Collecting is prohibited.
Between 1325 and 1365 AD a series of explosive events erupted creating Panum Crater, the youngest mountain on the continent and an archetypical example of a plug dome.
As shown on the informational panel at the trailhead, Panum Crater formed in a series of events. It began with magma rising up through the earth. Eventually the lava came into contact with the ground water where the water explosively boiled creating a crater. The magma then explosively erupted creating a ring of pumice, cinders, and ash. Lava then slowly exuded up into the crater creating the dome. As the dome cooled and contracted, the final lava was squeezed out into spires that later toppled.
Panum Crater is typical of a plug dome. The dome forms as the thick lava cools before it flows away from where it erupted building up upon itself. It is also considered a plug because the lava cooled within the vent through which the lava traveled to the surface.
As you walk up to the coordinates, examine some of the rock that make up the outer crater until you find a piece of rock that appears filled with holes. This rock is pumice. It is formed by gas filled lava that cools so fast the gas bubbles are frozen within it. Compare this with the rock at the coordinates.
The rock at the coordinates is obsidian. This rock is actually chemically identical to pumice but differs in how it cools. Obsidian cools a bit slower than pumice allowing the gas to escape, but fast enough so that crystals don’t form. It acts like glass and was in important resource for the native populations of the area.
The rock pillars at the coordinates also show flow banding. Flow banding forms from flowing lava that allows bubble- and crystal-rich layers to form on the surface. Multiple flows build upon each other to create the multiple lines within the rock.
This entire chain of volcanos is only dormant. These volcanos could erupt at any time, but don’t worry, they should give plenty of warning before the erupt again.
Logging questions:
- The text "GC3JK36 The Youngest Mountain on the Continent" on the first line.
- The number of people in your group (put in the log as well).
- What type of lava erupted at Panum Crater and what percentage was silica?
- Describe the weight difference between the pumice found on the rim and the obsidian found on the dome.
- Describe the thickness of the flow banding found at the coordinates and patterns it makes. What might account for that pattern?
The following sources were used to generate this cache.
- National Forest Service informational panel
- USGS Panum Crater, http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/panum_crater.html
- Renee Murdock, June 17, 2004, Geology of the Sierra Nevadas, Mono Basin Volcanism USGS Volcano Hazards Program, Long Valley Observatory, http://lvo.wr.usgs.gov/gallery/30714277-001_caption.html