In Late Cretaceous times, the site of the Oregon Cascades was largely if not entirely occupied by a shallow sea. During the Eocene period, widespread uplift drove the coast westward beyond the line of the Cascade Range, never to return. Volcanism began in eastern Washington during the early Eocene and gradually spread southward over Oregon. By late Eocene times, volcanoes were active throughout the present Cascade belt and on the plateau of central Oregon. The climate of Oregon was then warm temperate or subtropical, and a low plain spread far inland. During Oligocene and Miocene times, volcanism continued on a grand scale. Farther east, the volcanic deposits of the John Day formation had been laid down over an extensive area and had been buried during the Middle Miocene by enormous out pourings of Columbia River plateau basalt, erupted from swarms of fissures. Meanwhile the climate had become cooler and a temperate, redwood flora had replaced the warmer floras of the Eocene. Still, however, no high mountain range divided eastern from western Oregon.At the end of the Miocene period, renewed earth movements took place. At the same time, the volcanics of the Western Cascades were intruded by an approximately north-south line of dioritic stock. Simultaneously, north-south fractures opened along and near what is now the crest of the Cascade Range. An important effect of these disturbances was the elevation of a mountain barrier which shut off eastern Oregon from the supply of moisture-laden winds. For the first time, the floras on opposite sides of the Cascades began to show the marked differences which they exhibit today.

Figure 30.

The northeastern wall of Crater Lake. The Palisades at left, Roundtop in middle, and Wineglass at right. Timber Crater lies in middle distance and Mt. Thielsen in background. The Palisades and Roundtop are thick andesite flows. lgnimbrite overlies pumice in Wineglass. (Oregon State Highway Dept. Photograph 6290)During the Pliocene period, the disturbed rocks of the Western Cascades suffered rapid erosion, and along the summit of the range they were buried by the products of High Cascade volcanoes. These volcanoes were less explosive than those which had formed the Western Cascade series, and their products were less diverse. Whereas the older lavas range from rhyolite to basalt, those of the High Cascade cones are almost entirely composed of olivine-bearing basaltic andesite and basalt. Many of these younger flows poured for long distances down canyons cut across the Western Cascades.By the close of the Pliocene period, the crest of the Cascade Range had become a high plateau surmounted by overlapping shield-shaped cones of basic lava. In the succeeding Pleistocene period, a narrower, north-south belt of giant andesitic volcanoes commenced to form on the basaltic plateau. These continued to erupt and grow until Recent times. Today they form the crowning peaks of the Cascade Range. The bulk of this report deals with the rise of one of these andesitic cones, Mount Mazama, and the manner in which its summit was destroyed.

Mount Mazama rose from a basement the elevation of which lay between 5000 and 6000 feet (1525 and 1830 m). By the time the volcano reached full stature, the summit rose to a height of approximately 12,000 feet (3660 m). The main cone was built chiefly by quiet effusions of hypersthene andesite. Explosive activity was relatively unimportant. Eruptions took place from a number of conduits, the positions of which changed from time to time. Accordingly, Mount Mazama was never a simple, symmetrical cone; it was, rather, a complex of overlapping cones.Toward the close of the period of andesitic eruptions, flows of dacite escaped from fissures far down the south and east flanks of the volcano, and explosions of dacite pumice alternated with flows of andesite from the summit vents.

Perhaps from the beginning, Mount Mazama supported many glaciers. Even the oldest visible lavas are underlain by glacial moraines. In many places, the caldera cliffs reveal layers of bouldery till and fluvioglacial sand interbedded with volcanic rocks. The constructional forces building the volcano struggled incessantly with the erosive force of ice. Clearly, the glaciers advanced and retreated many times. Their greatest advance came after the dacitic eruptions mentioned above. At that time, many tongues of ice were more than 10 miles (16.1 km) long, and one extended 17 miles (29.4 km) from the summit. In some of the canyons, the thickness of the glaciers exceeded 1000 feet (305 m). Save for a few projecting aretes, the whole of Mount Mazama was mantled by an uninterrupted sheet of ice.After the period of maximum glaciation, when the ice had retreated from the divides and was confined to the canyon bottoms on the upper slopes of the volcano, a semicircular arc of vents opened on the north flank, approximately 5000 feet (1525 m) below the summit. From this Northern Arc of Vents, which determines the position of the north wall of Crater Lake, viscous flows of andesite and dacite were erupted. About the same time, a cluster of acid andesite and dacite domes rose near the east base of Mount Mazama, and many basaltic cinder cones were active on the lower slopes. Explosions of dacite pumice, partly in the form of glowing avalanches (nuees ardentes), also took place from the summit region.

A long period of quiescence ensued. The glaciers retreated until on1 y three small tongues extended beyond what is now the rim of Crater Lake. Even the longest stretched less than a mile and a half (2.4 km) beyond the caldera rim. The slopes of the volcano were almost barren of vegetation.The climactic eruptions then began. At first they were mild, but soon they increased in violence. During the initial stages, fine dacite pumice was blown high above the summit vents, to be drifted eastward by the wind. As the explosions became more violent and the pumice lumps increased in size, the wind veered toward the northeast. No less than 5000 square miles (13000 km2) were buried beneath the ejecta to a depth of more than 6 inches (15 cm). The finer dust spread over a vastly larger area. So much pressure was thus released that the gases in the feeding magma escaped from solution with unusual rapidity. The pumice was no longer projected high above the vents, but escaped in prodigious amount, boiling over the crater rims and rushing down the sides of the volcano at a tremendous rate, flowing after the manner of glowing avalanches. Most of this pumice was confined to the canyons, down which it raced for distances up to 35 miles (56.4 km). Where no canyons existed, the pumice flows spread as incandescent sheets. Those that swept down the east and northeast sides of the volcano deployed onto the flats bordering the Klamath Marsh. Such was their power that they traveled 25 miles (40 km) from their source, though half their journey lay across a plateau. Twenty miles (32 km) from the vents, the flows include bombs of pumice 14 feet (4 m) across. Just before the glowing avalanches ceased, the ejecta changed from dacite pumice to crystal-rich basic scoria. When the eruptions had come to an end, the glacial canyons were transformed into broad plains dotted with countless fumaroles. Each canyon had become a “Valley of Ten Thousand Smokes.” The abundance of charred logs within the deposits offers vivid testimony to the destruction of the forests.

When the culminating eruptions were over, the summit of Mount Mazama had disappeared. In its place, there was a caldera between 5 and 6 miles (8 and 9.7 km) wide and 4000 feet (1 220 m) deep. How was this formed? Certainly not by the explosive decapitation of the volcano. Of the 17 cubic miles (70 km3) of solid rock that vanished, only about a tenth can be found among the ejecta. The remainder of the ejecta came from the magma chamber. The volume of the pumice fall which preceded the pumice flows amounts to approximately 3.5 cubic miles (1 5 km3). Only 4 percent of this consists of old rock fragments; 10 to 15 percent consists of crystals, and the rest is made up of pumiceous glass. The volume of the pumice flows is approximately 8 cubic miles (33 km3). Of this amount, only 15 to 20 percent consists of old lava fragments. The remainder represents new magma in the form of crystals and glass. Weak, dying explosions deposited approximately a quarter of a cubic mile of fine ejecta, chiefly crystals and minute chips of rock.

 

Accordingly, 11.75 cubic miles (49 km 3) of ejecta were laid down during these short-lived eruptions. In part, it was the rapid evacuation of this material that withdrew support from beneath the summit of the volcano and thus led to a profound engulfment. The collapse was probably as cataclysmic as that which produced the caldera of Krakatau in 1883.Some process other than the expulsion of magma from the feeding chamber must also have operated. Whereas 17 cubic miles (70 km3) of the volcano disappeared, at most 11.75 cubic miles (49 km3) of ejecta were laid down. Moreover, by far the bulk of these ejecta consists of vesicular glass. The equivalent volume of liquid magma was less than half as much. Accordingly, it must be concluded that during or immediately before the great eruptions, large volumes of magma were injected into fissures at depth. It was by a combination of deep-seated intrusion and explosive eruption that the magma chamber was drained to make room for the collapse of the peak of Mount Mazama. Doubtless the caldera floor also subsided by cooling and solidification of the magma left below.The collapse of Mount Mazama was eccentric with respect to the former summit, for this lay well to the south of what i s now the center of Crater Lake. This eccentric collapse was controlled by the preexisting semicircular line of weakness along which the Northern Arc of Vents was formed.The formation of Crater Lake took place approximately 5000 years ago. The discovery of artifacts beneath the pumice deposits shows that man already inhabited this part of Oregon and was a distant witness of the catastrophe.After a period of quiet of unknown duration, activity commenced anew. Close to the western edge of the caldera rose the cone of Wizard Island, the final act of which was to erupt a rugged sheet of blocky lava, perhaps no more than a thousand years ago. As intra-caldera eruptions went on, rain and snow formed a lake on the floor, and this continued to gain in volume until it reached a depth of almost 2000 feet (610m).

 Note by the Editors

 

 

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