A tephra ring and dome, plus pumice and banded obsidian.
The name Panum Crater refers to a crater surrounded by a ejecta ring, with a dome in the middle. At Panum Crater the dome didn't completely fill the crater or overrun the ring (as often happens) providing an opportunity to explore all three structures. Panum Crater, part of the North Mono eruption episode, slightly preceeded the Inyo episode of 1350 C.E. and is the northernmost and youngest vent of the Mono Craters eruptions.
The geologic history of Panum is complex but a generalized summary follows (modified from Sieh and Bursik, 1986 and personal communications with Bursik). A phreatic (steam) eruption blew out an initial crater. The material that was thrown into the air by the steam, mainly old lake bottom sediments, was deposited around the new vent in little mounds. The explosion was most likely caused by magma moving towards the surface and heating the groundwater to steam. In addition to heating anything the magma comes in contact with as it rises, the reduction in pressure during ascent causes the gas within the magma to exsolve (bubble out of the liquid), much like CO2 comes out of a bottle of seltzer or champagne when opened. These gases drive explosive volcanic eruptions where magma is blasted out of a vent to varying heights depending on the gas pressure.
At Panum, a pyroclastic eruption (new magma explosively fragmented into the air) followed the phreatic(steam) eruption. During a pyroclastic eruption, the gas within the magma continues to expand and escape as the magma is thrown into the air and cools. The resulting deposits included ash (particles <2mm in size) and pumice. The pumice is frothy preserving the frozen gas bubbles.
Did you Know?
The word pumice refers to frothy magma and does not imply a size. If you want to use technical terms to describe the size of a volcanic fragment, you would use ash for particles smaller than 2 mm (0.08 inches) in diameter, lapilli or volcanic cinders for particles between 2 and 64 mm (0.08 and 2.5 inches) in diameter, and volcanic bombs or volcanic blocks for fragments that are larger than 64 mm (2.5 inches) in diameter.
After the explosive eruption, magma rose out of the vent forming a dome (imagine toothpaste coming out of a tube). It is likely that the extruding lava formed large spines that became unstable and collapsed. The dome collapse caused a block and ash flow (flow of ash and angular rock fragments larger than 256 mm or 10 in) towards Mono Lake. The dome we see today was erupted later, after the tephra ring was deposited.
Strombolian Eruption Forms Ejecta Ring
The block and ash flow was followed by a strombolian eruption (see image of arcs of lava). The strombolian eruption deposited the ejecta ring that we see today. After the ejecta ring was built, the dome we see today was extruded in four parts. The erupted materials are all rhyolite (high silica volcanic rocks) consisting of light gray pumice and black obsidian (see image of flow banding).
The Ejecta Ring
A path from the parking area leads onto and around the ejecta ring. Another path leads from the ejecta ring down into the crater and up to the summit of the dome. The ejecta ring is made up of small bits of pumice, ash, obsidian fragments, and well-rounded granitic pebbles (which were part of the surrounding rock and not formed during the eruption) that were ejected during the final explosive stage of the eruption. The ejecta ring path provides views of the outside of the dome and of Mono Lake. Walking on the ejecta ring is a little like walking on a beach with views of Mono Lake, the crater, and the exterior of the dome.
The Lava Domes
The central lava dome was erupted from degassed material and is made up of pumice and obsidian of the same composition. The difference between the two has to do with gas escaping as the magma cooled. The magma that created the dome had dissolved gas in it, like a bottle of seltzer water. As the magma rose towards the surface where there was less pressure on it than at depth, the gas expanded producing the holes (bubbles) you see in the pumice. The magma that remained pressurized while it cooled quickly or that had already lost its gas, formed the obsidian.
Flow banding containing both obsidian and pumice is common at Panum Crater. Another common texture, called breadcrust, can also be seen in the dome. Breadcrust textures form when the inside of a cooling rock is still hot with gas escaping from it while the outside surface has already cooled. As the gas expands from the inside, the outside surface cracks to allow the gas to escape. The obsidian from Panum was used by the Pauite Indians for arrowheads and trade.
Sieh, Kerry, and Bursik, Marcus, 1986, Most Recent Eruption of the Mono Craters, Eastern Central California, Journal of Geophysical Research, Vol. 91, No. B12, p. 12,539–12,571.
Field Stop Location: Panum Crater
Quadrangle: Lee Vining, California 7.5 minute topographic quadrangle
Coordinates: 37°55.537' N, 119°02.923' W (hand held gps)
Approximate Elevation: 6,830 ft (about 2,082 m)
Directions to Panum Crater:
|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 U.S 395 towards Lee Vining and Mono Lake.||Go 20.3 miles|
|2. Turn right onto CA–120 E.||Go 3.1 miles|
|3. Turn left at the Panum Crater sign onto a dirt road.||Go 0.9 miles.|
|4. Park in the parking area and follow the trail onto the tephra ring.|