Eruptive and Geomorphic
Processes at the Lathrop Wells
Scoria Cone Volcano
Greg. A Valentine, Donathan J. Krier,
Frank V. Perry, Grant Heiken
•DEM of Yucca Mountain
•Pliocene Volcanoes
TM Thirsty Mountain
BM Buckboard Mesa
PCF Pliocene Crater Flat
•Volcanoes in Crater Flat
~1 million years
LC Little Cones
RC Red Cone
BC Black Cone
MC Makani Cone
LBP Little Black Peak (~.32Mya)
HC Hidden Cone (~.37 Mya)
Lathrop Wells Volcano (.08Mya)
•Repository Footprint
•Lathrop Wells Volcano
Trachybasaltic lava and Scoria
Mg~54 indicates fractionalization of
parent magma
Mostly porphyritic with plag ground
mass
2 lobes of early lava fields and early
cone deposits have plag crystals up to
1 mm long.
Later flows are depleated and
enriched
Distribution of Volcanic Products at
Lathrop Wells
Inferred Fallout Distribution
•Contours represent fallout
isopachs. (cm)
•Lettered stations are stratographic
columns (Figure 8)
•Inset shows possible regional
extent based on thin ash remnants
Relative timing of emplacement of scoria cone,
fallout and lave fields at Lathrop Wells
•Cone building deposits associated with Strombolian phase
activity.
•Western Part of South Lava
Field
•Lobate flow margins (solid line)
•Pressure ridge and squeeze
up features (dashed lines)
A) Mound of broken masses of partly welded
agglutinate. Notice narrow squeeze up or rootless
dyke along the leading edge.
B) Pyroclastic mound composed of intact tilted block
of bedded spatter and agglutinate (Eastern margin
of south lava field)
C) Internal structure of intact pyroclastic mound with
steeply tilted bedding of partly welded agglutinate.
Notice thin layer of spatter near stick. This was
covered by fallout lapilli beds from Strombolian
eruptions at the cone
A) Cross stratified ash deposit interbedded with scoria lapilli deposits in upper zone
B) Grain size distribution for cross stratified ash deposit ( 074) and overlaying scoria
lapilli bed (076)
Fallout deposits from ten sites up to
2 km from the vent.
Inferred with a common datum
Scanning Electron
Photomicrograph of ash from
the laminated and cross
laminated sequence of fallout
deposits `200-300 m NW of the
cone (column D).
Note abrasion of some grains.
Medial and distal parts of the northeast lava field,
illustrating lobate nature of flow margin.
Dotted pattern indicates possible ripply surface
ridges along with what might pf been a main
surface lava channel.
A) Down stepping lobes representing successive tube
fed breakouts of lava from the toes of lobes along
eastern margin of lava field.
B) Stacking of the flow units, dashed lines indicate
outer margins of two flow units. Lower unit formed a
100m wide, 3-5 m high platform where contacting a
hill of Miocene ignimbrite.
C) Low lying flow units on the northern edge of the
northeast lava field partly buried by violent
Strombolian fallout deposits, sediment accumulation
from lava dams to the north, and eolian sands.
Arrows show tops of flow edges or squeeze ups that
protrude above the pyroclastic and sediment units.
Xenolith concentration (volume fraction) plotted against elevation of the Lathrop cone.
Lucero volcanic field (Mexico) and Hopi Butte fields (Arizona) used for comparison.
1987 Lathrop Wells aerial
photo showing erosional
rills extending toward and
away from the crater.
Dark colored features are
garlands produced by
creep of loose scoria
deposits.
Light colored area on the
southeast slope of cone
is caused my
accumulation of eolian
sand and silt at a high
rates.
Cross section to illustrate
evidence for the
erosional history of the
Lathrop Wells cone.
A) Through the crater
center
B) Cross section through
western crater rim.
Dashed line extrapolates
basal contact of black
tephra unit above the
modern rim.
Mechanical weathering of a
small lava crag on the northeast
lava field.
Fragment produced by freeze
thaw and thermal splitting of the
lava, especially on the south
side of the crag.
The eroded clasts accumulate
around the base and will
eventually turn into desert
pavement.
In interpretation of the effusive and
explosive processes that formed
the Lathrop Wells volcano.
Earliest plagioclase phenocryst
bearing lava flows to the southwest
of the developing cone were part of
Strombolian explosions that ejected
ballistic bombs that formed welded
deposits.
Strombolian eruptions continued
through construction of the south
lava field where material was rafted
on top of the flow.
Additional strombolian activity
produced multiple eruption columns
and buried the south lava field and
early parts of the northeast field.
The north lava field continued to
form after strombolian activity had
ceased.
Conclusion
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1.
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4.
Nearly all eruptive products have been modified by a variety of
geomorphic processes.
If a future volcanic event occurs at Yucca Mountain and intersects the
proposed repository (250-350 m deep) bad things will happen based on
the processes that formed Lathrop Wells Volcano.
Total volume of radioactive eruptive products will be around .1 cubic
kilometer, with eruptive columns a few kilometers high.
The event will be fed from one main feeder dike and conduit up to 20-25
m in diameter at the depth of the repository with shallow lateral
breakouts.
The violent strombolian nature of many of the eruptions we can anticipate
the explosivity of the initial interactions with the repository tunnels and
subsequent eruptive dispersal could be high if the trachybasaltic magmas
have significant volatile content.
Fallout tephra contaminated with entrained radioactive waste will be
spread across the land, water and atmosphere. We will be screwed
BAD IDEA!