Pictures were taken while hiking to the top of Eagle Rock on the west shore of Lake Tahoe. Eagle rock is a volcanic vent so the geography there is different from the surrounding area.

1. Eagle Rock Tuff Cone
WEST SHORE LAKE TAHOE
SPRING 2020 GEOLOGY 103
BY CHANDRA FULLER
Aerial views of the north view via video (Eagle Rock Lake Tahoe 2014, 2014)
View from the top

2. Aerial view looking East over Eagle Rock and across Lake
Tahoe (Eagle Rock HikingTrail, n.d.)
Eagle Rock is a
250-M-Diameter
promontory that
rises above the
floor of the canyon
of Blackwood
Creek on the West
Shore of Lake
Tahoe.
It has endured
three major
geological
changes

3. The tuff cone formed
at an explosive
volcanic vent
(Kortemeier et al.,
2018). New data
establishes that
basaltic volcanism
occurred 2.3-2.0
million years ago.
Geochemically diverse
alkali basalts erupted,
forming subaerial and
water-contact lavas at
Eagle Rock
(Kortemeier et al.,
2009)

4. Eagle Rock was
overridden by
glaciers in the
Blackwood Creek
Canyon. The
glaciers created
scouring on the
top of the vent
(Kortemeier et al.,
2018).

5. Eagle Rock was
exposed to
giant splash
waves caused by
the McKinney Bay
megalandslide.
The splash waves
washed away
most of the tuff
cone and left only
subvertical tuff
beds and an intra-
vent tuff breccia
(Kortemeier et al.,
2018).

6. GEOLOGY AND ROCKS OF THE TAHOE BASIN
Andesitic rocks made from subduction-related volcanoes are
exposed in the west and northwest parts of the basin.
Small volcanoes in the northwestern and western parts of the basin
erupted and produced basaltic and trachyandesitic lavas, cinder
cones, breccias, pillows, and tuffs that locally overlie and invade the
contemporaneous lake sediments.
These photos were taken in Tahoe City NOT
on Eagle Rock

7. On my adventure up Cave
Rock, I saw some neat
stuff and some pretty
cool rocks. They include (but
are not limited to) tuff, pink
andesite, and scoria...

8. Tuff is an igneous rock that
forms from the debris
ejected by an explosive
volcanic eruption . The
eruption blasts rock, ash,
magma and other materials
into the air. If the material is
cemented into rock, it
becomes tuff (Tuff, n.d.). Tuff
is found near the vent, as is
the case at Eagle Rock.

9. This pink andesite is an
igneous rock with
porphyritic textures; thus it
is designated
andesite porphyry.
These rocks had a
complex cooling history
and may have been made
by magma cooling
beneath the
surface followed by an
eruption and rapid cooling
at the surface (Monroe &
Wicander, 2005). A later
event embedded the
andesite into the bigger
breccia rock.

10. This dark-colored igneous
rock is called scoria. It is
recognized by the
vesicles. Scoria forms
when magma containing
abundant dissolved gas
flows from a volcano or is
blown out during an
eruption. As the molten
rock emerges from the
Earth, the pressure upon
it is reduced and the
dissolved gas starts to
escape in the form of
bubbles creating the
vesicles (Scoria, n.d.).

11. This is a vertical basaltic dike in the
vent area, looking northeast.
Dikes are quite common and range
from a few centimeters to more than
100 m thick. They intrude into
preexisting fractures or where fluid
pressure is great enough for them to
form their own fractures as they move
upward into county rock (Monroe
& Wicander, 2015).
The age of the dike is older than
the country rock based on the principle
of cross-cutting relationships. This
principle states that an igneous
intrusion or a fault must be younger
than the rocks that it intrudes or
displaces (Monroe & Wicander, 2015).

12. I had hoped to see at least one unconformity around Eagle Rock, but I
didn’t. This makes sense because unconformities require sedimentary
rock and we simply do not have much sedimentary rock in the Tahoe
Basin. I am guessing there may be some unconformities hidden
underneath the lake water where gravity-flow deposits have built up over
igneous rocks and with all the fault action that occurs they may very well
be angular unconformities. If I could decide what these
unconformities would look like, I would choose these guys...
Angular unconformity at San Lorenzo Canyon
near Socorro, New Mexico (Timmons, 2017)
The Great Unconformity in the St. Francois
Mountains, near Missouri’s Mark Twain
National Forest (Erasing a Billion Years, 2018)
Angular Unconformity at Telheiro
Beach, Portugal (Angular
unconfomity, 2015)

13. I also saw some living organisms
including butterflies, plants, and
lizards.
Butterflies are part of the Order
Lepidoptera. They have a
proboscis; an elongated mouth
part used for sucking. Biologists
have presumed that co-evolution
between angiosperms and
pollinating insects explained the
development of the proboscis.
New information reveals that the
proboscis was developed before
the evolution of flowering plants
during the Triassic-Jurrasic
transition (Eldijk et al., 2018).
This could mean that members
of the Lepidoptera Order
contributed to the success and
evolution of the first angiosperm.
Vanessa virginiensis commonly known as American Lady.

14. Angiosperms: evolved during the late Cretaceous Period, about 125-100
million years ago and include all flowering plants, shrubs, and trees.
Amelanchier allnifolia
Saskatoon serviceberryChorispora tenella
Blue mustard
Ceanothus jepsonii
Jepson ceanothus

15. Throughout the history of angiosperms there have been gene mutations such
as repetitive DNA loss, chromosome rearrangements and complex patterns of
gene loss that are collectively termed ‘diploidization’ (Dodsworth et al.,
2015). Diploidization may have led to the global speciation of angiosperms and
created new niches and food sources for many organisms.
Senecio integerrimus -
Lambstongue ragwort
Ribes nevadense
Sierra currant

16. Megachirella wachtleri is
classified as the oldest
known stem
squamate (Wachtler,
n.d.) making it an
ancestor to Sceloporus
occidentalis; the
Western Fence
Lizard who lives on
Cave Rock.
Above: fossil of Megachirella wachtleri Right: Illustration of what it
may have looked like ( both pics: Wachtleri, n.d.)

17. ~240 mya later...
The Western Fence
Lizard (WFL) has
evolved
a couple interesting
traits different from
other lizards...

18. The WFL has
evolved a protein
in it's blood that
kills Borrelia
burgdorferi, the
bacteria that causes
Lyme Disease (Lane
et al., 2006). Not
only does this make
the WFL immune to
Lyme Disease, it
also cures the tick
that carries it.
WFL during Lyme Disease study (Lane, 2011)

19. The WFL was studied in
Yosemite and
determined to have
evolved adaptive
phenotypic changes
associated with
increases in body size
and ventral
coloration when
at higher elevations
(Leaché et al., 2010).
Big WFL in Yosemite (UW Biology,
n.d.)

20. There are many awesome things in the Lake Tahoe Basin
making it impossible to sum up all of them in one
presentation. I have picked some of my favorites for you to
enjoy.
Chandra
2020
All pictures without references were taken by me or my grandson, Lucas

21. Angular Unconformity at Telheiro Beach, Portugal. (2015). Retrieved from
https://epod.usra.edu/blog/2015/01/angular-unconformity-at-telheiro-beach-portugal.html
Dodsworth, S., Chase, M. W., & Leitch, A. R. (2015). Is post-polyploidization diploidization the key to the
evolutionary success of angiosperms? Botanical Journal of the Linnean Society, 180(1), 1–5. doi:
10.1111/boj.12357
Eagle Rock Hiking Trail (Homewood) - 2020 All You Need to Know BEFORE You Go (with Photos). (n.d.).
Retrieved May 25, 2020, from https://www.tripadvisor.com/Attraction_Review-g32503-d12692643-Reviews-
Eagle_Rock_Hiking_Trail-Homewood_Lake_Tahoe_California_California.html
Eagle Rock Lake Tahoe 2014. (2014, July 9). Retrieved May 26, 2020, from https://youtu.be/TV34x5IqK4Q
Eagle Rock, Homewood, CA Great short hike. (With images): Tahoe, Eagle rock, House in the woods. (n.d.).
Retrieved from https://www.pinterest.com/pin/493918284112295319/
Eldijk, T. J. B. V., Wappler, T., Strother, P. K., Carolien M. H. Van Der Weijst, Rajaei, H., Visscher, H., &
Schootbrugge, B. V. D. (2018). A Triassic-Jurassic window into the evolution of Lepidoptera. Science
Advances, 4(1). doi: 10.1126/sciadv.1701568
REFERENCES

22. Erasing a Billion Years of Geologic Time Across the Globe. (2018). Retrieved May 27, 2020, from
https://eos.org/articles/erasing-a-billion-years-of-geologic-time-across-the-globe
Kortemeier, W. T., Moore, J. G., Schweickert, R. A., & Calvert, A. T. (2009, December). Ar-Ar Ages of Lake
Tahoe Basalts Confirm Several Eruptions at 2.3 to 2.0 Ma and Establish 0.92 Ma Activity. Retrieved May 25,
2020, from https://ui.adsabs.harvard.edu/abs/2009AGUFM.V41B2177K/abstract
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doi: 10.1645/ge-738r1.1
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fence lizards (Sceloporus occidentalis) in the Sierra Nevada Mountains. Biological Journal of the Linnean
Society, 100(3), 630–641. doi: 10.1111/j.1095-8312.2010.01462.x
REFERENCES

23. Monroe, J. S., & Wicander, R. (2015). The changing earth: exploring geology and evolution. Stamford, CT: Cengage
Learning.
Scoria. (n.d.). Retrieved May 26, 2020, from https://geology.com/rocks/scoria.shtmlTimmons, J. M. (2017). mysteries
of the great unconformity, a journey in deep geologic time. Retrieved May 27, 2020, from
https://geoinfo.nmt.edu/publications/periodicals/earthmatters/17/n1/em_v17_n1.pdf
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UW Biology. (n.d.). Retrieved May 26, 2020,
from https://www.biology.washington.edu/news/newsletters/newsletter/article/summer-fieldwork-updates
Wachtler, M. (n.d.). Highlights: Megachirella wachtleri. Retrieved May 26, 2020,
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from https://news.berkeley.edu/2011/02/15/ticks-lizard-lyme-disease/
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