How often do volcanoes erupt, and why is Mauna Loa erupting now?

For the first time in a generation, lava began flowing from Mauna Loa at the heart of Hawaii’s Big Island late Sunday. No one knows when it will stop, or what, if any, damage it might cause.

But there is a lot we do know about the more than 700,000-year-old volcano, and others like it.