Interbedded Andesitic Pyroclastic Rocks

Although the tuffs and breccias on the walls of Crater Lake are largely masked by talus, the tendency has been to exaggerate the amount of explosion debris. This tendency is easily explained, for it is only on close inspection that most of the red and brown layers between the lava flows, which seem at first sight to be of explosive origin, are seen to be merely the oxidized tops and bottoms of flows. Possibly the autobrecciated bases of many flows have also been mistaken for pyroclastic breccias.

Layers of pyroclastic debris are most abundant on the south and southeast sides of the caldera. Varicolored, coarse tuff breccias may be seen to advantage on the cliffs below the Sinnott Memorial, where they form more than half of the caldera wall. A convenient place to study similar fragmental ejecta is on the trail to Garfield Peak, where one of the layers of tuff breccia is 100 feet thick and carries blocks of andesite up to 6 feet in diameter in a matrix of solfatarized tuff. The bold face of Eagle Crags is striped with many thinner bands of similar ejecta; so also is the slope leading from Applegate Peak to Sun Notch. It must be emphasized, however, that these layers of pyroclastic material thin rapidly as they are traced away from the caldera rim. A mile or two away only a few thin bands persist. The inference is, of course, that the explosions which produced them were not particularly violent.

No part of the walls of Crater Lake is more impressive than the tremendous Dutton Cliff, which rises almost 2000 feet above the water. Here the somber lava flows are separated by brown and brick-red layers, mostly a few feet thick though reaching a maximum thickness of more than 80 feet, which seem at first sight to be coarse tuff breccias. Careful study shows that all but a few of they brighter layers are the slaggy, autobrecciated tops and bottoms of the flows themselves. It is enough to see how the laws grade imperceptibly into the breccias and to note how they send irregular fingers upward into them to be convinced that these are not breccias of explosive origin, but a product of quick chilling and subsequent shattering of lava crusts.

True explosion breccias are not lacking on Dutton Cliff, but they are readily distinguishable by reason of their stratification and depositional relations to the lavas beneath. The most conspicuous of these pyroclastic bands is one composed of pale sage-green tuff breccia, which reaches a thickness of 50 feet in places and carries angular blocks of andesite up to 3 feet across. It outcrops a short distance below Sun Notch and continues thence eastward toward Kerr Notch, as shown in plate 6.

Farther down Dutton Cliff, and on Phantom Ship, pyroclastic deposits are more abundant. These are products of the Phantom cone and have been discussed already (page 30).

Coarse, red agglomerates and breccias occur as irregular lenses between the lavas above Danger Bay, but between Sentinel Point and Pumice Point pyroclastic deposits are both rare and thin. In the lower part of Pumice Point, however, there occurs a layer of tuff breccia 150 feet thick. Thin bands of similar ejecta may also be found between the lavas on the walls of Steel Bay and beneath Llao Rock.

From what has been said it may be concluded that although explosive activity continued at intervals throughout the growth of the main cone of Mount Mazama, it was far subordinate to effusive activity. Such explosions as there were must have been principally at a low temperature, the ejecta being blown out for the most part in a solid state as angular blocks in a matrix of ash. True scoria (cinders) and rounded bombs are conspicuously absent; in other words, none of the explosions seems to have been of the Strombolian type, in which the ejecta are blown out while still sufficiently viscous to have their forms modified by flight through the air.

The nature of the pyroclastic deposits provides evidence that the feeding magma chamber or explosion focus lay at shallow depth, for despite a careful search no fragments of the Western Cascade lavas or of the underlying rocks have ever been found among them. Indeed, fragments referable to the older Pliocene lavas beneath Mount Mazama are extremely rare, the ejecta being almost entirely composed of typical Mazama andesites. On other volcanoes in the southern Cascades there is a similar absence of explosion debris from the underlying basement. Presumably the tops d the magma chambers lay close to the surface; that of Mount Mazama may have been well above the level of the Miocene lavas and probably lay among the upper flows of the Pliocene High Cascade basalts. If this inference is justified, it helps to explain why the earlier explosions of Mount Mazama were not especially violent, for magma at shallow depth cannot hold enough gas in solution to cause explosions of great intensity.

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