It really depends on the kind of volcano and the type of eruption. There are famously continuous lava flows on Big Island in Hawaii that have been going on for

It really depends on the kind of volcano and the type of eruption. There are famously continuous lava flows on Big Island in Hawaii that have been going on for decades. These are technically eruptions, and they're a _very_ interesting subject for a photographer. Because they've been ongoing for so long, there are more-or-less reliable ways to get fairly close to the flows and back. You're right for more violent eruptions, like St Helens, though!