Perfect for throwing trash into, right? But not all volcanoes are cooperative like that. The ideal trash incinerator would be a slow-erupting volcano that gradually spews lava out onto the surface of the earth, like the volcanoes in Hawaii, called shield volcanoes. But the majority of volcanoes on Earth are stratovolcanoes which occasionally have lava flows like Kilauea, but also have the unfortunate tendency to explode when the pressure of hot gas and magma inside the volcano gets to be too much. Long story short, you don't want to throw trash into an explosive volcano (think Mount St. Helens) when it's erupting. If you're close enough to throw trash into the exploding mass of molten rock, ash, and gases, you're already dead.