What Is a Composite Volcano Made up Of?

Composite volcanoes, one of the two main types of volcanoes along with shield volcanoes, are composed of alternating layers of lava and ash or other volcanic debris. These volcanoes are usually cone shaped with steep sides that are built up by multiple eruptions over hundreds or thousands of years.

Composite volcanoes usually experience more violent eruptions than shield volcanoes, as they contain thicker basaltic lava that is more likely to clog the volcanic vent. This causes pressure to build up inside the volcano that can only be released by an explosive eruption. These eruptions usually release a pyroclastic flow containing a mixture of lava, ash, rock, dust and steam, which can flow extremely quickly. On the other hand, shield volcanoes contain a less viscous andesite lava, which flows slowly over great distances to give these volcanoes a distinctive domed shape.

Most composite volcanoes are found in chains that occur along convergent plate boundaries, where an ocean tectonic plate is sliding underneath a continental plate. The biggest concentration of these volcanoes occurs along the so-called Pacific Ring of Fire that spans most of the boundaries of the Pacific Ocean. This includes Mt. Rainier, Mt. St. Helens, Mt. Fuji and other famous volcanoes.