Geological Survey Professional Paper 3 The Geology and Petrography of Crater Lake National Park

MOUNT MAZAMA.
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LAVAS OF MOUNT MAZAMA.
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BASALTS.

Unlike the andesites, the basalts are limited to the outer slope of Mount Mazama, and do not approach nearer than about a mile of the crest. None of the basalt flows came from the central vent of Mount Mazama. They all issued from adnate cones upon the lower slopes of that great volcano. The northern part of the area mapped contains the principal basalt masses, which issued from four vents—Timber Crater, Desert Cone, Bald Crater, and Red Cone.

TIMBER CRATER BASALT FLOW.

Timber Crater, 5 miles northeast of Crater Lake, is the peak next in size to Scott Peak. It rises 1,500 feet above the surrounding plain and has a somewhat eccentric conical form with gentle slopes covered largely, to an elevation of 6,700 feet, by pumice from the final eruption of Mount Mazama. At 6,900 feet a reddish vesicular flow of basalt (165) issued from the southwest slope. Another basalt stream forms a prominent spur to the northwest. The lava is fresh, with all the peculiarities of a recent flow. Above this point the slope is made of lapilli, with a light covering of pumice. That the upper portion of the mountain is a cinder cone is not seen by the traveler until he reaches the summit, where there is a well-defined crater 50 feet deep and 250 yards in diameter. The crater is double, or rather there are two craters of equal size side by side. One is nearly north of the other, and the two were most likely active at different times. With the progress of the eruption the vent shifted slightly parallel with the range. This is a common feature in many volcanic fields, but is rather unusual in the Cascade Range. The coating of pumice from Mount Mazama is spread upon the slopes of the cinder cone of Timber Crater, showing that its activity had closed before that of Mount Mazama.

The view of Crater Lake and its rim from Timber Crater is especially fine, and to the northward Mount Thielsen, the "lightning rod" of the Cascade Range, sometimes also called the "Matterhorn" of the range, stands out conspicuously.

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.

BALD CRATER BASALT FLOW.

Bald Crater has a well-defined pit on the summit of its cinder cone. Some solid flows of dark basalt (192, 193) are exposed, but red and black lapilli are most abundant. Its crater, being bare and prominent, affords a fine view, and when seen from neighboring points stands out conspicuously above the deep-green forests.

CRATER PEAK BASALT FLOW.

Crater Peak is the center of a small area of basalt on the divide extending south from Castle Crest and Vidae Peak between Anna and Sun creeks. The hill rises over 700 feet above the general level of the platform of andesite on which it rests. At the northern base of the hill is a mass of reddish-brown basaltic tuff. The general layer of pumice extends far up the slope, showing that, as in other cases, Crater Peak was not active after the final outburst from Mount Mazama. The pumice layer is decidedly darker than the material of which the peak (179-181) is composed. The cone is largely if not wholly fragmental, and there is no definite flow from it unless it extends to the northwest. Among the prevailing dark fragments of basalt there are occasional andesitic fragments hurled out from the underlying andesites. The summit of Crater Peak is a well-defined crater 100 yards in diameter and 25 feet deep, draining to the southwest. Some good-sized firs grow on it, and it contains a bank of snow as late as August.

OTHER FLOWS.

Toward the southwestern corner of the park there is a large area of basalt, in which there is considerable variation, and the mass may have been built up from a number of vents. It forms a bluff near the road southwest of Anna Creek from Pole Bridge Creek toward the summit. It ranges in color from gray and reddish to almost black and some is vesicular. North of Union Peak it rises to over 7,000 feet and is associated with basaltic tuff. The divide at this point is a ridge of basaltic lapilli, indicating the proximity of a volcanic vent, although no well-defined crater was seen. The rocks are well glaciated and the original form of the cinder cone may have been greatly modified thereby. The ridge of lapilli affords a fine view of Union Peak a few miles farther south on the crest of the range. Union Peak is rugged and composed largely of andesite, which came from a vent which was more ancient than that from which the basalts issued. Upon the northern border, as elsewhere, gray (164) and reddish (166) colors are common, but on the whole the darker colors (182, 187, 163) are most abundant, and in places, especially near the Rogue River road, the rock is decidedly platy.

In the southeastern corner of the park is a hill of scoriaceous basalt with lapilli overlying dacite, and west of this, in the flatter country near the border of the park, there are bowlders and bluffs caused by streams of basalt which may have descended from Crater Lake, although the connection was not observed. The falls of Anna Creek below the forks are over basalt, but the exposure is very small. Small areas of basalt occur along the road between the falls and Pole Bridge Creek, and basalt may cover much of the country marked andesite in the southwestern portion of the park.