What happens when you get really viscous rhyolite (high silica content which makes it very sticky) magma rising to the surface? Well, it either stops, produces a really big bang, or oozes. When it stops below the surface it forms granite, which we see a lot of nearby in Yosemite. A build up of gasses that produces very high pressures can result in an explosive eruption, like certain eruptions that have occurred in the past at Yellowstone and Long Valley calderas. When the conditions aren't right for an explosive eruption, a more quiet 'oozing' of lava occurs at the surface that creates some really fantastic looking rocks! If you want to see a great example of rocks where you can see how they moved, head over to the Inyo domes volcanic chain near Mammoth mountain in California. The Inyo domes are near the edge of the Long Valley caldera, Yellowstone's less infamous cousin, west of the Mono domes chain.
The Inyo chain is a group of rhyolitic domes and flows that was erupted about 550 years ago (see Miller, 1985 for more information) with a series of phreatic (steam) explosions and thick lava forming domes and thick flows. Two different batches of magma mixed together to give an array of intermingling textures, resulting in an incredible place worthy of a geological or touristic trip - with beautiful views!
The only recent obsidian flows that have been observed actually moving can be seen here with work done by volcanologist Hugh Tuffen at Cordon Caulle volcano, Chile, in January 2012.
Here are a few of the textures I saw at the Inyo domes, you can see different bands of black, obsidian and lighter grey pumice containing feldspar crystals (phenocrysts). When you look at the textures and patterns you can imagine the lava flowing up the volcanic conduit and on to the surface where it sits today.
These next few images are from Obsidian Dome/Flow:
|Vesicles (bubbles) within the grey pumice, with the black glassy obsidian.|
|Some neat banding patterns.|
|Vesicular and crystal-rich pumice band.|
|You can see the mixing of two batches of magma here with the lighter and darker greys.|
|Spot the evidence left by previous geologists studying these rocks!|
|Banding around a lighter grey zone (brown spots are biological, not rock).|
|Thicker obsidian bands with the light grey pumice.|
|A lens of light grey pumice.|
|Concentric patterns in the rhyolite.|
|The columnar basalt of the Devils Postpile.|
Miller, C.D., 1985. Holocene eruptions at the Inyo Volcanic chain, California: Implications for possible eruptions in the Long Valley caldera, Geology, 13, 14-17, 1985.