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Newberry Volcano: The Big Obsidian Flow
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Ricci Keller, Earth Science Major
INTRODUCTION
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
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.
Figure
1
shows
the flow
front.
Name: Riccilee Keller
Organization:
WOU Earth Science
Email:
[email protected]
Website: www.wou.edu/~rkeller06
The
Big
Obsidian
Flow
Lidar,
USGS
THE BIG OBSIDIAN FLOW
FOLDING AND GAS CAVITIES
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).
Figure 2 shows the tectonic setting of Newberry
CONTACT
Surface
Crater
Eruption
Vent
ABSTRACT
30m
Flow
Ridges
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 NNWNNE 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 5 shows folding and
surface buckling (Ref 6)
Figure 6 shows a large crater left
from an exploded gas cavity (Ref 1).
MAFIC INCLUSIONS
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.
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.
REFERENCES
1. 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): 313330. Print.
2. GVP http://www.volcano.si.edu/world/volcano.cfm?vnum=1202-11&volpage=erupt
3. 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.
4. 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..
5. 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.
6. Castro, Jonathan M. "Textural and Structural development of Obsidian
Lavas." A dissertation (1999): 79-152.
7. http://www.wou.edu/las/physci/taylor/g407/Oshkosh_Presentation_final.ppt