28 Desert Cone and Red Cone Basalt Flows

The Geology and Petrography of Crater Lake National Park, 1902

 PART I.

MOUNT MAZAMA.

LAVAS OF MOUNT MAZAMA.a

BASALTS.

DESERT CONE AND RED CONE BASALT FLOWS.

Desert Cone and Red Cone are volcanoes in line with a number of others which form a decided ridge, practically the crest of the range. They are evidently due to a number of vents on one fissure, and the material erupted is essentially the same throughout, although the volcanoes were not active at the same time. The oldest is to the northward, and the youngest is Red Cone. At the eastern base of the ridge, near the northern limit of the portion that appears upon the map, the basalt is vesicular (188), but farther up on the slope is compact and holocrystalline (189). This spur is plainly a flow to the east. The lava is often rough upon the surface and has lost little by weathering, presenting an aspect of newness not found on the associated lavas. The summits of the two most northern hills of the ridge were once craters, but the loose material has been swept away by subsequent erosion, exposing in the cliffs of the crest the solid lava (190, 191) of which the central portion of the cones is composed.

Desert Cone, near the southern end of the ridge, next to Red Cone, is a cinder cone with rough, chiefly reddish, basalt (167). The summit has an imperfect crater broken away toward the northwest. Its slopes are very steep.

Of all the small volcanoes which have furnished basalt in the northwestern part of the area mapped, Red Cone is the best example. It is well preserved, its lavas have the freshest look, and, all things considered, it appears to have been active later than any of the craters about the great central vent of Mount Mazama.

Red Cone is composite. The basal portion or pedestal is made up very largely of basalt flows, and the upper 500 feet is a cinder cone composed almost wholly of lapilli, sand, and slaggy chunks of red and gray basalt. Much of the reddish lava is vesicular, but the gray is not vesicular. The rim on the south side is 50 feet above the bottom of the crater, which drains to the northeast. The crater contains numerous fragments of dacitic pumice, like that of the final eruption of Mount Mazama. The fragments of dacite are so abundant in the crater and upon its slopes as to leave no doubt that Red Cone had closed its career before the final eruption of Mount Mazama. The radiating flows which form the base of the cone spread far beyond the limits of the cinder cone. To the east they are well exposed, and some of the flows are vesicular. The elongated cavities are flattened and lined with hyalite. The gray basalt (156) is often rich in olivine. From the base of Red Cone a great sheet of basalt spreads westward in the flat, forested country, where underlying rocks are concealed by a layer of pumice. Here and there, however, domes of the basalt rise through the pumice. The fresh vesicular basalt is like that of the base of Red Cone, from which it was derived. Its surface is well glaciated, and near the western border of the area mapped becomes irregular and rugged on a small scale, with many striated ledges and occasional meadows due to lava dams formed in a narrow part of the valley. Rarely (184) the basalt is platy. How far down Rogue River these flows extend is unknown. Basalt occurs along the Rogue River road for many miles, but it is a darker lava and much more vesicular than that of Red Cone.

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