CLOSE to the top of the caldera wall, between Pumice Point and Grotto Cove, lies a sheet of peculiar dacite tuff underlain by coarse lump pumice. Throughout most of its length of 4 miles, the tuff makes a conspicuous & up to 50 feet in height (plate 15, figure I; plate 25). It forms the dark brim of the Wineglass and the brick-red layer near the top of the long slide above Grotto Cove.

From Diller's account, it is plain to see that he was much intrigued and puzzled by the singular characters of this deposit. Though he referred to it as lava, he obviously had some doubt, for he notes that "it is altogether unlike the other flows of dacite and appears to be intermediate between them and tuff." In another place he remarks that "it is decidedly like a tuff and might well be so considered were it not for the stringers of black glass intermingled with a reddish groundmass containing fragments of other material and imparting a decided fluidal structure to the mass" On the geological map, he designates it "tuffaceous dacite." When Diller wrote, the manner of deposition of such material was quite unknown, and it is not surprising that he debated whether to call it tuff or lava. Rather, he deserves credit for recognizing that it presents features of both.

Since Diller wrote, similar deposits have been found around other volcanoes, and the clue to their mode of origin has been given by Fenner's study of the tuff erupted in 1912 in the Valley of Ten Thousand Smokes, Alaska. It is now apparent that magma may be erupted and flow over the surface in all degrees of comminution, from the coarse, blocky state seen in many lavas to the form of ultra-pulverized dust seen in certain glowing avalanches (nuées ardentes). The expansion of gases liberated from incandescent particles of magma gives to these glowing avalanches an amazing mobility by reducing internal friction, so that they flow more freely than the most liquid lava. Long after the avalanches come to rest, the constituent fragments continue to give off gas, gradually adhere to one another, and become firmly welded, while &e larger lumps of viscous glass are flattened by the weight of material above. The final products are thus hardly to be distinguished from banded lava.

Diller regarded the Wineglass tuff as the latest lava on the walls of Crater Lake, for he saw that on Rugged Crest it partly overlies the Cleetwood dacite. How he reconciled this observation with his theory concerning the origin of the celebrated "backflow" in Cleetwood Cove is not clear. The present survey shows, however, that the tuff was erupted long before the summit of Mount Mazama disappeared and that in places it is capped by glacial moraines. Unfortunately, its age relative to the dacites of Grouse Hill, Redcloud, and Llao Rock remains in doubt. Presumably it is slightly younger. If so, then the Wineglass tuff was the product of the last eruption preceding the great pumice explosions which led to the destruction of Mount Mazama.

The type section at the Wineglass. One of the best and most easily accessible sections of the tuffaceous dacite flows and of the underlying pumice forms the brim of the Wineglass. To reach the type exposure from the Rim Road, it is necessary to descend over 50 feet of loose pumice and scoria, products of the culminating eruptions of Mazama, and then over an irregular layer of bouldery glacial debris up to 20 feet in thickness. Beneath these deposits, the flat-topped sheet of tuffaceous dacite forms a cliff between 20, and 25 feet high (plate 15, figure I).

Most of the dacite consists of compact and finely streaked glass in shades of pink, red, orange, brown, and gray. Here and there are short, thin lenses made up of angular lithic blocks of andesite up to 2 feet across. Black strings of obsidian accentuate the banding of the varicolored matrix. Some of. these measure a foot long though less than half an inch in thickness. Others are paper thin and have frayed ends. They do not seem to be any more flattened near the base of the cliff than at the top. Within 2 to 5 feet from the base, they disappear. If no more of the section were seen than that just described, one would not hesitate to classify the rock as dacitic lava. But in the lower-most part of the cliff, the massive, streaked glass passes within a foot or two, into pinkish, pulverulent containing many chips of andesite. Seeing only this part of the section, one would say, with no more hesitation, that the material was tuff, for it is crushed and flattened granular pumice. Yet the part which seems to be tuff grades imperceptibly into that which looks like lava. Suspicion that the main body is not really lava is increased when the streaked glass is traced laterally, for on the same horizon it is possible to follow all gradations between dense, varicolored obsidian and almost incoherent lump pumice. The conclusion is then inescapable that one is dealing with a pyroclastic flow compacted and welded to various degrees owing to differences in temperature, gas content, and thickness.

This conclusion gains weight when the Wineglass deposit is compared with welded tuffs recently examined in Japan, New Zealand, and California. Around the giant calderas of Kyushu, notably around &at of Aso, there are tremendous sheets of glassy ejecta almost identical with the Wineglass tuff. Mention may also be made of the welded tuffs on the "Rhyolite Plateau" of the North Island of New Zealand. Here, in Pliocene times, great volumes of fine tuff were poured from swarms of fissures and flooded an area of no less than 10,000 square miles. So closely do the tuff sheets resemble rhyolitic lavas that their origin has only lately been recognized. Marshall,¹ who discussed them in detail under the name ignimbrites, has shown that, characteristically each sheet has a basal layer composed of quickly chilled and comminuted particles of glass. Above this, the tuff becomes increasingly welded, the glass shards are flattened, and the pumice lumps have collapsed. Near the top, because of the smaller load and quicker chilling, the welding gradually diminishes and the topmost layers may be as incoherent as those at the bottom. Gilbert² has given a splendid account of comparable welded tuffs in the Owens Valley, California.

The Wineglass welded tuff is therefore regarded as the product of glowing avalanches, erupted as an emulsion of finely divided magma spray enclosing larger incandescent clots of viscous glass in a medium of highly heated and compressed gas. The mass must have been extremely fluid, and its path was controlled by pre-existing hills and valleys. Where it came to rest in shallow depressions, as at the Wineglass, it accumulated to greater thickness than elsewhere and therefore retained its heat and gases for a longer time. Cooling slowly in these depressions, the particles of fine, plastic glass firmly adhered to one another while the larger and hotter lumps slowly collapsed and spread under the material above. On the low divides between the depressions, conditions were quite different. There the tuff was thinner, cooled more rapidly, and lost its gas earlier. For these reasons, the constituent fragments show neither welding nor flattening, but remain incoherent, and the deposits clearly betray their pyroclastic origin. In a word, the nature of the tuff is determined by the topography over which it flowed. In the valleys it resembles streaked lava; on the higher ground, it is thinner and the fragmental character is unmistakable.

Beneath the welded tuff forming the brim of the Wineglass lies a layer of loose, coarse lump pumice, the exposed thickness of which is approximately 20 feet (plate 15, figure I). Probably similar ejecta extend downward beneath the talus for another 100 feet, to the neck of the Wineglass. The bulk of this deposit is made up of lumps of pumice between I inch and 2 feet across, characterized by extreme vesicularity and a shredded, silky appearance. Mixed with the pumice, and comprising about 5 per cent of the deposit, are angular lithic blocks of andesite; these were doubtless torn from the sides of the conduits as the frothy magma was expelled. In its lower part the pumice is pale cream or white, but within 3 or 4 feet of the overlying welded tuff it becomes pink or brick red. At first one is likely to attribute this discoloration to reheating by the welded tuff, since the pulverulent base of the tuff is also pink. Closer examination and comparison with other pumice deposits which can never have been reheated in this way show that this interpretation is false. Similar reddening may be seen close to the top of the latest pumice deposits filling the canyons around Crater Lake. It is apparently caused by near-surface oxidation of iron-bearing fumes escaping from the pumice as it slowly cools. This explanation is supported by the fact that within the white pumice, far below the base of the welded tuff, there are many bombs with pink or brown crusts and others which show a concentric pink layer within a millimeter or two of the surface. Tsubo³ has described similar pumice lumps among the products of the 1929 eruption of Komagatake and has shown that the pink color may be produced artificially when the pumice is kept at 730° C. for a period of I to 4 hours.

With regard to the Wineglass section, one more question calls for, an answer. Was the welded tuff erupted immediately after the coarse lump pumice, or did a long interval of quiescence intervene? The pumice, as we have noted already, is incoherent. It would therefore have been particularly liable to erosion. Yet throughout a length of 4 miles, the contact with the welded tuff is perfectly smooth and comfortable. There is no hint of any channeling. Presumably, therefore, the eruption of the welded tuff followed immediately on that of the lump pumice. Possibly the pumice was still hot and giving off gas when it was buried by the tuff, and this may explain why the pulverulent base of the latter shares the pink color of the topmost pumice.

Sections northwest of the Wineglass. It is instructive to make a traverse along the rim of the caldera wall in either direction from the Wineglass, if only to observe the remarkably rapid lateral variations that take place in the welded tuff. As the tuff and underlying lump pumice are traced uphill toward Roundtop, the pumice thins to approximately 50 feet and the tuff thins to about 10 feet, at the same time losing its welded character. The tuff is no longer streaked with black obsidian, but gradually merges into a yellow, orange, or brown, friable pumiceous lapilli tuff. If the tuff was once continuous over the entire top of the Roundtop lava, it has since been stripped from the higher marginal parts and is exposed only in the saddle-shaped depression over the center, where it is approximately 8 feet thick and is overlain by bouldery drift. The underlying silky lump pumice, on the other hand, can be followed uninterruptedly over the Roundtop flow, resting directly on the lava near its margins and on an intervening lens of morainic debris in the lower, central part.

In the descent from Roundtop to the shallow depression separating it from the Palisades flow, the tuffaceous dacite begins at once to thicken and become compacted again, and within a short distance it shows the same firmly welded, streaky appearance which it displays in the brim of the Wineglass. In the center of the valley it forms a 30-foot cliff underlain by some 50 feet of incoherent, white lump pumice. The two layers then continue westward over the top of the Palisades flow.

Beyond Palisade Point, on the rise toward Rugged Crest, both the pumice and the welded tuff rest on the margin of the Cleetwood dacite, but they thin rapidly and are soon lost to view. Over the center of the Cleetwood lava there is no trace of either. Near the western edge of the lava, however, they reappear and are splendidly exposed along the Rim Road. Here the tuff rests directly on the red, oxidized crust of the Cleetwood lava and consists of incoherent, pumiceous lapilli, pink and orange at the base and paling upward to yellow and buff. Were it not that elsewhere similar deposits can be traced laterally into welded tuff, the identity of the material might be doubted. A short distance to the west, the varicolored, incoherent ejecta cross the highway and descend the caldera wall, still above the Cleetwood lava, but now separated from it by the usual layer of lump pumice. Once more, as the tuff falls in elevation, it takes on a welded appearance and simulates banded lava. From the west edge of the Cleetwood flow to the east base of Pumice Point, almost a mile away, the thickness of the cliff-forming, lava-like tuff varies between 20 and 50 feet, and in many places lenticular patches of andesite blocks are embedded in its lower part. The lump pumice below thickens to 100 feet, and locally shows a distinct stratification, as shown in plate IX B of Diller's monograph. Finally, on the rise toward Pumice Point, the welded tuff thins, becomes incoherent, and disappears, though the lump pumice continues, as shown in the panorama, plate 27.

Plate 27. Panorama of the caldera wall: Pumice Point

Section south of the Wineglass. In the opposite direction from the Wineglass, the dacite tuff may be followed for 2 miles to the top of Skell Head, where it comes to an end. The underlying lump pumice, on the other hand, continues over the summit of Redcloud Cliff. Immediately the tuff rises southward from the Wineglass, the firmly welded character and the streaky alternation of gray and black obsidian bands disappear. Once more the deposit becomes a weakly compacted mass of buff, pink, and orange pumiceous dust, lapilli, and bombs. Mixed sparingly with the pumice are fragments of andesite. Throughout most of its course the deposit varies in thickness between 4 and 6 feet, reaching a maximum of 12 feet at the head of the "Champagne Glass" slide, just before it comes to an end. Unfortunately, talus obscures the relations at the north edge of the Redcloud lava. Presumably the steep margin of the Redcloud flow prevented the tuff from spreading farther south.

The extent and nature of the tuff were largely determined, as we have seen, by the preexisting topography. But the coarse lump pumice erupted before the tuff must have been blown high above the vents and, - settling from the air in showers, covered hill and valley alike. For this reason, it does not end at the margin of the Redcloud lava, but buries the top of the flow completely, and continues uninterruptedly almost as far as Kerr Notch. The relations on this rim of the caldera merit detailed description.

For a distance of 3 miles along the caldera rim, south from Skell Head, three conspicuous layers can be recognized at once (see plate 10; plate 14, figure 2; plate 23), and a fourth can be distinguished by closer inspection. These layers are as follows:

I. A topmost layer of line pumice, scoria, and crystal ash, products of the culminating explosions of Mount Mazama, immediately before the formation of the caldera. The thickness varies between approximately 10 and 100 feet.

2. A coarse block layer, 40 feet thick on top of Sentinel Point, and between 20 and 70 feet thick above the Redcloud dacite. Though most of the blocks are angular and suggest the products of low-temperature explosions, there are many unmistakable glacial erratics, up to 8 feet in length. The balance of evidence suggests that this block layer is not of pyroclastic origin, but represents an extensive glacial moraine.

3. Below this, a third layer of coarse, silky lump pumice, usually about 40 feet in thickness and containing abundant bombs, many as much as 6 inches in diameter. Between 10 and 15 per cent of the deposit consists of angular lithic blocks of andesite, mostly less than 3 inches across, but occasionally 5 feet in maximum dimension.

4. A patchy bottom layer of bouldery glacial till resting on striated lava.

Of these four layers, the third, namely the coarse lump pumice layer, seems to be equivalent to the lump pumice beneath the welded tuff of the Wineglass. The only notable difference is the greater proportion of large lithic blocks. This correlation is supported by the fact that on the north rim of the caldera all four layers can be distinguished, though the glacial deposits are only preserved in patches, and the welded tuff intervenes between the lower pumice and upper till.

Finally, it may be suggested that the coarse lump pumice which outcrops in Kerr Notch to a thickness of 50 feet and in Sun Notch to a thickness of m feet, beneath the latest glacial moraines, is also part of the same deposit that underlies the Wineglass tuff.

Summary. After the andesite and dacite eruptions from the Northern Arc of Vents had come to an end, glaciers readvanced down the slopes of Mount Mazama. The tops of the Watchman, Hillman Peak, Llao Rock, and the Cleetwood and Redcloud lava flows remained bare, but elsewhere the ice descended a short distance beyond the present rim of the caldera.

The ice then retreated, leaving most of the rim bare. Subsequently, powerful explosions took place and a thick stratum of coarse lump pumice containing abundant lithic blocks was deposited over most of the cone, but particularly on the north and east slopes. Immediately afterward, glowing avalanches swept down the northeast flank of the volcano. Where these came to rest in shallow valleys, they left a remarkable deposit of streaky, welded tuff; where they covered higher ground they left a weakly compacted buff, pink, and orange layer of pumiceous lapilli tuff. There is no means of determining how far these avalanches continued down the slopes, for, except at a point about a mile east of the Wineglass, their deposits are entirely concealed beyond the caldera rim by the products of later explosions.

Once more the glaciers advanced. Along what is now the north wall of Crater Lake they left patches of till on top of the welded tuff; along the east wall they deposited an almost continuous sheet of blocky detritus on top of the coarse lump pumice; in Sun and Kerr notches they covered the pumice with bouldery moraines.

A considerable interval of quiescence must have followed. Everywhere except in Sun, Kerr, and Munson valleys the glaciers withdrew above the present rim of the caldera. Within the magma chamber fundamental changes were going on preparatory to the climactic explosions which were to destroy the summit of Mount Mazama.

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