Now that residents of the Cook Inlet region of Alaska (including Anchorage) are dealing with volcanic ash falls from Redoubt, I’ve read a bunch of tweets warning folks not to use their windshield wipers to push dry ash off their cars windshields as it may scratch the windshield. Indeed, this recommendation is found among the recommendations in the USGS Ashfall Preparedness website.
And yet, something initially struck me as odd about this recommendation. Volcanic ash is primarily composed of tiny glassy shards of quenched magma, or obsidian. Obsidian, like windshield glass, has a Mohs hardness of 5-5.5. Normally, objects with the same or very similar hardness will not noticably scratch each other. No doubt, the jagged ash fragments are likely to tear up the rubber windshield wipers, but not the windshield itself. At least, that was how I thought about it at first.
Of course, I was overlooking the fact that most erupting magmas are not aphyric (crystal free). And a bunch of new SEM photos of minerals in the Redoubt ash now makes that clear to me. It is these minerals, with Mohs hardnesses in the 6-7 range, that will actually abrade your windshield if you aren’t careful.
Among the minerals identified in the SEM photos are amphibole (with reaction rims and nice cleavage), plagioclase (showing concentric compositional zoning and a sieve-textured core), orthopyroxene, calcic pyroxene (augite?), and oxide microlites. I haven’t yet read or heard of any completed chemical analyses of the ash, but this mineralogy is consistent with the andesite-dacite composition of magmas erupted in 1989-1990. As much as I’ve enjoyed following the webicorders and webcams as the eruption was ramping up, it’s good to finally see some petrologic/petrographic data.