Tephra is unconsolidated pyroclastic material produced by a volcanic eruption. Volcano Island sank from as a result of the eruption. One possibility is that ash from a volcanic eruption was generating the lightning. A major volcanic eruption at Mount Mazama around 4860 BC formed Crater Lake. This was destroyed in 1997 by the volcanic eruption. Primary pollutants are usually produced by processes such as ash from a volcanic eruption. A lava flow is a moving outpouring of lava created during a non-explosive effusive eruption. Lava flowed but less than the 1766 eruption. The new dome did not rise above the caldera created by the 1980 eruption. There was initially some doubt as to whether the earthquakes were precursors to an eruption. Fine ash from the eruption reached Norway. The last major eruption occurred around 1655. It had roughly four times the energy of the 1883 Krakatoa eruption. Another hypothesis builds on the flood basalt eruption theory. The last major eruption occurred in 1994.