How Volcanoes Work

 Image Described in Caption Fumarole pipes, Real Grande ignimbrite, Argentina -- These dark, lithic-rich pipes are gas segregation structures that provide direct routes for the degassing of the ignimbrite. The escaping gases cause fragments of different sizees and denisities to jostle apart from one another. The largest fragments in the pipes are ~20 cm in diamter. Most of the finer material, however, has been blown out of the pipes (elutriated) by the escaping gas. The ignimbrite was derived from an eruption 4.6 million years ago, associated with the Cerro Galan caldera. Courtesy of Peter Francis.