Petrology

The rocks of the Crater Lake region, like those of all the High Cascade volcanoes studied thus far, belong to the calcic igneous series as defined by Peacock.⁸ The alkali-lime index, that is, the silica percentage at which the content of lime equals that of the alkalis combined, is approximately 62 (see figure 32). For the lavas of the Lassen region, the index is 63.9; for those of Mount Shasta, 63.7; and for those of Mount St. Helens, 63.2.

According to the scheme proposed by Niggli,⁹ the magma types of the Crater Lake region are mainly quartz dioritic (peleitic), dioritic, and trondhjemitic. Burri,¹⁰ who was struck by the resemblance of the Crater Lake rocks to the Tertiary lavas of the Sierra Nevada, grouped them together as the Sierra Nevada (effusive) type of the Pacific Province. However, the Crater Lake lavas are more similar in many respects to those of the Lassen region than they are to the effusive rocks of the Sierras. Thus, the values for al in the Crater Lake series are consistently closer to those of the Lassen lavas. So are the values for alk and al-alk among the basic lavas (si<200). The values for c in the Crater Lake rocks, on the other hand, are closer to those of the Sierran lavas.

The principal differences between the Lassen and Crater Lake series have already been shown in an earlier paper.¹¹ Briefly, they are as follows: lime is consistently higher among the Lassen rocks; potash is also higher except among lavas with less than 56 per cent SiO₂; soda is distinctly higher among the rocks of Crater Lake; so is iron, except in the most basic flows; and finally, magnesia is higher among the basic lavas of Crater Lake, but lower among lavas with more than 55 per cent SiO₂. Noteworthy is the wider range in silica content of the Lassen series.

Among the acid rocks of the Lassen region (si>300) the k value, that is, the molecular ratio of K₂O to total alkalis, may reach as high as 0.5, though in the Crater Lake series the k value never exceeds 0.28 (see figure 33). This low potash content of the Crater Lake lavas is reflected in the absence of orthoclase and the extreme rarity of biotite in even the most siliceous types. The acid dacites of the Lassen country, on the contrary, contain biotite in abundance. The acid effusive rocks and the Mesozoic intrusives of the Sierra Nevada also show high k values, the magma types tending toward the granodioritic. If assimilation has played any role in the differentiation of the Lassen magmas, their high k values may imply contamination with rocks of the granodiorite basement. Presumably the upper parts of the magma chambers beneath the Crater Lake region lay far above such a basement and among the thick series of calc-alkaline lavas which outcrop in the Western Cascades. Causes other than assimilation, however, may well account for the observed differences.

In their low k values, the Crater Lake lavas resemble those of Mounts Shasta and St. Helens. They are not, however, so poor in potash as the lavas of Montserrat.¹² Nevertheless there are strong similarities between the Crater Lake rocks and those of the Lesser Antilles in general.

In brief, the Mazama and pre-Mazama flows of the Crater Lake region belong to the same calcic magma type as those of Lassen Peak and the Lesser Antilles, though exhibiting many similarities to the Tertiary lavas of the Sierra Nevada. That they also bear a notable resemblance to the Mesozoic batholithic rocks of the Sierra Nevada suggests the persistence, over a very long period and with little modification, of similar magmas beneath the Sierra Nevada-Cascade orogenic belt.

We may now inquire concerning the Eocene-Miocene rocks of the Western Cascades and the igneous rock series-in the region east of the High Cascades. Thayer¹³ has shown that, compared with the rocks of the Western Cascades, the Pliocene and younger rocks of the High Cascades are richer in A1₂0₃ in the range above 53.5 per cent SiO₂, in lime above 55 per cent SiO₂, and in Na₂O above 65 per cent and below 56 percent SiO₂, and are generally richer in magnesia. The Western Cascade lavas are consistently richer in iron oxide and potash. From the available evidence, it is clear that the Western Cascade lavas belong to the calc-alkaline series, for the alkali-lime index is 58.6. Since the High Cascade lavas are generally richer in alumina, lime, soda, and magnesia and poorer in potash and iron, it is possible that they were derived by crystal differentiation of a magma common to both, for the settling of early-formed feldspars and magnesian minerals would readily account for the differences. Certainly a sharp break seems to have occurred in the magmatic history of the Cascade volcanoes at the close of the Miocene, and perhaps this sudden change was related causally to the strong orogenic disturbances which took place at that time.

To the east, on the other hand, in the Medicine Lake Highlands of northern California, and at the Newberry volcano of central Oregon, calc-alkaline magmas were being erupted simultaneously with the calcic magmas of the High Cascades.

Throughout the High Cascades, at least in Oregon, the dominant lavas are either olivine basalts or olivine-bearing basaltic andesites such as tho4 which immediately underlie Mount Mazama and form the bulk of the volcanoes to the south. Indeed, scarcely any other type of lava was erupted in the High Cascades during Pliocene times. Subsequently, at certain centers on and near the crest of the range, differentiation led to the eruption of hypersthene andesite and of more acid types. At these centers, large composite cones, including Shasta, Mazama, Hood, Adams, Rainier, and Baker, were built. Yet even while these were active, olivine basalts and basaltic andesites continued to escape from neighboring vents. Thayer has noted a similar relationship in the vicinity of Mount Jefferson :

The occurrence of large andesitic cones literally in the midst of small basaltic cones suggests that the more acidic lavas were erupted from comparatively small pockets or cupolas in the main magma chamber. The localization of differentiation may have been due to local cooling, shown by the porphyritic nature of the andesites and associated basalts in the high peaks as compared to the uniformly fine-grained textures in the basalts of the smaller cones.¹⁴

The activity of Mount Mazama itself began, as we have seen, with the eruption of hypersthene andesite, and during most of its history the volcano erupted no other type of magma. At the northern end of the Cascade Range, on Mounts Rainier and Baker, this is the only type of magma represented. Southward, however, the variety of magmas erupted by the major cones of the High Cascades shows an almost regular increase. This southward increase in the diversity of the products of the Pleistocene and younger cones of the High Cascades has been emphasized repeatedly since it was first detected by Hague and Iddings. Its cause remains unknown.

The hypersthene andesites of Mount Mazama, like those of the circum-Pacific volcanoes in general, are characterized by an abundance of porphyritic feldspar. In this respect they contrast strongly with most of the pre-Mazama basalts and basaltic andesites. This difference implies more advanced crystallization of the hypersthene andesite magma prior to eruption, and helps to explain why it was more explosive than the earlier, more basic magmas.

Hornblende andesite lavas are rare throughout the High Cascades. In the Crater Lake region, only two examples are known. Between Mount Mazama and Mount Shasta, only one occurrence is known, namely in the dome on the summit of Rustler Peak. On Mount Shasta, hornblende andesite is best developed in the dome of Black Butte. On Mount St. Helens, the plugs (domes) invariably carry hornblende. In brief, hornblende andesite lava is almost confined to quickly chilled viscous domes erupted from parasitic vents.

On the other hand, hornblende is extremely abundant in the basic scoria flows of Mount Mazama. It is also common in the dacite pumice and almost ubiquitous, though in small amount, among the glassy dacite domes and flows erupted from the Northern Arc of Vents. Yet among the holocrystalline, pilotaxitic dacites of Mazama, the mineral is scarcely ever present. Hence, rapid cooling appears to be necessary to prevent complete resorption of the mineral in magmas of shallow origin.

After the main cone of Mazama had been constructed by long-continued emission of hypersthene andesite, the magma changed to dacite on the one hand and to olivine-bearing basaltic andesite and olivine basalt on the other. More the culminating eruptions which led to the formation of the caldera, the basic magma escaped from parasitic cinder cones far down the sides of the mountain, and the acid magma was also erupted in the main from parasitic vents, either in the form of viscous flows and domes or as pumice.

The culminating eruptions were likewise characterized by the presence of opposed types of magma, but now they escaped from the summit vents instead of from fissures on the flanks of the cone. The acid magma was erupted as dacite pumice, essentially like the lavas erupted from the Northern Arc of Vents, but somewhat richer in hornblende. On the other hand, the basic magma was blown out in the form of scoria exceptionally rich in hornblende and basic feldspar and relatively poor in pyroxene and olivine. Yet despite the abundance of hornblende in the products of these final scoria eruptions, the chemical character of the magma is almost identical with that of the hornblende-free parasitic cinder cones and the Pliocene, pre-Mazama lavas.

From a petrological standpoint there is perhaps no more striking feature of the Crater Lake rocks than this extraordinary abundance of hornblende in the basic scoria of the culminating eruptions and the paucity of the mineral among all the earlier products of the volcano. Apparently before the culminating eruptions began, the upper part of the magma chamber was composed of dacite in which the stable ferromagnesian minerals were mainly hypersthene and augite. In this pan, hornblende was rare. At greater depth, the reverse was the case.

At Montserrat, MacGregor was led to the conclusion that "the pumiceous and most glassy rocks characterized by green hornblende . . . represent highly gas-charged magma that initiated great volcanic explosions." He pictured this magma as overlain by semiconsolidated material in which the hornblende was brown. This was incapable of initiating great explosions. He suggested further that the pyroxene lavas represent hornblendic magma in which the amphibole has been completely or almost completely altered. Finally, he noted that in general the hornblende of the basic autoliths is brown and more or less resorbed, and he therefore supposed that they were derived from the semiconsolidated upper part of the magma chambers and were blown out with the green hornblende-bearing pumice rising from greater depth.

To what extent are these deductions applicable at Crater Lake? In several respects there appear to be close analogies. In the first place, as we have seen, the hornblende-rich scoria of the culminating eruptions is chemically almost identical with the olivine-bearing basaltic andesites erupted by the parasitic cinder cones. From the distribution of these cinder cones, it may be supposed that they were fed from fissures connected with the main reservoir. Perhaps small satellitic chambers underlay each one. In such shallow chambers, the hornblende which had been stable at greater depths in the principal chamber may have suffered complete breakdown to pyroxene and olivine.

In the second place, it should be noted that in the basic inclusions hornblende is rare and largely replaced by granular pyroxene and ore. Resorbed crystals of oxyhornblende are also typical of the basic inclusions in the dacites of the Lassen region. By far the commonest type of inclusion in the Mazama lavas consists of a crisscross felt of hypersthene and labradorite crystals with accessory augite, a little glass, and either tridymite or cristobalite. Hornblendic inclusions are unknown in hornblende-free lavas. Similarly, the hornblende-rich dacites which form the main dome of Lassen Peak are crowded with basic inclusions characterized by abundant resorbed hornblende, whereas the 1915 lava of Lassen Peak, which presumably rose from very shallow depth, is almost devoid of hornblende and such as there is shows extensive resorption. Further, the basic inclusions in the 1915 lava, which came from still shallower depths and presumably represent fragments of the semiconsolidated roof of the chamber and walls of the conduit, are quite devoid of hornblende. These occurrences lead us to agree with MacGregor's view that close to the surface hornblende breaks down unless the magma is drastically and suddenly chilled. The occurrence of plentiful tridymite or cristobalite, or both, in the basic inclusions is also in accord with the idea of a shallow origin, for siliceous vapors would normally be concentrated at the roof of the magma chamber and in the overlying conduit.

A further analogy may be drawn with the magmas of Montserrat, for at Crater Lake also the explosive magma of the final eruptions was characterized by paucity of any but fresh, green hornblende.

When the climactic eruptions began, the magma in the upper part of the chamber was of dacitic composition and the ratio of crystals to melt was approximately 3 to 7. What little hornblende floated in the magma was still in a stable condition. Below this acid fraction lay more basic magma, of the composition of basaltic andesite, which had crystallized to an even greater extent. At these depths, hornblende was by far the dominant ferromagnesian constituent. Between these two types of magma there was little or no material of intermediate composition. Intermediate magma, hypersthene andesite, like that which had built the main cone of Mazama, was either absent altogether or present in such small amount that it has not been recognized among the ejecta. What agencies brought about such a clear differentiation is not understood. Yet they must have operated earlier, for the basic inclusions in the dacite flows erupted from the Northern Arc of Vents have approximately the same composition as the hornblende-rich scoria. This implies that the semicrystallized roof of the dacite magma chambers had virtually the same composition as the hornblende-rich scoria. Olivine-bearing basaltic andesite, erupted from parasitic cinder cones, basic inclusions in the andesite and dacite flows, and the hornblenderich scoria of the culminating eruptions are therefore merely heteromorphs.

After the summit of Mount Mazama disappeared, new eruptions took place on the floor of the caldera. Unfortunately, only the products of the youngest of these are visible for inspection. Nothing can therefore be said of the trend of post-caldera differentiation. The visible products on Wizard Island consist of hypersthene andesite. Thus, as its final act, the volcano reverted to eruption of an intermediate type of magma essentially identical with that which had characterized its growth from the beginning until the time it reached maturity.

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