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What's So Tuff About Rhyolite? EarthCache

Hidden : 01/23/2009
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1 out of 5

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Geocache Description:

This earthcache takes you to a developed park with a wheelchair-accessible display of Castle Rock Rhyolite.

About 36 million years ago, there was a catastophic volcanic eruption near Mt. Princeton in the Collegiate Range, about 100 miles to the west. The eruption created a superheated cloud of pumice, ash, and rock that traveled to this area within about an hour. As it landed, it melted together under its own weight and formed a welded tuff, a glassy rock that has become known as the Castle Rock Rhyolite or Wall Mountain Tuff.

Castle Rock Rhyolite has been found throughout Douglas County, up to 20 feet thick in places. Nearby Quarry Mesa was mined for many years and the rhyolite was shipped throughout Colorado and neighboring states.

Park trails lead to the top of Quarry Mesa where you can see what is left of the original deposit. Park developers have placed some large specimens of rhyolite on display near the park entrance.

The earthcache is located at one of the rhyolite specimens. Examine the specimen and notice some of the features.

On the east side of this specimen, there is a small cavity lined with white crystals, similar to the inside of a geode. Lower down, you can see a former cavity that is completely filled with ochre-colored silica. There are also open cavities visible with no crystals. These cavities are bubbles of fluid or gases that were trapped when the rhyolite solidified.

If you look closely, you can see small specks of crystals. These were formed in the magma before it erupted and traveled downwind along with the molten ash. The tiny white needles are feldspar and the small black specks are biotite. These are the same minerals that you would find in larger crystals in Pikes Peak Granite.

This rhyolite is ideal for a building stone because of its uniformity, durability, and light weight. The glassy matrix does not have cracks that weaken many types of rock. When it does break, it breaks in conchoidal fractures similar to patterns in broken glass or other non-crystalline volcanic rock like obsidian.

To log this cache, answer the following questions in an email to the owner. Do not post the answers in your online log.
1. Estimate the size range of the cavities you can see in this specimen.
2. Estimate the size of the feldspar and biotite crystals in this specimen.

Post a photo of yourself and your GPS in front of the rock specimen.

Additional Hints (No hints available.)