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
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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
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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. Geometry of rising magma in a and b is highly activity ceased upon extrusion of lava. schematic. Vesicles seen in photomicrographs of the tephra are elongated and parallel to each explosive "throat-clearing" leads to a new rhyolite appears as domes over the vent other. Pumice lapilli have fracture surfaces tephra layer; or magma may move laterally areas, with finely vesicular pumice on the oriented perpendicular to the direction of and erupts at a nearby center, a process that upper flow surface. Obsidian forms the vesicle elongation, suggesting that fragmen- may form a chain of coalescing domes and core, and coarsely vesicular pumice is found tation occurred through the stretching and flows. at the distal margins and at the base of the pulling apart of the vesicular magma in the vent (Heiken, 1978). This model implies that flow stratigraphy flow. Movement and cooling of the lava should consist of a basal layer or layers of causes the upper surface to fracture into Contacts between the tephra and the tephra, overlain by coarsely vesicular pum- blocks that cascade off the flow front and overlying lava-flow units are concealed by ice, obsidian, and finely vesicular pumice. become buried by advance of the lava; talus in most of the flow fronts. The unit Toward the end of the eruptive cycle, crys- hence the stratigraphy also includes basal that most commonly occurs directly over talline rhyolite forms a summit dome, and if and surface layers of breccia. Repetition of the talus is a coarsely vesicular, brownish to the flow cools slowly enough, crystallization any of these stages during an eruption will greenish-gray pumice. In cross sections of occurs in the flow interior. Consequently, cause variations in the above stratigraphy. other flows that show similar stratigraphy,
Figure 4. Coarsely vesicular pumice unit, (a) Hand specimeni,. (b) Thin section (25x enlargement).
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such as the Banco Bonito Flow in the Valles lites. The needle-like crystals are generally creases. Figure 7b illustrates this gradation Caldera of New Mexico and the Roches aligned parallel to each other within a layer, microscopically for the outcrop shown in Rossa Flow in Lipari, Italy, coarse pumice although they need not be parallel to the Figure 7a. passes downward into a brecciated zone boundaries of the layer. Layers may extend Contacts between the coarse and fine that overlies the basal tephra, with no continuously for several metres or may be pumice are rare because these units are gen- intervening soil horizons. disrupted by small faults of a few centime- erally separated by a zone of obsidian. In The coarsely vesicular pumice (Fig. 4a) tres displacement. These layers, as well as some places, blocks of fine pumice perch has the lowest bulk density of any of the the interlayered contact zone between the unconformably on outcrops of coarse pum- textural types (sp. gr. = 0.8 to 2.0 g/cm3) obsidian and coarse pumice, may be con- ice (Fig. 8). These erratic blocks may rest a and is consequently mined preferentially centrically or isoclinally folded. few metres above the mean flow surface in the quarries found on many rhyolitic Obsidian outcrops in flow fronts are usu- without any nearby fine pumice outcrops of obsidian flows (Chesterman, 1956). Vesicles, ally between 2 and 15 m thick. They grade comparable height. Similarly, isolated which generally make up more than 50% by upward into a finely vesicular pumice unit blocks or extensive bands of fine pumice volume of this pumice, are ovoid to highly that forms a carapace over much of the flow may lie in the middle of a large expanse of elongate and range from less than one mil- surface. This fine pumice differs from the coarse pumice without an obvious source. limetre to a few centimetres in diameter, obsidian only in its higher concentration of Both of these relationships apparently with the most distorted bubbles developing vesicles, which may be as much as 30%. represent stratigraphic anomalies caused by a highly filamentous appearance. Vesicles Bubble shapes range from spherical in sam- deformation of the flow surface during or seen in thin section are commonly flattened ples that have less than about 25% vesicles after its emplacement (Fink, 1980a). and folded, giving the pumice a close by volume to elongate in more vesicular The sequence described above (tephra resemblance to the tephra (Fig. 4b). samples; vesicles are rarely as distorted overlain by coarse pumice, obsidian, and Coarse pumice grades upward across a as those in the coarsely vesicular pumice. fine pumice) is observed in the flow fronts zone less than a few metres wide into a Bubble size is commonly less than one and over much of the surface of Little Glass prominent black obsidian unit that is millimetre in diameter, but ranges up to Mountain and other rhyolitic obsidian largely free of vesicles and phenocrysts a centimetre in some places. Like the ob- flows in the Medicine Lake Highland and (Fig. 5). The lava ranges from homogene- sidian, this unit exhibits banding defined elsewhere. Near the vent areas, however, ous coarse pumice, to pumice with scattered by crystallite concentrations, but it also another unit predominates, which resembles glassy blebs and layers, to an equal mixture contains layers defined by bubbles. the fine pumice. This rock is a uniform gray of pumice and obsidian, to obsidian with Contacts between this pumice and the to dark bluish-gray rhyolitic glass with pumiceous inclusions, and finally to pure obsidian are of two types: interlayered and numerous gabbroic xenoliths and widely obsidian. Within the pumice, some round gradational. Interlayered contacts parallel - scattered pyroxene and feldspar pheno- glassy inclusions have elongated tails, and foliations as in the coarse pumice-obsidian crysts. It is thoroughly banded and has a some glassy layers widen locally into nearly contacts. Inclusions of finely vesicular specific gravity of 2.1 to 2.3. Foliations are circular structures, illustrating a transition pumice are stretched into layers up to 20 cm defined by feldspar microlites, which are from isolated blebs to continuous layers thick. Gradational contacts are typically suspended in a glassy matrix. The unit (Fig. 6). seen in vertically foliated outcrops where differs from the obsidian and fine pumice Ubiquitous layering within the obsidian, obsidian near the base becomes increasingly primarily in its distinctive waxy luster, its ranging from tens of microns to several cen- bubble-charged upward. The color of the higher concentration of crystallites, and its timetres in thickness, results primarily from outcrop passes upward from black to gray strong tendency to develop deep smooth varying concentrations of feldspar crystal- to white as the bubble concentration in- fractures that cut across foliations. The
Figure 5. Obsidian unit. Hand specimen (compare with thin Figure 6. Interlayered contact zone between coarsely vesicular section in Fig. 7b). pumice and obsidian units.
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rhyolite unit has only limited contacts with reflects both differences in the thickness of the other three lava textures. It contains a the original deposit and the irregular height finely vesicular surface crust as much as 1 m of the concealing talus apron. Figure 10a thick that grades downward into bubble- shows the southern flow front of the north- free rhyolite. Coarsely vesicular pumice ernmost (Inyo) obsidian dome on Glass outcrops interrupt continuous foliations in Creek, north of Long Valley Caldera, Cali- the rhyolite, especially where exposed in fornia (Wood, 1977). A prominent black valley-like cracks as much as 10 m deep obsidian layer can be seen below the white (Fig. 9). Blocks of rhyolite perch uncon- surface layer of finely vesicular pumice. formably on top of areas of fine pumice and A second, lower layer of coarsely vesicular obsidian, but these contacts are difficult to pumice is visible toward the right-hand side identify due to local similarities between of the photo; measured thicknesses for fine pumice and rhyolite. a portion of this outcrop are shown in Figure 10b. Distribution of Textures Much of the upper flow surface is covered by finely vesicular pumice, with outcrops of The general distribution of lava textures coarsely vesicular pumice concentrated on Little Glass Mountain is typical of many toward the flow fronts. Obsidian outcrops rhyolitic obsidian flows. Mapped exposures are relatively restricted and occur sand- in flow fronts show a relatively uniform wiched between the upper-flow surface and thickness of obsidian wrapping around the the flow front. Outcrops of rhyolite are re- flow, overlain by finely vesicular pumice stricted to the central vent areas, which and underlain by coarsely vesicular pumice. commonly are dome shaped with steeper Figure 7a. Gradational contact between The coarse pumice layer has a more variable slopes than the rest of the flow surface. obsidian and finely vesicular pumice units. thickness than the obsidian; this variation Scattered outcrops of coarsely vesicular
Figure 7b. Thin sections of samples taken from different positions within gradational contact, showing progressively greater disruption of obsidian foliation by bubbles in finely vesicular pumice (25* enlargement).
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 Figure 8. Unconformable "erratic" block of finely vesicular pumice unit resting on large Figure 9. An 8-m-deep fracture in rhyo- outcrop of coarsely vesicular pumice. lite unit intruded by an ~ 2-m-high outcrop of coarsely vesicular pumice.
40 FINE PUMICE
OBSIDIAN
TALUS Figure 10. (top) Southern flow front of northernmost Glass Creek (lino) obsidian dome, eastern California, (bot- tom) Tape and compass map of portion of flow front of Glass Creek dome. 10 m
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pumice occur at the base of rifts up to 10 m flow structure. Near the top of the cliff face, to be continuous, with the transition occur- deep in the rhyolite. the foliation becomes predominantly verti- ring when tensile stresses associated with cal, which may correspond to the finely the rise of magma fall below the effective Implications of Rhyolite Stratigraphy vesicular surface of Little Glass Mountain. tensile strength of the vesicular magma. Not much structure is preserved on the sur- Subsequent lava is volatile-depleted and The stratigraphic arrangement of Little face of this flow. Other Holocene flows, more glassy; postemplacement vesiculation Glass Mountain can be observed in other such as Sugarloaf Mountain in the Coso forms the carapace of finely vesicular pum- Holocene rhyolitic obsidian flows, includ- Volcanic field (100,000 yr old; Bacon and ice. The thin interlayered contact zone ing Big Glass Mountain and the Crater others, 1980), have very few exposed cross between coarsely vesicular pumice and Glass Flows in the Medicine Lake High- sections or surface outcrops. Nonetheless, obsidian implies a discrete zonation of the land, Big Obsidian Flow in Newberry Vol- scattered road cuts allow distinction of parent magma body. Furthermore, the cano, Oregon, and the Roches Rossa Flow three textural types that may correspond to presence of groups of domes and flows of on the island of Lipari, Italy. Each of these obsidian and coarse and fine pumice. Older similar age and textural makeup in the flows has well-preserved surfaces where tex- flows, such as the Tertiary Andy's Dome Medicine Lake Highland (Heiken, 1978), tural relations can be discerned. On Roches (Moyer, 1982) in northwestern Arizona, near South Sister Volcano (Williams, 1944), Rossa, outcrops are covered by an orange- also contain good cross sections with strati- in Long Valley Caldera (Bailey and others, colored lichen that obscures the original graphic information still discernible (Fig. 1976), Inyo Craters (Wood, 1977), the Coso textures, but when fresh surfaces are ex- 11). This 50-m-thick dome and flow com- Volcanic field (Bacon and others, 1980, posed, the same relationships as those on plex is cut through by Burro Creek, which 1981), Newberry Volcano (MacLeod and Little Glass Mountain are observed. reveals an obsidian core overlying 2 to 3 m others, 1981), and elsewhere suggests that In order to see obsidian-flow cross sec- of basal breccia but lacking a coarsely each group of extrusions taps a larger body tions, one must examine flows old enough vesicular pumice zone. The upper few of magma that is capable of regenerating its to have been dissected, yet young enough to metres of the flow have upturned foliations stratification by continued exsolution of have their glass preserved. The 100,000-yr- and lighter color than the horizontally volatiles after each eruption. banded central and basal portions. old Banco Bonito Flow in the Valles Cal- The stratigraphy of Little Glass Moun- dera, New Mexico, has been deeply The close association between these tain is not typical of all rhyolite flows but is dissected along one margin by San Antonio small-volume extrusions and tephra erup- most common among relatively dry flows of Creek. Here one may locally observe I to tions may be a genetic one. Eichelberger low volume. Instead of the asymmetric ver- 2 m of basal breccia overlain by a similar and Westrich (1981) have suggested that the tical zonation seen in Little Glass Moun- thickness of coarse pumice and as much as water content of obsidian fragments in tain, most larger flows show concentric 10 m of obsidian. Most of the flow is hori- tephra deposits reflects the volatile content layers overlying and underlying a crystalline zontally banded, although large pods of of the magma body. In the present model, core: glassy, spherulitic, pumiceous, and glassy obsidian show prominent vertical eruptions of tephra and lava are considered brecciated zones (Christiansen and Lipman,
Figure 11. Cross section of rhyolitic Tertiary Andy's Dome, exposed by Burro Creek near Kaiser Spring area of northwestern Arizona. Light-colored tuff cone underlies darker obsidian flow. Wedge of basal breccia visible on left side of photo. Foliations in feeder dike (center) cut.through tuff cone and are continuous with those in the overlying lava flow. Foliations in flow are primarily horizontal except for upper 3 to 5 m where they are vertical. Cross section shown is ~ 1.5 km long and 50 to 75 m high.
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1966). There are at least two possible expla- (Shaw, 1963; Friedman and others, 1963; (a,), that is, r 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 370 J. H. FINK 1.0 2.0 3.0 250 500 750 io7 10® 1011 1013 SURFACE BRECCIA FINE PUMICE OBSIDIAN ^ COARSE PUMICE \— BASAL BRECCIA v- - TEPHRA DENSITY TEMPERATURE VISCOSITY (g/cm3) <°C) (g/cm - sec) Figure 13. Rhyolitic obsidian flow profiles, (a) Stratigraphy for a typical 35-m-thick rhyolitic obsidian flow, (b) Density profile, based on measured densities of samples of coarse pumice, obsidian, and fine pumice. Note density inversion at contact between obsidian and coarse pumice, (c) Temperature profile, assuming constant internal temperature. Note steep surface-temperature gradient, (d) Viscosity profile, based on (b), (c), and laboratory viscosity measurements (Friedman and others, 1963). Note rapid decrease of viscosity with depth near the upper surface. should lead to a slight downward increase lar pumice account for a large fraction of pumice outcrops are surrounded by a band of density through the basal breccia and the distal flow surface of Little Glass Moun- or zone of obsidian a few metres wide, tephra. tain and may occur singly or in groups of as which grades outward into finely vesicular The density inversion between the obsid- many as 30 adjacent domes and anticlines. pumice. For example, Figure 14b shows a ian and coarse pumice layers provides the Individual domes range from about 3 to coarse pumice dome bisected by a fracture. basis for a gravitational instability. Hence, 10 m in diameter and up to several metres in Moving outward from the fracture plane, we may expect coarsely vesicular pumice to height. On some flow lobes, these outcrops one encounters an obsidian zone separating rise buoyantly through the overlying obsid- exhibit a regular spacing which has been coarse pumice from fine pumice. In some ian and fine pumice. If the rate of ascent is related to the gravity instability inherent to locations, coarse pumice domes rise up high enough relative to the forward velocity the flow stratigraphy (Fink, 1980a). through the overlying fine pumice crust and of the lava and the cooling rate, then these The two most useful criteria for identify- lift blocks of the surface unit that remain as diapirs could reach the flow surface. Fink ing pumice diapirs are their foliation pat- perched erratics (Figs. 9 and 14a). Where (1980a) has described surface outcrops of terns and stratigraphic setting. Foliations several adjacent outcrops of coarse pumice coarsely vesicular pumice that were inter- generally define the domal or anticlinal are exposed, the bordering obsidian may preted to be such diapirs. These domes and shape of the outcrops (see Fig. 14a), except enclose the entire area (see maps, Figs. 20 plunging anticlines cored by coarsely vesicu- where obscured by fractures. Most coarse and 21a). The pervasive nature of this con- Figure 14. Coarsely vesicular pumice diapirs. (a) Two coarse pumice diapirs on Northwest Lobe, Little Glass Mountain. Note anticlinal foliation patterns and fine pumice in synclinal trough between diapirs. Flow direction was from right to left, (b) Single fractured coarse pumice diapir near front of Northeast Lobe. Opposing halves of diapir spread apart after fracturing. Man in fracture for scale. Coarse pumice is separated from fine pumice by 2- to 5-m-wide zone of obsidian. 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These stresses will thus crops out near the intersection of the flow the flow surface should lead to inward cause the orientation of some fractures to be front and the upper flow surface, the struc- propagation of cracks, whenever the ther- perpendicular to flow fronts, although their ture is better revealed. Figures 15a and 15b mal stresses exceed the tensile strength of initiation may be caused by thermal con- show a plunging anticline cored by coarsely the lava. The tensile strength of pumice is traction. vesicular pumice at the distal margin of the not reported in the literature, but using In addition to cooling and spreading of Northwest Lobe of Little Glass Mountain. values for trachyte (Jaeger and Cook, 1976), the flow, a third source of fracture is the rise Approximately 2 m of obsidian drapes over we can place an upper bound at 30 to 400 of coarsely vesicular pumice diapirs to the the exposed coarse pumice core of the fold. bars. Thermal stresses at the surface of the flow surface. As a diapir of coarse pumice The orientations of large stretched vesicles flow may be expressed as: approaches the surface, it creates tensile are consistent with movement of this coarse stresses in the overlying crust. The rising pumice mass outward and upward from the a„ = (a E A0) I (1 -n) diapir becomes split by a downward propa- flow interior. The subhorizontal layer of gating fracture with the halves migrating obsidian in the upper limb of the fold is where a is the linear coefficient of thermal laterally (Fig. 17). When two or more paral- overlain by finely vesicular pumice, which expansion, E is Young's modulus, A6 is the lel coarse pumice diapirs are spreading in forms a small syncline whose axis parallels temperature drop, and n is the Poisson's this manner, lava in the zone between the the crest of the flow front. Moving up onto ratio (Timoshenko and Goodier, 1963). two fractures may become compressed and the flow surface, one encounters another The elastic properties of vesicular rhyolite uplifted. Figure 18a shows a 4-m-high out- band of obsidian followed by a large anti- are also unreported in the literature, but crop of fine pumice crust sandwiched clinal region of coarse pumice (Fig. 16). using a range of estimates for obsidian, between two coarse pumice diapirs. Figure Temperature Profile and the Inward welded and unwelded tuffs, and nonvesicu- 18b shows foliations, and Figure 18c is an Growth of Fractures. Temperature values lar rhyolite (Clark, 1966), we can approxi- interpretation of the structure. The dis- within an obsidian flow can be estimated mate the tensile strength to be: tances separating the opposing walls of a only approximately. The surface layer of fracture increase from zero near the base to as much as 10 m at the coarse pumice- finely vesicular pumice insulates the interior axx = (0.01 to 10.0) (A0 bars/ °C). so that the obsidian core should maintain a obsidian contacts. Fractures may cut uniform temperature close to that of its Taking minimum values for the elastic through a series of coarse pumice anticlines, eruption temperature over most of the parameters, we find that a temperature drop forming a continuous valley several metres length of a flow. If we assume that the of a few hundred degrees is sufficient to deep and tens of metres long. These valleys ambient temperature is about 25 °C and the cause the surface to fracture. Such cracks commonly trend perpendicular to the flow initial ground temperature is about 100 °C propagate inward until they reach a level front (Fig. 19), with their opposing surfaces (due to heating by the overlying lava), then where the stress drop due to cooling is less showing foliation patterns that are mirror we might expect a temperature profile like than the tensile strength of the lava. images of one another. These walls are smooth, rounded, and outwardly convex. that shown in Figure 13c. The main features On Little Glass Mountain, fractures pene- of this model are the steep temperature gra- trate to a maximum depth of about 8 m, Viscosity Profile and Surface Folding. Figure 15. Overturned anticline of coarse pumice exposed in front of Northwest Lobe, Little Glass Mountain, (a) Obsidian outcrop wraps around coarse pumice core of fold, which dips about 30° into flow, (b) View to south of same fold. Large vesicles stretched by outward flow of coarse pumice. 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 372 J. H. FINK ness of the viscosity profile and hence to the thickness of the stiff, nearly congealed crust Figure 16. Schematic profile of the flow. The steeper the gradient, the of flow at location of Figure thicker the crust and the wider the spacing 15a, viewed to north. T = talus, of ridges (Fink and Fletcher, 1978). o = obsidian, c = coarse pu- Surface folds can be recognized by both mice, f= fine pumice. Thickness detailed foliation patterns (flow banding) of obsidian outcrops slightly measured in the field and by the continuity exaggerated. of topographic ridges (Fig. 20). Based solely I 1 on the stratigraphy, surface folds should be 20 METERS found exclusively in the finely vesicular The viscosity profile (Fig. 13d) reflects both As the flow advances, it will be sub- pumice crust. Although such folds do occur, the density and temperature profiles de- jected to compression and extension due the original subvertical foliation patterns in scribed above. Here again we estimate only to changes in topography and flow-lobe the fine pumice make them difficult to iden- the approximate shape of the curve. The configurations. The rapid decrease in vis- tify. More commonly these folds stretch viscosity of the obsidian core will depend cosity associated with the steep surface- across the flow surface as parallel ridges upon the temperature and water content. temperature gradient may lead to a surface- that cut contacts between the different tex- Viscosity of the coarse pumice will be modi- folding instability at sites of compression tural units, as seen in aerial photographs fied by variations in both temperature and (Fink and Fletcher, 1978; Fink, 1980b), (Fig. 2) and on maps (Figs. 20 and 21). The porosity. Decreasing temperatures and resulting in surface folds or ridges oriented synclinal portions of these folds are rarely lower density toward the base of the flow transverse to the flow. Regular spacings of defined by foliations; more commonly they tend to increase viscosity. these ridges may be correlated to the steep- are identified as continuous bands of finely vesicular pumice lying in troughs parallel to and in between the more prominent anticli- (ine pumice nal ridges (Figs. 14a and 20). The spacing of obsidian surface folds ranges up to several tens of EXTENSION • EXTENSION metres on rhyolitic obsidian flows. coarse pumice Combinations of Diapirism, Folding, and Fracturing Most deformation of obsidian-flow sur- faces occurs near the flow fronts where low- density pumice is concentrated and both tensile and compressive stresses are highest. Here we consider maps of two distal por- tions of Little Glass Mountain that have had relatively little deformation, so that much of the flow surface is composed of light-colored, finely vesicular pumice (Fig. 20). Discontinuous outcrops of darker coarse pumice and obsidian become more extensive adjacent to the flow front. Some of these coarse pumice outcrops are split by 1- to 2-m-deep fractures with the halves hav- ing spread laterally by as much as 20 m. These fractured areas are topographically lower than the surrounding fine pumice. Surface folds may be delineated by anticli- nal and synclinal foliation patterns within the coarse pumice and obsidian, and as topographic ridges and troughs within the fine pumice. Fold wavelengths are between 8 and 15 m, and amplitudes range from 1 m within the center of the coarse pumice out- coarse pumice obsidian coarse pumice Figure 17. Schematic diagra"j. m showing ~ fine pumice obsidian - obsidian simultaneous rise of coarse pumice diapirs fine pumice coarse pumice and inward propagation of fracture. Frac- obsidian - ture axis corresponds to former anticlinal coarse pumice axis and lies parallel to flow direction. 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 373 crops to 3 m near the flow front. Continu- ous ridges are not well developed. This portion of the flow was apparently undergo- ing north-south extension and east-west, flow-parallel compression as the lava ad- vanced slowly across a gentle slope. The predominance of ridges over fractures in this area suggests that expansion of the flow was restricted by topography or by a decrease in lava supply. The part of the Northwest Lobe mapped in Figures 21 and 22 has undergone more extensive diapirism and extension than that in Figure 20. The shapes of coarse pumice outcrops vary from subcircular near the vent to elongate near the flow front, and spacings between outcrops show a corre- sponding decrease from about 70 m to 20 m near the flow fropt. The area mapped in Figure 21a consists of several coalesced coarse pumice domes that have been folded and fractured. Spreading across fractures was extensive. As shown in cross section N-S (Fig. 21b), separation of opposing fracture surfaces is as much as 50 m on indi- vidual fractures and more than 100 m total. Obsidian that was originally positioned over the coarse pumice was broken and pushed apart by the rising diapirs. Between parallel fractures, this obsidian became compressed into tight synclines (Fig. 21b; compare with Figs. 17 and 18). On the basis of underlying topography, it appears that the flow direction was mainly to the west, although a small volume of lava moved northward at a late stage of advance. This final movement might have been asso- ciated with spreading and lowering of the flow surface by 1 to 3 m. The extensive exposure of coarse pumice in this area illus- trates a type of stratigraphic inversion resulting from gravity instability that is common in the distal portions of these flows (compare with Fig. 17). The area mapped in Figure 22 lies imme- diately south of that mapped in Figure 21. Here a single large outcrop of coarse pumice forms a 10-m-high, 150-m-long ridge whose structure consists of two anticlines and a syncline all trending parallel to the flow front. These folds are cut nearly at right angles by a series of fractures with separa- EUf tions that widen toward the flow front. This area has apparently undergone less exten- sion than the area to the north, and so the width of separation along the individual fractures is not more than about 10 m. Foli- ations in the entire coarse pumice outcrop dip under surrounding adjacent outcrops of Figure 18. This 3-m-high outcrop shows a fine pumice crust which has been sandwiched fine pumice, indicating a broad anticlinal between two parallel, fractured, coarse pumice diapirs. (a) Photograph; (b) foliation structure, but most of the contacts are ob- attitudes; (c) schematic representation. Arrows indicate directions of spreading and sites of scured by blocks that have fallen off the compression. ridge. 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 374 J. H. FINK Figure 19. 3-m-deep fracture cutting through a series of coarse pumice outcrops on the Crater Class Flow. Fracture axis is parallel to flow direction. Vent Area of the Northeast Lobe Constructs such as Little Glass Mountain commonly consist of two parts: outlying flows with gently sloping upper surfaces and steeper-sided domes over the vent area. The rn Talus vent area of the Northeast Lobe (Fig. 23) is I I Coarsely vesicular pumice a broad dome elongated along a N30°E axis, made up almost entirely of smooth- I • | Obsidian sided blocks of microcrystalline rhyolite. I. | Finely vesicular pumice The dome is cut by deep fractures that have a radial pattern near the center of the dome. The deepest fractures in the summit area trend about N60°W. Foliations are gener- ally steeply dipping and are truncated nearly everywhere by the smooth surfaces of the rhyolite outcrops. Shallow dips are found only near the bases of some of the fractures, and these gradually steepen up- ward, becoming vertical at the top of the fracture (Fig. 24). Strikes of foliations in the vicinity of the deepest fractures in the sum- mit trend approximately parallel to them (N60°W). whereas foliations farther from the summit are oriented parallel to the elon- gated axis of the dome (N30°E). Stereoplots of poles to foliations (Fig. 25) clearly dem- onstrate these relations. Extending down the east side of the dome, there is a series of regularly spaced ridges as much as 50 m wide, 1 to 3 m high, with a spacing of 3 to 5 m, which trend perpendicular to the downslope direction (Fig. 26). Foliations strike parallel to the crests of these ridges, dip steeply, and are Figure 20. Map of portion of Northeast Lobe. In this and following maps, structures were determined from detailed measurement of foliation orientations and from topo- graphy. Folds are indicated by standard symbols. Axes of fractures are shown by solid lines with single or double cross bars, depending upon amount of lateral spread- ing associated with fracture growth. Single bar = less than 5 m spreading; double bar = greater than 5 m. Farthest extent of fracture surfaces marked by pairs of dotted lines. Contacts dashed where obscured by fallen blocks. 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 375 Figure 21. (a) Map of part of Northwest Lobe. Location of measured profile indicated by "N-S." (b) Measured profile across part of Northwest Lobe. Obsidian (ob) outcrops originally overlie and are subsequently pushed apart by coarse pumice (cp) diapirs. No vertical exaggeration. truncated by the smooth surface of the flow. foliations indicate that the smooth surfaces preserve the vertical dip, and the viscous Three aspects of the foliation pattern are fractures. The observed pattern could material beneath could flow laterally, com- require explanation: the steep dips, trunca- result if fracturing accompanied magma pressing the surface into folds like those in tion by smooth surfaces, and the change of upwelling (Fig. 27). The two opposing sur- Figure 26. strike direction away from the central vent faces of the fracture could then rotate away The strike directions of foliations can area. As flow banding generally forms from the crack axial plane until most of the be explained by a two-stage emplacement. parallel to a fixed surface like a conduit wall exposed foliations had near-vertical dips. The lava initially came up through dikes or underlying topography, the truncated Solidification of the fracture surface would oriented N30°E, parallel to prominent 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 376 J. H. FINK tion (N60°W). The change in prevailing tend to devitrify, accompanied in some foliation direction could reflect changes in cases by the growth of spherulitic textures. the state of stress of the volcano during Vesicles in pumice units can be filled by the course of a single eruption or during a secondary minerals, but the glassy septa hiatus between two successive extrusive also undergo devitrification. However, some events. Alternatively, this change could vestiges of textural differences may still be indicate modifications in the vent geometry discernible on the bases of color, specific caused by explosive events or obstruction gravity, and degree of crystallization. Folia- by congealed lava. tions are commonly preserved in older Preliminary examination of other domes flows, and these can aid greatly in the inter- near Big Glass Mountain in the Medicine pretation of flow deformation. Although Lake Highland and near South Sister Vol- the detailed foliation patterns on a flow can cano in Oregon shows similar concentra- be exceedingly complex due to the interac- tions of foliation orientations. At both of tion of diapirism, folding, and fracture, these locations, more than a dozen domes mapping the gross distribution of these are arranged along linear trends. The pre- structural markers can give a general indica- ferred orientation of foliations in the domes tion of the flow direction and, in some lies parallel to those directions that corre- cases, the vent location. This recognition is spond to either a local or regional structural facilitated by comparison with fresh flows trend. Furthermore, relatively low lying where the outcrops are nearly continuous smooth areas in the centers of the domes are and the original textures can still be also elongated parallel to the distribution of identified. the domes. These preliminary observations Rhyolite flows and domes can signify suggest that the structure of some individual either the beginning or the end of a particu- domes may reflect regional structural lar cycle of volcanic activity. They are trends and/or the shape of the magmatic commonly found interlayered with mud- conduit system. On the basis of surface flow and pyroclastic-flow deposits in structure, most of these domes appear to caldera-filling sequences, as in the Supersti- have been extruded through elongated vents tion Mountains of Arizona (Sheridan, rather than from pipe-like point sources of 1978). In other locations, such as the Coso magma. Volcanic field (Bacon and others, 1980), Mapping foliations exposed on the sur- domes are thought to be indicators of incip- face and within fractures in fresh domes ient explosive volcanism. In either case, it is such as those in the crater of Mount useful to identify the proportions of pum- St. Helens might help delineate the geom- iceous to nonvesicular lava in different etry of their plumbing systems. The distri- domes within a field, in order to determine bution of earthquake epicenters in the vicin- how the relative volatile contents of extru- ity of Mount St. Helens in the three months sions increase or decrease in time and space. following the May 18, 1980, eruption Furthermore, the recognition of vent loca- 0 10 20 30 40 50m (Weaver and others, 1981) showed a strong tions and alignments can help specify the 1 i I i i i preference for the same north-south trend tectonic stress fields in effect during the on which the domes align. Recognition of eruptive activity. Finally, mineralization of Obsidian changes in orientation of foliations and rhyolitic terranes is commonly associated fractures in lava domes with time could with the migration of magmatic volatiles. | | Coarsely vesicular pumice indicate the directions in which future Understanding the distribution of the var- extrusions were likely to occur or where ious textures in young flows and their rela- Figure 22. Map of ridge immediately near-surface magma bodies might reside. tion to volatile partitioning can assist in the south of area shown in Figure 21. Flow evaluation of ore deposition in older rocks. moved from right to left. Interpretation of Older Flows SUMMARY ground cracks and a series of vents that The surface structural information de- supplied Little Glass Mountain and the rived from Little Glass Mountain was Little Glass Mountain is a 300- to 1,100- Crater Glass Flows several kilometres to the obtained under conditions of optimum yr-old, chemically homogeneous, rhyolitic north. As this lava emerged, its foliation exposure. Even so, it was first necessary to obsidian dome and flow complex that has was impressed with the strike direction of recognize the initial configuration of the been subjected to negligible postemplace- the vent, and this orientation was preserved flow and then identify various deforma- ment alteration. The distribution of textural as the lava moved away laterally. At a later tional processes that disrupted the original rock types and the foliation patterns ob- stage, new fractures opened perpendicular stratigraphy. On older flows, both of these served on the flow surface reflect both the to the first, and the last lava to erupt devel- steps are made difficult by weathering initial stratigraphy of the flow and its sub- oped foliation striking parallel to this direc- effects. Over millions of years, glassy lavas sequent deformation during emplacement. 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 Figure 23. Map of vent area, Northeast Lobe, showing a sampling of foliation attitudes. Topographic break separates dome where rock type is primarily microcrystalline rhyolite, from outlying flow where the surface is composed mostly of fine pumice. Dashed line separates inner region where folia- tions are influenced by N60°W fracture, from outer, earlier- emplaced region where orientations are controlled by N30° E fractures (see Fig. 25). 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 Figure 24. (A) View along N60°W- trending fracture in vent area of Northeast Lobe. Dips of foliations gradually decrease from near vertical at tops of outcrops to horizontal near the base. View to the northwest. Largest blocks are about 5 m high. (B) Tracing of foliations from photo above (A). Figure 25. Stereoplots of poles to folia- B tions from large outcrops in vent area of Northeast Lobe. One measurement taken near top of each outcrop, (a) Plot for inner zone surrounding prominent N60°W frac- tures. (b) Plot for outer zone, showing preferred orientation around N30°E. Figure 26. Folded smooth-frac- ture surface near vent area of Northeast Lobe. Fold spacing about 3 m, amplitude about 1.5 m. Flow direction from right to left. View to south. 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 379 areas of the dome became covered by these fracture surfaces. Subvertical foliations in CD these summit domes partly reflect the geometry of the fractures, which in turn may correspond to the orientations of the feeding vents. Mapping these foliations in older flows may help in the interpretation of the eruptive history of a particular extru- sion and in the evaluation of likely sites of future activity and residual hot magma. ACKNOWLEDGMENTS Much of this work was completed as part of the author's Ph.D. thesis at Stanford University and was supported by a National Science Foundation graduate fellowship and grants from the Geological Society of America. S. Wood, D. W. Peterson, and R. L. Coats provided helpful comments on the manuscript. Spencer Wood's review was Figure 27. 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REVISED MANUSCRIPT RECEIVED MAY 11, 1982 Medicine Lake Highland, California: Evi- Sibree, J. O., 1933, The viscosity of froth: MANUSCRIPT ACCEPTED MAY 17, 1982 Primed in U.S.A. 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