Access to Obsidian Dome is along a couple of mile dirt road that is not plowed in the winter.
Eruptions at this location began in the late summer of 1350 based on tree ring data. As the magma moved toward the surface along a dike, it encountered the groundwater causing a phreatic eruption. In this type of eruption, the groundwater flash boiled, exploding material up and out. The eruption continued with gas filled magma projecting material up into the air in a pyroclastic eruption. In this part of the eruption ash and pumice was ejected out around the vent. Once most of the gas had left the erupting magma, viscous lava oozed out of the dike forming a dome. The high viscosity of the lava did not allow it to flow far before it cooled and stopped flowing. The dome built up in places 300 feet high.
The dome, as its name implies, is made of obsidian, but also includes porphyritic rhyolite. Obsidian is a glassy volcanic rock that cools so quickly that crystals can not form. Some portions of the obsidian still retained some gas so it cooled with air bubbles in it. The air bubbles are called vessicles. Within the obsidian, you can see flow-banding that is the result liquid lava moving past the solid outer shell of the flow. In areas that cooled a little more slowly, crystals grew, forming rhyolite. Because some crystals grew larger than others the rhyolite has two distinct sizes of crystals, which is called porphyritic.
Logging questions:
- The text "GC3KRJD Obsidian Dome - Long Valley Caldera" on the first line.
- The number of people in your group (put in the log as well).
- Can you find flow banding in the obsidian? How thick are the bands?
- Can you find the porphyritic rhyolite? What is the difference in size between the two crystal sizes?
- Can you find the vesicles? Do the number of vesicles change the color of the obsidian?
The following sources were used to generate this cache.
- USGS Volcano Hazards Program, Long Valley Caldera Field Guide – Obsidian Dome http://volcanoes.usgs.gov/volcanoes/long_valley/long_valley_sub_page_19.html
- David R. Jessey. 2007.Cenozoic/Mesozoic Volcanism of the Eastern Sierra Nevada