Near the posted coordinates you should see a noticeably different rock layer along the shore of the reservoir. This fossilized rock layer is from the famous Rockland Ash Layer, a volcanic eruption from a volcano near Mt. Lassen about 600,000 years ago!
Outcrops of an ash bed at several localities in northern California (including Scotts Flat) and western Nevada belong to a single air-fall ash layer, the informally named Rockland ash bed. Relations between initial thickness of the Rockland ash bed and distances to eruptive source suggest that the eruption was at least as great as that of the Mazama ash from Crater Lake in Oregon.
Volcanic rocks have a wide variety of textures and appearances, as can be seen in the image below:
•Glassy: smooth, glassy; indicates very rapid cooling; typical example is obsidian
•Aphanitic: fine, individual grains not visible to the naked eye; indicates rapid cooling; examples are rhyolite and basalt
•Phaneritic: individual grains visible to the naked eye; indicates slow cooling
•Pegmatitic: coarse; very large grains, often over 2 cm; indicates very slow cooling
•Porphyritic: contains crystals of varying sizes; formed via two-stage cooling whereby large crystals form slowly at depth and fine crystals form quickly at the surface
•Vesicular: porous, filled with cavities; develops when gas bubbles exsolve as magma rises to lower pressures (at higher pressures, the bubbles are kept in solution), and the lava then solidifies around these bubbles
•Pumice: vesicular rock of felsic composition
•Scoria: vesicular rock of mafic composition; compared to pumice, typically darker in color and denser, with thicker walls and larger vesicles
•Poikilitic: larger crystals contain small pieces of other minerals within them; typically indicates long cooling periods, allowing time for materials to diffuse into the center of the crystal.
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1. Xtra Flammin Hot (Red Sauce my Ash!) - GC6TYKG
2. What colors do you see amongst the volcanic rock visible here?
3. Based off the above chart, and the colorations in the ash layer, which volcanic rock type would you consider is at this location?
4. What does this tell you about how quickly or slowly this volcanic material cooled?
5. How does this rock layer feel to the touch?
Vg fubhyq pbire n jvqr nern nybat gur rqtr bs gur erfreibve.