In our present concern with possible volcanic activity in the Cascades, it seems appropriate to summarize what we know about past volcanic activity in the Portland area and its possible structural significance. A recent paper (Allen, 1974) noted that several vents and lava tubes on the west side of the Portland Hills represented the westernmost group of large Plio-Pleistocene centers of volcanic activity in the Northwest.
This, however, by no means suggested the total extent of late volcanism in the Portland area. Within a 13-mile radius of Kelly Butte there are over 32 volcanic vents; within a 20-mile radius centered at Troutdale there are 90 volcanic centers. Most of these were originally small cinder cones like Pilot Butte and Lava Butte near Bend, Oregon, but some of them, such as Mount Sylvania in southwest Portland, Highland Butte 10 miles southeast of Oregon City, and Larch Mountain south of the Columbia River Gorge, were low, broad lava domes of the type called "shield volcanoes".
The densest concentration of volcanic vents lies west of the town of Boring, where 20 centers occur within an area of about 36 square miles. Because of this grouping near Boring, Ray Treasher (1942) first gave the name "Boring lava" to the lava, cinders, and ash which emanated from volcanic centers in the Portland area within a time span of from perhaps 10 million to less than 1 million years ago (Trimble, 1963). Some like Bob's Mountain in Washington, may be very young indeed.
Trimble (1963) mapped the areal extent of the Boring lava in the Portland area and mentioned (p.36-42) that it erupted from 30 centers, but gave the exact location of only a few vents. Geomorphologic study of the new 7 1/2 -minute quadrangles (not available to Trimble) allows fairly accurate location of many of these and also other vents. The degree of assurance attributed to the identification given is indicated by the legend symbols (certain, probable, possible) used on Plate 1.
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Plate 1: Location Map and Table: Location and elevation of 95 vents, including multiple vents, in the Portland area -- Modified from: Allen, 1975, Ore-Bin no.37, no.9 |
I wish to thank my colleagues at Portland State University for their suggestions while I was writing this paper and for their careful review of it.
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