How Volcanoes Work

 Image Described in Caption
 Phonolite lava erupting from Jabal Shalman, a basaltic scoria cone on the Harrat Kishb lava field of western Saudi Arabia -- The phonolite is most likely post-Neolithic in age based on the lack of Neolithic monuments commonly developed on adjacent lava flows. This exceptionally thick flow is 2.5 km long, 1.0 km wide, and 40 m thick. It contains flow ridges and a coarse blocky surface. The light-colored flow to the left is an older , thicker (100 m) phonolite which extruded from a separate scoria cone. Courtesy of John Roobol.