Know before you go: Posted coordinates are for parking. Use the provided waypoint (located several feet away) to help answer the logging tasks.
Safety Information:
As per the NPS at Grand Canyon, it is crucial to
- “Stay on designated trails and walkways and always keep a safe distance if at least six feet (2 m) from the edge of the rim.
- In areas where there is a railing or fence, do not climb over the barrier.
- Keep an eye on all of the people in your group, especially small children. Make sure that your travel companions have both feet firmly planted on pavement or developed trails at all times.
- Know where the edge is. Watch foot placement and look for trip hazards.
- Do not run, jump, or perform physical stunts when near the rim.
- Do not back up without first looking where you are going”
In addition, it is illegal to collect rocks, fossils, and minerals from any National Park unit.
For more safety information, please visit https://www.nps.gov/grca/planyourvisit/safety.htm.
What are geodes?
Geodes are spherical rock structures with an internal cavity lined with minerals. They have a durable outer wall that is more resistant to weathering than the surrounding bedrock. This allows the geode to survive intact when the surrounding bedrock weathers away.
Image 1: Example of a geode
Image 2: A collection of geodes embedded in rock
How are geodes formed?
Volcanic geodes are the most widely known and sought after geodes and are formed as the outcomes of volcanic eruptions. Magma that erupts from volcanoes contains a significantly high percentage of dissolved gas with an incredible amount of weight. As the lava rises to the surface, the gas expands in proportion to the pressure reduction, and while most of it escapes, some of it gets trapped in the lava. This leads to the creation of voids or gaps in the lava. Similar voids are also created as liquid lava flows out from partially solidified lava, producing small lava tubes. Hydrothermal water or groundwater erodes and delivers minerals like opal, agate, quartz, and others, which accumulate in the voids of solidified basaltic lava flows. Over time, these mineral deposits in long lava tubes develop into some of the largest and longest geodes. Many of these have the geometry of long tree branches, having a diameter of a meter and stretching several meters in length. Most volcanic geodes are found in parts of California, Nevada, and southern Arizona.
Minerals commonly found in geodes:
Image 3: A geode containing amethyst
Amethyst is discovered in the form of thousands of smaller crystals in huge and hollow geodes known as cathedral geodes. It ranges in color from a light violet pink to deep purple.
Image 4: A geode exhibiting calcite crystallization
Calcite is the most abundantly found mineral in geodes. Large calcite crystals form when magma cools slowly once reaching the surface of the earth and there is a large supply of mineral-rich groundwater. Dolostone that contains 50% of the mineral dolomite, also contains calcite crystals. Its color is white.
Why aren’t geodes found naturally at the Grand Canyon?
The Grand Canyon is part of a larger geologic formation known as the Colorado Plateau. This plateau was formed by the process of uplift. This process involves pressure from inside of the earth moving rock fragments toward the surface. Once at the surface, these weathered fragments of sediment and debris begin to accumulate and form sedimentary rock through the processes of sedimentation and compaction. In this part of the United States, it is extremely rare for sedimentary rock to produce geodes. At the GZ, the geodes that are found at this viewpoint of the Grand Canyon are embedded in a rock wall consisting of igneous rock from southern Arizona. Naturally-occurring rock at the Grand Canyon is found in different layers of rock (also known as strata).
Image 5: This is a stratigraphic column of the Grand Canyon. Note that sandstone, limestone, and shale are all types of sedimentary rocks.
Image 6: A map of the United States depicting the distribution of igneous rock (extrusive and intrusive types). Notice that the approximate location of this EarthCache is in green.
Logging Tasks:
- At the GZ, take a look at the rock wall. You should observe geode crystals in the rock. What is the size, texture, and appearance of the geodes?
- Using information you’ve learned in the description, why do you think that the geodes in the rock wall at the GZ are not naturally formed here at the Grand Canyon?
- What mineral seems to be found in the geodes?
- As of June 2019, earthcaches now contain required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Sources:
https://www.backyardnature.net/g/rox-ign.htm
https://nayturr.com/types-of-geodes/
https://geology.com/articles/geodes/
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