Not all volcanoes are the familiar cone-shaped mountains with lava spewing out of the top. Sometimes you can get a very slow eruption, underneath the ground, where a mound of earth rises up above the surroundings. These are called lava domes.
Lava domes form when you have slow eruption of very thick lava, or multiple eruptions of different kinds of lava. Instead of breaking through to the surface, the material builds up underneath the ground, causing the huge lava dome mound to form. It’s so thick that the lava can’t flow very far at all, it just hardens quickly, extruding new material around the vent. They can grow for months or years, eventually building up to several hundred meters in height. Large composite volcanoes can have hundreds of lava domes across their flanks.
The sides of lava domes are very steep, and can cause rockslides around because of the extrusion of new lava. This can bury the surrounding landscape, and even start forest fires if the rocks are still hot. The surfaces of lava domes are rough and blocky because of the fragmentation of rock shell as the dome grows.
As you can probably guess, this buildup of lava and gas can be dangerous. Although people can avoid the growth of a lava dome, they can be a problem for buildings that can’t be moved. But an even more dangerous problem with lava domes is the fact that they can explode violently, without warning, releasing pyroclastic flows of ash and mud for more than 20 km around the dome itself. In 1902, an eruption from a dome on Mont Pelee in Martinique killed 30,000 people with high-velocity ash flow and poisonous gasses.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.