An extensive volcano glossary can be found at volcanoes.usgs.gov. Above left: Mt. St. Helens, a grande dame among andesitic volcanoes, erupts on July 22, 1980.

Lava flows come in a great variety of shapes and sizes. Nyiragongo is considered one of the most dangerous volcanoes in the world because of its particularly fast-moving lava, which can flow more ...