Volcanic plateaus are produced by volcanic activity. The mountains formed from the remains of the magma chambers which fed volcanic eruptions. Tephra is unconsolidated pyroclastic material produced by a volcanic eruption. It is one of several volcanoes which form the Campanian volcanic arc. Iceland has hundreds of volcanoes with about 30 active volcanic systems. These spots are thought to represent lava freshly released by volcanic eruptions. El Salvador has a long history of destructive earthquakes and volcanic eruptions. Primary pollutants are usually produced by processes such as ash from a volcanic eruption. The gas phase is extremely important in propelling volcanic eruptions. The volcano is monitored by a team of scientists at the Goma Volcanic Observatory. Glass can form naturally from volcanic magma. Cocos Island is an oceanic island of both volcanic and tectonic origin. The VEI itself is inadequate for describing the atmospheric effects of volcanic eruptions. These terranes consist of oceanic crust, volcanic and plutonic rock. It is often found in association with volcanic or basaltic sands.