Two questions about Mt. St. Helens 1980 eruption?

Question

One of the major dangers of debris flows and pyroclastic avalanches is not by lahars which they generate, but by lahars which develop in the hours and days after such events. How do these develop, and how was this development illustrated in the 1980 eruption of Mt. St. Helens?

Answer 1

Secondary lahars are often formed when rain mixes with ash from the eruption.  This causes instability.  The formation of these lahars often occur within 30 minutes of as little as 10-15 mm of precipitation falling on the loose ash.

For images of Mt St. Helens' secondary lahars destruction, go to:

http://vulcan.wr.usgs.gov/Volcanoes/MSH/...