153 Microscopic Petrography – Pumice and Scoria of the Climactic Eruptions – Dacite Pumice

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.

     Dacite Pumice

Except for the degree of sorting and the size of the fragments, the granular pumice that fell from the air during the first stages of activity does not differ from the more voluminous pumice erupted later in the form of glowing avalanches. Together, these pumice deposits constitute more than seven-eighths of the total volume of fresh magma erupted.

Both mineralogically and chemically, the dacite pumice is almost identical with (the dacitic lavas erupted at an earlier date from the Northern Arc of Vents. In other words, the pumice is composed of glassy hypersthene-augite dacite with accessory hornblende. Variations in crystal content and in the ratio of light to dark minerals have already been discussed.

The glassy base of the pumice is highly vesicular. Indeed, the density of large bombs may be as low as 0.50. When the pumice is pulverized artificially, its density may not exceed 2.0, showing that even the fine pumice dust is minutely vesicular. So frothy is some of the pumice and so elongated are the vesicles that the fragments appear like delicately shredded glass.

Typically, the glass is colorless and has a refractive index between 1.500 and 1.510. Occasionally it is pale yellow or buff, and in large bombs with oxidized crusts it may appear pale pink by reflection, perhaps because of dusty hematite. Trichites and belonites are exceptional.

The content of crystals varies between wide limits, the average lying between 10 and 15 per cent by volume. In some samples, the content falls below I per cent; in a few, it rises as high as 60 per cent. Such irregular concentration is only to be expected among ejecta derived by eruption of magma undergoing crystallization.

Few crystals exceed 2 mm. in length, and few measure less than 0.2 mm. By far the majority range in size between 0.5 and 1 mm. Plagioclase is almost invariably more abundant than all the ferromagnesian minerals combined. Most of the feldspars are intensely zoned in an oscillatory fashion, and some show a range in composition from labradorite at the center to oligoclase at the rims. Equally striking is the abundance of glassy inclusions. These are more plentiful than in the feldspars of the dacitic lavas, and may be so numerous as to give the crystals a spongy appearance. Characteristic also is the highly cracked nature of the feldspar.

Of the dark constituents in the pumice, hypersthene is usually the chief. Rarely it is exceeded in amount by pale-green augite, and still more rarely by hornblende. Occasional crystals of hornblende may have a deep-red color and low extinction angle, but typically the mineral varies in color from olive green to brownish green and has an extinction angle, Z to c, of about 15°. Resorption of hornblende and consequent formation of magnetite rims, a feature characteristic of the hornblende which occurs sparingly in the basic inclusions and andesites of Mount Mazama, is completely absent in the dacite pumice. Olivine has not been observed. Magnetite, on the other hand, is ubiquitous, though sparsely distributed. Tridymite and cristobalite, though plentiful among the dacitic lavas, have not been detected in the pumice.


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