142 Microscopic Petrography – Timber Crater Lavas

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Microscopic Petrography

Timber Crater Lavas

If the presence of olivine is regarded as a criterion for the recognition of basalts, then certainly most of the lavas of the Timber Crater volcano must be classified as such. Nor is there much doubt that the field geologist would agree to the use of the name, for the lavas are dark-gray to black, vesicular flows in which little but olivine and minute glassy feldspars can be recognized with the hand lens. Yet the only specimen analyzed from this volcano (no. 12) must be classed chemically as an andesite. The best solution of the problem is perhaps to refer to the rocks as olivine-bearing basaltic andesites. As such they resemble the lavas of the Union Peak volcano and the pre-Mazama flows in the northwest corner of the park. In other words, the pre-Mazama type of lava continued to escape long after the hypersthene andesite cone of Mazama had begun to develop.

Patton described the Timber Crater lava as a “fluidal-interstitial basalt” with almost no olivine and with hypersthene distinctly the dominating pyroxene, but apparently he examined only a single thin section. More extensive study shows that whereas a few flows contain no olivine, by far the majority do include this mineral and in some flows it constitutes as much as 10 per cent of the volume.

The olivine-free lava from the southeast base of Timber Crater has the following content: stumpy, glass-charged, and embayed laths of labradorite, 55 per cent; euhedral prisms of hypersthene, some of which have jackets of augite, 15 per cent; brownish-black glass stippled with granules of ore, 30 per cent.

Contrasted with this are the pale-gray flows on the north slope of the volcano. Though these are also rich in deep-brown glass (15 per cent), they carry abundant phenocrysts of olivine. Hypersthene is present as smaller euhedral microphenocrysts, but augite is restricted to minute, ragged granules. In the “cinders” of the summit cone, augite is altogether absent, whereas hypersthene is unusually abundant. Here, as in the quickly chilled “cinders” of other cones, glass makes up almost half the total volume.

Finally, in the analyzed specimen (no. 12) from the southwest flank of the volcano, olivine phenocrysts constitute 3 per cent, hypersthene is by far the dominant ferromagnesian, and augite is restricted to the dense, intergranular base. The feldspar varies little in composition from medium labradorite. Cristobalite, which is rarely absent from any of the Timber Crater flows, is plentifully sprinkled on the walls of vesicles.


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