Menan Buttes

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Menan Buttes

• 7-6-2007

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The north Menan Butte as seen from westbound Main
St. (HWY 33). Notice the difference between
images A and B.
Image A was taken from within the floodplain,
before crossing the Henrys Fork River while image
B was taken west of the Henrys Fork River amongst
the basalt produced by a nearby shield volcano.
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The parking area. Looking south.
The parking area. Looking north.
Hiking the trail.
The first stop. TAs looking south.
The Henrys Fork to the northeast.
The South Menan Butte.
The view south from the first stop.
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30
Angle of Repose The angle of repose for a loose
material is the maximum slope or angle at which
it remains in place The type of material and the
shape of the material are factors determining the
angle of repose. The angle of repose for the
basaltic tuffs that make up the buttes is about
30-35.
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5
The buttes are made of fine grained volcanic
material that readily weathers becoming loose
rock (seen here as loose gravel). This fine
pyroclastic material is called _______ (read next
slide). Larger solid fragments, such as the
vesicular basalt seen here, originated prior to
the formation of the butte and is called a
xenolith. The basalt is essentially the same
chemical composition as the fine grained rock
that makes up the butte. The difference in their
appearance is the result of how they were erupted.
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Hot magma, working its way towards the surface,
encountered the prolific Snake River plain
aquifer (water). The heat of the magma converted
the ground water to steam which produced an
explosive eruption, blasting the basaltic magma
to the surface as small fragments. The resulting
rock is identified as a tuff. The water of the
aquifer was located in older layers of river
gravels and layers of basaltic rock. The
explosiveness of the eruption carried pieces of
these materials to the surface as the volcano
grew, incorporating them into the volcano as
xenoliths.
Menan Buttes
Menan Buttes info
(1,4,8)
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The second stop. A view of the crater looking
northeast.
Erosion and Weathering on the Butte
Pathway eroded by water and wind.
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Erosion by wind and water yields a honeycomb
appearance.
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Things to look for 1) Caliche White calcium
carbonate deposits on the rock as water
evaporates. 2) Lichen A fungus and an algae
growing on the rock, enhancing weathering. 3)
Plant roots Biological/mechanical weathering.
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When these volcanoes erupted, the pyroclastic
material was ejected uniformly up and out into
the air. If not for the constant wind of the
Snake River plain, the material would have come
to rest as a symmetrical ring around the central
vent. Most basaltic volcanic vents are much
smaller because they are producing lava flows.
The craters encountered in these buttes are so
large because of the violent reaction of the lava
with the water during the eruption.
NE
wind direction
SW
The answer to this question (5) is easier to
realize while standing on the butte. The concept
of uniformitarianism (the present is the key to
the past) is also helpful. Its very windy up
there today. Most likely, it was windy at the
time of eruption.
(4,5)
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Other Kinds of Volcanoes on the Snake River Plain
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West of Rexburg, visible on the horizon from
US-20 and the top of the Menan Buttes on clearer
days numerous volcanic features can be seen. Big
Southern and East Buttes are rhyolite lava domes
while Middle Butte is an uplifted block of
basalt, likely uplifted by rhyolitic magma.
Big Southern Butte
Go to this hyperlink and read the text and play
the Quicktime video.
A Shield volcano located northwest of the North
Menan Butte. This is likely one of the volcanic
vents that produced the basalts located north and
west of the Menan Buttes. No lava flows came from
the buttes. It is also viewable from westbound
ID-33 (Main St.) at about 2 oclock on the
horizon from Beaver-Dick Park.
Shield volcanoes literally cover the Snake River
Valley. Any broad gently sloping structure on the
horizon is most likely a shield volcano.
West rim of the North Menan Butte.
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Yellowstone Caldera info
Click on the link to see a view of the
Yellowstone Caldera
The smaller Island Park Caldera is barely visible
from the Menan Buttes, located to the northeast,
best seen on exceptionally clear days.
Island Park Caldera western wall
Sand dunes
North rim of North Menan Butte.
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The Menan Buttes may have formed along a fault.
That could explain why they and the smaller Anis
Buttes, line up so well (including the smaller
third cone in the middle). Some studies report up
to 6 tuff cones at the Menan Buttes location.
Menan Buttes
Look for the linear appearance of the young vents
from the link above.
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www.inl.gov/geosciences/earthquakes.shtml
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XENOLITHS xeno foreign lith rock
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Xenoliths are rocks that were not produced from
the original magma that erupted, rather they were
incorporated with the tuffaceous material during
the eruption of the volcano.
Basalt (all dark black material exposed at the
surface)
River gravels mostly quartzite and other
metamorphic rocks
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To simulate actually being there and seeing some
of the features on the butte yourself, cross your
eyes just a little at these two images and make a
third image in the middle. Focus your eyes on the
middle image and you might see the bowl on the
southwest rim in 3D!
Walking the rest of the way to the top of the
west rim.
More weathering erosion across the bowl.
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Eroded/incised middle butte
Lava fields to the northwest.
The view from the top of the west rim of the
North Menan Butte, looking west.
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After leaving the flood plain, the soils on the
benches of the Rexburg area are the product of
wind. Without this wind-blown soil, the green
vegetation would not be able to establish itself
in this area. This picture was taken near the
southwest corner of the McKay Library during
excavation prior to installing new storm drains.
The zone where roots are sticking out is the
wind-blown soil called loess. Notice the columnar
basalt below the soil and just how thin the soil
is. Blasting allowed excavation of the basalt.
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History of Yellowstone Hotspot
Click on the link above to access information
about the Yellowstone Hotspot.
Though most of the volcanism on the Snake River
plain is quite old, the Menan Buttes are
relatively young. The age of the Buttes is
approximately 100,000 years.
(2,10)