volcano
A volcano is a doorway in the Earth's crust where molten rock, ash, and gas push up from the hot interior and break out at the surface. Think of the planet as having a thin, cracked shell over a fiercely hot inside; a volcano is one of the places where that inside leaks out, building a mountain from its own erupted material, layer upon layer.
Volcanoes are not scattered at random. Most of them line up along the edges of the giant slabs called tectonic plates, where one plate dives under another or two pull apart, and a few sit over deep, stationary "hotspots" far from any boundary, like the chain that made Hawaii. This is why a world map of volcanoes traces the same seams as a map of earthquakes.
One common myth is that the glowing orange rock pours straight up from the planet's molten core. It doesn't. The melt comes from the upper mantle, only tens of kilometres down, and it usually rises because of falling pressure or added water, not because the rock below is simply liquid waiting to spill.
The Pacific is ringed by hundreds of volcanoes where plates collide — the so-called Ring of Fire.
Plate boundaries and volcanoes follow the same lines.
The name comes from Vulcan, the Roman god of fire, by way of the Italian island of Vulcano. Geologists call the same molten rock magma while it is underground and lava once it has erupted into the open air.