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The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Post-caldera Eruptions

 

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Wizard Island

Wizard Island (see plate 20) consists of a perfectly symmetrical cinder cone almost completely encircled by dark, rugged flows of lava. The summit of the cone rises 763 feet above the level of the lake, and the area of the cone and lavas exposed above water approximates a square mile. Were it not for the sparse covering of trees, one might suppose that the eruptions had only lately ceased, so fresh is the appearance of both the cone and the flows. Set in the placid blue waters of Crater Lake, the somber island leaves an indelible picture in the mind.

Aerial view of the cone of Wizard Island, Crater Lake NP

Plate 20. Aerial view of the cone of Wizard Island. (Photograph by U. A. Army Air Corps.)

The cone. The cone itself is little more than 1/2 mile in diameter and consists chiefly of black, scoriaceous lapilli and blocks. Near the summit these are mixed with red and brown "cinders." Fine ash is notably scarce. Though most of the fragments measure more than an inch across, there are few large pieces. Near the base of the cone bombs 2 feet in diameter may be found, and on the crater rim there are blocks of lava more than 6 feet in diameter. Well rounded bombs are exceptional. Most of the ejecta are angular or subangular and must have been blown out when practically solid. Some of the fragments were undoubtedly formed by explosive shattering of lava flows and temporary conduit plugs. Eruptions of Strombolian type characterized only the final stages of activity, for near the crater rim there is much agglutinate, indicating that the latest ejecta were still sticky enough when they fell to adhere to adjacent fragments.

Not all the cone consists of fragmental material. During its growth there were several subterminal eruptions of lava. These were scarcely more than an oozing through the sides of the cone.

At the summit there is a bowl-shaped crater, approximately 300 feet across and go feet deep (plate 20). On its floor, a patch of black, scoriaceous lava, crowded with foreign inclusions, represents the filling of the central conduit.

The main lava Paws. After the cone had been built, lava escaped from fissures at the base. Soundings to the east of Wizard Island suggest that flows from this source cover an area of approximately 3 square miles, but whether these hidden lavas were erupted after the building of the cinder cone, or, as seems more probable, form the basement, cannot be determined. The visible post-cinder-cone flows spread chiefly toward the west, in which direction they extend for 1/2 mile. The tongue of lava projecting into Skell Channel is marked by concentric curved ridges up to 60 feet in height, separated by troughs many of which are occupied by jade-green pools floored with diatomaceous earth. As seen from the Watchman, the wavelike surface of the flow is quite distinct (plate 21, figure I). The lava composing this western flow is of the block type. As it moved, the surface layers broke into sharp-edged, angular fragments, many more than 10 feet across and some even 30 feet in maximum dimension, with smooth, glassy skins.

The southeast wall of the caldera, and Wizard Island

   Plate 21. Fig. 1. The southeast wall of the caldera, and Wizard Island. From left to right: Mount Scott; Kerr and Sun notches with Dutton Cliff between; Applegate and Garfield peaks, with the Chaski slide between. Farther to the right, in the background, rises Crater Peak, a parasitic cinder cone. The arcuate block ridges on the flow from the Wizard Island cone are emphasized by snow banks in the intervening hollows. (Photograph by H. B. Taylor.)

After the coarse, block lava had been erupted, a second pulse of magma broke through the northwest side of the cone. In this later flow there are no large blocks; indeed, few measure more than 3 feet across. It is also much more scoriaceous and weathers to brown crusts. In general, the boundary between the two flows is sharp. Locally, the front of the younger one forms a steep wall as much as 40 feet in height; elsewhere, well defined tongues of the finer, scoriaceous lava poured down channels in the earlier blocky flow (plate 21, figure 2). In other places, however, the flows merge in a broad intervening zone. Hence, they may represent a continuous emission of magma increasingly rich in gas.

Lava flows on Wizard Island, Crater Lake National Park

   Plate 21. Fig. 2. Lava flows on Wizard Island. The figure stands at the junction between the coarse block lava and the finer, scoriaceous lava.

On the east side of Wizard Island, lavas form only a narrow strip, rising between 20 and 200 feet above the lake. Here they support a thicker growth of vegetation and have weathered to a thin soil. Judged by these features, they may be older than the lavas on the opposite side of the island.

Not all the flows of Wizard Island issued from a single vent. Those on the east escaped from radial fissures, as is suggested by the presence of lava ridges above the general level. On the southwest slope of the cone the principal vent lies about 300 feet above the lake, but doubtless other vents are buried beneath slides of scoria. Only by assuming a number of source fissures is it possible to account for the form and distribution of the flows.

Mention should be made, in conclusion, of an interesting feature to be seen through the water of the lake in the bays on the southwest side of the island. Here, the lake floor is pitted by many deep cavities, more or less lined with diatomaceous earth. Several measure 3 or 4 feet in diameter and are roughly cylindrical; others are irregular and much larger. Considerable doubt attaches to their origin. Some observers maintain that they may be related to fumarolic activity. More probably, the cylindrical pits represent molds of tree trunks that were enclosed by lava, and the larger depressions are cavities between blocks on the surface of the flow.

Age of Wizard Island. Considering the perfect symmetry of the cone, the well preserved form of the crater, and the freshness of the lava surfaces, the eruptions seem to date back only a few centuries. But if we assume that the oldest trees on the island began to grow only after the eruptions had ceased, then, according to H. H. Waesche, activity must have ended approximately 800 years ago. These were the last eruptions in the Crater Lake region. If fresh magma still exists beneath Wizard Island, it must be at considerable depth, for there are now no signs of solfataras, mofettes, or hot springs in the vicinity. It would be rash to assert, however, that Mount Mazama is quite extinct.

 

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