How Do Geodes Form?
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Description
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Geodes are spheroid-shaped, hollow rocks that have an inner lining of small crystals projecting inward. Geodes are natural phenomena formed out of millions of years of geological evolution. While experts don't know for certain how geodes are actually formed, there are theories among geologists about the processes that take place. There are generally two types of geodes. One geode is made out of lava. The other is made from dolomite, a limestone-like sedimentary rock. Each geode is created through different processes. All geodes, no matter how they are formed, must contain hollow bodies in which crystals can form.
How Geodes Are Formed From Lava
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Some geodes are formed out of molten lava rocks. After the molten rocks cool, they create gasses that, when dissolved, turn into bubbles. These bubbles turn into cavities once the rocks harden. The early stages of these cavities are called thunder eggs. According to experts, the minerals that form into crystals may have flowed into these cavities in two ways. Either the minerals could have come from the hot water flowing through the cracks in the lava rocks as they cooled, or they could have been deposited much later as mineral groundwater. However the convention, minerals such as quartz are introduced into the thunder egg through a flow of water, thus creating the opportunity for crystals to form.
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How Geodes Are Formed From Dolomite
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The formation of geodes from dolomite is much more complicated that those formed out of lava rocks. The theory of how dolomite geodes are formed came from geologist Robert Maliva in 1987. His theory goes that the geode cavity formed from a small, hard spheroid that later dissolved and became a cavity, a process that began over 350 million years ago in the warm, shallow waters that once covered what is now the central United States region. Lime sediments grew in the warm saltwater. When the saltwater interacted with sediments rich in calcite, the calcite metastasized into the minerals dolomite and anhydrite.
The anhydrite then turned into hard spheroid bodies inside the dolomite. Meanwhile, the dolomite changed from a sediment into a hard rock. Anhydrite will dissolve even in the slightest amounts of acid in water. Thus, after acidic water was introduced, outer areas of the anhydrite broke away and were replaced with quartz, which were in the beginning only small fibers of crystal. The centers of the anhydrite bodies were then dispersed completely. Quartz was fed through the rock from water rich with minerals, forming the crystals that grow inside. Over time, the rocks disintegrated, leaving the dolomite to open the elements and causing it to dissolve. The hardened quartz then separated from the dolomite, turning into geodes formed on the ground.
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Zea just busted open his first geode today! After checking online for explanations of how geodes formed I checked a book on "hydroplate Theory". In the light of a global flood involving massive geological forces, involving super critical water and liqufication it seems slightly more clear. Check out Walter Brown PHD book for more.... Aloha to all gh.
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I understand the cavity being called a "thunder egg". Perhaps I'm being obtuse when the sentence sounds like it's saying the rocks produce gasses AFTER cooling and then the gasses DISSOLVE and turn into bubbles. My perception has always been that molten rock contains dissolved gasses, that create bubbles upon the release of pressure, resulting from the magma (molten rock) moving from an area of high pressure (deep under ground) to an area of lessened pressure (Earth's surface.)
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Thunder eggs
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That's right Bob, and I could cite numerous examples like-"After the molten rocks cool.they create gasses, when dissolved, turn into bubbles." where try as I might, simply can't fathom what that might even mean.
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I'm not sure what you originally objected to, Gerald, but I found it to be muddled, horribly unclear, and presumptive of previous knowledge. Not good primer material at all!
