Here's an unusual volcanic ash sample. This is rhyolite ash from a subduction zone stratovolcano in southern Chile. The Chaitén Volcano erupted in May 2008 and its wind-blown ash got deposited in Chile and Argentina. The color of the ash is very pale yellow to cream - it's one of two volcanic ash samples I have that are this light-colored (a Lava Creek B ash-fall sample from Oklahoma that was erupted 640,000 years ago from the Yellowstone Hotspot is just a bit paler yellow than this Chaitén sample).
What makes this pale yellow rhyolite ash sample from Chile so unusual? Well, subduction zone stratovolcanoes typically have andesite or dacite ash eruptions. Andesite and dacite are intermediate extrusive igneous rocks, having total silica contents between 52% and 65%. Rhyolite ash has a total silica content over 65%. The Chaitén 2008 erupted material has a 73% to 76% silica content - very high for a subduction zone.
Published research on the May 2008 Chaité Volcano indicates that the rhyolite magma ascended very rapidly for a felsic magma, at about 1 meter/second, before having an explosive ash eruption. This is the quickest ascent of rhyolitic magma ever recorded - four hours elapsed for the magma ascent, starting from an estimated >5 km depth, and reaching the near-surface. Rhyolitic melt is famous for typically having a high viscosity. Some rhyolitic lava outcrops have the appearance of toothpaste oozing from a vent. Chaitén Volcano's rhyolite melt broke the rules (see Castro & Dingwell, 2009 for details).
Location of Chaitén Volcano: ~10 km northeast of the town of Chaitén, southern Andes Mountains, southern Chile (42º 50' 18" S, 72º 39' 01" W).
Rhyolite ash (ash fall deposit) (field of view ~3.4 cm across), erupted early May 2008 from Chaitén Volcano, southern Chile. Collected between 3 May & 16 May 2008 from a farm in the town of Esquel, northwestern Chubut Province, southern Argentina.
Some info. from:
Castro, J.M. & D.B. Dingwell. 2009. Rapid ascent of rhyolitic magma at Chaitén Volcano, Chile. Nature 461: 780-784.