Pisgah Crater is a young basaltic cinder cone surrounded by lava flows that spread across the Mojave Desert. Although mining has altered parts of the cone itself, the surrounding basalt preserves clear evidence of volcanic processes. This EarthCache focuses on columnar jointing, a fracture pattern that forms as molten basalt cools and solidifies. By closely examining the rock at this site, visitors can observe how cooling lava responds to stress and why columnar basalt does not always form perfect hexagons.
Cooling of Basalt and Columnar Jointing
The lava erupted from Pisgah Crater was basaltic, meaning it was very hot, fluid, and able to flow easily across the landscape. Once the lava stopped moving, it began to cool from its exposed surfaces inward. As the basalt cooled, it contracted. This contraction created internal stress within the rock, and the stress was relieved by the formation of fractures. Rather than cracking in straight lines, the fractures spread in multiple directions and organized themselves into polygonal patterns that evenly distributed the stress. As cooling continued, these fractures deepened and extended downward, forming the beginnings of column-like structures oriented roughly perpendicular to the cooling surfaces.
Why the Basalt Here Is Not Perfectly Hexagonal
Columnar basalt is often shown as neatly formed, six-sided columns, but those ideal shapes require very slow and uniform cooling conditions. At Pisgah Crater, cooling conditions were more variable. Differences in lava thickness, uneven heat loss to the air and the ground, and changes in cooling speed caused fractures to adjust as they formed. Because of this, the polygons visible at this site commonly vary in size and shape, with a mixture of four-, five-, six-, and even more-sided forms. Some fractures curve or shift slightly as they respond to changing stresses during cooling. The irregular patterns visible in the basalt are therefore a realistic record of how the lava cooled, rather than an imperfection.
At Pisgah Crater, and especially in the basalt visible at this EarthCache, the columns are often:
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Irregular in size
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A mix of 4-, 5-, 6-, or more-sided polygons
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Slightly curved, offset, or incomplete
These variations occur because cooling conditions were not uniform. Factors such as:
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Changes in lava thickness
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Uneven heat loss to air and ground
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Differences in cooling speed
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Later cracking or minor movement
All influence how the fractures form and evolve. The result is a more chaotic — but very realistic — example of columnar jointing.
Works Cited
https://www.nps.gov/subjects/volcanoes/columnar-jointing.htm
https://www.geologyin.com/2015/10/mystery-solved-how-these-rocks-got.html
1. "Pisgah Crater - Columnar Basalt" on the first line of your message AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirit of earthcaching.
2. Take a photo of your favorite basalt column using some sort of scale. Geologists often use coins, GPS', pens, shoes, notebooks, ect. You might want to be creative in what you use for your scale, however, the scale that you choose is entirely up to you. Make sure your photo is close enough to capture the mineral grains which should be clearly seen! This photo MUST be attached to your "found it" log.
3. Describe the columnar basalt by telling me their (a) range of colors, (b) textures, (c) average heights or thickness, and (d) average widths.
4. On average, how many sides (or faces) do these columns have? Are they truly hexagonal or are they a different shape? Explain.
5. Based on your answer to number 5, how would you describe the cooling rate of the basalt? Was it fast or slow?