82 Flows down the North Slope

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

The Climax: Culminating Explosions of Pumice and Scoria


Detailed Description of the Individual Flows

Having enumerated the general features of the glowing avalanches, we may now pass to an account of the individual flows.

     Flows down the North Slope

On the north side of Mount Mazama there were no deep glacial canyons to concentrate the flows. The consequence was that once they had passed through the gap between Red Cone and Grouse Hill, they spread out in wide sheets. The momentum of the earlier pumice flows was enough to carry them across the basin of the Pumice Desert and over the saddle to the north, but the later scoria flows failed to escape from the desert in that direction. Consequently the surface of the Pumice Desert consists chiefly of crystal-rich scoria and ash, among which lie many bombs. One of these measures 12 by 12 by 3 feet, but most of the others are less than a yard across. Many are exceptionally rich in hornblende, and the fine ash is crowded with glistening prisms of this mineral. Despite the large size of the bombs and the proximity to the source, fragments of old lava more than 1/2 inch in diameter are notably rare. What the volume of the pumice and scoria deposits is in the Pumice Desert there is no means of determining accurately, though their thickness may well be 200 feet. The pronounced red and pink blotches revealed by road cuts through the upper part of the scoria show that fumaroles must have been abundant and long-lived in this region. A well drilled adjacent to the highway where it crosses the lowest part of the desert passed through 106 feet of smoke-gray, hornblende-rich scoria without penetrating the underlying white dacite pumice. Samples consist almost entirely of lithic fragments and discrete crystals of feldspar and hornblende. With these are much less numerous crystals of pyroxene, and a little brown glass. The content of lithic fragments increases, though not regularly, with depth. Near the bottom they are far more abundant than crystals. An interesting deduction may be drawn from these observations. Although the Pumice Desert is thickly covered by ejecta, particularly in its central part, it still has the form of a shallow basin. Hence, a deep valley must have existed there before the final eruptions of Mount Mazama began. Yet when the first glowing avalanches of dacite pumice swept across the valley they did not convert it into a flat plain. Instead they rushed across with such vehemence that they left it much as it was and continued over the divide to the north. The later avalanche of scoria and crystals lacked either the momentum or the mobility of the dacite avalanche, for little of it escaped across the northern divide. Most of it came to rest in the valley itself, and some made its way out via Desert Creek and so continued to the plateau beyond.