference
In-Depth Information
eruptions that have really global effects. The eruptions from Mt. Asama in Japan in
1783 or Krakatau in 1883 (Winchester
2003
) threw out vast amounts of dust, rock
and pumice into local areas, killing thousands of people in their region and destroy-
ing the productivity of surrounding agricultural areas. Krakatau's explosion was
reputedly heard over 3,000 miles away and was estimated to have had a destructive
capacity 13,000 times the Hiroshima atomic bomb. But its biggest effect lay in its
dust output that eventually blanketed the earth. This has been estimated to have af-
fected the world climate for 5 years, because the dust reflected the sunlight back,
reducing the temperature by an average of 1.3 % C, leading to bad harvests around
the world, with similar consequences to those after Asama's eruption.
One of the most worrying things about the effect of many of these hazards upon
settlements is their
speed of impact
. Most of the geological events occur rapidly, with
no or little warning, and often produce major loss of life if they affect a populated
area. Weather-related events take more time to develop into life or property-threat-
ening major hazards and usually affect wider areas, which means more warnings are
often provided by modern weather monitoring systems. Most of these hazards are
also time-limited in their
duration
, in the sense that their most intense direct effect is
measured in hours or days rather than weeks, although their effect on the disruption
of life in settlements may be measured in years. However, some natural hazards,
such as droughts affecting large areas, may last for years, having huge effect on
the size and density of settlement patterns. This cause—aided by poor agricultural
practices—created the so-called Dust Bowl in the American mid-West in the 1930s.
It must also be stressed that many of these natural extreme events do not have only
direct effects; many create what amount to
related and indirect effects
. For example,
many large storms produce rain, high winds and floods; droughts cause vegetation
to dry-out and become more vulnerable to fire; high winds help to rapidly spread
these fires. Similar related effects occur with many geological events. Earthquakes
and landslides into oceans or lakes can push up or replace large columns of water,
leading to an increase in both the size and length of waves radiating out from the
impact centre which increase in amplitude once they reach the shore, causing the
flood waves known as tsunamis, the largest in recent history being around 30 m.
Also there should be more recognition of what has been summarized as Na-Tech or
Hybrid origins of many disasters, where a dam breaks due to seismic activity and
floods an area that was developed because it was thought the dam would protect the
region from river floods (Smith and Petley
2009
). The list goes on and on.
These examples of indirect effects can be complemented by other historical cases
that have
societal effects
, not just local ones, such as the rainfall decrease, smaller
crops and political instability that led to the eventual collapse of the impressive
city-states system of the Maya in Yucatan and surrounding areas from the sixth
century (Kennett and Breitenbach
2012
). Of course, even the most extreme of these
examples pale into insignificance beside the effect of a large asteroid impact on the
earth which could wipe out life as we know it. Most of the millions of meteoroids
that enter the earth's atmosphere every day are small, burn up quickly and have little
impact. Very occasionally a large one occurs, but few have been experienced and
documented in our human memory. In 1908 a stony, iron-rich meteor of 30-50 m in
