Geology 103 01 Lab/Field Assignment

1. Lassen County,
California
By April Scarpa
Photo Credit: A.
Scarpa

2. In the north east corner of California is the county of Lassen. Lassen county consists of
part of the Sierra Nevada mountains, and the high desert terrain, like that of Nevada on
the east. Lassen county is at the northeastern most part of the Sierra Nevada mountains
and at the most southern end of the Cascades that take in much of Oregon and
Washington State. Lassen county is the home of Mt. Lassen and has experienced much
volcanic activity.
Photo Credit: A.Scarpa

3. Photo Credit: A. Scarpa
Thompson Peak
Overlooking the Honey Lake Valley of Lassen, is Thompson Peak. Thompson Peak is part of the Diamond
Mountains, which is a subrange of the Sierra Nevada Mountains. The Diamond Mountains are the Northeastern
end of the Sierra’s. “The Sierra’s Susanville terminus is a mass of Sierran granodiorite, cut or capped in various
places by a gamut of volcanic rocks and conglomerates, towering more than 3000 feet above Susanville on the
valley floor, with a high point at Diamond Mountain summit (7738 feet)”. (U.S. Forest Service, n.d., p2)
South of Susanville, Thompson Peak towers over the small town of Janesville, CA., at it’s base. The peak is
composed of a blend of Tertiary volcanic rock, on the upper portion of the mountain, while the bottom portion
and surrounding areas is mezoic granitics.

4. As I was researching Thompson Peak, I noticed
that the soil on the lower portion of the peak
consisted of mostly decomposed granite. The top
photo is an outcropping of granite on the east
side of the peak. It sits about one-third of the way
up the peak. As you can see in the middle photo,
the decomposed granite, excluding the large
piece, is coarse and rough, with pieces of granite
as large as 10 millimeters. However, in the bottom
picture, which is in my backyard, at the base of
Thompson Peak, the decomposed granite is fine
and sand like. It is easily moved by water. You can
also see where the current rain has removed the
sand and deposited a flow of black iron oxides. As
the decomposed granite has eroded and moved
down the mountain, the pieces have become
smaller and less coarse.

5. Photo Credit: A.Scarpa
Granite
This piece of granite was found on the western side of Thompson
Peak, near Antelope Lake. It is a flat piece of granite measuring
only about a half an inch thick on one end and about an inch and
a half on the other. The granite has a very rough surface. It is
white, with specks of gray crystals and black which suggests that
it has feldspar, biotite, and quartz in it. (Monroe & Wicander,
2012, p 75) The crystal in the granite tells me that this is a
metamorphic rock that has been formed by either heat, pressure,
or both.

6. Vesicular basalt
Vesicular basalt is an igneous rock commonly known as lava
flow. Basalt is a dark-colored, fine-grained, igneous rock
composed mainly of plagioclase and pyroxene minerals.
(Basalt, n.d., p 1) It has holes in it from bubbles of gas
trapped within it as the fluid lava cooled. Basalt is usually
dark grey or black and has small mineral crystals that are
not visible to the naked eye. This piece of basalt was found
in the high desert area of Lassen County just northeast of
Susanville. The area is called Rice Canyon and has numerous
amounts of basalt rock, which suggest that this was an area
where the lava was surfacing through cracks or crevices in
the ground.
Photo Credits: A.Scarpa

7. Mount Lassen
Photo Credit:
A.Scarpa

8. Mt Lassen is near the Northern edge of the Sierra-Nevada mountains and sits in the southern most part of the
Cascades. Mt Lassen is a volcano. Lassen last erupted in 1917. There were more than 400 eruptions between the years of
1914-1917. The most significant and greatest was on May 22, 1915. This event altered the appearance of Mt. Lassen. Mt.
Lassen, at 10,457 feet elevation, is the largest volcanic dome in the world. The primary rock is dacite, although flows of
basalt and andesite have added to the complex rock mosaic. (Webmaster, 2015, p5) About 5 million years ago, as new
volcanic activity was increasing, Mt. Lassen began to emerge. It emerged from the base of Mt Tehama, a large composite
volcano that began erupting around 600,000 years ago, just south of where Mt Lassen sits today...
Photo Credit: USGS

9. This picture of the May
1915 eruption gives us a
glimpse of what Mount
Lassen looked like prior to
the eruption.
Will it erupt again?
Photo Credit: USGS

10. “In May 1915, Lassen Peak, California, the southernmost active volcano in the
Cascade Range, erupted explosively. Avalanches, mudflows, and flows of hot ash
and gas devastated nearby areas, and volcanic ash fell as far away as 200 miles to
the east. The Lassen area remains volcanically active, and the volcano hazards
demonstrated in 1915 still can threaten not only nearby areas but also more
distant communities.”
(Volcano Hazards of the Lassen Volcanic National Park Area, California: USGS
Volcano Hazards Fact Sheet, n.d., p1)
This map, from the USGS, shows what is
likely to happen if, and when, an eruption
occurs within the Lassen Volcanic Center.

11. Lassen County is home to a wide variety of
wildlife, from the majestic bald eagle, the
white-tailed deer, and antelope, to the bobcat,
mountain lion and grey fox, to name just a few.
Of all the animals that live in the area, the Black
Bear stands out as the most problematic.
Photo Credit: gohunt
n.d.). photograph. Retrieved
from https://gohunt-
assets.s3.amazonaws.com/medi
a/Large-black-bear-OG.jpg

12. California Black Bear
Scientific Classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Ursidae
Genus: Ursus
Species: U. americanus
Subspecies: U. americanus
californiensis
(American black bear, 2020, p1)
Photo Credit: (n.d.). photograph. Retrieved from https://bear.org/wp-
content/uploads/2018/02/Faith-4-8-11-768x768.jpg

13. Modern bears appeared in Eurasia
around 5 million years ago during
the late Miocene Epoch. These
bears were relatively small
animals, but some types
eventually grew quite large. They
diversified and spread through
Europe, Asia, and North America,
eventually reaching South
America. Fossils indicate that
bears once lived in Africa, with
one large primitive species found
as far south as present-day South
Africa. (Bear Evolution, 2019, p2)
Photo Credit: (n.d.). photograph. Retrieved from https://bear.org/wp-
content/uploads/2008/10/bear_ready_to_climb_tree-768x543.jpg

14. Not only has the black bears been a problem for Lassen County, but have become a nuisance throughout most of
the California mountains and foothills. “California's black bear population has increased over the past 25 years. In
1982, the statewide bear population was estimated to be between 10,000 and 15,000. Presently, the statewide
black bear population is conservatively estimated to be between 30,000 and 40,000.” (Black Bear Population
Information, n. d., p1) There are two subspecies of black bear recognized in California, the California black bear
(Ursus americana californiensis) and the northwestern black bear (Ursus americana altifrontalis). The California
black bear is what populates the area of Lassen in which I reside. It is differentiated regionally, into the Sierra
subpopulation.
Credit: Black Bear Population Information. (n.d.).
Retrieved from
https://wildlife.ca.gov/Conservation/Mammals/Black-
Bear/Population

15. Black bears of California have
become more of a problem for
people in recent years. As humans
have moved further into their
natural habitat, the bears have
become more adaptive to the
accessibility of food from human
garbage. They have learned that it is
much easier to forage through trash
or raid homeowners of their
livestock feed. In my case, our
chickens, as seen by the damage
done in this picture.
Photo Credit: A. Scarpa

16. Columnar Basalt Formations
Along the Bizz Johnson Trail in Lassen County are
these towering walls of rock. These are columnar
basalt rock formations. The basalt rock is an
example of lava flow and is a direct result of
volcanic activity. The orange color also suggests
that the rocks contain iron. Along the Bizz Johnson
Trail there are two examples of lava flow,
Pleistocene lava, which is approximately 1 million
years old and Tertiary lava that is approximately 10-
11 million years old.
In the photo’s, the Susan River flowed between me
and the rock walls of the canyon. The erosional
force of the river has cut through these lava flows,
exposing the basalt and andesite formations.
(Mincher, n.d., p4)
Photo Credit: A. Scarpa

17. Disconformity and Horizontality
Although they are not in Lassen County, this picture is an example of disconformity. While visiting the Petrified
Forest National Park, I took this photo of the “Tepees”. “As part of the Blue Mesa Member in the Chinle Formation,
they consist of thick deposits of grey, blue, purple, and green mudstones and white minor sandstone beds, and are
colored by iron, manganese, and other minerals”. (BournDesigner, 2018, p4) The Chinle Formation was formed
from deposits during the Triassic Period over 200 million years ago. This photo is also an example of Steno’s law
principle of horizontality. (Monroe & Wicander, 2012, p418) Because this area was covered in water, and with the
help of gravity, sediment settled in horizontal layers, as depicted in the photo.

18. References:
• (n.d.). photograph. Retrieved from https://gohunt-assets.s3.amazonaws.com/media/Large-black-bear-OG.jpg
• (n.d.). photograph. Retrieved from https://wildlife.ca.gov/portals/0/Images/Game/BlackBear/BearPopulationEstimates1992_2012_thumb.gif
• (n.d.). photograph. Retrieved from https://bear.org/wp-content/uploads/2008/10/bear_ready_to_climb_tree-768x543.jpg
• (n.d.). photograph. Retrieved from https://bear.org/wp-content/uploads/2018/02/Faith-4-8-11-768x768.jpg
• American black bear. (2020, May 18). Retrieved from https://en.wikipedia.org/wiki/American_black_bear#Taxonomy_and_evolution
• Basalt. (n.d.). Retrieved May 18, 2020, from https://geology.com/rocks/basalt.shtml
• Bear Evolution. (2019, March 28). Retrieved from https://bear.org/bear-evolution/
• Black Bear Population Information. (n.d.). Retrieved from https://wildlife.ca.gov/Conservation/Mammals/Black-Bear/Population
• BournDesigner, J. (2018, July 21). The Tepees At Petrified Forest National Park. Retrieved from https://inspiredimperfection.com/adventures/tepees-petrified-forest/
• Mincher, E. (n.d.). Welcome to Rocky Star Geology. Retrieved from http://www.rockystar.com/home/geology/bizzgeol/bizzgeol.html
• Monroe, J. S., & Wicander, R. (2012). The changing earth: exploring geology and evolution. Pacific Grove, CA: Brooks/Cole Cengage Learning.
• U.S. Forest Service. (n.d.). Retrieved from https://www.fs.fed.us/wildflowers/regions/Pacific_Southwest/DiamondMountains/index.shtml
• Thompson Peak (Diamond Mountains). (n.d.). Retrieved from https://www.summitpost.org/thompson-peak-diamond-mountains/794813
• Volcano Hazards of the Lassen Volcanic National Park Area, California: USGS Volcano Hazards Fact Sheet. (n.d.). Retrieved from https://pubs.usgs.gov/fs/2000/fs022-00/
• Webmaster. (2015, March 2). Mt. Lassen – a Geological Must-See. Retrieved from https://www.gsoc.org/news/2015/3/2/mt-lassen-a-geological-must-see