Timber Crater - The Geology of Crater Lake NP, 1942

Plate 14. Fig. 1. Timber Crater and the rim of Crater Lake, from Mount Bailey. Mount Scott on the extreme left. the U-shaped valley beheaded by the far (south) wall of the caldera in Sun Notch. The bare patch on the extreme right is part of the Pumice Desert. In the middle distance is the symmetrical shield volcano of Timber Crater, with its summit cinder cone. (Photograph by Victor Duran.)

IN THE northeast corner of the park, there is a well preserved shield volcano capped by pyroclastic cones. From the perfection of its form, this volcano may be judged at once to be among the youngest of the region. See plate 14, figure I.

The shield is approximately 5 miles in diameter, and the volume of material erupted approximates 5 cubic miles. The shield is composed essentially of flows of olivine basalt and basaltic andesite. The only notable variation is in the texture and color of the lavas and in the degree of vesicularity. As on other shield volcanoes in the Cascades, the pale-colored lavas are less vesicular and poorer in glass than the darker lavas, and where both types occur in a single sheet, the crust is darker and contains more vesicles.

Some of the flows show surface forms suggestive of recent eruption, and several of the spurs radiating from the summit region, though covered with pumice, seem to represent flows little modified by erosion.

Capping the lava shield are the remains of two cinder cones, aligned in a north-south direction. Little is preserved of the older cone, which forms 6889 Hill, north of the summit. There is no trace of a crater. All that can be seen beneath the pumice are a few small outcrops of bright-red scoria with vesicular bombs up to 3 feet across and patches of agglutinate.

The younger cone is complex, and only slightly sculptured by erosion. It is oval in plan, and extends for a mile in a north-south direction, rising approximately 600 feet above the lava shield. Here and there small exposures of well bedded scoria may be found, though rounded bombs are extremely rare. Most of the ejecta consist of angular blocks and lapilli of dense, black vesicular basalt, presumably already solid when erupted. Accompanying the explosive eruptions were short and sluggish flows of lava.

Within the summit cone three craters may readily be distinguished. The northernmost is indicated only by a remnant of the northern rim. The other two coalesce to form a depression between 50 and 100 feet deep, measuring about 1/4 mile from north to south and 250 yards across at the widest place. Probably these two vents were active at the same time, though the deeper, southern crater may have continued to erupt after the other became extinct.

We may conclude, therefore, that during the explosive eruptions which brought to an end the activity of the Timber Crater volcano, there was a southward migration of the vents, parallel to the general trend of the Cascade Range, from 6889 Hill to the present summit region, more than a mile away.

Activity at the Timber Crater volcano had certainly ended before the last pumice explosions of Mount Mazama. Its closing eruptions were probably contemporary with those which formed the parasitic cinder cones on the sides of Mount Mazama. A little gravelly drift on 6889 Hill indicates the former presence of a small icecap at that point, but the summit cone of Timber Crater and the sides of the lava shield do not appear to have been covered by glaciers. The final activity of this volcano may therefore be assigned to post-Pleistocene times.