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TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line of your email.
2. The number of people in your group.
3. Estimate or measurement of: a) the angle of repose for Golden Gate Hill (2 measurements - one for each slope; the number of degrees between 0 (horizontal) and 90 (vertical)); and b) the angle of repose for Tunnel Peak (also 2 measurements - one for each slope; the number of degrees between 0 (horizontal) and 90 (vertical)).
4. Do the lava domes have a higher angle of repose than other slopes nearby?
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples (1-7) are relevant (list)?
7. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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GEOLOGY OF THE TERTIARY VOLCANIC DOMES
This EarthCache teaches more about the geology of the Tertiary volcanic domes, or buttes, in the Paloma area, Calaveras County, California. The domes were mapped by Robert L. Rose, in the late 1950’s. EarthCachers will make observations, then estimate the steepness of the slopes of 2 of the domes as a geologist might do, to assess slope stability for landslide hazard, or other purposes.
There are 5 volcanic domes near this EarthCache. They include from southwest to northeast: 1) Golden Gate Hill (4.73 Ma (Ma = "mega annum" = million years old)); 2) Tunnel Peak (4.12 Ma); 3) Hamby Dome (3.73 Ma); 4) McSorley Dome (3.76 Ma); and 5) Jackson Butte (3.78 Ma) (see Figure 1). This area is generally composed of steeply dipping, slightly metamorphosed sedimentary and volcanic rocks of the subjacent series - the Western Sierra Nevada Metamorphic Belt – overlain by younger volcanic and volcaniclastic rocks (volcanic sediments) of the superjacent series – the Valley Springs and Stanislaus Group (generally known as the Mehrten Formation). The 5 domes have been identified as either dacite, or rhyodacite, extrusive volcanic rocks, which have moderate amounts of silica. They would have erupted as thick, pasty lava, building up dome-shaped structures, similar to what is forming in the crater of Mt. St. Helens today. Dacite typically forms when younger oceanic crust subducts under thicker, felsic (silica-rich) continental crust and the subducting plate metamorphoses, then partially melts, causing minerals such as talc, serpentine, mica, and amphiboles to break down. The magma then rises and becomes even more enriched in sodium and silica, crystalizing as viscous dacite lava domes at the surface, rich in the minerals plagioclase, quartz, and hornblende.
Plate tectonic theory did not formally exist in the 1950’s, therefore no tectonic model was proposed for their formation at that time. The domes erupted during the Pliocene epoch (~5.3 to 2.6 Ma), following activity in the Ebbetts Pass Arc Volcanic Center (~6.3–4.8 Ma) at the Sierra Crest, east of Mokelumne Hill. Volcanic activity occurred as the region was tectonically pulled apart in the Walker Lane Shear Zone (see Figure 4 on the "Dragoon Gulch EarthCache"), releasing magma in vents along what are called “transtensional stepover faults”. These types of faults have both transform (sliding-past) and tensional (pull-apart) components of motion. This is significant because along these faults were the locations of multiple volcanic vent systems between Sonora Pass and Ebbetts Pass, where magma formed volcanoes, with some similarities to the volcanic domes near Mokelumne Hill. The migration of the Mendocino Triple Junction past this part of the Sierra Nevada microplate, approximately 3 to 4 million years ago, may have played a role in opening the Melones Fault Zone to release magma, forming the domes.
Figure 1: Location of the 5 Miocene volcanic domes mapped by Robert L. Rose.
The Tertiary Volcanic Domes EarthCache is located at the junction of Highway 26 and Paloma Road. There is enough room to park safely in a triangular area between the roads. Volcanic domes are defined as:
“a steep-sided mass of viscous lava extruded from a volcanic vent, commonly circular in plane view and spiny, rounded, or flat on top. Its surface usually is rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.”
To complete the EarthCache you will observe the slopes formed by 2 of the volcanic domes (Golden Gate Hill and Tunnel Peak) and compare and contrast their morphology. To do this, you must understand the concept of the angle of repose, which is "the maximum slope, measured in degrees from the horizontal, at which loose solid material will remain in place without sliding" (see Figure 2). To estimate this angle you will need to assume that horizontal surfaces have an inclination of 0 degrees and vertical surfaces are oriented at 90 degrees. The angles will vary somewhat, depending on which slopes for each dome you choose, so give a range of estimates (i.e. 20-30 degrees, or 30-35 degrees, etc.). If you are inclined, try downloading a clinometer app to your smartphone and use it to help, otherwise you should be able to give a crude field estimate without a clinometer or smartphone app. Golden Gate Hill is directly to the south of the EarthCache location – the tall hill in front of you. Assuming the rock composing the dome is “loose solid material”, estimate the angle of repose for Golden Gate Hill. Then turn around and look to the north. You will be facing Tunnel Peak, another of the 5 domes. You may need to move to see the sides of the dome, depending on where you parked. Once you have a good view, safely estimate the angle of repose for the slope of Tunnel Peak. Compare, contrast, and record your estimates.
Figure 2: Image showing the angle of repose (36 degrees, measured upward from horizontal) behind the person in the foreground.
Good luck!
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EARTH SCIENCE BIG IDEAS
The Earth Science education community put together 9 “Big Ideas” for the Earth Science Literacy Initiative (ESLI), shown below. Their purpose was to highlight the main concepts and ideas a person should understand to be literate in the earth sciences:
An Earth-science-literate person:
• understands the fundamental concepts of Earth’s many systems
• knows how to find and assess scientifically credible information about Earth
• communicates about Earth science in a meaningful way
• is able to make informed and responsible decisions regarding Earth and its resources
Which of these Big Ideas below do you think are most relevant to this EarthCache?
Earth Science Literacy Project:
1. Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet.
2. Big Idea 2: Earth is 4.6 billion years old.
3. Big Idea 3: Earth is a complex system of interacting rock, water, air, and life.
4. Big Idea 4: Earth is continuously changing.
5. Big Idea 5: Earth is the water planet.
6. Big Idea 6: Life evolves on a dynamic Earth and continuously modifies Earth.
7. Big Idea 7: Humans depend on Earth for resources.
8. Big Idea 8: Natural Hazards pose risks to humans.
9. Big Idea 9: Humans significantly alter the Earth.
For more details see: Earth Science Literacy Initiative
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GEOPRINCIPLES
There are several fundamental principles, developed over time, that guide geological reasoning and critical thinking, listed below. Read each short description, then use your best judgement to determine which principle, or principles, best relate to this EarthCache.
7 Principles in Geology:
1. Superposition – the oldest strata are at the bottom of the sequence
2. Original Horizontality - layers of sediment are originally deposited horizontally
3. Lateral Continuity - layers of sediment initially extend laterally in all directions
4. Faunal Succession - fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances
5. Law of Intrusive Relationships - the geologic feature which cuts another is the younger of the two features
6. Uniformitarianism - the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe
7. Catastrophism - the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope
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LOGISTICS AND SAFETY
This EarthCache is located on public roads and there is no need to go onto any private land. Take all precaution when making observations and estimating angle of repose - pay close attention to any traffic hazards.
Visitors to this site should plan ahead and prepare by:
• Knowing the regulations and special concerns for the area you are planning to visit (obeying laws that prohibit collection or destruction of artifacts);
• Carrying a map and a GPS unit and/or compass;
• Staying on existing roads and trails;
• Staying away from any/all mine shafts and adits;
• Planning for extreme weather, hazards, and emergencies;
• Being aware that cell phones DO NOT usually work in the rural areas away from the major highways;
• Leaving your travel plans with a responsible party, including the date and time of your return;
• Being aware of any natural hazards associated with the region (e.g. poison oak, rattlesnakes, mosquitoes, cliffs/steep slopes, etc., etc);
• Carrying a full-size spare tire, extra food, water, and warm clothing;
• Following the “Tread Lightly" and "Leave No Trace” philosophy.
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TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line of your email.
2. The number of people in your group.
3. Estimate or measurement of: a) the angle of repose for Golden Gate Hill (2 measurements - one for each slope; the number of degrees between 0 (horizontal) and 90 (vertical)); and b) the angle of repose for Tunnel Peak (also 2 measurements - one for each slope; the number of degrees between 0 (horizontal) and 90 (vertical)).
4. Do the lava domes have a higher angle of repose than other slopes nearby?
5. Which Big Ideas (1-9) are connected (list)?
6. Which GeoPrinciples (1-7) are relevant (list)?
7. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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Note: For a brief summary of the geologic history of the Central Sierra, see this EarthCache:
Dragoon Gulch EarthCache
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REFERENCES
1. “Angle of Repose”, www.dictionary.com, accessed on 05-12-2017.
2. Busby, Cathy J., Andrews, G.D.M., Koerner, A.K., Brown, S.R., Melosh, B.L., and Hagan, J.C., “Progressive derangement of ancient (Mesozoic) east-west Nevadaplano paleochannels into modern (Miocene–Holocene) north-northwest trends in the Walker Lane Belt, central Sierra Nevada”, Geosphere 12, p. 135-175, 2016, http://www.geosphere.gsapubs.org.
3. Busby, Cathy J., Koerner, Alice, Hagan, Jeanette, and Andrews, Graham, 2012, “Sierra Crest graben: a Miocene Walker Lane Pull-apart in the Ancestral Cascades Arc at Sonora Pass”, in, N. Hughes and Garry Hayes (eds), “Geological Excursions, Sonora Pass Region of the Sierra Nevada”, Far Western Section, National Association of Geoscience Teachers field guide, p. 8-36.
4. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
5. Kluge, Steve, Image of Talus Slope in the Huerfano Valley, Colorado, 2000.
6. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
7. Portland State University, “Glaciers of California”, last updated 08-04-2011, http://glaciers.research.pdx.edu/glaciers-california.
8. Putnam, Roger, (pers. comm.), May 2017, Professor of Earth Science, 11600 Columbia College Drive, Sonora, CA, 95370.
9. Rohlen, Ginger, (pers. comm.), June 2017, Teacher, Sierra Waldorf School,19234 Rawhide Rd., Jamestown, CA., 95327.
10. Rose, Robert L., “Tertiary Volcanic Domes near Jackson, California”, Special Report 50, Department of Natural Resources, California Division of Mines, Ferry Building, San Francisco, 1959.
11. Schweickert, Richard, 2006, “Accretionary Tectonics of the Southern Part of the Western Sierra Nevada Metamorphic Belt” (modified from a 1999 guidebook article by Schweickert, Girty, and Hanson), in J. Tolhurst (ed), “Geology of the Central Sierra”, National Association of Geoscience Teachers Far Western Section Fall Conference field guide, p. 55-95.
12. Turrin, Brent, “Potassium-Argon Dates and Stratigraphy of Pliocene Volcanic Domes near Mokelumne Hill”, Stanford University Department of Geology, in California Geology, October 1982/Volume 35/ Number 10, p. 220-222, October 1982.
13. “Volcanic Dome”, https://en.wikipedia.org/wiki/Lava_dome, Wikipedia, accessed on 05-12-2017.
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