Illustrated Volcano Glossary

Lava domes are accumulations of highly viscous lava above or near the vent(s) where they were erupted during slow flow, like a sticky glue squeezed out or oozing out from a hole in its container.
Lava domes often take the form of a rounded hill, resembling a dome, but come in many other shapes as well.

Lava domes are very similar to thick lava flows, with the difference being that the lava once erupted from the vent almost doesn't flow at all, and piles up above the vent instead of flowing away downhill or to the side. In fact, there is a transition between thick, sticky lava flows and lava domes, depending on how viscous the lava is, how steep the terrain, the flow rate and other factors. Sometimes, lava domes look like short, but thick sausage-like flows. This type is sometimes called a coulée (from the French word meaning "flow").

The stickier the magma, the taller usually the dome can grow. At the lower end of viscosity, dome-forming lava might often looks like a flat pan-cake with a rugged top. Excellent recent examples include the lava domes that have been forming in the craters of Popocatepétl volcano in Mexico and Santiaguito in Guatemala.