15 Andesites

The Geology and Petrography of Crater Lake National Park, 1902

 PART I.

MOUNT MAZAMA.

LAVAS OF MOUNT MAZAMA.a

ANDESITES.

WATCHMAN ANDESITE AREA.

The area of which The Watchman is part contains a number of separate flows, but upon the map they are not distinguished. They are particularly well exposed by numerous cliffs, but in much of the area they are covered by glacial material. Although the flows are much alike when compared with one another, there is considerable variation within each flow. The holocrystalline gray forms, like No. 60, pass into the type like No. 6—dark, porous, soft, and crumbly, but rich in amorphous matter—and No. 26, which is decidedly vitreous.

The composite character of the flows may be best seen upon the inner slope, where their sections are exposed. The andesites are generally gray, and may be reddish. They are seldom as vitreous as No. 26. No. 60 has a decidedly platy structure, which is locally well marked, although absent at other points. The plates are usually about one-half inch to an inch in thickness, and strike N. 70° E. nearly parallel to the spur, with a dip of 25° NW. The thickness of the plates at each outcrop is rather uniform, but it varies between the outcrops.

At The Watchman and Glacier Peak 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 in 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. On the inner slope of Glacier Peak (Pl. VII, A) are numerous columns, one of which is over 100 feet high. To call this Glacier Peak is a misnomer, for the glaciation here is much less distinct than on the hills farther south along the rim.

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