How would an eruption of Mount Rainier compare to the 1980 eruption of Mount St. Helens?
Eruptions of Mount Rainier usually produce much less volcanic ash than do eruptions at Mount St. Helens. However, owing to the volcano's great height and widespread cover of snow and glacier ice, eruption-triggered debris flows at Mount Rainier are likely to be much larger -- and will travel a greater distance -- than those at Mount St. Helens in 1980. Furthermore, areas at risk from debris flows from Mount Rainier are more densely populated than similar areas around Mount St. Helens.
Mount Rainier - Living Safely With a Volcano in Your Backyard; USGS Factsheet 2008-3062