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Structure and Emplacement of a Rhyolitic Obsidian Flow: Little Glass Mountain, Medicine Lake Highland, Northern California

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Structure and Emplacement of a Rhyolitic Obsidian Flow: Little Glass Mountain, Medicine Lake Highland, Northern California Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain, Medicine Lake Highland, northern California JONATHAN H. FINK Department of Geology, Arizona State University, Tempe, Arizona 85287 ABSTRACT studies that relate morphology to flow pro- vesicular core of the flow. Interactions cesses and lava rheology (Wentworth and among these three processes lead to the Many rhyolitic obsidian flows show con- Macdonald, 1953; Peck, 1966; Swanson, complex surface patterns observed on Little sistent stratigraphic relations among tex- 1973; Fink and Fletcher, 1978; Peterson Glass Mountain and other rhyolite flows. tural units exposed in the flow fronts of and Tilling, 1980). Comparable work on In this paper, the textural units and their undissected flows and in cross sections of rhyolite flows has been hindered by a lack distribution will first be described and older flows. The stratigraphy of the Holo- of observations of active flows and by the interpreted, using an emplacement model cene Little Glass Mountain rhyolitic obsid- seemingly random distribution of blocks on for rhyolitic obsidian flows. Maps and ian flow consists of (from bottom to top): the surfaces of young, undissected flows. photographs of the surface structure will air-fall tephra deposits, basal breccia, However, close examination of Little Glass then be explained by a deformational model coarsely vesicular pumice, obsidian, finely Mountain, a Holocene rhyolitic obsidian that takes account of the mechanical prop- vesicular pumice, and surface breccia. flow in northern California, shows the sur- erties of the different textural units. Finally, Slightly crystalline rhyolite occurs near the face to be composed of four principal tex- the structure of domes will be used to inter- vent areas. A model of obsidian-flow em- tural units; these vary in color, specific pret local and tectonic stress patterns at the placement based on these textural relations gravity, and degree of vesiculation. Map- time of extrusion. is presented. This stratigraphy may reflect ping the distribution of these units reveals the distribution of volatiles within the that the surface structure results from three Geologic Setting magma source region, with the interlayered deformational processes: compression dur- contact between coarsely vesicular pumice ing advance of the lava, fracturing due Little Glass Mountain lies on the south- and overlying obsidian indicating stratifica- to cooling and radial expansion of the flow, west flank of the Medicine Lake Highland tion of volatiles in the magma body. and diapirism caused by the presence of shield volcano in northern California, The distribution of pumiceous and glassy a light pumiceous base beneath the non- near the southern end of the Cascade vol- zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows. This complex structure, as mapped on the Little Glass Mountain flow, reflects both the initial flow stratigraphy and subsequent deformational processes. During emplace- ment of the lava, three processes disrupt the initial flow configuration: the rise of coarse pumice diapirs from the base of the flow, the inward propagation of fractures in areas of extension, and surface folding in sites of flow-parallel compression. Subvertical flow banding in vent areas indicates that fractur- ing accompanies the emergence of lava; most of the observed upper surface of a dome originates as a fracture plane. The structure of domes that form over vent areas may reflect the orientation of dike-like conduits as well as the local state of stress NEVADA during extrusion. Figure 1. Map showing location of Medicine Lake Highland Volcano and the major Cascade INTRODUCTION volcanoes. Heavy line indicates outline of volcano. Dotted line shows margin of caldera. Dashed lines The surface structure of basaltic lava represent prominent faults. BGM = Big Glass Mountain Flow; LGM = Little Glass flows has been the subject of numerous Mountain Flow; ML = Medicine Lake. Geological Society of America Bulletin, v. 94, p. 362-380, 27 figs., March 1983. 362 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/3/362/3444643/i0016-7606-94-3-362.pdf by guest on 30 September 2021 RHYOLITIC OBSIDIAN FLOW, CALIFORNIA 363 A Model for Obsidian Flow Emplacement Consider the emplacement of a low volume (less than 1 km3) of relatively dry (less than 1.0% volatiles by weight) rhyolitic magma. As this body approaches the sur- face, volatiles exsolve and rise to its top. Lower confining pressure leads to frothing of the volatile-rich cap as the stratified mass works its way upward (Fig. 3a). Contact with ground water also contributes to the formation of a gas-rich carapace. This vesiculation may generate one or more explosive eruptions of tephra (Fig. 3b). The remaining volatile-rich magma reaches the surface as a highly inflated pumiceous lava (Fig. 3c). The transition from discrete pumice particles to a continuous lava flow occurs when gas pressure in the vesicles is no longer sufficient to overcome the tensile strength of the magma. Once extruded, the lava flows due to the relatively low viscosity of the glassy septa between vesicles; such motion causes the bubbles to be flattened and distorted. As the eruption continues, bubble-free obsidian flows out over the earlier emplaced coarsely vesicular pumice, compressing and shearing the latter into a basal layer of irregular thickness (Fig. 3d). This lava is slightly less viscous but more dense than the coarse pumice. The transition from coarse pumice to obsidian is gradual, so that some Figure 2. U.S. Forest Service photograph of Little Glass Mountain, showing locations of vesicular material continues to be extruded Figures 20 through 23. Light color of surrounding area is due to presence of tephra. Scale with the obsidian, forming blebs of different bar = 500 m. North is to top. sizes, which are stretched into layers parallel to the flow direction. As the obsidian rides over the coarse canic chain (Fig. 1). The flow is nearly cir- been described elsewhere (Anderson, 1933, pumice, some volatiles continue to evolve cular in plan with a radius of about 2 km 1941; Eichelberger, 1975; Mertzman, 1977; from the cooling upper flow surface, form- (Fig. 2). It contains at least two discrete Mertzman and Williams, 1981). ing a finely vesicular carapace that then vent areas which produced penecontempo- Although chemically homogeneous, the insulates the flow interior (Fig. 3e). Bubbles raneous overlapping flow lobes. Flow fronts lavas of Little Glass Mountain contain a in this layer are smaller and less distorted range from 15 to 60 m in height, and maxi- wide variety of textures that are distin- than those in the lower pumice unit, due to mum relief is about 120 m from the base to guished on the bases of color, specific grav- the lower temperature, higher viscosity, and the summit of the flow. Total flow volume is ity, and the size, shape, and abundance of lower shear stresses at the flow surface. about 0.3 km3. The vents of Little Glass vesicles. Five textural types can be delin- Most contacts between obsidian and finely Mountain align with four other obsidian eated, both in the field and in thin sections: vesicular pumice are gradational, although flows, a series of surface cracks, and several (1) fragmental pumice (tephra) deposits, early-formed masses of fine pumice may basaltic cinder cones, all located along a (2) coarsely vesicular pumice flow unit, break off in zones of high strain and become north-northeast trend on the west side of (3) nonvesicular glassy obsidian, (4) fine- stretched parallel to the obsidian, locally the volcano. ly vesicular pumice, and (5) crystalline giving interlayered contacts. The rocks of Little Glass Mountain have rhyolite. All units except the tephra are Magma remaining in the conduit has a calc-alkaline rhyolitic composition, typi- intimately flow banded. A similar classifi- more time to crystallize. The final material cal of glassy rhyolite flows in the Cascades cation scheme can be applied to the rocks of to erupt piles up over the vent rather than (Shepherd, 1938; Higgins, 1973) as well as many other Holocene rhyolite flows. Con- advancing outward, due to its higher crystal in the Long Valley Caldera (Bailey and oth- sistent contact relations between the four content and viscosity and lower tempera- ers, 1976) and the Yellowstone Caldera flow units (tephra excluded) aid in mapping ture (Fig. 30- Eventually, lava plugs the (Boyd, 1961). The petrology of all of the the structure on upper surfaces of the flows vent. New cracks then open in the dome, lavas in the Medicine Lake Highland has and in flow fronts. through which additional lava extrudes; an Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/3/362/3444643/i0016-7606-94-3-362.pdf by guest on 30 September 2021 364 J. H. FINK STRATIGRAPHY OF LITTLE GLASS MOUNTAIN It may seem unusual to discuss the "stra- tigraphy" of a single lava flow. However, the textures found on Little Glass Mountain and other flows occur in discrete layers with consistent spatial relations. This sequence is most easily understood in terms of a strati- graphic arrangement. In aerial photographs of Little Glass Mountain, the surrounding forests appear to have a white soil. This is actually an air- fall pumice deposit that underlies the lava flow. The rhyolitic pumice consists of poorly sorted gray lapilli in a coarse gray ash (Heiken, 1978). Isopach maps indicate that the pumice erupted from a vent or vents now buried by Little Glass Mountain. The volume of this tephra constitutes about TEPHRA OBSIDIAN J ¡ FINE PUMICE 9% of the volume of the subsequent lava COARSE PUMICE |V>1 RHYOLITE flows. No tephra was found on the surface or interlayered in the flow fronts of Little Figure 3. Diagram showing emplacement of small rhyolite lava flow with concurrent Glass Mountain, indicating that explosive development of flow stratigraphy.
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