Lewis, S.R., Yntema, E.R., and Brunstad, K.A

Lewis, S.R., Yntema, E.R., and Brunstad, K.A

Crystallization History of the 1 Ma Tieton Andesite Lava Flow, Washington, USA Lewis, S.R., Yntema, E.R., and Brunstad, K.A. Department of Earth & Atmospheric Sciences State University of New York - Oneonta Over 1 Ma in the Cascade Mountains, Goat Rocks Volcano produced on of the largest andesitic lava flows reaching 80 km in length. The flow had a minimum volume of 2 km3 and a thickness of up to 60 m. The flow flows through a valley pooling in the Naches Heights valley near Yakima, WA. Pine Grass Ridge Naches Heights SiO2 60.65 62.47 Ridge In this study we measure the crystal content, distribution of different sized crystals (crystal-size distribution, CSD), Plagioclase 80 87 and chemical composition of the Tieton andesite lava flow to determine the affects of crystal growth and average Pyroxene 15.6 8 chemistry have on the viscosity and heat budget of the lava flow. The goal of this study was to determine how fast Figure 3: Average values of Silica from XRF, Plagioclase, and Pyroxene phenocryst content minus groundmass based on hand sample observations for Pinegrass Ridge and Naches Heights groupings, outliers were excluded from average value data crystals grow before and during the lava flow movement. CSD analysis were used to determine how fast the initial 1A.26vi15 1B.26vi15 formation and growth of minerals occur. The geochemical data was used to calculate the viscosities of the crystal- Pinegrass melt. The subject of crystal-size distribution is still new and little data is available to compare the results with other 2.03vii15 findings. We hypothesis that the production of heat during crystal growth may counteract the ability for the lava Initial results show the highest silica content (61-63% SiO ) in the lava flow is down valley near flow to cool resulting in a lava flow that traveled further than all other reported cases of andesitic lavas flows, <10 2 Yakima and the lowest silica content (59-61% SiO ) is up valley closest to the source area. Two km. 2 basaltic andesite samples with silica content of ~55% SiO2 found on Pinegrass Ridge showing part of the flow boundary. Phenocrysts of plagioclase, pyroxene, hypersthene and magnetite comprise on Pinegrass Ridge Outlier average 25 – 35% and reside in a glassy groundmass. Initial conclusions about the Tieton andesite flow include: (1) the length of the flow appears to have been controlled by heat loss, effusion rates, and topography, (2) rock chemistry distribution points to a two chambered system with a mafic injection eruption, and (3) multiple smaller volume lava flows erupted during the waning stage of Naches Heights Naches 1A.03vii15 1C.03vii15 the eruption. Rimrock Lake Naches Heights area 1B.03vii15 Stratigraphic sections were sampled through the lava flow, bottom to top, from near the source on Pinegrass Ridge to its terminus near Yakima. The samples were analyzed in hand sample and scanned thick section, and chemically using X-Ray Fluorescence and Inductively Coupled Plasma Pinegrass Ridge area Mass Spectrometry. Source Vent Goat Rocks Wilderness Figure 1: Map of the Tieton Andesite flow indicating major chemical groupings of the Pinegrass Ridge and the Naches Heights area Waning Stage Waning 2A.25vi15 2B.25vi15 2C.25vi15 Crystal growth within melts have an affect on a lava flow's viscosity during solidification. Heat is released by the formation of crystals influencing the cooling rate of the lava flow, or heat budget. These characteristic changes Figure 4: The crystal sizes can be seen for the two main flows, Pinegrass Ridge and Naches Heights, and two of the waning flows found near the source vent. strongly control how the lava flow behaves, like viscosity, and what its final shape and dimensions will be. Marsh (1981) concluded that the crystal contents of a lava that are above 50-60% will restrict flow by lowering the viscosity. Cheng (1984) and Metzner (1985) in separate studies showed that a large suspension of crystals within the Figure 6: Field photos of the Tieton Andesite from the Pine Grass Ridge Section and Naches Heights melt have a viscosity dependent on the magma composition, eruption temperature, gases, and physical condition of the flow. Past lava flow models have been developed and used on the main assumption that the length of the flow is controlled by cooling (Pinkerton & Sparks, 1978; Guest et al., 1987; Crisp & Baloga, 1990), and the amount of lava CONCLUSIONS (Walker, 1973; Harris & Rowland, 2009). However, few attempts have been made to document the changes in crystal growth, the connected effects of the temperature at which crystals form on the interior of a lava flow, and Hypothesis A: Goat Rocks contains a single stratified magma chamber. Naches Heights (NH) flow how the magma's viscosity changes. erupted first and flowed down the Tieton river valley toward Yakima. The river then began to down cut through the NH flow. Pinegrass Ridge (PGR) erupted second flowing through the river Figure 5A Figure 5B Figure 5C down cut of NH flow creating inverse topography. The inversion of topography indicated by the outlying data point 15037. Sample 15037 matches the composition of the Pine grass Ridge flow but was taken from the Oak Creek area within the Naches Heights region. The NH flow is not seen Pinegrass Flow up valley on PGR due to the PGR lava flow covering the Naches flow. Inverted topography is evidence for multiple eruptions. It is also believed that the NH flow erupted first because of the higher silica content. The higher silica content down valley is also evidence for a stratified magma Waning Phase chamber where higher silica content is typically found near the top of the magma chamber. Hypothesis B: Goat Rocks volcano contained multiple magma chambers with one more mafic Figure 5D Figure 5E Figure 5F than the other. The mafic magma was injected into the less mafic melt driving the eruption. Figure 5A shows Pinegrass Ridge has a slightly higher aluminum than Naches Height KEY consistent with the higher calcium in Pinegrass Ridge in Figure 5B and the higher Increased iron and magnesium concentrations in the Pinegrass Ridge section and increased potassium in Naches Heights in Figure 5C. Figure 5C shows higher magnesium within - Columbia River Basalt the Pinegrass Ridge flow indicates it is more similar to the source melt. Figure 5D shows pyroxene in hand samples indicated that the magma was more mafic and therefore drove the - Pine Grass Ridge a generally constant rate of potassium. Figure 5E shows a higher concentration of - Clear Fork titanium in Pinegrass Ridge which is consistent with the higher amount of magnetite, eruption. Mafic magma injection into a more felsic andesite as evident by the xenoliths present - Pine Grass Ridge and pyroxene crystals found in hand samples. Figure 5F gives support for a more mafic - Naches Heights magma injection evident from the higher amount of iron and magnesium that may be within the hand samples. The xenoliths may be the result of crystals being scraped off the walls of - Pine Grass Ridge driving the eruption.. the magma chamber, or brought from the more mafic magma chamber. Evidence for this hypothesis is currently being processed with an Electron Microscope. 3 - 1 References Cashman, K.V. (1988) Crystallization of Mount St. Helens 1980-1996 dacite: a quentitative approach, Contrib. Mineral. Petrol., v. 50, 194-209. Cashman, K.V. & Marsh B.D. (1988) Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization, II makaopuhi lava lake, Contrib. Mineral. petrol. v. 99, 292-305. Cheng, D.C.-H (1984) Further observations on the rheological behavior of dense suspensions, Powder Technol., v. 37, 255-273. Crisp, J. & Baloga, S. (1990) A model for lava flows with two thermal components, J Geophys. Res. v. 95, 1255-1270. Guest, J.E. et al. (1987) The evolution of lava flow-fields: Observations of the 1981 and 1983 eruptions of Mount Etna, Sicily, Bull. Volcanology, v. 49, 527-540. Harris, A. & Rowland, S. (2009) Effusion rate controls on lava flow length and the role of heat loss: a review, in: Thordarson, T. et al. (eds) Studies in Volcanology: The Legacy of George Figure 2: Effusion rate controls on lava flow length and the role of heat loss, Harris and Rowland (2009) shows projected effusion rate based on flow length, the TA plots with an effusion rate of Walker. Spec. Publ. of IAVCEI, v. 2, 33-51. 3 -1 3 -1 Marsh, B.D. (1981) on the Crystallinity, probability of occurrence, and rheology of lava and magma, Contrib. Mineral. Petrol., v. 78, 85-98. ~200 m s for the entire flow which pools near Yakima. The Pinegrass lava flow ends near the Naches Heights flow as an outlier, ~44 km in length and an effusion rate ~50 m s . The waning Marsh, B.D. (1988) Crystal size distribution (CSD) in rocks and kinetics and dynamics of crystallization I. Theory, Contrib. Mineral. Petrol., v. 99, 277-291. stages of the flow are ~7 km in length with an effusion rate of ~0.3 to 40 m3s-1. Other andesite's have been recorded to have effusion rates of less than 100 m3s-1 and a maximum length of 10 km. Metzner,A.B. (1985) Rheology of suspensions in polymeric liquids, J Rheol., v. 29,739-775. Sparks, R.S.J. & Pinkerton, H. (1978) Effects of degassing on rheology of basaltic lava, Nature, v. 276, 385-386. Swanson, D.A. (1978) GeologicMap of the Tieton River area, Yakima County, south-central Washington, scale 1:48000. USGS-MF-968. Walker, G.P.L. (1973) Lengths of lava flows. Phil. Trans. R. Soc. Lond. A., v. 274, 107-118.

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