The summit of Sunset Crater has been closed to hikers since 1973, and getting access to the fumarole deposits requires special permission. Sarah Hanson, a professor at Adrian College, has worked for the Park Service for the past 5 summers at the Monument and is lead author on a new report describing the mineralogy of the fumarole deposits. Sarah sent us a copy of Rocks & Minerals magazine, Nov-Dec issue with an 11-page article filled with SEM photos of the minerals.
The six fumarole deposits are narrow, elongate and parallel to the crater rim. Sarah and co-authors conclude the fumarole vents were active in the waning stages of volcanic activity (which began during the period of 1073-1090 A.D.) with cooling vapors and increasing oxygen fugacity, resulting in a zoning of minerals from gypsum-hematite-magnetite to gypsum-earthy hematite to gypsum-sulfur.