"The Pinnacles: When the very hot ash and pumice came to rest near the volcano, it formed 200-to-300-foot (60 to 90 m) thick gas-charged deposits. For perhaps years afterward, hot gas moved to the surface and slowly cemented ash and pumice together in channels and escaped through fumaroles. Erosion later removed most of the surrounding loose ash and pumice, leaving tall pinnacles and spires."
Ian