Mt. St. Helena Volcanics EarthCache
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This earthcache is located just northeast of St. Helena California at a roadcut roadside pulloff (Please be careful...this pulloff is on a sharp corner, so be sure to COMPLETELY and safely pull off!). The pulloff and earthcache location are EAST of the road.
Mount Saint Helena, part of the now extinct Sonoma Volcanics chain, erupted at least once in ages past, in a lateral blast similar to that of Mt. St. Helens in 1980. This blast leveled the trees in the Petrified Forest near Calistoga, and eruptions within the same time period (all things being relative of course) also created the Obsidian found on “Glass Mountain” near the towns of Deer Park and Angwin, east of St. Helena, CA.
Obsidian is one of a group of silica minerals (Rhyolite and Granite being two others with a similar chemical composition) formed by rhyolitic volcanic activity. These minerals require a highly viscous (sticky like peanut butter), silica-rich magma with a large amount of water. The difference is the location and placement when these chemicals cooled. Granite-forming magna cools well below the surface, providing ages of time for crystals to form from the silica in the presence of cooling water. Closer to the surface, Rhyolite forms, with much smaller crystals, largely because the water escapes so much faster, mostly through steam. Obsidian is the result of the rhyolitic magma that has lost a lot of its water content as the magma reaches the surface. As a result, Obsidian is not technically a mineral, since it contains no crystals (non-crystalline) – the water didn’t cool slow enough for it to form any. However, the purity of Obsidian is also a benefit of this rapid process – time was short and thus few mineral or other impurities had a chance to mix with the viscous silica rich magma! When small amounts do “mix-in” (such as iron), the result are different colors of obsidian. Interestingly, the percentage of silica & water and the amount of impuries present in a bed of Obsidian are a geological “fingerprint” as it were, of the Obsidian, meaning that any piece of Obsidian can be chemically trace back not only to the area it came from, but even to the eruption that caused it. Obsidian is typically dark, though black, gray, and reddish hues all exist. In addition, rainbow, green, and gold “sheens” are seen and desired by rockhounds.
Because of its lack of mineral structure, obsidian has been used for thousands of years to make weapons, tools, and sharpened points. The rock is able to be fractured to almost a molecular thinness! Throughout this area one occasionally finds obsidian arrowheads that demonstrate this use.
In the case of Glass Mountain, the Obsidian is present in layers, presumably from several eruptions, but is largely embedded in “chunks” in the “host rock.” The Obsidian is dark shiny black, though it will take a moment or two for you to likely see any, since the weathering by the elements, and the light covering of the host rock make it hard to see the black “glass like” shine. When observing (or collecting) Obsidian, you are looking for “black glass” that splits in a concoidal fracture pattern. Since there are no crystals, there is no “flat break” nor dependable point of fracture. Thus, to find Obsidian at this location (its is present all over in front of and underneath you!), you should probably first look down and look for dark shiny black pieces that have been fractured (broken) by the wheels of vehicles using the pull off.
Mohs scale of relative hardness list a scale of 1 (Talc—think powder) to 10 (Diamond) of relative Mineral hardness. Now look at a piece of the lighter host rock and the darker Obsidian and observe their relative hardness to each other. You can determine which is harder by taking a piece of each in hand, and attempting to “scratch” one with the other. A “SCRATCH” if left by the “scratching” rock is the harder one! Conversely, if the "scratching" rock leaves rock fragments instead of a scratch, it is the softer rock.
To log this cache:
1. (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, your GPS, and the ROCK FACE. Post it with your log.
2. Tell me which is the “harder” rock (host rock or Obsidian)
3. Why do you think it is harder?
Logging: Since the advent of the "new" Message the Owner feature, I prefer messages through that venue. DO NOT post these answers to your log or it will be deleted!
I will only respond if you have incomplete logging requirements. Go ahead and log your cache
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