Volcano-ice Interaction at Mount Rainier and Mount Baker, Washington:
Lessons and Examples
[D.T. Lescinsky] (Dept. of Earth Sciences, University of Western Ontario, London,
ON, Canada, N6A 5B7; ph: 1-519-661-2111 ext. 86063; fax: 1-519-661-3198; email:
dlescins@uwo.ca)
Mount Rainier and Mount Baker of the Cascades volcanic range, Washington are
large stratovolcanoes that have undergone extensive glaciation during recent ice ages
and still retain large volumes of ice. In addition, both volcanoes have experienced
abundant and diverse styles of eruptive activity. Not surprisingly, Mount Rainier and
Mount Baker display numerous examples of volcanic interaction with glacial ice.
During the past ten years, these volcanoes have been extensively studied (e.g., Sisson
et al., 2001; Hildreth et al., 2003) revealing how geologically dynamic glaciated
volcanoes can be.
Evidence of volcano-ice interaction ranges from fairly obvious to fairly ambiguous
due to formation processes, and aspects of preservation and erosion. Ice contact
features in lava flows offer compelling evidence of glacial interaction. Features
include: glassy, closely spaced polygonal joints, with and without water penetration
fractures, and having curving to subhorizontal orientations. Spectacular examples of
these features are visible at the terminus of Burroughs Mountain lava flow and along
the margins and at the terminus of Mazama Ridge lava flow at Mount Rainier, and
near the end of Kulshan Ridge at Mount Baker. The fracture orientations are
indicative of a vertical cooling surface and the great thickness of these flows is
attributed to ponding against ice. The Mazama Ridge lava flow is notable for multiple
“table-like” portions, where the advancing lava intersected and ponded against
tributary glaciers.
Exceptionally thick lava flows (>100 m thick) are scattered around Mount Rainier and
Mount Baker. While their great thickness is related to ponding, at times it is unclear
whether the flows have been emplaced within valleys with rock walls or with ice
walls (or one of each). The inverse topography commonly associated with these lava
flows can be produced in either situation. Extensive erosion caused by glacial
reoccupation of drainage valleys and ice sheet advance during subsequent glaciations,
can remove the margins of the flows and any evidence of ice contact. Hildreth et al.
(2003) note examples of stranded lava flow remnants, such as Lookout Mountain,
where a gorge >500 m deep separates the lava flow from its likely source. The rapid
incision rates required highlight the difficulty in evaluating older ridge-capping and
valley-wall lava flows.
The exceptionally thick lava flows at Mount Rainier and Mount Baker correspond
primarily to discrete periods of high effusion rates. Lava flows erupted outside of
these periods are thinner, less voluminous, and number in the hundreds. These lava
flows mantle the upper slopes of both volcanoes and show localized evidence of
interaction with ice: glassy textures, and small diameter polygonal joints. Small
block-and-ash flows were formed by collapse of such lavas on the steep upper slopes.
traveled out onto the glacial ice. These pyroclastic flows commonly traveled out onto
the glacial ice and are likely responsible for a large number of debris flows and
several of the non-vesicular tephra layers found at Mount Rainier.
Both Mount Rainier and Mount Baker have experienced sector collapse and the
generation of large volume clay-rich debris flows (i.e., the Osceola Mudflow at
Mount Rainier). Detailed study of the debris flow deposits indicates that they tend not
to be associated with large eruptions, but instead with small phreatomagmatic
eruptions or seismic events (Sisson et al., 2001). In fact, Mount Rainier and Mount
Baker have experienced few large explosive events. However, the record of activity
may be somewhat influenced by a preservation bias towards lava over volcaniclastics.
This is highlighted by the removal of almost the entire proximal deposit of the
Kulshan caldera ignimbrite (1.15 ma; Mount Baker) by glacial erosion.
*References:*
Hildreth, W., J. Fierstein, and M. Lanphere (2003), Eruptive history and
geochronology of the Mount Baker volcanic field, Washington, Geol. Soc. Amer.
Bull., 115 (6), 729-764.
Sisson, T. W., J. W. Vallance, and P. T. Pringle (2001), Progress made in
understanding Mount Rainier's hazards, Eos Trans. AGU, 82 (9), 113, 118-120.
INVITED ORAL
CORRESPONDING AUTHOR: D.T. Lescinsky