Fumaroles may persist for decades or centuries if located above a persistent heat source; or they may disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools. The Valley of Ten Thousand Smokes, for example, was formed during the 1912 eruption of Novarupta in Alaska. Initially, thousands of fumaroles occurred in the cooling ash from the eruption, but over time most of them have become extinct.