| Abstract |
- 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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