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. In this study we measure the crystal content, distribution of different sized crystals (crystal-size distribution, CSD), and chemical composition of the Tieton andesite lava flow to determine the affects of crystal growth and average chemistry have on the viscosity and heat budget of the lava flow. The goal of this study was to determine how fast crystals grow before and during the lava flow movement. CSD analysis were used to determine how fast the initial formation and growth of minerals occur. The geochemical data was used to calculate the viscosities of the crystal-melt. The subject of crystal-size distribution is still new and little data is available to compare the results with other findings. We hypothesis that the production of heat during crystal growth may counteract the ability for the lava flow to cool resulting in a lava flow that traveled further than all other reported cases of andesitic lavas flows, <10 km.

Pine Grass Ridge Naches Heights SiO2 60.65 62.47 Plagioclase 80 87 Pyroxene 15.6 8

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

Figure 6: Field photos of the Tieton Andesite from the Pine Grass Ridge Section and Naches Heights

CONCLUSIONS Hypothesis A: Goat Rocks contains a single stratified magma chamber. Naches Heights (NH) flow 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 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 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 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 than the other. The mafic magma was injected into the less mafic melt driving the eruption. Increased iron and magnesium concentrations in the Pinegrass Ridge section and increased pyroxene in hand samples indicated that the magma was more mafic and therefore drove the eruption. Mafic magma injection into a more felsic andesite as evident by the xenoliths present within the hand samples. The xenoliths may be the result of crystals being scraped off the walls of the magma chamber, or brought from the more mafic magma chamber. Evidence for this hypothesis is currently being processed with an Electron Microscope.

Figure 5A Figure 5B Figure 5C

Figure 5D Figure 5E Figure 5F

Figure 5A shows Pinegrass Ridge has a slightly higher aluminum than Naches Height

consistent with the higher calcium in Pinegrass Ridge in Figure 5B and the higher

potassium in Naches Heights in Figure 5C. Figure 5C shows higher magnesium within

the Pinegrass Ridge flow indicates it is more similar to the source melt. Figure 5D shows

a generally constant rate of potassium. Figure 5E shows a higher concentration of

titanium in Pinegrass Ridge which is consistent with the higher amount of magnetite,

and pyroxene crystals found in hand samples. Figure 5F gives support for a more mafic

magma injection evident from the higher amount of iron and magnesium that may be

driving the eruption..

KEY

- Columbia River Basalt

- Pine Grass Ridge

- Clear Fork

- Pine Grass Ridge

- Naches Heights

- Pine Grass Ridge

3 - 1

Pinegrass Flow

Waning Phase

Source Vent

Goat Rocks Wilderness

Pinegrass Ridge area

Naches Heights area

Pinegrass Ridge Outlier

Rimrock Lake

Figure 1: Map of the Tieton Andesite flow indicating major chemical groupings of the Pinegrass Ridge and the Naches Heights area

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 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 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 (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 how the magma's viscosity changes.

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 Mass Spectrometry.

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

~200 m3s-1 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 m3s-1 . The waning

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.

Initial results show the highest silica content (61-63% SiO2) in the lava flow is down valley near Yakima and the lowest silica content (59-61% SiO2) is up valley closest to the source area. Two 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 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 the eruption.

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 Walker. Spec. Publ. of IAVCEI, v. 2, 33-51. Marsh, B.D. (1981) on the Crystallinity, probability of occurrence, and rheology of lava and magma, Contrib. Mineral. Petrol., v. 78, 85-98. 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. 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. Warren, W.C. (1941) Relation of the Yakima basalts to the Keechelas Andesite Series. Journal of Geology, v. 49, 795-814.

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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.