154 Microscopic Petrography – Pumice and Scoria of the Climactic Eruptions – Basic Scoria

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

Microscopic Petrography


Pumice and Scoria of the Climactic Eruptions

In the culminating eruptions of Mount Mazama, two radically different types of magma were involved. Throughout most of the activity, pale-gray, buff, and white dacite pumice was erupted; in the concluding stages, the ejecta consisted of dark-gray and black basic scoria.

     Basic Scoria

Immediately after the dacitic part of the magma chamber had been erupted, it was followed by much darker, more crystalline, and more basic scoria, in the form of nuées ardentes. The change, it should be emphasized, was sudden. The silica content of the dacite pumice ranges from 66.38 to 69.85 per cent; that of the basic scoria ranges from 53.94 to 56.85 per cent. In fact, the composition of the basic scoria approximates closely that of the pre-Mazama basaltic andesites and that of the parasitic cinder cones, though the mineral content is strikingly different.

The typical basic scoria is marked by an abundance of dark-brown, vesicular glass and by plentiful crystals. Two scoria bombs were selected for analysis (nos. 26 and 27). One of these came from the walls of Llao’s Hallway, a narrow chasm tributary to Castle Creek. This has the following content: broken and glass-charged phenocrysts of zoned andesine-labradorite, 40 per cent; prisms of hornblende, showing pleochroism from pale yellow, X, through yellowish green, Y, to deep olive green, Z, and extinction angles up to 23° (Z to c), and occasionally with cores of fresh olivine, 15 per cent; granular ore, 3 per cent; vesicular brown glass, with a refractive index of 1.531±0.002, 42 per cent. Other scoria bombs from the same locality carry up to 4 per cent of hypersthene and 1 per cent of augite. The ratio of the various minerals differs within wide limits, the percentage of feldspar occasionally falling below 10 and that of hornblende sometimes rising to 30.

The second bomb analyzed came from the deposits in Sun Creek canyon. This is of an unusual type. The glass is paler than in most bombs and has a refractive index of 1.528 to 1.532. Approximately 60 per cent consists of intensely zoned plagioclase ranging in composition from andesine to medium bytownite. Small prisms of augite and hypersthene, in roughly the same proportions, together make up 10 per cent of the whole. Granular ore constitutes another 5 per cent. The principal feature of the bomb is the unusual paucity of hornblende, less than 1 per cent.

Rarely, bombs may be found which show a strongly banded structure, streaks of white pumice from a hair’s breadth up to an inch across running through the black scoria. Specimens were found in Annie Creek canyon and on the Pumice Desert. The light and dark parts are sharply delimited. To judge from the refractive indices, 1.507±0.002 for the light bands and 1.530±0.002 for the dark, it seems that the bombs are composed of both dacite and basic andesite. In the dark bands the phenocrysts are basic feldspar, hornblende, accessory hypersthene, augite, and ore; in the light bands the same minerals recur in roughly the same proportions, but are less than half as numerous. Apparently both types of magma were for a time erupted simultaneously without mixing.