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Long Valley Caldera Field Guide - Glass Creek Flow

Example of two magmas that mixed during an eruption.

Geologic Summary

Large Glass Creek rocks showing mixed magmas....
Large Glass Creek rocks showing mixed magmas.(Public domain.)

The north margin of Glass Creek Dome provides an excellent example of mixed magmas. Both magmas are rhyolites (high silica content), but they come from different storage regions and have physical characteristics that make them easy to distinguish one from the other. One is a crystal–poor black obsidian and the other is a crystal–rich light-gray pumice. The minerals in the crystal–rich pumice are plagioclase (white - Ca Na feldspar), sanidine (K feldspar), hornblende (dark blades), biotite (black shiny sheets when magnified), quartz, orthopyroxene (dark), clinopyroxene (dark), magnetite (magnetic), ilmenite, apatite (the mineral that makes up your teeth), and zircon (small) (Samson and Cameron, 1987). The zone where the two magmas are comingled (gently mixed) looks like a marble cake. If you look at the marble cake-like structures you may notice that the obsidian seems to flow around the pumice. The flow structures record the mixing process between the viscous (sticky) pumice and the relatively fluid (less viscous) obsidian. An analogy for the formation of this rock would be the combination of equal amounts of maple syrup and oatmeal stirred briefly.

Studies of the two Glass Creek magmas to determine their origins suggest that they came from different magma storage areas and erupted in 1350 C.E. (See the Obsidian Dome Field Guide page for more information about the 1350 C.E. eruptions). As it erupted, the lava flowed both north and south, the latter steeply down the caldera wall. The more fluid obsidian probably originated from the same Mono Craters storage system that fed the Obsidian Domeeruption. The more crystalline pumice most likely originated from the same magma storage system from which Deer Mountain erupted 115,000 years ago. The crystal-rich component is therefore the most recent eruption of material from the Long Valley storage system. It is currently believed that magma from that system has cooled and crystallized too much to erupt on its own.

Glass Creek Dome seen from highway 395, Long Valley Caldera....
View of Glass Creek flow from Highway 395 looking toward the west; the crest of the Sierra Nevada towers above. The flow descends the steep northwest wall of the Long Valley caldera.(Credit: Venezky, D. Y.. Public domain.)

Many rocks preserve evidence of mixing processes but often they are more subtle and can only be found by analyzing the compositions of the individual phenocrysts (minerals). The Glass Creek magmas most likely mixed during the eruption and did not have the time or ability (temperature, viscosity) to homogenize into new magma.


Sampson, Daniel E. and Cameron, Kenneth L., 1987, The Geochemistry of the Inyo Volcanic Chain: Multiple Magma Systems in the Long Valley Region, Eastern California., Journal of Geophysical Research, Vol. 92, No B10, p. 10,403–10,421.

Field Stop Location: Glass Creek Dome

Quadrangle: Mammoth Mountain, California 7.5 minute topographic quadrangle
Coordinates: about 37°44.810' N, 119°01.305' W
Approximate Elevation: 8,218 ft (2505 m)

Directions to Glass Creek Dome:

Obsidian Dome is accessible from a 2.7 mile dirt road. This stop provides access to Glass Creek Dome (a mixture of obsidian and pumice), Obsidian Dome (large flow with different obsidian textures), and explosion craters (large holes in the ground formed by a blast of steam, generally generated by gases from rising magma). Paths lead to the tops of the domes providing access to unique views of the domes. If you do not want to drive the full 2.7 miles, you can stop and view a part of Obsidian flow from a parking area on the left side of the road. At this stop you can see large chunks of obsidian with holes (vesicles) in them from the escaping gas and small minerals.

Directions from Mammoth Lakes exit U.S. 395 and CA-203 Go this distance
1. Zero your odometer at the intersection of Highway 395 and Highway 203. Head north on US-395 towards Lee Vining and Mono Lake. Go 11.0 miles
2. Turn left onto Obsidian Dome road Go 0.3 miles
3. Continue onto Glass Flow Road. 0.6 miles
4. Continue left onto Obsidian Dome Road at Obsidian Flow and Hartley Springs Campground turnoff. From here you will drive around the dome to its south side. 0.6 miles
5. Slight left at 2S10 0.7 miles
6. Continue onto Glass Creek Rd 0.3 miles
7. At the fork in the road, stay to the left to park near Obsidian Flow among the large Jeffrey pines (about 37°44.990' N, 119°01.292' W).
  • Walk back towards fork in road.
  • Take the middle fork and walk along the road across Glass Creek and up to the left around an explosion crater.
  • The base of Glass Creek flow will be ahead to the right and you will see the large marble-cake blocks.
If you have a four-wheel drive vehicle, you could take the middle fork and drive across the river to Glass Creek flow.