A new model for the formation of Crater Lake Caldera, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bn999920q

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  • During the Pleistocene, an andesitic volcano named Mount Mazama grew to a probable elevation of 3000 meters in south- central Oregon. Near the end of the Pleistocene, three diverse magma types appeared in the eruptive products associated with Mount Mazama: l)High-alumina basaltic andesite magma associated with an early plateau and with coeval cinder cones; 2) Hypersthene andesite magma similar in composition to the andesite which had formed the main cone; 3) Dacite magma, erupted both as pyroclastic material and as flank lava flows after formation of the andesitic cone. Approximately 12, 000-13, 000 years ago, eruption of andesite and then dacite occurred along a semicircular fracture now represented by the north wall of the caldera. The eruption of dacite was a surface expression of a shallow chamber of dacite magma emplaced beneath Mount Mazama. Before and after emplacement of the dacite magma chamber, basaltic andesite erupted upon the lower flanks and plateau surrounding the volcano, The last such parasitic activity occurred at Forgotten Crater (on the western flank of Mount Mazama) where lava and pumice of intimately mixed dacite and basaltic andesite appeared. Approximately 6600 years ago, explosive eruption of dacite pumice began from central vents. The magma of these initial eruptions was characterized by discrete gas nuclei which expanded rapidly, disrupted the enclosing liquid into fine ash, and provided the energy to propel this ash high above the summit. As eruption proceeded, magma was tapped in which gas nuclei were actively forming and were thoroughly dispersed through the magma. At this stage, the erupted material behaved as a fluidized mass, frothed over the crater rim, and rushed down the slopes as glowing avalanches. Immediately following eruption of fluidized dacite, a more mafic magma was erupted in the form of basic scoria flows. This basic scoria was chemically, similar to par3sitic eruptions of basaltic andesite, and was not nearly as mobile as the earlier dacite ash-flows. The waning stages of eruption were characterized by crystal-rich basic scoria containing abundant fragments of andesite and granodiorite. Following the eruption of crystal- and lithic-rich basic scoria, the summit of Mount Mazama collapsed into the underlying magma chamber, producing the Crater Lake caldera. Soon after collapse, eruption of mafic magma occurred on the caldera floor, probably forming a lava lake. The last eruption formed an intra-caldera andesitic cone called Wizard Island Wizard Island lavas are chemically similar to the andesites which formed the main cone of Mount Mazama. Thus, a sequence of volcanic activity culminating in violent eruption and collapse, returned to its initial condition. Calculations of Mazama ash volume are presented and it is concluded that there is no discrepancy between volume of missing Mount Mazama and volume of erupted material. Crystal-rich basic scoria flows containing melted granitic fragments (torn from the margins of the dacite magma chamber) indicate that the chamber was essentially emptied during the culminating eruptions. It is suggested that basic scoria magma occupied the bottom portion of the dacite magma chamber. This mafic magma assimilated granodiorite which had crystallized around the margins of the dacite magma. A new model for the formation of the Crater Lake caldera is presented. In this model the magma types recognized at Crater Lake ascended through the crust as independent bodies. Whenever two of these magma types came into marginal contact, limited eruption of mixed magmas was possible. However, prior to the culminating eruptions, a high-temperatures basaltic andesite magma was injected into the lower portion of the volatile-saturated, cooling dacite magma. Energy transferred from the mafic magma triggered exsolution of gas in the silicic magma until vapor pressure exceeded confining pressure. Once eruption began, it did not stop until the chamber was depleted of its gas-rich magma. The Crater Lake caldera was formed when the unsupported summit of Mount Mazama collapsed into this sub-volcanic vacancy. Many other calderas, long recognized as a special (Krakatoan) type, were produced following the eruption of more than one magma. It is suggested that the process described above is an important caldera-forming mechanism.
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