LATEST PLEISTOCENE AND HOLOCENE GLACIER FLUCTUATIONS AT MOUNT RAINIER, WASHINGTON

Mount Rainier is a large stratovolcano that reaches a height of 4,393 meters (14,410 feet), making it the highest peak in the Cascade Range. Mount Rainier is flanked by a greater volume of glacier ice than any other mountain in the conterminous United States, and as a result provides an exceptional opportunity for the study of glacier fluctuations.

It has been argued that the magnitude and timing of late Quaternary glacier fluctuations on northern Cascade volcanoes, including Mount Rainier, Mount Baker and Glacier Peak, differed from that recorded elsewhere in the central North American Cordillera. However, recent work done on Mount Baker and Glacier Peak suggests that glacier fluctuations on these volcanoes did not vary significantly from fluctuations elsewhere in the central Cordillera; earlier inconsistent interpretations may have resulted from incomplete mapping and/or varying interpretations by different field parties and/or inadequate dating control. A study in progress that involves a re-examination of proxy evidence from Mount Rainier may provide new insight into whether anomalous glacier fluctuations occurred at this location.

A study of Little Ice Age (LIA) lateral moraines, constructed of tills from multiple advances of Nisqually Glacier, has generated radiocarbon dates that constrain the timing of advances of the last two millennia. An age of 480 ± 70 ¹⁴C yr BP [1300 – 1630 AD] yielded by fossil wood recovered 8 m below the right-lateral moraine crest, along with other samples from about that depth not yet dated, indicates that the maximum LIA advance began after that time, and that only the upper several meters of the “LIA moraine” date from the LIA. In the left-lateral moraine, a discontinuity between tills, that is approximately parallel to the proximal moraine flank, is marked by a sand layer and several fossil wood fragments. An age of 1670 ± 50 ¹⁴C yr BP [250-530 AD] from one of these wood fragments indicates that the glacier advanced shortly after this time during the “First Millennium Advance.” The emerging record of late Holocene glacier fluctuations at Mount Rainier matches the record recently interpreted at Mount Baker.