Friday, September 23, 2016

Geo 898: Three Fingered Jack

From a quarry on the north side of Hogg Rock, the view to Three Fingered Jack is excellent. Three Fingered Jack is described on OSU's Volcano World page as a stratovolcano in the leading basic information, then in the first sentence as "a deeply eroded basaltic shield volcano." I don't know which it is, but I guess I'd lean toward shield rather than stratovolcano. The Oregon Cascades were heavily glaciated during the Pleistocene, and the fact the volcano is so deeply incised shows that it hasn't been active- or at least hasn't had any major lava-producing eruptions- since the end of the ice age. Mt. Jefferson, the next major peak north of it, shows the same style of faceted glacial erosion.

Route 20 winds around the west and south side of Hogg Rock, a tuya (a sub-glacial eruptive landform, where the lava erupted under ice), much too close to get any decent photos, and the rock here in the quarry seemed too nondescript to bother with. The best photo of Hogg Rock on this trip was the one I took at Lost Lake. As noted in the post at that link, the pass area burned a few years ago, killing many, many trees.

Photo unmodified. September 7, 2016. ZoomEarth location.

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