Mauna Loa Volcano, Hawaii: Number 8 of America's Ten Most Dangerous Volcanoes

The largest volcano on Earth, Hawaii's Mauna Loa has erupted 33 times since its first documented eruption in 1843, but has been relatively quiet since 1984. (Video: Hawaii's Mauna Loa.)

Mauna Loa's general ooziness is due largely to the fact that "the Pacific crust is sinking down below the continents," the USGS's Eichelberger said. "That introduces a lot of water into the hot area of the mantle, which causes melting—and then you get volcanoes."

The molten rock is less dense than solid rock, so it tends to rise to the surface, as at Mauna Loa, where lava constantly flows into a beleaguered nearby community. Source