IF THE WEATHER forecasters are right, some of the remains of the top of Mount St. Helens will float over Washington today. The dust will do us no harm and may provide some much-needed inspiration. Sunrises and sunsets are likely to be spectacular as long as the dust lingers in the sky. But in the Northwest, particularly if Mount St. Helens continues to burp, the dust and the rest of the aftermath of last Sunday's eruption may make for a long, hard summer.

Volcanoes are both devastating and renewing. They can kill, as this one did in Washington, and they can destroy farm crops and plant life, as this may do if the dust keeps rolling out. But they also build land and replenish the soil, which is one reason so many of the earth's 500 or so active volcanoes are in areas of fairly dense population. That, of course, is a long-term proposition. It will provide little comfort to the farmers in the Northwest if their crops are suffocated by volcanic ash or if the growing season is cut short by a haze in the sky.

It was fortunate, perhaps, that nature chose Mount St. Helens and not one of the other volcanoes in the Cascades as a reminder of its capacity for fantastic violence. A similar explosion inside Mount Hood or Mount Rainier -- both dormant volcanoes -- might have been far more devastating. Moreover, the loss of life at Mount St. Helens, tragic as it was, is small when measured against the toll taken by Vesuvius in Italy, Krakatau in the East Indies and Pelee in the Caribbean. The force of the explosion, though monumental, hardly compares with what happened when St. Helens' neighbor, Mount Mazama, threw nearly 12 cubic miles of ash and pumice into the air 7,000 years ago and then collapsed, forming the caldera now known as Crater Lake.

Knowledge about volcanoes has advanced greatly since mankind worshipped Vulcan, Pele and other gods of fire. But many questions are still unanswerable. Each volcanic eruption, be it an explosion like that at Mount St. Helens or a lava fountain like those frequently presented by Kilauea in Hawaii, provides a close-up glimpse of the imperfectly understood forces still at work shaping the earth.

The event Sunday at Mount St. Helens was not like the explosion of an atomic bomb -- the unfortunate comparison made by many who saw it. Its destructive power was tempered by creativity, not heightened by malice, and its violence was a natural part -- an unusually dramatic one -- of a changing world. Rather than raising questions about the wisdom with which mankind has used the knowledge it has, Mount St. Helens raised questions about the knowledge mankind has yet to acquire.