20 Another Large Volume or Caldera Forming Eruption?

Volcano and Earthquake Hazards in the Crater Lake Region, Oregon 

Events of High Consequence but Low Probability

Another Large Volume or Caldera Forming Eruption?

The climactic eruption of Mount Mazama, during which Crater Lake caldera collapsed, took place ~7,700 years ago (calendar years, based on radiocarbon age of 6,845±50 14C years B.P.; Bacon, 1983). This was the largest eruption in the Cascades in the last ~400,000 years, explosively venting ~50 km3 of magma during perhaps only a few days. The products of the climactic eruption are dominantly rhyodacite pumice and ash. Perhaps 10 percent of the total is andesite and crystalrich “scoria” largely ejected late in the eruption. The compositionally-zoned eruption products indicate that relatively low-density rhyodacitic magma overlay hotter, denser andesitic magma and accumulated crystals deeper in the climactic magma chamber.

Tephra fall from the climactic eruption reached into southern Canada and pyroclastic flows traveled down the Rogue and Umpqua Rivers, and other drainages, as much as 70 km from Mount Mazama. The maximum extent of the pyroclastic-flow deposits of the climactic eruption, not to be confused with a modern hazard zone boundary, is shown on plate 1. The area devastated as a result of the eruption exceeds that bounded by the limit of pyroclastic-flow deposits shown on plate 1. The eruption began with hydromagmatic explosions leading shortly thereafter to a high plinian column from a single vent in what is now the northeast quadrant of the caldera, north of the summit of old Mount Mazama. A major pumice fall deposit extended in a northeast direction, downwind at the time. As the eruption proceeded, the eruption rate increased, causing the high column to eventually collapse as it ceased to be buoyant in the atmosphere. At this time, at least four valley-hugging pyroclastic flows descended the north and east flanks of Mount Mazama and left a deposit known as the Wineglass Welded Tuff. This phase of the eruption ended as the caldera began to collapse and multiple vents opened around the subsiding block. From these vents, eruption columns fed highly-mobile pyroclastic flows that descended on all sides of Mount Mazama, partially filling all valleys and spreading out across lowlands (plate 1). The result of the climactic eruption was transformation of the volcano from a large, snowcapped composite cone to a 1,200-m-deep caldera basin, drastic modification of all drainages nearby, and annihilation of all life forms for at least 30 km in all directions from Mount Mazama.

In the 200 years prior to the climactic eruption, there had been two smaller rhyodacitic plinian eruptions, each followed by sluggish emplacement of a thick rhyodacitic lava flow (Llao Rock and Cleetwood flows). The younger of these flows, Cleetwood, was still hot when the climactic pumice fell on its surface. Although there would have been vigorous seismicity before each of these eruptions and the climactic eruption, the magnitude of the climactic event might not have been anticipated at its onset. The stage was clearly set for a voluminous eruption, however, as the geologic record indicates only rhyodacitic eruptions from the general area of Mount Mazama in the preceding 20,000-25,000 years. The eruptive history thus records growth of the shallow magma chamber approximately beneath the present caldera.

Is a shallow magma chamber still present and is another caldera-forming eruption likely in the next few centuries? The geologic evidence suggests that most of the gas-charged rhyodacitic magma was ejected in, or crystallized following, the climactic eruption. Virtually all of the postcaldera lava is andesite which probably would not have been able to erupt had a large amount of lower density rhyodacite remained molten in the subsurface. The small postcaldera rhyodacite dome appears to be related to cooling and crystallization of the magma batch which had earlier produced the postcaldera andesites of the central platform, Merriam Cone, and Wizard Island (fig. 3) rather than being left over from the climactic chamber. Rhyodacitic magma apparently accumulated in the climactic magma chamber at a rate of ~2 km3/1,000 yr. If the postcaldera rhyodacite reflects the cooling of the last magma emplaced in the upper 10 km of the crust, then sufficient magma for a voluminous, explosive eruption will not accumulate for many thousands of years. A less likely situation would be that the postcaldera rhyodacite represents the onset of silicic magma accumulation, in which case as much as 10 km3 of magma might have accumulated in the last ~5,000 years. This amount would be sufficient to feed a major pyroclastic eruption (e.g., Mount Pinatubo, 1991) but probably would not lead to caldera collapse. In conclusion, we consider the annual or 30-year probability of a major silicic pyroclastic eruption to be low and the probability of a caldera-forming eruption to be negligible.

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