Newberry Volcano: The Big Obsidian Flow

1. Newberry Volcano: The Big Obsidian Flow Flow Ridges INSERT YOUR ORGANIZATION’S LOGO HERE Surface Crater The Big Obsidian Flow Lidar, USGS Eruption Vent Ricci Keller, Earth Science Major ABSTRACT THE BIG OBSIDIAN FLOW FOLDING AND GAS CAVITIES INTRODUCTION Newberry volcano is bimodal. Outer flanks of the volcano erupt basalt-type magmas, while obsidian and pumice associated with rhyolitic eruptions, dominate caldera events. The Big Obsidian Flow is the youngest found in Oregon and emplaced 0.13km³ of material. According to Castro, this effusive eruption first spewed out .32km³ of tephra, followed by pyroclastic flows and finally deposited an obsidian layer 30m thick and 1.8km long (Figure 1 & 2). Flow ridges concave to the vent show flow direction and indicate high viscosity magma. The behavior and shape of gas bubbles in magma plays an important role on structure of emplacement and influences features like surface folds caused by buckling. Explosion craters are commonly associated with voids that fill with gas (Figures 6 & 7), formed by surface folding caused by buckling of extremely viscous magma. Mafic inclusions in rhyolite are direct evidence of variations in magma compositions. It is important to study volcanic processes involved with Newberry because they provide recent volcanic deposits from a very old volcano that is still active. Newberry is also associated with hot rocks which have the potential to provide a hydrothermal energy resource for Oregon, however, volcanic behavior must first be understood. Newberry Volcano is a broad shield volcano 20 miles southeast of Bend, Oregon, to the east of the Cascade Range. It is one of the largest volcanoes in the United States and is still active today. Newberry produces mostly basalt lava flows and more recent, Holocene caldera events, erupt mostly pumiceous tephra and obsidian flows. Volcanic activity is centered on 3 major fault zones and began nearly 600,000 years ago. The most recent eruption at Newberry is the Big Obsidian Flow (BOF), which occurred about 1300 years ago within the caldera. Cavities on the flow surface represent either folding during emplacement or are large gas voids, suggesting exsolution of volatiles during flow. Rheology of bubbles in rhyolitic magma plays an important role in flow structure. Plagioclase rich, mafic inclusions in rhyolites are evidence of compositional variation in the pre-eruptive magma chamber. The Big Obsidian Flow (BOF) erupted from the south caldera wall . Volcanic activity began with a plinian eruption that deposited the Newberry Pumice associated with tephra fall, followed by a small pyroclastic flow associated with Paulina Lake and finally laminar emplacement of the BOF (Rust 2007). Viscosity of magmas is increased with increasing bubble content as well as silica content. Obsidian flows typically exhibit three different textures: finely vesicular pumice (FVP), coarsely vesicular pumice (CVP), and obsidian (Castro 1996 See Figures 3&4). The primary difference in these textures is vesicularity, which shows evidence of bubble content in the magma before it cooled. Vesicles can be spherical or elongate; both features have varying affects on shear stress in the magma. Bubbles however, have decreasing affects on increasing volumes of magma (Manga et al). According to Castro 2002, the upper 15m of the BOF shows stratigraphy of basal breccia, CVP, obsidian, and brecciated FVP. The southern most portion of the BOF has a suite of vent-facies rhyolite that forms a domal protrusion indicating the vent. Mesoscopic structures, found on the surface of the Big Obsidian Flow, provide evidence of surface folding that leads to formation of cavities. According to Castro, cavities range in size from 10-25m in diameter and can fill with exsolved gasses. These gases eventually cause the fold surface to rupture. Flow banding and vesicle lineations are interpreted to form during advance of viscous lava during flow and are very common (Castro 2002). The upper 10m of the flow cools much master than the massive internal flow. This allows the upper layer to buckle and detach from the flow interior along planes of varied vesiculation (Figure 5). As the flow progresses, cavities created during folding, grow due to continued progression of the flow. Volatiles, also known as bubbles, in the magma are released into the cavities and they eventually explode, leaving behind large craters. (Castro 1999 & 2002, See Figures 6,7,8). Figure 7 shows folding and cavity growth processes (Ref 6 & 1). ERUPTIVE HISTORY AND TECTONIC SETTING CONCLUSION Figure 3 shows obsidian from the BOF (Ref 2). Figure 4 shows the different textures of pumice found (Ref 2). The tectonic setting of Newberry is largely responsible for the range of volcanic process observed. Surface features like flow ridges and craters can be attributed to high viscosity magma. Bubbles help to increase viscosity and allow the formation of large gas cavities. Felsic volcanism dominates caldera events making it unusual to observe mafic material in deposits from the Big Obsidian Flow. The relationship between these two compositions needs further research to test ways in which the internal magma system at Newberry could be interacting with volcanic eruptions. Microlite crystallinity was not discussed but should be included in further research. The Big Obsidian Flow Figure 2 shows the tectonic setting of Newberry Newberry is a rear-arc volcano covering over 500 mi² and surrounded by multiple fault systems: The Walker Rim Fault (WRF), The Brother’s Fault Zone (BFZ), and the Tumalo Fault Zone (TFZ)(Figure 3). Newberry lies west of the High Lava Plains and east of the Cascade’s causing geologic features associated with both extension and subduction. According to GVP, volcanism at Newberry began around 0.73Ma with ash flows and air fall tuffs. The primary modes of eruption are central vent eruptions forming cinder cones, radial fissure eruptions, plinian eruptions, pyroclastic flows, and lava flows (GVP). Major ash flows, emplaced 0.3-0.5Ma are believed to have caused the initial caldera collapse. Gradual formation of fissures and over 400 cinder cones, associated with basalt to andesite flows, post date the initial caldera collapse. Silicic lava domes also formed which produce obsidian, pumice, and rhyolite/dacite (Castro 2002). Most of the cinder cones and fissure vents on the flanks of Newberry trend NNW-NNE following the BFZ-WRF fault zone (GVP). A rhyolitic magma chamber is present in the caldera and appears to be most juvenile in a “younging” trend of volcanics. Figure 6 shows a large crater left from an exploded gas cavity (Ref 1). Figure 5 shows folding and surface buckling (Ref 6) MAFIC INCLUSIONS REFERENCES According to Linneman and Myers, mafic inclusions are found in all parts of the Big Obsidian Flow, but are concentrated near the vent. Inclusions exhibit a wide variety of mineralologies and textures ranging from fine to coarse grained. Coarse grained inclusions suggest cumulate origin while fine grained inclusions are suggestive of quench methods. Plagioclase dominates all inclusions with varying amounts of other mafic minerals such as, but not limited to, amphibole and augite and rarely olivine. Presence of these mafic inclusions are evidence of a second type of magma interfering with the host rhyolite chamber. The most fitting model to describe this phenomena includes injection of a mafic magma into a pre-eruptive rhyolite chamber. Some suggest the magma chamber is zoned because of the complex nature of inclusions (Linnemann and Myers, 1990). A rhyolite chamber impeded by a mafic dike makes a much better model for inclusions. Further research is needed to complete a model for understanding magma mixing. Figure 1 shows the flow front. • Castro, Jonathan, Katherine Cashman, Nick Joslin, and Brian Olmsted:. "Structural originof large gas cavities in the Big Obsidian Flow, Newberry Volcano." Journal of volcanology and geothermal research 114 (2002): 313-330. Print. • GVP http://www.volcano.si.edu/world/volcano.cfm?vnum=1202-11-&volpage=erupt • Manga, Michael, Jonathan Castro, Katherine V. Cashman, and Michael Loewenberg. "Rheloogy of bubble bearing magmas." Journal of volcanology and geothermal research 87 (1998): 15-28. • Rust, A C., and K V. Cashman. "Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the 1300 B.P. eruption of Newberry Volcano, USA." Volcanology 69 (2007): 825-845.. • Linnemann, S R., and J D. Myers. "Magmatic Inclusions in the Holocene Rhyolites of Newberry Volcano, Central Oregon." Journal of geophysical Research 95.B11 (1990): 17677-17691. • Castro, Jonathan M. "Textural and Structural development of Obsidian Lavas." A dissertation (1999): 79-152. • http://www.wou.edu/las/physci/taylor/g407/Oshkosh_Presentation_final.ppt 30m CONTACT Name: Riccilee Keller Organization: WOU Earth Science Email: rkeller06@wou.edu Website: www.wou.edu/~rkeller06