The volcanoe erupted suddenly after years?

Answer 1

Many volcanoes have a period where they are dormant for years, decades, or even centuries, and then have erupted suddenly. As a matter of fact, I would say, that volcanoes being dormant and then erupting, (after a long period where they did nothing), is much more likely than a volcano in a constant state of eruption. Most volcanoes erupt, use up all the material they have stored up inside, and much later, (after building up more energy), erupt again.