This earthcache will bring you to St Andrews State Park. There is an $8 entrance fee per vehicle to enter the park. Permission was granted to place the earthcache here. You will learn about geodes and send some answers to questions to me. You will be going to the limestone jetty and observing the rocks.
What Are Geodes?
Geodes are spherical to subspherical rock structures with an internal cavity lined with mineral materials. They have a durable outer wall that is more resistant to weathering than the surrounding bedrock. This allows the geode to survive intact when the surrounding bedrock weathers away. The mineral lining the cavity is often a scintillating druse of tiny quartz crystals underlain by multiple bands of translucent gray and white agate. Many are lined with more spectacular treasures.
The Public’s Love Affair With Geodes
Most geologists enjoy geodes. However, the general public has a love affair with them. They are delighted and amazed that an uninteresting rock can contain a beautiful cluster of gemmy crystals, or a colorful lining of banded agate, or both of those in the same cavity. Broken open, or sawn and polished, people who have never taken a geology course buy thousands of tons of geodes each year. They buy them because they enjoy them. They love tiny geodes as jewelry, sawn and polished geodes as bookends, and spectacular amethyst geodes as items of home or office decor.
In several parts of the world, geode localities have spawned profitable industries that collect them, prepare them for market, and ship them to destinations where they are purchased as items of science, natural art and enjoyment. Brazil, Uruguay, Mexico, and Namibia are four examples of countries where geodes have become a local industry.
Geologic Occurrence and Formation
Geodes are not found randomly here and there. Instead they are usually found in large numbers in areas where the rocks have formed in a special geochemical environment. Most geodes localities are in A) stratified volcanic deposits such as basalts and tuffs; or B) stratified sedimentary carbonate deposits such as limestones and dolomites. A diversity of other environments yield a small number of geodes.
Geodes form in many different ways, and there are a number of valid theories about their formation. The intent of this article is not to provide a single or a comprehensive coverage of the many ways that geodes might form.
Sedimentary Geodes
Geodes in sedimentary rocks are usually found in limestones, dolomites, and calcareous shale. In these deposits a gas-filled void can serve as the opening for geode formation. Shells, tree branches, roots and other organic materials often decay away to leave a void for the formation of mineral materials. These cavities can be filled with quartz, opal, agate or carbonate minerals. They are generally smaller than the geodes formed in volcanic rocks.
Geodes are most easily collected when their host rocks have weathered away. This can occur because basalt, limestones, dolomites, and shales weather much more readily and rapidly than the quartz and chalcedony that typically form the outer layer of a geode. The host rock weathers away and the geodes are left on the surface, washed into a stream, or stranded in a residual soil. In these situations the geodes are easily found and collected. Some geodes are produced by mining the host rock, but that method is difficult, costly, and often damages the geode.
Information from Geology.com
To get credit for this earthcache, please email or message me the answers to these questions:
1. What color is the insides of the geodes in the limestone blocks?
2. Describe in your own words how geodes form.
3. Are the geodes the same size that you see in the limestone blocks?
4. Post a picture of yourself (face not required) or a personal item at the jetty.