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Dave and Jan's travels:
Pinnacles in Crater Lake

First a pyroclastic flow dumps the yellow stuff everywhere. Then a slower avalanche of molten rock adds the distinct grey layer. Finally, hot gas below seeking a vent comes up through cones called fumaroles. The rock immediately around a fumarole is exposed to great heat (400 degrees centigrade/750 fahrenheit) and fuses into something very strong indeed. The usual erosion strips everything else off exposing the cones. The really amazing thing about all this is that, as you'd imagine from this process, the fumaroles are often hollow!