The Hillman Cone, Crater Lake National Park

The highest point on the caldera rim is the summit of Hillman Peak, formerly known as Glacier Peak (see plate 11, figure 2), which rises to an elevation almost 2000 feet above the waters of Crater Lake. Here, as Diller long since pointed out, the layers of lava have a decided upward curve when viewed from the lake, and suggest that the volcanic vent from which the lavas of that portion of the rim issued was not central over the lake, but much closer to the western border. This view is fully borne out by the character of the igneous material of Glacier Peak. It is composed in small part of darker slaggy andesites and much red, yellow or whitish fragmental material which is highly colored, as if by the escape of hot volcanic gases near the vent. From the lake these colored patches are brilliant in the morning light.

Plate 11. Fig. 2. Hillman Peak from the south. The spires on the caldera wall consist chiefly of cinders and agglomerate; the summit consists of andesite flows. The vent filling of the former Hillman Cone lies among the crags about halfway down the visible part of the caldera wall, and is mainly shadow. (Photograph by Elmer Aldrich.)

Clearly, Hillman Peak is the remnant of a parasitic cone on the side of Mount Mazama. When the caldera was formed, the eastern half of the Hillman cone disappeared, revealing a perfect cross section through the central conduit.

In its original state the cone measured approximately 3/4 mile across, and it rose to a height of perhaps 1000 feet above the adjacent slopes of Mount Mazama. The lower half of the cone is composed of dark-gray and black "cinders" and scoriaceous tuff breccias (plate 29). The upper part, on the contrary, is made up of dark flows of andesite interbedded with coarse breccias. Much of the lava is kaolinized and propylitized, and some of the fissures are coated with opal and hematite.

In brief, the Hillman cone was built first by explosions of viscous scoria, and later by alternating outflows of lava and more violent, low-temperature eruptions. The flows were confined to the immediate vicinity of the cone, and probably activity came to an end before the Watchman andesite and the neighboring dacites were extruded from the Northern Arc of Vents.

Fortunately, the conduit of the Hillman volcano is perfectly exposed a short distance east-northeast of the summit pinnacle, and is easily accessible. In plan, it is approximately oval, measuring 1/3 by 1/5 mile, and is elongated in a direction more or less radial with respect to the former summit of Mount Mazama (plate 29). Most of the conduit consists of massive andesite devoid of conspicuous banding. Toward the center, this andesite is generally of a pale-gray color, is traversed by widely spaced, steeply dipping joints, and is notably vesicular. Intruded into the pale lava is a denser, darker, and less vesicular andesite in which the jointing is more closely spaced. Similar relations have been noted in the conduits of the Union Peak, Mount Thielsen, and Howlock Mountain volcanoes.

Along the margins of the conduit there is commonly a thin skin of coarse breccia, deeply reddened by the action of gases. Within the conduit are much larger bodies of massive, unbedded breccia, likewise affected by solfataric action. Close to the exposed top of the conduit, near the western edge, the unbedded breccias pass upward gradually into well stratified ejecta that were probably laid down at the surface.

The margins of the conduit seem to have offered ready passage for gases and solutions rising from the chamber below. Not only are the rocks of the conduit itself largely altered, but the enclosing lavas are tinted in shades of orange, red, pink, brown, purple, and green by the development of chlorite, hematite, and limonite, or are bleached to whitish, crumbly masses of kaolin and opal.