81 Other Flows down the West Slope of Mazama

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.

     Other Flows down the West Slope of Mazama

The flows directed through the depression between Llao Rock and Hillman Peak left only a patchy cover of material on top of the hummocky glacial moraines between the rim of the caldera and the headwaters of National Creek. Apparently most of the flows which passed across this part of Mazama were made up of basic scoria and crystal-rich ash. In the canyon of National Creek the deposits are mainly composed of gray scoria and ash, mixed in the upper stretches with abundant glacial debris. The flows had traveled 10 miles before they tumbled into the canyon, and it is not surprising that they had lost much of their heat and gas and therefore show no signs of fumarolic action.

The flows directed down the west slope between Hillman Peak and the Watchman found their way into the glacial valleys of Copeland and Bybee creeks, though for a distance of about 3 miles beyond the caldera rim they left scarcely any trace of their passage. Indeed, most of the fine ejecta thinly sprinkled on these upper slopes of the volcano was probably laid down after the flows had passed beyond.

In the upper parts of both Copeland and Bybee canyons most of the deposits left by the flows consist of incoherent gray scoria and ash, much admixed with glacial material. Farther down the canyons, the scoria gives place to the older dacite pumice, which continues downstream to the confluence with the Rogue River. Lumps of pumice as much as 18 inches across are not uncommon, though pieces of old lava rarely measure more than 4 inches. The total content of lava fragments amounts to between 5 and 10 per cent of the whole.

 

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