Columns of the Giants Fault Zone and Source EarthCache

This EarthCache teaches more about the geology of the Columns of the Giants Geologic Site, Stanislaus National Forest, Tuolumne County, California. You will visit and learn more about the probable source of the lava flow, the age, and the likely fault zone that exists at the vent location. Then you will observe a dike and estimate its physical and geometric characteristics.

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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: 1) the horizontal orientation (strike) of the geologic dike feature (number of degrees between 0 (north) and 360 (also north)); and 2) the vertical orientation (dip) of the dike feature (number of degrees between 0 (horizontal) and 90 (vertical), including the dip direction).
4. Is the rock similar to the rock at the base of Columns of the Giants? (Yes or 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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COLUMNS OF THE GIANTS FAULT ZONE AND SOURCE

The Columns of the Giants lava flow, named by Carol Virgin, wife of Terry Virgin, Forest Naturalist and Interpreter (retired), Stanislaus National Forest, in the late 1960’s, after visiting the Devil’s Postpile National Monument and comparing the columns at each site. Columns of the Giants (COG) is located in the Stanislaus National Forest on Highway 108, just east of Dardanelle, California. An age of 150,000 +/- 30,000 years was obtained by Brent Dalrymple in 1964

“…from an interglacial flow about 11 miles west of Sonora Pass in Secs. 29 and 30, T. 6 N., R. 20 E. (Dardanelles Cone quadrangle, U.S. Geological Survey, 1956 ed.). The flow crops out in the bottom of the canyon of the Stanislaus River and rests on what appears to be a glacial surface. Till and glacial striae occur on the upper surface of the flow.” (See Figure 1).

Studies on the glacial history of California and the central Sierra Nevada region were being done at that time and the lava flow covers an older glacial deposit of till (loose boulders, cobbles, gravel, sand, and silt) on the Eureka Valley floor. It also shows evidence of a younger glacial event since there are glacial striations (grooves made by rocks in the sole, or underside, of a glacier that permanently scratched, or gouged, the surface) in the bedrock lava. Additionally, there are glacial erratics, boulders of dissimilar rock types (granitic rock) transported by glacial ice from upslope, lying on top of the older, hardened, lava flow. The dating of the lava helped identify timing of the different ice age events during Pleistocene time (approximately the last 2 million years). 

Figure 1: Oblique view of Columns of the Giants, T6N, R20E, Secs. 29 and 30 where an age date of 150,000 +/- 30,000 years was obtained (Dalrymple, 1964).
 

When the lava erupted from its source (likely a series of dikes/vents mapped by Columbia College field geology students in 2012, upstream of the COG site) it flowed down the paleo-Stanislaus River channel. Somehow the lava ponded, allowing it to cool slowly and form joints, or cracks, that penetrated downward from the top of the flow, upward from the bottom, and inward from the sides of the flow in the valley as it existed at that time. Geologists hypothesize that the flow was likely dammed by some natural feature downstream that allowed for slow cooling and hardening of the columns seen (see Hughes, 2012, for possible models). They most likely formed as the cooling fronts intersected one another from the top down and bottom up. Columns tend to form when the lava cools differentially, loses some volume, and contracts. Contraction leads to tensional (pulling apart) stresses relieved by cracking, or jointing, usually at 120 degree angles that propagate away from the cooling front (See Figure 2). This is how the columns form – the joints, or cracks, grow away from the cooler air (downward), and the ground (upward), or even from other cooler surfaces such as a vertical slope or wall, a stream bank, etc. (horizontally or some other angle). They then intersect with one another, forming the structures seen at COG. At the end of the trail from the parking lot (west of the EarthCache location) there appear to be two main sets of columns. However, it is likely these are just one cooling feature that formed what is called an entabulature and colonnade structure (see Figures 3 & 4). The lava erupted into a river valley, then was emplaced below its own cinder accumulation and likely had joints (cracks) that allowed water to enter and cool the lava, distorting the columns into non-vertical shapes (see Figure 5). Glacial ice may also have been present at the time of the eruption, contributing water to the processes forming the entabulature. 

Figure 2: Two conceptual models of contraction for a hot lava flow. The whole-sheet mode (#1) is unlikely; the localized contraction mode (#2) is the most likely explanation of how columnar joints form (Railsback, 2001).

Rock samples from the lower and upper sections of the columns were sent to CSU Fresno’s geology department for analysis by Dr. Keith Piturka. The type of rock he identified for both sections is called a basaltic trachyandesite. Basalt, higher in iron and magnesium composition, is the common black lava seen in Hawaiian lava flows on the Big Island. Andesite is grayer, contains more silica, is more viscous, and tends to flow less easily – it is more pasty. The amount of silica in lava is the primary control on the viscosity, or resistance to flow, that the lava has. Basaltic andesite has a silica content between basalt and andesite and can flow readily, ponding and cooling. The “trachy” prefix indicates the rock is comparatively enriched in potassium and sodium, suggesting that it is undersaturated in silica, so quartz, the most common mineral on earth, is virtually absent from COG rocks. Finally, according to Piturka and Busby (2007), "high-K magmas record the onset of Walker-Lane style transtensional stresses". The significance of this will be discussed below.

Figure 3: Two-tiered columnar jointing in the basalt at Hloduklettur, NE Iceland. The lava is ponded in a river valley, and the irregular top is due to emplacement below its own cinder accumulation. 

Figure 4: Stages in the growth of Figure 7: L: liquid lava; F: master joint allowing ingress of water coolant; C: colonnade; E: entablature. Note how the cracks/joints grow perpendicularly away from the cooling surfaces, or fronts.

Figure 5: Image of the Columns of the Giants entabulature and colonnade.

Columns of the Giants is located west of 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. The 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 during Miocene time. The landscape in this region 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. As the 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. The MTJ currently is located off the coast of Northern California, near Eureka. It is presumed to be contributing to the formation of the Lassen Volcanic Center today, no longer influencing the Sierra Crest – Little Walker Volcanic Center (see Figure 5 in the Dragoon Gulch EarthCache, link here). 

What then, could be the tectonic explanation for the formation of Columns of the Giants, which is only ~150,000 years old? This is currently a geologic enigma – no one truly knows. We understand how Columns of the Giants formed, but not why. Since COG is in the Walker Lane Shear Zone, one possibility may be that the transtensional fault system responsible for the volcanic vents near Sonora Pass, also cuts through the Eureka Valley east of Columns of the Giants. Up to 7 basaltic-andesite dikes have been mapped by Columbia College field geology students (in 2012) along a NW-SE trending lineation (possibly a transtensional fault), within granitic rock, just east of the lava flow (see Figures 6 and 7). It seems likely the dikes were the source of the COG lava, though more geochemical and geochronological work needs to be done to confirm the hypothesis. The COG, part of the Sierra microplate, may have resulted from tensional pull-apart tectonic forces, releasing magma along a possible transtensional fault zone, proposed here to be named the “Columns of the Giants Fault Zone”, trending NW-SE through the Eureka Valley. Future geologic studies may help confirm, or deny, this hypothesis. An interesting aspect of the youth of the Columns of the Giants lava flow is wondering whether earthquake and volcanic activity will occur here anytime in the near future (wouldn’t it be amazing to have hot springs here?!).

Figure 6: Topographic map of the Columns of the Giants region.

Figure 7: Topographic map of the proposed “Columns of the Giants Fault Zone” shown in red. The vent that produced the Columns of the Giants lava flow may have opened up along this fault zone. Subsequent glaciation in Eureka Valley would likely have removed any evidence of the vent by erosion.

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 Columns of the Giants interpretive trail trailhead. To complete this EarthCache, you will estimate or measure several physical and geometric characteristics of a volcanic dike at the GPS location east of the trailhead, up Highway 108. A volcanic dike is a vertical sheet of rock that formed in a younger fracture within other rock (in this case granitic rock). As you locate the dike, carefully watch for traffic if you park on the north side of Highway 108, then cross over to the GPS location. You should be able to find the feature that looks similar to the dike shown in Figure 8 below. The COG dike cuts across granitic rock in a direction toward the Middle Fork of the Stanislaus River. BE VERY CAREFUL here since the river is commonly at a high stage, with potentially dangerous runoff from meltwater from the spring and summer snowpack! 

Figure 8: Students examining a 3 meter (~9 feet) wide intrusive basaltic dike at Acadia National Park in Maine.

To complete the EarthCache, if may help if you ask yourself the following questions. What is the color of the dike and how does that compare to the color of the columns at the end of the trail to the main Columns of the Giants site? Could the dike be the same type of rock? What is the grain size of the minerals in the rock composing the dike? Are grains visible (macroscopic) or invisible (microscopic)? Macroscopic crystals would indicate the lava cooled slowly, while microscopic grains would indicate the dike cooled rapidly and the crystals did not have time to grow big enough to see. Geologists measure the attitude – called strike and dip – of geologic features in order to try to infer what has happened to that feature during its geologic history. This is a significant skill in volcanic hazard mapping, where features that cut across others can be inferred to be younger, helping geologists piece together a sequence of events for a volcano backward through time, which can help us to understand the potential for eruption (or not). The diagrams below (see Figures 9 & 10) can help you to assess the horizontal and vertical orientation of the dike feature. Once you have measured the strike and dip for the dike, record each.

Figure 9: Strike (horizontal direction of the rock feature) and Dip (vertical direction of the feature).  The value for the strike of the layers in this diagram is east-west, which is 90 degrees. North is always 0 (or 360) degrees, while east = 90 degrees, south = 180 degrees; and west = 270 degrees. The approximate dip of the layers in this diagram = 45 degrees toward the north.

Figure 10: Using the right hand rule: 1. Place your right hand on the surface (if the feature you are interested in is visible). 2. Rotate your hand so that your four fingers point downward, toward the dip direction. 3. Finally, extend your thumb on the same plane. The direction which your thumb is pointing to is the direction of the strike.

Geoscientists have to be skilled in several different areas. They need to be somewhat proficient in: math, physics, chemistry, biology, surveying, geospatial technologies (GPS, GIS, remote sensing, and webmapping), lab technologies (microscopes, spectrometers, etc.), computer use, and more. Career opportunities in the earth sciences include: geologist, hydrologist, mapping/surveying and geospatial technologist, watershed analyst, mineralogist/mining engineer, petroleum geologist, environmental scientist, natural resources scientist, and more. If you are interested in learning more, take a course at your local community college to get started.

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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 site can be reached via Highway 108 to Dardanelle, then continuing another 1.6 miles to the parking area off of Hwy 108, to the right. The EarthCache is located another half a mile (0.5 mi.) east on Hwy 108. You can park in the dirt turnout area on the north (left) side of the road. Be very careful to watch for traffic on Highway 108 when crossing back over (south) to find the EarthCache GPS location. The geologic feature is next to the Middle Fork of the Stanislaus River, which, during spring season, can experience high runoff, so BE VERY CAREFUL! The dike’s 3D orientation can be measured away from the river’s edge, so no need to get too close to the water here. You can estimate the strike and dip from just below (20 feet or so) the roadway. The area is very hot and dry in the summer time and the best time of year to visit may be during spring when the wildflowers are in bloom.

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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