Lava domes are rounded, steep-sided mounds built by very viscous magma, usually either dacite or rhyolite. Such magmas are typically too viscous (resistant to flow) to move far from the vent before cooling and crystallizing.

Facts and Figures: Of the 1,511 volcanoes known to have erupted in the past 10,000 years, only 40 are classified as independent lava domes not associated with another volcano. Lava domes are often erupted, however, on the top and sides of stratovolcanoes.

Although lava domes are built by nonexplosive eruptions of viscous lava, domes can generate deadly pyroclastic flows. Recent eruptions of lava domes at Unzen volcano in Japan and at Soufriere Hills in Montserrat have forced thousands of people from their homes.

Some domes erupt obsidian, which is volcanic glass that may form in rhyolite or dacite lava flows. Most obsidian is black, but red, green, and brown obsidian is known. Obsidian forms when magma is cooled so quickly that individual minerals cannot crystallize.

Dome within the crater of Mount St. Helens, Washington, USA. Photo: J. Alean