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

The Formation of Mount Mazama Just before Its Destruction

 

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THE remnant slopes of Mount Mazama show the concave profile typical of composite volcanoes, and it may be assumed with confidence that the missing slopes were steeper than those which remain. Unfortunately nothing is known of the manner in which the missing part was constructed, so that there is no means of determining just how steep they were. Nor is there any evidence to indicate the size of the former summit crater. Accordingly, estimates of the original height of the volcano can only be approximate. Nevertheless, in order to reach a conclusion as to how Crater Lake was formed, it is essential to determine as closely as possible how much of Mount Mazama has disappeared.

Profiles through Mounts Shasta, Rainier, and Adams, and Crater Lake.

   Fig. 13. Profiles through Mounts Shasta, Rainier, and Adams, and Crater Lake.

Conclusive proof of the former existence of a high cone is the fact that large glaciers, one of them at least 17 miles long, once swept down the sides of Mount Mazama. Glaciers of such size, even during the Pleistocene period, must have originated on a cone far higher than the present rim of Crater Lake. This much follows from a study of the size of Pleistocene glaciers on neighboring Cascade volcanoes.

We are aided in forming an estimate of the original size of the cone by comparing its remnant profiles with those of other Cascade volcanoes formed of similar materials, and we are assisted further by picturing the formation on these volcanoes of calderas of the same size as Crater Lake and noting the elevations of their imaginary rims (see figures 13 and 14).

Consider the rim of Crater Lake. Certain precautions must first be taken. In order to estimate the original height of the peak, the eminences on the rim which are due to parasitic, flank eruptions must be ignored and account taken only of what we have termed the "primary slopes," namely, those built by eruptions from the central, summit crater. For example, if sections be drawn across the volcano through the Watchman, Hillman Peak, Llao Rock, Rugged Crest, or Redcloud Cliff, they give a false impression, for each of these high points owes its origin to eruptions not from the summit crater, but from the flanks of Mount Mazama. That is why the profiles of Mazama seen from a distance are deceptive. They all suggest that the peak must have been higher than it probably was.

The "primary slopes" were formed by the thin-bedded andesites which make up the bulk of the caldera walls. These slopes reach their greatest elevation on the south rim of the caldera between Dutton Cliff, at a height of 8150 feet, and Garfield Peak, at 8060 feet. Along the western rim of the caldera, the primary surface varies in height between 7000 and 7500 feet. Thence it falls eastward, passing beneath the Cleetwood flow and reaching lake level, 6177 feet, near Palisade Point. Reappearing above lake level close to the Wineglass, it rises southward to heights between 7000 and 7500 feet. In brief, the primary surface of Mount Mazama is approximately 8000 feet high on the south rim of the caldera, 6000 feet on the north, and at intermediate elevations on the other walls.

forms of Mounts Shasta, Rainier, and Adams compared with the remnant Mount Mazama

   Fig. 14. The forms of Mounts Shasta, Rainier, and Adams compared with the remnant Mount Mazama. The coarsely stippled areas on the maps of Mount Rainier and Crater Lake indicate the basement rocks. The finely stippled areas on the map of Crater Lake and the black areas on the caldera wall indicate the products of parasitic eruptions. These did not increase the height of the main cone. Areas enclosed by dotted lines are glaciers. On Mounts Rainier, Adams, and Shasta, the heavy lines mark the rims of imaginary calderas equal in size to Crater Lake. The slopes outside the imaginary calderas may be compared with the remnant slopes of Mount Mazama. Section lines aa, bb, etc., correspond to sections in figure 13. Contour interval, 1000 feet.

It is instructive to compare these heights with those of imaginary caldera rims on other Cascade volcanoes. For instance, if the summit of Mount Rainier were to disappear and a caldera as large as Crater Lake were to be formed, the rim would vary between 6500 and 9000 feet. More than ha& of the rim would lie at an elevation of about 8000 feet. Not only would most of the rim of the imaginary Rainier caldera be higher than that of Crater Lake, but the remnant slopes would be considerably steeper. Hence, if the original Mount Mazama had a crater commensurate with that of Rainier, its summit must have been at a much lower elevation, or well below 14408 feet.

A similar conclusion is reached when comparison is made with an imaginary caldera on Mount Shasta (see figure 14).

Consider next Mount Hood, the summit of which reaches an elevation of 11,225 feet. If a caldera as large as Crater Lake were produced by the disappearance of its upper part, the rim of the depression would lie mainly between 5000 and 6000 feet, and the remnant slopes would not be unlike those of Mount Mazama. Accordingly, if the summit crater of Mazama was small it was probably higher than the top of Hood:

Several other Cascade volcanoes were examined with the same idea in mind, and the conclusion was reached that Mount Adams offers the closest comparison. Hence, Mount Mazama rose to a height of approximately 12,000 feet. The summit lay about a mile south of the center of Crater Lake.

Even if the 'foregoing argument be accepted, it does not follow that Mount Mazama was 12,000 feet high immediately before the formation of the caldera, for after the volcano had reached this height there may have been profound collapses on its northern side at the time of the pumice explosions from the Northern Arc of Vents. The summit may also have been lowered by glaciation.

As for the shape of the volcano, it was far from being a symmetrical cone. The thick deposits of glacial debris on the northeast wall of the caldera imply the existence of deep ice-filled cirques on that side of the cone. The great U-shaped canyons on the south side must likewise have headed back into amphitheaters.

The symmetry of the cone was also marred by parasitic cones and domes associated with the Northern Arc of Vents (see figure 15). Had the lost part of the volcano been perfectly symmetrical, the glacial scratches on the caldera rim would be directed radially. In many places, such is far from being the case In particular, as figure 31 shows, the striae on the summit of Roundtop and near the Wineglass, instead of pointing toward the center of the caldera, converge toward a point a little to the west of Skell Head. Perhaps the semicircular form of Grotto Cove indicates the former existence of a parasitic cone in that vicinity, just as the scalloped margins of the caldera of Santorin are related to old cones that once rose above them and the semicircular Cleetwood Cove is related to the vent of the Cleetwood flow.

sketch panorama of Mount Mazama from the southwest

Fig. 15. Panorama of Mount Mazama from the southwest, sketched from the slope of Castle Point. In the foreground, the pumice-filled canyon of Castle Creek. The two peaks on the west (left) rim of Crater Lake are Hillman and the Watchman; the peak on the right is Garfield.

The glaciers which once choked the canyons and enveloped all save the topmost ridges had retreated to the upper part of the cone and were confined to the bottoms of the canyons. Only at three points did they extend beyond the present rim of the caldera, namely in Munson, Sun, and Kerr valleys on the south side of the mountain, where they lay in deep glacial troughs. A small corrie glacier may also have existed on the northwest side of Mount Scott, but the Union Peak volcano, if it was not entirely bare, supported only very small tongues of ice on the shady side of the summit pinnacle.

The slopes of Mount Mazama were almost barren of vegetation. A few tall trees grew on the moraines near the caldera rim, but thick forests were restricted to the foot of the volcano, below elevations of about 4500 feet. The sheet of pumice left by the eruptions from the Northern Arc of Vents had been almost wholly removed by erosion, leaving slopes of lava littered with moraines and fluvioglacial outwash.

 

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