Image: Jeff Marsh

Mount Rainier is 100 square miles of active volcano. Armed with just a rock hammer and a handheld magnifying glass, geologist Tom Sisson set about reading its rocks like tea leaves. After a dozen years, off and on, of scouring the peak, Sisson created an info map for the U.S. Geologic Survey, a kind of personality profile of Rainier. His hard-won samples trace the history of thousands of years of lava flows and geologic hiccups and help us determine how the mountain will erupt. 

Rainier, Sisson explains, used to be compared to an Eskimo Pie—crusty on the outside and soft underneath, where rotten bits of rock were hydrothermally compromised. But science has disproved that gooey and delicious analogy (darn), finding that the mushy bits are few and visible on the volcano’s surface. Scientists have determined that the upper west flank is most likely to collapse in the event of an eruption—so watch out, Orting and Sumner. 

But anyone fearing (or secretly hoping for) a St. Helens–style blowup will be disappointed. “Rainier doesn’t erupt in a big way like that,” says USGS seismologist Seth Moran. “What it does have is lava flows and small-scale explosions that put ash down locally, just in the park. Not nearly the same scale as Mount St. Helens.” 

Rainier has more monitoring stations measuring seismologic activity than any Cascade mountain but the always-burping St. Helens, so we’ll probably know when it’s about to blow. But when magma bubbles close to Rainier’s surface, rather than worry about an ash cloud, we’ll need to watch for a lahar, a muddy flood wall that could barrel down the Carbon, Puyallup, and White River valleys. Orting even has five lahar alert sirens and instructs residents to get 80 feet above the valley floor within in 40 minutes of them ringing.

A thousand years after Rainier’s last minor eruption and some 5,600 years since a big lahar, it’s hard to say how or when Rainier will blow. “Volcanoes don’t erupt in a linear way,” says Moran. And even with Sisson’s decade-long mapping of Rainier’s geologic history and personality, there are a few blank spots—he couldn’t examine every square inch of the mountain, what with the glaciers and spots that were too dangerous to visit. “I can’t say to someone who works for me, ‘Why don’t you run under that ice cliff?’ ” he says. “If they’re crushed, I’ve got a mountain of paperwork and a lifetime of guilt.” We’ll just have to live with the uncertainty.