Volcanic Ash on the Windshield

Ashfall on a pickup truck.  Photo by Kristi Wallace.Now that residents of the Cook Inlet region of Alaska (including Anchorage) are dealing with volcanic ash falls from Redoubt, Cleaning volcanic ash from windshield wipers 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.SEM image of Redoubt ash.  Photo by Tom Kircher.

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.

BSE image showing amphibole grain with reaction rim in sample AT-1605. Jessica Larsen, AVO/UAF-GI.

Where on (Google) Earth #165?

Paul Arndts (aka Felsic Intruder) correctly identified the Kinmei Seamount at the bend of the Hawaiian-Emperor Seamount chain. Since Paul has no blog, I’m going to post his WoGE #165. To earn the honor and glory that accompanies all WoGE solutions you merely need to identify the location of the Google Earth screen capture below (latitude and longitude will do nicely) and explain the geologic significance of the landforms visible in the image. There’s no Schott Rule this time, but I won’t be participating since I already know the location. :-)

Where on (Google) Earth #165.

Good Luck!

Ajo, Arizona and Organ Pipe Cactus National Monument

You may recall that on my return trip from AGU last December weather patterns had forced me to change plans in order to keep GigaPanning. Well, after the Calico Hills I proceeded south through the Mojave with a brief stops at Pisgah and Amboy Craters and Dish Hill. I can’t show you the GigaPan I shot at the Pisgah Lava Field, unfortunately because the GigaPan Stitching software has a lingering bug that prevents me from stitching my big 360 degree panoramas. By the time I got to Dish Hill the sun was already setting, so I didn’t even have much time to look for xenoliths. (For all you WoGE fans, be sure to note that Peter Luffi has a new article out in JGR that has some very interesting insights into the nature of the lithospheric mantle based on studies of these xenoliths.)

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After a night in Blythe, CA and a flat tire near Gila Bend, AZ on Christmas Eve, I finally got about as far south as I could go and camped two nights in Organ Pipe Cactus National Monument. Christmas Day was pretty grey and chilly so I spent most of the day reading in my car. Finally, on Boxing Day the weather began breaking and I was able to get out and shoot some GigaPans again.

The first one is taken on the loop drive in Organ Pipe Cactus NM. The rocks are Tertiary volcanics and the sky was still pretty grey. But look at all the pretty cacti!


Tertiary Volcanics, Ajo Range, Organ Pipe National Monument
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Noticing that the high deck of clouds was clearing from the north, I decided to go back up to Ajo, Arizona to see if I could GigaPan the open pit copper mine there. Unfortunately the only public viewpoint of the pit was looking south into the sun, and the view wasn’t great anyhow. Unable to GigaPan the pit, I decided to do the next best thing by GigaPanning the piles of colorful overburden (technically not the tailings) that were removed to create the pit.

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In searching for a spot to get an overview of the open pit, I had passed a couple of places that had a decent vantage point of the town of Ajo. Since the light wasn’t right for a shot in the downtown plaza, I decided to go for an overview of the town instead. It turns out that there was a lot of good detail to see, making this one of my more popular GigaPans for exploring and bookmarking.

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Finally, late in the day, I decided to make one more brief foray down to Organ Pipe NM to catch the setting sun illuminating the west face of the Ajo Range. More Tertiary volcanics dominate this view, thought I believe there are some Paleozoic & Mesozoic sedimentary rocks somewhere in the neighborhood.

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The Organ Pipe area is a lovely corner of Arizona with some wonderful Basin and Range geology. Although it wasn’t on my original itinerary, I’m glad to have had the chance to get back this way, almost two decades since my first visit (on a Colgate University January Term geology trip). Hopefully it won’t take me another two decades to get back again.

Where on (Google) Earth #164?

I discovered Peter’s devilish Dagestani deformation, so it’s my turn again to see if I can stump the Geoblogosphere. Actually, I’m picking a big fat wet one for all of you. Shouldn’t be too hard to find since it covers a huge area, but the real challenge (after you’ve identified its latitude and longitude) will be in identifying the geologic significance of this spot.

Where on (Google) Earth #164.

Once again, I’ll forgo the Schott Rule, but I’d ask recent winners to give the newbies maybe a day or so. (Post time: 23:58 CDT). Good luck!

Calico Folds

There’s been an overwhelming and uniformly positive response to the Google Earth Geology Layer post I made a week ago. I’m still trying to digest all of the feedback, but as soon as I have a chance to I’ll do a follow up. Meanwhile, back to my regularly scheduled blogging…

The Calico Folds, just northeast of Barstow, California was the next geological stop on my journey back to Hays from AGU last December. The GigaPan below is one of four I shot at this locality. You can explore the whole set in their “natural habitat” in Google Earth. This one’s for all of the structural geologists out there.

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Remarkably little is written up about this locality in guidebooks of the region, despite the fact that these spectacular folds are featured in a number of textbooks. Remarkably, though I’ve passed through this region at least twice in my geological career I was unaware of the location of these spectacular folds until I did a little sleuthing on Flickr this past year. This leads me to believe that they are a “secret” location visited by many classes and field camps, though I have no direct knowledge of that. If anyone knows of any guidebook descriptions of this locality that may have escaped my attention or of any classes that visit this locality for fieldwork I’d be interested to know about them.

It’s remarkable to me that such a spectacular and generally accessible locality could be such a well kept secret – well, no more!