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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. What is the rock’s texture? a) phaneritic; b) porphyritic; c) aphanitic; d) aphantastic.
4. Is this ridge an example of inverted topography? a) Yes; b) No.
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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SONORA PASS GEOLOGY
This EarthCache is on public land managed by the U.S. Forest Service (USFS). Please follow the rules and regulations posted on their entry signs at the Pacific Crest Trail trailhead (see Figure 1). Sonora Peak is located in the Sierra Crest - Little Walker Arc Volcanic Center, that began to form approximately 12 million years ago as the tectonics in this area changed due to migration of the Mendocino Triple Junction (MTJ) past this latitude. This region was affected by being pulled apart along what are called transtensional faults, which have both transform (sliding-past) and tensional (pull-apart) components of motion. This is significant because along these faults were the locations of volcanic vent systems, where magma reached the surface, forming volcanoes. The landscape here is covered with the deposits of those volcanoes. They filled the valleys they erupted in with large volumes of volcanic rocks - lava, debris flow deposits, volcanic ash, etc. Those valleys have since been disrupted by faulting and tectonic uplift as well as weathering and erosion by glacial ice and rainfall. The landscape here today looks different than when the volcanic centers were active. As this MTJ passed by this latitude starting 12 million years ago, it signified the beginning of the formation of a new plate tectonic boundary (first proposed by Dr. Cathy Busby) in the Sonora Pass region!
Figure 1: This map shows the trailhead at the Sonora Pass parking area and the Pacific Crest Trail to the EarthCache site.
As you hike to the EarthCache site (staying on the Pacific Crest Trail the whole way), you will see layer upon layer of different volcanic deposits along the trail and outcropping in the sides of the mountains high above you. Colors range from red, orange, gray, brown, tan, yellow, purple, to black. In the valleys below, you will likely be able to see mostly lighter colored, granitic rock exposed in places. The landscape is like a geological pastry in some ways, with younger, darker-colored volcanic “frosting” on older, lighter-colored, granitic “cake”. The volcanic rocks here are approximately 14 to 6 million years old, while the granitic rocks are between roughly 120-80 million years old. There are several types of volcanic deposits, mapped in detail here by Dr. Cathy Busby and her graduate students from U.C. Santa Barbara between 2006 and 2016, that show how this area formed. When a continental arc volcano, higher in silica composition and lower in iron, erupts, it tends to be more explosive and leave deposits behind that look like those seen at Mt. St. Helens. Volcanic rocks with larger, blocky chunks, form closer to the vent; medium sized particles are deposited at moderate distances; then rocks with smaller and smaller particles are found further and further away from where they were erupted. Ash deposits can travel miles away from the main vent system. When lava flows emanate from a continental volcano, it is usually because the magma is either very hot, or the lava travels quickly from deep in the earth to the surface. Additionally, the chemistry can control how viscous, or sticky, the lava is – the more silica and less iron, the more explosive the volcano; less silica, more iron generally means more effusive, less explosive volcanoes form at the vent.
Dozens and dozens of lava flows have been mapped coming from the Sierra Crest - Little Walker Arc Volcanic Center, just east of Sonora Pass (see Figure 4 in the Dragoon Gulch EarthCache). One of these layers, visible in Sonora Peak, is part of the Table Mountain Latite lava flow. The flow erupted east of here (while Sonora Pass was being uplifted) and partially filled, then overtopped, a tectonically formed valley called a graben. The lava then flowed all the way down to where the town of Knights Ferry is located, along Highway 108. It travelled a minimum of 80 miles, hardening in the paleo Stanislaus River channel. Subsequent erosion of the more easily weathered surrounding rocks, left the Table Mountain latite higher than the landscape it flowed through, thus forming Table Mountain, an example of “inverted topography”. What was once the low spot on the land is now the higher feature topographically.
The EarthCache site is located at the base of Sonora Peak, on the Pacific Crest Trail, at a ridge overlooking the Wolf Creek Valley to the north (see Figure 2). Carefully observe the dark rock at the GPS coordinates. What do you notice? In order to assess the type of volcanic rock here and the history it preserves, you will need to understand that when magma crystallizes deep inside the earth, the crystals that form grow large enough to be visible with the naked eye. The rocks are said to have a phaneritic (coarse-grained) texture. If the magma reaches the surface quickly, then solidifies, the crystals do not have enough time to grow large and are, therefore, microscopic and cannot be observed without the aid of a hand lens. This texture is called aphanitic (literally “without grains”). Sometimes, however, things are not so simple and a magma can be cooling deep within the earth, forming crystals big enough to see, when the system changes quickly, allowing the magma to rise to the surface, then erupt as lava. This causes a situation where the resulting rock has both large and small crystals – some macroscopic, some microscopic. The texture is called porphyritic – with 2 general sizes of crystals observed.
Figure 2: This view looks NNE toward Wolf Creek and the Spring Mountains in the background. The EarthCache is to the right of the photo.
To log this part of the EarthCache, answer the following questions:
- What is the rock’s texture?
- Phaneritic
- Porphyritic
- Aphanitic
- Aphantastic
- Is this ridge an example of inverted topography?
- Yes
- No
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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 approximately 2.5 miles up the Pacific Crest Trail (PCT) at approximately 10,500 feet elevation and is closed for half of the year or more, depending on the amount of snow in the area. The area is rugged and without cell phone service. EarthCachers should take great care in terms of safety while enjoying this cache.
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 ( and ).
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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. What is the rock’s texture? a) phaneritic; b) porphyritic; c) aphanitic; d) aphantastic.
4. Is this ridge an example of inverted topography? a) Yes; b) No.
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. 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.
2. 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.
3. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
4. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
5. Portland State University, “Glaciers of California”, last updated 08-04-2011, http://glaciers.research.pdx.edu/glaciers-california.
6. Putnam, Roger, (pers. comm.), May 2017, Professor of Earth Science, 11600 Columbia College Drive, Sonora, CA, 95370.
7. 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.
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