Re: why might a lahar (mud flows) affect a community more severely than lava flow?

Dear Adrian,

As you may know, lava flows are streams of molten rock that pour or ooze 
from an erupting volcano. A lahar is a hot or cold mixture of water and 
rock fragments that flows down the slopes of a volcano.  Lava flows are 
less dangerous than lahars primarily because lava flows move more slowly.  
For example, people can often out-run or even out-walk the lava flows 
occurring in the State of Hawaii.  In 1990, the village of Kalapana, Hawaii 
was destroyed by lava flows over seven months.  Fortunately, there was 
enough time to allow Kalapana's inhabitants to evacuate safely and even 
take many of their belongings with them - including some buildings! 

Lahars, on the other hand, are much more dangerous to a community because 
they travel very fast, can be very large, and occur with little or no 
warning.  Some lahars can travel several tens of meters per second.  For 
example, if a lahar moves:

	10 meters per second = 36 kilometers per hour or 22 miles per hour (thats 
too fast to outrun!)
	50 meters per second = 180 kilometers per hour or 111 miles per hour 
(faster than many cars can go!)

Lahars can also be very large - some are hundreds of meters wide (wider 
than a football field) and tens of meters deep (twice as deep as a swimming 
pool).   

Lahars are not just made up of mud - they contain rock fragments of all 
sizes - clay fragments to large boulders.  They can also carry trees, soil, 
buildings, and cars that are in their way.  When lahars begin to slow down 
they deposit their debris, burying rivers and fields or streets, buildings 
and even villages.  Such unpredictable and widespread destruction can be 
truly devastating to a community.

If you are seeking more information or photographs of lahar damage I 
recommend visiting the US Geological Survey's web site on Volcano Hazards: 
 http://vol
canoes.usgs.gov/Hazards/What/Lahars/lahars.html 
(The background information for this answer was obtained at their website, 
too.)

Information about the village of Kalapana can also be found at: http://volca
noes.usgs.gov/Hazards/Effects/Kalapana.html


Thank you for the interesting question and best of luck to you in your 
studies