28 Interbedded Andesitic Pyroclastic Rocks

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

The Main Andesite Cone of Mount Mazama

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.