March 29, 2011

Field photo Set #2

Late March - early April is the end of semester period, thus I'm swamped with term papers, labs, and presentations. I'm not one to procrastinate, since I've been burned by it in the past and learned my lesson. But activities like blogging must take a back seat, and I'm sure most reading this blog are experienced in what the last month of a semester is like.

So to keep things simple, for me and for you, but mostly for me, I have another pair of field photos to show, replete with explanation for the features' what/when/where/how. This time, a pair of volcanically-derived features in Oregon, both derived from eruptive activity from the legendary Mt. Mazama complex in what is today Crater Lake National Park.

The Pinnacles fumarole features, looking west.
The V-shape valley shows river erosion exposing them
To the right is a snapshot of the elegant Pinnacles, located just a couple of kilometers to the east of Crater Lake. These spires were ancient fumarole conduits of Mazama's gaseous content (SO2, CO2, H2S), exposed by fluvial erosion from a radial stream, but remaining resistant to that erosive force. Prior to the finale of Mazama's VEI 7 eruption 7.7 Ka ago, a nuée ardente flowed down Mazama's east flank, carrying scoria. Gasses escaped from the settling scoria through fumarole vents, and the mineral content, given the extreme heat, welded loose pumice to the sides of the fumaroles. Thus the pinnacles seen to the right are hollow, and are resistant to erosion from the inside-out, which is atypical of common geologic thought.

When viewing the pinnacles along the 2-3 km path (42° 51.056'N 122° 0.558'W), I noticed that some of the spires had puncture holes in them, in which I could see through. The keen eye will also notice that the base the pinnacles stand on is a lighter color, which is due to the more silica-rich rhyodacite ash falls that preceeded the scoria-laden pyroclastic flow.

This field photo is of a roadcut along the North Umpqua highway, not far from Watson Falls (43° 14.553'N 122° 21.486'W). Catching this roadcut out of the corner of my eye made me glad my car has a low center of gravity. This hillface, ~35km from Crater Lake, showcases silica-rich ashfall from Mazama during its major eruptive phase 7.7 Ka ago. The several-meters thick deposit is a testament to the volume of tephra ejected by the monster eruption (~60 km3), and its coverage across the northwest is found much further afield as well (Mount Baker slopes have a few cm thick of Mazama ash deposit, and it's over 600km from Crater Lake). Tephrochronology analyzes in the region are easily guided by Mazama ash, as the distribution of the ash from the centroid is quite ideal, making it a prime stratigraphic marker for 7.7 Ka.

The white color stems from sanidine feldspar content within the silica-rich ash, and darker grey portions contain a greater percentage of ferromagnesian minerals. The grainsize is quite fine, looks & feels almost silty, with a gritty abrasiveness, but not too harsh and not as hard as sheer-faced plutonic rocks. You can jab this rockface and it feels somewhat padded.

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