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
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.
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.
- Mount Mazama and Crater Lake: Growth and Destruction of a Cascade Volcano
- Cascades Volcano Observatory - Mazama Ash
- Ellen Morris Bishop - Hiking Oregon's Geology
- Journal of Volcanology and Geothermal Research - Eruptive history of Mount Mazama and Crater Lake Caldera, Cascade Range, U.S.A.
- Charles Mandeville et al - Stable isotope and petrologic evidence for open-system degassing during the climactic and pre-climactic eruptions of Mt. Mazama, Crater Lake, Oregon