The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Origin of the Caldera
Smith and Swartzlow’s Explosion Theory
For more than thirty years, Diller’s opinion passed unchallenged. In 1936, Smith and Swartzlow advanced the radically different view that the top of Mount Mazama was destroyed by explosions of unusual violence, during which 17 cubic miles of the original cone were removed by blasts from within. So revolutionary was their opinion that it seems advisable to discuss their principal arguments in some detail.
I. Their main reason for adopting the explosion theory was the belief that Diller had grossly underestimated the amount of old debris on the outer flanks of the volcano. It seemed to them that the mantle of pumice conceals vast quantities of coarse, blocky detritus, ample in fact to account for the missing part of the mountain.
This conclusion can no longer be entertained. A thorough study of the products of the final eruptions shows that only about 1 1/2 cubic miles of lithic detritus were removed from the summit of the volcano. The remainder consisted of new magma in the form of pumice, scoria, and crystals erupted from the underlying reservoir. In the canyons on the outer slopes of the mountain, these ejecta rest directly on old lava flows or on unmistakable glacial moraines. Nor is there any trace of a layer of lithic detritus beneath the vast sheet of granular pumice which encircles the volcano. Here and there along the caldera rim a thin layer of coarse fragmental material does separate the pumice from the underlying lavas, but this is far from being the general rule. More than half a mile from the rim there is no hint of such a layer. Even on the brink of the caldera, blocks of old lava more than 3 feet across are surprisingly rare; in the pumice and scoria flows lithic fragments larger than 3 inches in diameter are extremely few; in the earlier pumice fall, only occasional fragments measure more than 2 inches in diameter. Surely an explosion of such gigantic proportions as to remove 17 cubic miles of solid rock must have left behind great piles of much coarser debris.
Observation of the distribution of the products of the culminating eruptions and analysis of their lithic content therefore leads us to accept with confidence the original views of Diller and Dutton. There is not nearly enough old lithic material among the ejecta to warrant the belief that Mount Mazama was decapitated by explosion.
2. The suggestion has been made that glacial and stream erosion may have removed much of the lithic debris. Even if this were true, the estimate that only 1 1/2 cubic miles of lithic material were erupted was based on the assumption that the present canyons had not been cut. Since the caldera was formed, a few thousand years ago, streams have cut narrow box canyons through the pumice flows, but apart from this the removal of ejecta has been insignificant.