Journal of Volcanology and Geothermal Research 65 ( 1995) 5 l-80 Surficial extent and conceptual model of hydrothermal system at Mount Rainier, Washington David Frank U.S.Environmental Protection Agency, 1200 Sixth Avenue, ES-098 Seattle, WA 98101, USA Received 8 January 1992; revised version accepted 10 February 1993* Abstract A once massive hydrothermal system was disgorged from the summit of Mount Rainier in a highly destructive manner about 5000 years ago. Today, hydrothermal processes are depositing clayey alteration products that have the potential to reset the stage for similar events in the future. Areas of active hydrothermal alteration occur in three representative settings: ( 1) An extensive area (greater than 12,000 m*) of heated ground and slightly acidic boiling-point fumaroles at 76-82°C at East and West Craters on the volcano’s summit, where alteration products include smectite, halloysite and disordered kaolinite, cristobalite, tridymite, opal, alunite, gibbsite, and calcite. (2) A small area (less than 500 m’) of heated ground and sub-boiling-point fumaroles at 55-60°C on the upper flank at Disappointment Cleaver with smectite alteration and chalcedony, tridymite, and opal-A encrustations. Similar areas probably occur at Willis Wall, Sunset Amphitheater, and the South Tahoma and Kautz headwalls. (3) Sulfate- and carbon dioxide-enriched thermal springs at 9-24°C on the lower flank of the volcano in valley walls beside the Winthrop and Paradise Glaciers, where calcite, opal-A, and gypsum are being deposited. In addition, chloride- and carbon dioxide-enriched thermal springs issue from thin sediments that overlie Tertiary rocks at, or somewhat beyond, the base of the volcanic edifice in valley bottoms of the Nisqually and Ohanapecosh Rivers. Maximum spring temperatures of 19-25°C and 38-50°C respectively, and extensive travertine deposits have developed in these more distant localities. The heat flow, distribution of thermal activity, and nature of alteration minerals and fluids suggest a conceptual model of a narrow, central hydrothermal system within Mount Rainier, with steam-heated snowmelt at the summit craters and localized leakage of steam-heated fluids within 2 km of the summit. The lateral extent of the hydrothermal system is marked by discharge of neutral sulfate-enriched thermal water from the lower flank of the cone. Simulations of geochemical mass transfer suggest that the thermal springs may be derived from an acid sulfate-chloride parent fluid which has been neutralized by reaction with andesite and highly diluted with shallow groundwater. The model may accomodate some of the thermal springs beyond the base of the edifice. Present heat flow from Mount Rainier is substantial relative to other Cascade Range volcanoes and does not appear to have diminished since at least the late 19th century. Evidence of older hydrothermal processes found in Holocene lithic tephra and debris avalanches record activity more extensive but similar in chemical composition to that of today. * This paper was initially part of the Harry Glicken Memorial special issue. At a later stage, after consultation with the Guest Editors, the Publisher has removed the paper from the special issue to be published in a regular issue. 0377-0273/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO377-0273(94)00081-6 52 D. Frank/ Journal of Volcanology and Geothermal Research 65 (1995) 5140 1. Introduction 2. Geologic setting Mount Rainier rises to 4392 m from a lOOO-m-high Like many of the other large stratovolcanoes in the base of deeply eroded, Tertiary rock west of the crest Cascade Range of western North America, Mount of the Cascade Range in western Washington (Fig. 1) . Rainier’s hydrothermal system produces a distinctive The Tertiary base consists of a 5000-m-thick, gently folded pile of Oligocene and younger volcanic flows set of mineralogical alteration products from primary and detritus ranging from basalt to rhyolite, though volcanic rocks. Concurrently, the hydrothermal system andesite and rhyodacite predominate (Fiske et al., overlays a chemical imprint on groundwater. Surface 1963). These volcanics were intruded, mainly during leakage of these fluids produces thermal activity in the the Miocene, by numerous dikes and sills and a major form of fumaroles and heated ground on the volcano’s granodioritic complex (Fiske et al., 1963; Mattinson, upper slopes, and thermal springs at lower elevations. 1977; Murphy, 1991). Hydrothermal activity opens a window into subsur- Early Pleistocene volcanic sediments of horne- face processes that can dramatically affect the course blende-hypersthene andesite (Lily Creek Formation) of future volcanic events, be they magmatic or phreatic. west of Mount Rainier may represent the earliest erup- Hydrothermally altered rocks or pressurized hydro- tions from a Rainier vent that has since been buried thermal fluids provide source areas for non-magmatic (Crandell, 1963b, pp. A21-A22; Mattinson, 1977, p. explosions and debris avalanches. With the addition of 15 12). The bulk of the present edifice is built of lava magma intrusion, hydrothermal systems can produce flows and brecciated lava flows, with lesser amounts of preferential pathways for structurally weakening vol- interlayered tephra and other pyroclastic deposits. canic cones. Also, heat stored in hydrothermal systems These products vary little in composition, consisting can help drive eruptions initiated by dike-related frac- mainly of hypersthene-augite andesite, though two turing. Future scenarios or localities of destructive, small satellite vents of olivine andesite occur on the hydrothermally influenced events may be anticipated north flank (Fiske et al., 1963). by constructing conceptual models of hydrothermal Mount Rainier has been one of the more active Cas- systems. cade volcanoes during Holocene time. Evidence of 11 This paper relies primarily on minerals, gases, and eruptions can be found in Holocene tephra deposits water found in surficial deposits to construct a concep- (Mullineaux, 1974)) with major eruptive episodes between 6500 and 4000 years ago and again between tual model for Mount Rainier that considers the follow- 2500 and 2000 years ago. Lava flows, flow breccia, and ing factors: explosion rubble that form a young summit cone at - Locations of hydrothermal leakage at the surface; least 400 m thick are no more than 2200 years old - Structures that provide permeable paths of fluid (Crandell, 197 1, p. 11). The most recent dated eruption egress to the surface; episode produced pumice sometime during the early to - Amount of excess heat discharge; middle 19th century (Mullineaux et al., 1969). - Composition of surficial thermal fluids; In addition to eruptive deposits, at least 55 large - Composition, guided by mineralogy, of subsurface Holocene mass-movement deposits from Mount Rai- thermal fluids. nier, including debris avalanches and lahars as large as Analytical data used as a basis for the model are from 3 X lo9 m3, have been identified (Crandell, 1971; Scott samples collected during field investigations in 1982- et al., 1992). Many of these events apparently accom- 1985 (Frank, 1985), whereas other field and remote panied eruptions. However, some of the larger mass- sensing observations span a longer time period up to movements occurred during time periods for which no the present. In addition, a critical overview of historical eruptive deposits have been recognized (Crandell, information on hydrothermal activity at Mount Rainier 197 1, p. 57)) and may have resulted from non-eruptive is provided as an important context in which to place a phenomena such as hydrothermal explosions, earth- conceptual model. quakes, or oversteepening from long-term erosion. D. Frank/Journal of Volcanology and Geothenn~l Research 65 (1995) 51-80 53 \ \ MT. ST. HELENS , 480 Fig. 1.Regional map of Mount Rainierlying between the crest of the CascadeRange and the densely populatedPuget Lowland (heavy outline). Black area denotes the extent of Mount Rainier lava flows. Shading shows approximatearea inundatedby the largest Holocenelahar~ 5000 years old or younger. Adapted from Swanson et al. ( 1992, fig. 1). 3. Thermal activity - The lower flank, 1000-3000 m altitude; - Adjacent to the base of the cone. Historical reports of visual observations, more recent field investigations, and aerial thermal-infrared surveys 3.1. Summit area provide a basis for mapping active thermal areas at Mount Rainier. The complete record of data on thermal Historical observations activity can be conveniently discussed with reference Boiling-point fumaroles and adjacent heated ground to four representative settings (Fig. 2), the first three cover an exposed area of 12,000 m2 along the rims of of which are on the cone: two overlapping craters at the volcano’s summit (Fig. - The summit cone, above 4000 m altitude; 3). Additional fumarolic activity occurs beneath the - The upper flank, 3ooo-4000 m altitude; ice cover that fills the basins of both craters. The first 54 D. Frank/Journal of Volcanology and Geothermal Research 65 (1995) 5140 Fig. 2. Map of localities of thermal activity at Mount Rainier. A large fumarole field occurs at the top of the young summit cone (large dot) ; fumaroles and heated ground occur at Disappointment Cleaver (DC) and, in some cases ephemerally, at 5 other similar but poorly accessible areas on the upper flank (small dots). Thermal springs (squares)occur on the lower flank of the volcano near Paradise (ES) and Winthrop (