Other Pre-Mazama Lava Within the Park

The lavas occupying the northwest corner of the park and the slopes that lead thence to the Rogue River are composed principally of olivine-rich basaltic andesites similar to those of the Union Peak volcano. Some of these lavas are related to Red Cone, Desert Cone, and Bald Crater, but most of them are much older. Although they are not exposed on the walls of Crater Lake, there is little doubt that they form the foundation beneath Mount Mazama and are of approximately the same age as the lavas of Union Peak. In other words, they belong to the older volcanic series of the High Cascades and antedate the great composite cones. Probably they were erupted in Pliocene and early Pleistocene times.

Generally there is little difficulty in distinguishing these lavas from those of Mount Mazama, for they are usually rich in granules of olivine and devoid of the large feldspar crystals that characterize the Mazama flows. Most of them are so fine-grained that only grains of olivine and pyroxene and specks of feldspar can be recognized by the unaided eye. In color they range from pale gray to black, and, as on Union Peak, the darker flows are usually more vesicular. Some types are holocrystalline and have an ophitic texture; others include much glass and are darker.

Diller referred to these lavas as the Desert Cone and Red Cone basalts on the assumption that they were erupted from those vents, but although similar flows do occur on these cones and in their immediate vicinity, most of the lavas in the northwest corner of the park are much older. This is not to deny that some of them may have been erupted from older vents beneath those cones; indeed, the attitudes of the flow planes point to sources thereabouts. Possibly the youthful Red and Desert cones were the result of renewed activity at vents which had lain dormant throughout the growth of Mount Mazama. (See plate 4, figure 2.)

Not all the pre-Mazama lavas of this region issued from vents near the parasitic cones just mentioned; many are related to the ruined volcano of Desert Ridge. The thick glaciers that once swept down the north side of Mount Mazama almost completely buried this volcano, subjecting it to severe erosion. There is now no trace of a summit crater. Nevertheless, the vague flow banding of the lavas suggests a vent on the north boundary of the park, near post 51. Apparently the volcano was built entirely of lava. Precisely similar flows form the younger volcanoes of Twin Knobs and Sherwood Buttes, respectively 4 and 6 miles to die northwest.

Similar lavas also form the pedestal beneath Bald Crater and the spur running northwest therefrom, and presumably Bald Crater, like Red and Desert cones, marks the site of a vent which burst into renewed activity about the close of the Pleistocene.

Finally, reference should be made to the olivine-bearing lavas forming the mesas on either side of National Creek. On Crescent Ridge these lavas are exposed to a thickness of more than 1,500 feet, and throughout this great pile there are no interbeds of fragmental debris. The source of the flows must lie to the east, probably in the vicinity of some of the ruined volcanoes already mentioned.

Hence, during the Pliocene and perhaps also during the early Pleistocene, before the cone of Mount Mazama began to form, the northwest corner of the park was occupied by a group of volcanoes the activity of which was marked by quiet effusion of liquid, wide-spreading, basic lavas, and an almost complete absence of pyroclastic explosions. Mount Mazama rose in the depression between this group of volcanoes on the north and the Union Peak volcano on the south.