Devitrified Obsidian EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(other)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
Obsidian is a glassy material that was used for many years by the native Americans to make the arrowheads, spear-points, and other tools that they usedin their daily lives. Many people locally refer to it as flint though it is not in the same category as true flint.
Obsidian is made up of the same material as Granite, or Rhyolite. Those have cooled slowly and formed crystals in the rock. Obsidian has cooled differently and is not composed of the mineral crystals like those, and is not a true rock. It is really a congealed liquid with tiny microscopic crystals and other impurities.
Granite cools very very slowly. The crystals that you can see it it form over hundreds even thousands of years. Creating large crystals.
Rhyolite cools at it approaches the surface. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra/tuff (ash) or of stones cast from the eruption.
Once and a while the lava arrives at the surface quickly, and very hot. The water molecules that are in the Lava evaporate out quickly as steam and the remaining quickly cooling material is the liquid obsidian, or glass. It is very thick and pasty. It then cools so rapidly that the crystals cannot form, or separate. Forming an amorphous solid. So you have a sheet of cooled black glass.
Obsidian is fairly unstable. As it gets older it changes from glass to a rock. The silica modules rearrange into a crystal. Leaving the black glass form behind, and changing into something else.
The quartz in it can begin to form larger crystals that look like snowflakes or white crystals in the obsidian.
They can also cause the obsidian to start to breakup and look like coal. Breaking into strange crumbly shapes when struck.
The process is referred to as devitrification. Obsidian will break like glass, forming very sharp surfaces, however when devitrification occurs it starts to change.
Look at the stone outcrop by the road. Take some pieces and try to break them and answer the following questions.
1-What direction do you think this obsidian magma flowed.
2-Does there appear to be many flows (layers)
3-Is this is obsidian devitrified, and in what manner? (Snowflake, crumbly, or both) Do see see any of the other nearby.
4-does any of this appear to not be devitrified? is it a small percentage of the outcrop or a lot?
Additional Hints
(No hints available.)