Franklin & Marshall College Franklin & Marshall College

Stanley A. Mertzman

Picture guide of recent fields of study

Looking west from Rye Spur volcano towards Mount McLoughlin one can see the result of a massive landslide, which removed much of the northern and eastern flanks of the volcano and deposited the material as a chaotic jumble further down slope.

Looking from the southeast to the northwest, you see Mount McLoughlin in the late summer, completely devoid of snow, with the parasitic vent, South Tip, readily apparent on the far left of Mount McLoughlin. South Tip and North Tip, not visible in the image, both erupted between 100,000 and 50,000 years ago and extruded porphyritic basaltic andesite lavas with approximately 55% SiO2 and 2 to 4 millimeter in diameter phenocrysts of plagioclase feldspar, pyroxene (both augite and hypersthene) and olivine.

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Looking south from Sky Lakes Wilderness, the central conduit region of Mount McLoughlin is apparent. The North Tip parasitic cone is visible on the right shoulder of the volcano and the Brown Mountain summit area visible to the far left. The central conduit region of Mount McLoughlin has been exposed by a large ladnslide which moved off to the north and east of the edifice.

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Looking west from a boat dock on the eastern shore of Lake of the Woods, in the foreground aligned parallel to the lakeshore is a forested ridge oriented north-south that marks a normal fault with the lake on the down dropped side of the fault. Brown Mountain is an andesitic composite volcano, SiO2 ranging between 59 and 61% and quite aphyric in hand specimen, in which lava flows greatly predominate over pyroclastics. A small summit crater is present on top of Brown Mountain.

Another view of Brown Mountain in which blocky lava flow surface can be seen peering out from the tree cover on the eastern flank of the volcano.

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A quite young, blocky andesite lava flow from Brown Mountain.