57 The Younger Cones – The Pumice Flat Cones

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

The Parasitic Scoria Cones of Mount Mazama

The Younger Cones

     The Pumice Flat Cones

On the southeast flank of the Union Peak volcano, immediately to the east of Pumice Flat, there are three well preserved scoria cones. These, together with the Goose Nest farther south, lie on a fissure which is radial with respect to the caldera of Crater Lake. For this reason, they are regarded as parasites fed from the magma chamber of Mount Mazama.

The northernmost cone forms 6545 Hill. No lava seems to have been erupted from this vent. The explosions must have been mild, for most of the ejecta measure only a fraction of an inch to a few inches across. In the top of the cone there lies a bowl-shaped crater breached on the east.

Adjoining this cone on the south is a crescentic rim of scoria partly enclosing an arcuate ridge of lava. The eastern part of this cone has likewise been destroyed.

Farther south, forming 6627 Hill, is a third cone consisting of scoria mixed with masses of agglutinate. This cone is particularly interesting because it has a double crater. The larger and older crater is clearly of explosive origin, and it was from this that the scoria cone was mainly if not entirely built. At the north end of the flat crater floor there is a deep, rectangular pit, 100 yards long and 50 yards across, with precipitous walls. From the floor of the older crater to the bottom of the pit the drop is go feet; the outer wall of the pit is a cliff of lava zoo feet high. The western and eastern sides of the pit are formed by slides of scoria above and lava below. If the slides of scoria were removed, the walls of this inner pit would consist entirely of lava. Clearly this cannot have come from the pit itself. Nor can the pit have been formed by explosion; its form belies such an origin, and the absence of fragmented lava in the vicinity also rules out the possibility. The pit must therefore have been produced by collapse, probably as the result of fluxing and stoping by magma rising from below. How, then, are the lava walls to be explained? Only by assuming that they are composed of much older flows belonging to the Union Peak volcano. After the first explosions had formed the older crater and scoria cone, magma rose under the north wall, undermining it and causing it to founder. The history is analogous to that of many pit craters in Hawaii.


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