I’m going to speculate a little bit about the future of Mt. St. Helens. My basic assumption is that the volcano has a future eruptive history; it is unlikely that it will become an extinct volcano anytime in the near future. I’ll give two scenarios for the next few hundred years of the mountain.
Scenario 1. The dome-building eruption will continue for decades. It may not be continuous—it could be extruding lava for a few years and then be quiet for a decade—but it could keep this up for a long time. The result would be a rebuilding of the mountain to something like its pre-1980 glory.
Scenario 2. The dome-building eruption stops sometime in the next few years, and then the mountain lies dormant for a century or even several hundred years. This would be consistent with its history over the past several thousand years. If this happens, it is likely that the crater will slowly fill with glacial ice. Even now, 26 years after the catastrophic eruption that created the present landscape at MSH, glacial ice reaches a depth of 200 m (600 ft) in areas between the crater rim and the lava domes in the center of the crater. The shape of the crater—a horseshoe facing north—is ideal for the formation of a large glacier, perhaps the largest individual glacier in the United States outside Alaska (this is my speculation; I haven’t looked into it closely).
Whether scenario 1 or 2 happens, it is highly probably that MSH will someday have another large, explosive eruption. If scenario 2 is the case, that eruption will be accompanied by an very large lahar, as volcanic ash mixes with melting ice to form a dense slurry that rapidly flows downstream.
Image of current glacial ice in the MSH crater, right up against the new lava dome. USGS/NASA
The crater of MSH is shaped like an ideal glacial cirque. Because the crater opens to the north rather than some other direction, the interior of the bowl does not receive as much direct sunlight as it otherwise would. USGS.
Grace and Peace, especially to whoever lives downstream from MSH 300 years from now if Scenario 2 comes to pass.