A lahar is generally described as a "volcanic debris flow." They can be triggered by volcanic activity, but active volcanism is not a necessary component.May 18, 1980 lahar covering highway at the Toutle River. The bridge was wiped out.
USGS Photograph taken July 1, 1980, by Lyn Topinka. (Large version, archived here)
So the above, from St. Helens 1980 eruption qualifies as a lahar, and so does the following, which was not associated with an eruption:Photo of the debris flow at Newton Creek, Mount Hood, 2006. This debris flow turned and flowed down Highway 35, destroying several miles of highway (photo from Bill Burns, 2006) (Large version, archived here)
Volcanic arcs, such as the Cascades, tend to have numerous composite stratovolcanoes. Those two modifiers- composite and strato- refer to the fact that the volcanoes are made of layers (strata) of differing rock types (composites). Volcanoes of this type tend to be tall and steep. Volcanic rocks tend to be heavily fractured, and to weather rapidly compared to many other rock types. Combine this with increased precipitation due to orographic effects, and it becomes obvious that lahars are a natural and inevitable consequence of volcanic landscapes. Eruptions can trigger them by shaking or by rapidly melting large amounts of snow and ice, or they can be triggered by something as innocuous as a fall rain (like the Mt Hood example) or spring snowmelt.
Debris flows are a well known hazard throughout the more rugged parts of the Pacific Northwest, but they are of particular concern in heavily populated areas on the western drainages from the Cascades. And nowhere is the concern greater than the northwest flank of Mt Rainier. A test of a debris flow monitor and alarm system failed there; it turns out the wrong audio file was loaded. The report says it has been fixed.
I say get a kid with an Ipod to double check (and maybe get his/her recommendations for an alarming audio file as well), then test it again. This one's too important to bollix it up.
This Week's Geo-Quiz: Other Planets
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