Just north of Alturas, California on the Modoc Plateau are a smattering of interesting looking pinnacles all formed from volcanic rhyolite ash. For ease of access I have guided you to a roadside pinnacle. There is a pull-out right next to this rock but be sure you pull ALL THE WAY OFF for your safety.
If you want to visit the famous and historic Chimney Rock you can take the short loop section of "old highway", a public road, to that site. Look for highway department signage about 3/4 mile south and 3/4 mile north of the cache coordinates. The north entrance might be muddy during wet weather but the south approach should always be passable. The interesting history of Chimney Rock is told in this Waymark.
Volcanic ash and pyroclastic flows (a combination of very hot rock, debris and ash flowing along ground or water surfaces) are very prevalent in the Modoc Plateau as are flows of dark basalt. Along and across the highway from here are exposures of white rhyolite ash of this formation and a few miles south we see a basalt flow on top of the rhyolite. The ash at this location was weathered or eroded into these conical shapes sometime in the distant past, perhaps from the effects of the ice age rivers that periodically flooded this valley.
Rhyolite is an igneous rock (in the form of rock, pumice, obsidian or ash) and it is the extrusive form of granite. Rhyolite and granite from the same source have the same chemical composition with the only difference being that granite forms and cools below the earth surface (intrusive, also known as plutonic) and rhyolite above (extrusive, aka volcanic). These are the lightest in color and density of the igneous rocks.
In geologic literature "The Rock Cycle" describes the three types of rocks of the earth's crust. Getting into that here is beyond the scope of this earthcache but you can click on the links in the reference section below for more information.
Basalt, rhyolite ash, pyroclastic and volcanic mud flows erupted in the Modoc Plateau for a long period of time during the mid to late Cenozoic Era and from four different sources: the older no longer active Western Cascades intruded from the north and the Ancestral Cascades (Sierra Nevada volcanic range) from the south; the modern High Cascades; the Basin and Range activity. The 2010 Geologic Map of California identifies this immediate area as "undivided Pliocene nonmarine" which I take to mean it has been generally dated but not fully studied. It does tell us that this ash deposit could be from any of the above volcanic periods except the High Cascades.
Rhyolite eruptions are interesting in that they can be either very violent or very tame. The difference is in how much gas (steam and carbon dioxide) is trapped in the magma and at what internal pressure. The higher the gas content and pressure the more explosive the eruption.
Rhyolite ash is a product of extremely violent volcanic explosions which can be far more disastrous than the primarily dacite eruptions of Mt. Saint Helens in the 1980s. Rhyolite eruptions can spread measurable ash fall for hundreds to thousands of miles and pyroclastic flows can travel many tens of miles.
Rhyolite can also erupt quietly into relatively low domes when compared to the tall composite volcanoes such as Lassen and Shasta. These domes typically include great quantities rhyolite rock, pumice and obsidian. They are generally symetrical but sometimes produce flows out and away from the dome but only for short distances.
The ash we see along the highway here has traveled far from the source volcanoes.
To log this cache:
Send the answers to #2-#4 to me through my geocaching profile. Since the advent of the Message the Owner feature, I prefer messages through that venue.
I will only respond if you have incomplete logging requirements. Go ahead and log your cache.
1. List the name “GC1VWJW Chimney Rock Earthcache” in the first line of your email. Also, list the number of people in your group.
2. Walk up to the pinnacle and observe the rock composition. Describe the rock consistency or luster where it is not covered by the dark growth. (Does it look granular, shiny, greasy, transparent, milky?)
3. Describe the type of rock you see. (Sedimentary, metamorphic, igneous?)
4. (Per current gc.com guidelines, photos are no longer allowed to be required. HOWEVER they are encouraged, since they can help clarify that you have visited the location if your other logging requirement answers are vague). Take a picture of yourself/group with a pinnacle clearly visible behind you. You may find it extra special to try to creatively include the "window" that is in the pinnacle.
Roadside Geology of Northern and Central California "Second Edition", 2016
(Igneous Rock)(Metamorphic Rock) (Sedimentary Rock) (Photos of Igneous Rocks)