Editor’s note: This is the latest installment in a series about the 30th anniversary of the the eruption of Mount St. Helens. See special features, including reader memories and Columbian coverage at www.columbian.com/volcano. JOHNSTON RIDGE OBSERVATORY — Visitors gathered beneath a gray, overcast sky to commemorate the day 30 years ago when Mount St. Helens destroyed what nature almost immediately began to rebuild.During a moment of silence, it might have seemed possible to hear the faint echo of geologist David A. Johnston’s final dispatch: “Vancouver! Vancouver! This is it.”Hundreds of people descended Tuesday on the spot where Johnston perished in the eruption on May 18, 1980. Another 56 died that day in the blast zone.Of all the people who gathered during the 30th anniversary commemoration, none may have known the mountain as thoroughly and as intimately as U.S. Geological Survey retiree Donal Mullineaux.