During some eruptions, lava flows cement these rocks together. Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lava either flows thorough breaks in the crater wall or from fissures on the flanks of the cone. Lava, solidified within the fissures, form dikes that act as ribs which help to strengthen the cone. They may rise as much as 8,000 feet above their bases.