Geography Reference
In-Depth Information
A
lahar
, a word of Indonesian origin, is a dangerous, fast-moving mudflow. During an erup-
tion, vast quantities of emitted steam may cool rapidly, fall as rain, mix with ash, and form
a mudflow. In the cases of very high volcanoes, such flows may be complemented by large
amounts of water from rapidly melted snow and glaciers. The resulting lahar may race down
flanking valleys for miles and miles from the volcano, burying everything in its path.
In November 1985, the destructive potential played out to its fullest following the eruption of
the Nevada del Ruiz volcano in Colombia. Lahars as deep as 150 feet raced down the side
valleys. Within four hours, locations as far as 65 miles away — seemingly well beyond the
volcano's reach — were under mud. Hardest hit was the town of Armero, where some 23,000
people were killed and another 5,000 injured.
In a way, a lahar is to a volcano as a tsunami is to an earthquake — a mechanism by which the
power of a major tectonic event may be fatally felt far away from the actual event itself. But
being far away, the possibility exists for early warning systems that can significantly lessen
the number of people who end up as statistics.
A mountain blows its top
I have two atlases that disagree mightily concerning the height of Mt. St. Helens. One
says the summit is 9,677 feet above sea level. The other gives an elevation of 8,363 feet
above sea level. That's a difference of 1,314 feet, which is close to the height of the Em-
pire State Building. The reason for the disagreement is one atlas was published before
the volcano erupted (May 18, 1980) and the other was published afterwards.
But “erupted” is a bit of a misnomer. The mountain literally blew its top. The U.S. Geolo-
gical Survey estimates 3.7 billion cubic yards of mountain got blown away. Another 1.4
billion cubic yards of ash got ejected, much of it in a cloud that reached 80,000 feet within
15 minutes. Accumulations of the inevitable ash fall averaged 10 inches 10 miles down-
wind, 1 inch 60 miles downwind, and 1/2 inch 300 miles downwind. Fifty-seven people
perished (some from the blast, others from the suffocating ash-fall), along with an estim-
ated 7,000 big game animals and 12-million Chinook and Coho salmon fingerlings. As
you can see, all kinds of numerical facts have been calculated and committed to print.
What's really amazing to me is that the volume of forest that got blown down (4 billion
board feet of timber) was enough to build 300,000 2-bedroom homes.
