Butte Fire Geology and Debris Flow Hazards EarthCache

TO LOG THIS EARTHCACHE INCLUDE:

1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. The type of rock observed at the GPS coordinates is: A) decomposed granitic rock; B) metamorphic rock (of the Calaveras Complex).
4. Which Big Ideas (1-9) are connected (list)?
5. Which GeoPrinciples are relevant (list)?
6. Include a photo or 2 if you're so inclined (optional).

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GEOLOGY OF THE BUTTE FIRE

The Butte Fire started on September 9, 2015, just downstream of this EarthCache, in the Mokelumne River watershed. The rocks underlying most of the burn area (~95%) are mapped as the Calaveras Complex (see Figure 1), which is mostly marine metasedimentary rock with minor metavolcanic rock, plus chaotic argillite (hardened mudstone) and subordinate chert (deeper water radiolarian plankton). Additionally, there are units of marine limestone and dolomite metamorphosed to marble. The Calaveras Complex is interpreted to be a Triassic-Jurassic subduction complex that formed from fine-grained (deeper water) ocean sediments and pieces of island arc volcanics, shoved into a subduction zone and metamorphosed approximately 200 million years ago. The marble formed from fringing reefs on a volcanic island system, evidenced by Amphipora fossils (similar to spaghetti coral) found. The metamorphic rocks of the Calaveras Complex are dense, crystalline rocks because the sediments that were metamorphosed lost their permeability and porosity when they were compacted and densified.


Figure 1: Geologic Map of the Butte Fire area. The Calaveras Complex is shown in brown with marble in blue and the granitic rocks are shown in pink.

The burn area is also underlain by a smaller percentage (~5%) of granite and granodiorite, with lesser amounts of quartz diorite, diorite, diorite gneiss, and gabbro, that intruded the older metamorphic rocks of the Calaveras Complex. Depending on mineralogy, granitic rocks can decompose, or break down through the processes of weathering. Granitic rocks are typically coarse-grained (phaneritic) and composed of quartz, feldspars, mica, and other accessory minerals. When feldspars are weathered, they can break down and transform into the clay mineral kaolinite, which can allow water to seep into its structure, further weakening the granitic rock. This causes granitic rocks to often break down into their mineralogical constituents forming what is known as “decomposed granite” (DG). DG is like gravel, but finer and generally more stable. The DG sold as landscaping material is typically composed of fine (<3/8”) particles, some no bigger than a grain of sand. The harder, relatively more resistant quartz grains are left behind, forming quartz-rich sand and gravel.

When wildfire rages through a watershed, geoscientists work with soil scientists and geospatial experts, who use remote sensing and field methods to assess post-fire hazards, such as flooding and debris flow potential. They do this by immediately going into the field with sensor systems called field spectrometers to measure soil characteristics, such as reflectance of light. Satellites are also taking photos of the burned area with special infrared and visible light sensors. The images and field measurements are used to create post-fire vegetation condition maps called Burned Area Reflectance Classification, or BARC maps. This product is then used as an input to the soil burn severity map produced by either the California (state level) Post Fire Emergency Watershed Response Team, or the Burned Area Emergency Response (BAER = Federal) team of which geoscientists are members. They then take that data and use it to produce debris flow Preliminary Hazard Assessment maps (see Figure 2). After the fire, the teams use the map products generated to develop emergency response warning and evacuation systems, for revegetation/ecosystem restoration decisions, for infrastructure protection, and more.

Figure 2: The map above displays estimates of the likelihood of debris flow (in %), potential volume of debris flow (in m3), and combined relative debris flow hazard. These predictions are made at the scale of the drainage basin, and at the scale of the individual stream segment. Estimates of probability, volume, and combined hazard are based upon a design storm with a peak 15-minute rainfall intensity of 24 millimeters per hour (mm/h). Note that the EarthCache is located in a higher probability (~60-80%) debris flow watershed.

The Calaveras Complex geology likely has played a role in reducing and minimizing the negative effects of debris flows within the burned area. The bedrock is relatively dense, doesn’t weather readily, and generally has shallow soils. The lack of loose, unconsolidated material may be the reason why minimal debris flows were observed during the winter following the fire. Three types of hazards are of concern in a burned watershed when intense rainfall occurs: 1) floods (<20% sediment in the streamflow); 2) hyperconcentrated flows (20-60% sediment concentration); and 3) debris flows (>60% sediment concentration in the flow). The type of rock controls whether a debris flow may occur. Rock with lots of loose, easily mobilized particles, on steeper slopes can generate debris flows under the right conditions.

For this EarthCache, once you have located the GPS coordinates, you will make observations of the geology along the roadway, in the burned sub-watershed, above the Mokelumne River. Most of the Butte Fire burned in rocks of the Calaveras Complex, with a smaller portion having burned granitic rocks. Your task here is to examine the bedrock at the GPS location in order to determine the role it played in erosion, sediment production, and mass wasting within the sub-watershed. Does the rock have any sort of layering visible? What do the grains look like – colors? Sizes? Shapes? Is the rock competent (is it solid, or can you break it apart in your hands)? What role do you think this rock played in determining whether this area might experience debris flow or flooding hazards? Do you think the rock is igneous granite or decomposed granite, or is the bedrock of the Calaveras Complex – harder, more dense, metamorphic rock? Finally, why didn't this watershed experience major debris flows following the Butte Fire (intense rainfall from a storm event in December did occur ). How might the geology here have played a role?

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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

To get to this EarthCache you will take Ponderosa Way off of Highway 26, east of Mokelumne Hill and Highway 49. There is a gate on the dirt road that is closed during times of the year. If closed, park in the parking area above the gate and walk approximately 3/4 (0.75) of a mile to the EarthCache. If open, proceed at your own risk down the single lane gravel road to just above the EarthCache site, where there is a turnout on the road. You may park at this wide spot and walk to the EarthCache.
 

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 (no need to go off trail for any measurements at this site);
• 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.
2. The number of people in your group.
3. The type of rock observed at the GPS coordinates is: A) decomposed granitic rock; B) metamorphic rock (of the Calaveras Complex).
4. Which Big Ideas (1-9) are connected (list)?
5. Which GeoPrinciples are relevant (list)?
6. 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. California Natural Resources Agency, “Butte Fire (CA AEU-024918)”, Post Fire Emergency Watershed Response Team Report: Incident File 1221_1956, October 12, 2015.

4. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.

5. Hill, Mary. 2006. Geology of the Sierra Nevada. University of California Press revised edition, Berkeley and Los Angeles, California 468 pp.

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. 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.

11. USGS Landslide Hazard Program, Butte Fire (Amador and Calaveras Counties, California), Preliminary Hazard Assessment Map, October 2015, https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=20.

12. Wagner, David L., Jeremy T. Lancaster, and Margie B. DeRose, 2012, “The Oak Creek Post Fire Debris and Hyperconcentrated Flows of July 12, 2008”, Inyo County, California: A Geologic Investigation, SPECIAL REPORT 225, Version 1.0, California Geological Survey, California Department of Conservation.
 

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