Nature of the Lavas

By far the greater part of Mount Mazama is composed of thin flows of hypersthene-rich augite andesite, similar to the lavas of Mounts Rainier and Baker and the bulk of Mount Shasta. Andesites rich in hornblende with or without biotite are almost completely absent. Invariably the lavas are characterized by abundant and large phenocrysts of. feldspar. Texturally, they show wide variation, even within a single flow, from holocrystalline to glassy. Rarely, however, are they as rich in glass as the dacites, nor do they show the hair-fine banding characteristic of the latter. Indeed, except on a broad scale, it is difficult to detect fluxion. In color, the andesites vary from black to pale gray, according to the content of glass, and many flows are pale brown, pink, or dark brick red where they have been affected by escaping gases. Nevertheless, flows which, for these reasons, seem in the field to be of widely different types are singularly alike under the microscope.

One of the most distinctive features of the Mazama andesites is the widespread occurrence of basic inclusions. Few lavas are entirely devoid of them, and in some they make up no less than a quarter of the volume. Generally they are more plentiful among the later flows, where many exceed a foot in length. Most of these inclusions are ovoid and range from I inch to 3 or 4 inches across. Compared with the enclosing lavas, they are much more porous and crystalline, and most of them stand out conspicuously in the darker matrix by reason of their pale-gray or pink color. They consist of crisscross prisms of hornblende, pyroxene, and feldspar separated by a small amount of interstitial glass. Similar inclusions are common among the lavas of other Cascade volcanoes; they represent fragments torn from the walls of the conduits and the roofs of the magma chambers as the lavas rose to the surface.

In the great cliff sections along the south, east, and west walls of the caldera, the Mazama andesites usually range between 10 and 80 feet in thickness, most of them measuring between 20 and 30 feet thick. They are considerably thinner than the andesites of Mount Rainier, for which Coombs¹ estimates an average thickness of about 80 feet. But a cursory examination shows that the Mazama flows pinch and swell with great rapidity and that the massive lavas commonly merge into red, slaggy breccias, like those plainly visible on the Castle Crags, east of the Park Headquarters.

As for the jointing of the andesites, columnar structures are rarely well developed. On the other hand, platy or slabby jointing both parallel and perpendicular to the flow planes is seldom absent, and may be so well marked and closely spaced as to give the lavas the appearance of stratified sediments when viewed from a distance. Elsewhere, particularly in Grotto Cove, the andesites are traversed by dose-set curved joints at high angles to the banding. The origin of these aberrant structures is nor understood.

True scoriaceous (aa), ropy (pahoehoe), or block lavas have not been observed on the walls of Crater fake. It seems unlikely that original surfaces were destroyed by weathering and erosion prior to burial by later flows and ashes; much more probably the crusts of the flows have always been as smooth as they are today.

In a general way, the lavas of Mount Mazama became thicker as activity progressed. Among the earlier lavas on the caldera walls, few reach 100 feet in thickness. The thickest sheets are practically confined to the upper part of the walls, as, for example, near Discovery Point and along the northern wall. These were erupted at the close of the andesitic period, not long before the main dacitic eruptions began.

The thickest andesites are those of the Palisades and Roundtop. These rest on a thick deposit of glacial debris and were erupted after a long period of quiescence. Presumably they did not issue from the central, summit vents of Mount Mazama, but from fissures far down the northern slope. If this be true, then the eruption of these flows added nothing to the height of Mount Mazama; they were "parasitic" or subterminal flows. We may go even farther and suggest that only the thinner andesites escaped from the central vents of the volcano. It was these that built Mount Mazama to its greatest height. Accordingly, we may speak of the slopes left by the youngest of the thin andesite flows as the primary slopes of Mount Mazama, and in trying to estimate how high the volcano formerly rose these are the only slopes to be considered, since the thick flows from fissures below the summit only served to widen, not to heighten the cone.

In places, the Roundtop flow is no less than 500 feet thick. Its summit is strikingly polished and scratched by glacial action. The base of the flow has the form of a broad W (plate 25), and rests oh coarse, bouldery till. Apparently the lava moved down a U-shaped valley occupied by a medial moraine. The bulk of the flaw consists of porphyritic andesite marked by pronounced slabby jointing parallel to the banding. Near the base, the lava becomes darker and increasingly glassy and finally passes into black obsidian. Much of this basal glassy layer is intensely brecciated, and locally it has been reddened by gas action. In crossing the moraines, the lava enveloped many glacial boulders, some as much as 5 feet in diameter.

Plate 25. Panorama of the caldera wall: Palisades and Roundtop

Immediately to the west of the Roundtop Bow is another thick sheet of andesite, forming Palisade Point. This also rests on glacial moraines (see panorama, plate 25). But the top, unlike the top of the Roundtop flow, is extremely irregular, bristling with monolithic crags and traversed by deep, steep-sided fissures, like those which characterize the tops of the later dacite flows. In the upper part of the Palisade flow, the jointing is closely spaced and curved, though generally almost vertical; lower down, the vertical joints are farther apart and produce the crude columnar structure which gives the flow its name. At the base, the lava becomes black, glassy, and autobrecciated. In this flow, ovoid inclusions up to a foot in length make up approximately 5 per cent of the bulk.

Other young, thick andesite flows form the topmost cliffs of the caldera wall overlooking Grotto Cove. These likewise rest on glacial deposits and dip outward at much lower angles than the thin andesites forming the "primary slopes" of Mount Mazama (panorama, plate 3). Probably they were erupted from fissures far below the summit of the volcano. The thick flow immediately south of the Wineglass certainly issued from a feeder at the Wineglass itself, for where the cup joins the stem of the glass, the arrangement of the columns shows that the lava rose through a steeply inclined conduit before spreading at the surface (plate 25). Still other thick andesitic lavas form the highest cliffs beneath the Watchman and on Sentinel Point.

The rise of Mount Mazama took place during the Pleistocene period, and as the volcano grew its flanks were mantled with glaciers. Accordingly, the lavas are interbedded in many places with glacial moraines and fluvioglacial sands, and elsewhere there are signs of strong glacial erosion between successive flows. Strangely enough, the flows that were erupted onto and under the glaciers show no peculiarities suggestive of rapid chilling. Moreover, there must have been recurrent mudflows on the sides of the volcano as the ice was melted by falls of ash and eruptions of lava, yet no deposits of such flows have been recognized on the walls of Crater Lake. They must have been swept to lower elevations and buried there by later ejecta.

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