In the analyses listed by Moore, the contrast between the “fines” in the flows and in the fall is very clear. Considering only the fraction less than 64 mm. in size, the averages are as follows:
| Less than 0.25 mm.
(%) |
0.25-1 mm.
(%) |
|
| Pumice flows
Pumice fall |
36
1 |
27.6
22 |
In other words, almost two-thirds of the pumice flow material below 64 mm. in diameter measures less than I mm, though in the pumice fall less than a quarter is below that size. Furthermore, by far the bulk of the fine dust in the flows is pulverized pumiceous glass, whereas much of the dust in the fall consists of broken crystals. Many samples collected in the area south and east of Klamath Marsh contain merely a trace of material less than I mm. in size, and this may consist entirely of crystals. Even in the finest pumice fall examined, from the Paisley Cave, 80 miles east of Crater Lake, the percentage of dust less than 0.25 mm. in size is only 3.
The reason for the much greater content of “fines” in the pumice flows is clearly related to the comminution of the material as it was carried along in the form of avalanches, the fragments continually bombarding one another as the glowing masses swept down the sides of the volcano. Among the particles in the pumice fall there was, of course, no such mutual attrition.
Nature of the pumice in the fall. Close to the surface, where the pumice has been weathered, the color is pale b d or brownish, but below it is usually white or pale gray. Where vegetation is heavy, the zone of weathering may be as much as 2 feet thick, but generally it is only a few inches. Soil is either extremely thin or altogether absent.
Most of the fragments are approximately equidimensional, but many are spindle- or disk-shaped and some are drawn out into long threads. In certain fragments the vesicles are ovoid; in others they are tubular, so that the pumice has a shredded, fibrous appearance. In the larger lumps the vesicles occupy much space, but they become progressively smaller as the size of the lumps diminishes, and some of the finest dust is made up of glass shards almost wholly devoid of pores. The difference results, of course, from the bursting of gas bubbles in the larger clots. Accordingly, small shards of glass may weigh as much as cellular fragments many times larger, and for that reason ejecta of quite different sizes settled from the air simultaneously over the same regions.
Unlike the large lumps in the pumice flows, which were rounded by abrasion during transport, the larger fragments in the pumice fall are either angular or subangular. Many of the larger pieces are pink throughout; many more show a narrow pink zone a few millimeters from the surface. Apparently the pink color results from atmospheric oxidation of iron-bearing gases given off by the pumice in a heated condition. The smaller pieces fail to show the color because they were chilled too rapidly and ceased almost at once to give off gas.
Though most, of the larger pumice fragments are extremely vesicular, there are a few, particularly within 20 miles of Crater Lake, which are dark gray, compact, and lithoidal. These represent clots of magma chilled quickly, probably prior to explosion.
Crystal content. Casual inspection gives the false impression that crystals form only a small fraction of the pumice fall. When the material is screened and examined with a hand lens, crystals are seen to be abundant. In size, they range from about 0.2 to 2 mm, by far the majority measuring between 0.5 and I mm. in longest dimension. In most samples, the fraction between those limits may be almost exclusively and is usually at least one-third crystals. In the larger fragments of pumice many crystals are still embedded in the glass, but in pieces smaller than a few millimeters across the separation of glass from crystals is almost complete. This separation is not the result of weathering and disintegration, but an original charmer, caused by liberation of the crystals from the enclosing droplets of viscous glass as they spun through the air.
