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The possible relation of lahars at Mount Rainier to volcanism was of paramount
concern in this study. The possibility of lahar formation would undoubtedly be
increased by the phenomena that are typically associated with eruptions. There
are a number of indications, or lines of indirect evidence, that link the
formation of some lahars to contemporary volcanism. The presence of fresh
volcanic bombs in a lahar suggests that the lahar was caused in some way by an
eruption. The eruption of pyroclastic material at the same time a lahar is
formed carries a strong implication that the lahar was caused by volcanism. The
occurrence of lahars within a general period during which pyroclastic deposits
were erupted is another, but less satisfactory, basis for inferring a causal
relation. Still another line of reasoning to support the association of lahar
formation with volcanism is the presence of complex assemblages of fluvial
deposits and lahars whose aggradation is best explained by repeated floods from
the volcano. These floods, in turn, are most readily explained by repeated
eruptions of hot pyroclastic debris or lava onto snow and ice. Finally, because
of their size and composition, some lahars could only have been caused by
tremendous slides from the volcano. The most probably causes of these slides
are volcanic explosions, earthquakes associated with volcanism, and possibly
lateral spreading or tilting of parts of the volcano caused by the rise of
magma; however, nonvolcanic earthquakes cannot be ruled out as possible
triggering mechanisms.
The lahars discussed below seem to have been triggered by eruptions or to have occurred during known periods of eruptive activity; lahars for which there is no independent evidence of contemporaneous volcanism are not mentioned here. Evidence of the age of lahars and eruptions before 6,600 years ago is too fragmentary to relate them with confidence. The oldest eruptions recorded by pyroclastic deposits are those that produced layer R more than 8,750 years ago. Lahars older than layer O have been found in the valleys of Van Trump Creek and the Nisqually River, but their age with respect to layer R is not known. The Paradise lahar is between 5,800 and 6,600 years old and thus was formed sometime during an interval of repeated pumice eruptions. Pyroclastic layer N, formed during this interval, is younger than the Paradise lahar, but any one of the eruptions that produced layers A, L, D, or S could have triggered the avalanche that caused the lahar. The Greenwater lahar, also formed by avalanches of rock debris, may have been caused by the same volcanic explosion that produced pyroclastic layer S. Most of the pre-Osceola lahar assemblage in the White River valley probably was formed after the Greenwater lahar occurred. The two bomb-bearing lahars in the assemblage evidently were caused by eruptions of hot rock debris and bombs. A detailed study of airlaid deposits contemporaneous with the Osceola Mudflow indicates that an eruption led to the formation of the Osceola. The last major episode of volcanic activity began with the formation of the block-and-ash flow in the South Puyallup River valley about 2,500 years ago. The evidence that it was hot clearly shows that the flow was associated with an eruption. A lahar older than pyroclastic layer C in the White River valley may have been formed at about the same time. Pyroclastic layer C was erupted next in sequence, after which the summit lava cone of the volcano was built and accompanied by aggradation of lahar assemblages in the valleys of the White River, Nisqually River, and Kautz Creek. Lahars probably also moved down the Muddy Fork, West Fork, and Van Trump Creek valleys at the same time. This attempt to assign individual lahars or lahar assemblages to certain eruptive episodes leads to the speculation that there is a genetic relation between kinds of volcanic activity and certain types of lahars. Lahars at Mount Rainier that lack a considerable content of clay and clay minerals probably resulted from eruptions that produced large quantities of blocks and lithic ash or from eruptions during which much ice and snow was melted by hot pyroclastic debris of lava flows. Lahars that contain a significant amount of clay (perhaps 5 percent or more) seem to be derived from old, hydrothermally altered parts of the volcano, and some may have been started by phreatic explosions. Such explosions may not necessarily have been accompanied by the eruption of new magma. |
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