Geodes: how are they formed?

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Geodes, the magic eggs of the universe. On the outside they seem to be ordinary rocks, but when broken or cut open reveal a cluster of wonders in the form of an inner crystal lining. Interestingly enough, the outer material of these rocks are far more resilient to their surroundings than the bedrock in or on which they are found. Makes a lot of sense considering the crystals inside often take thousands to millions of years to form (the smaller the crystal formations are, the younger the geode is). Think of the geode as a yolkless egg, with the shell being its host and the crystal clusters inside being the “membrane” which is made up of minerals building on top of each other. So now that we know what a geode is – if we didn’t already – how are they formed and where can we find them?

The formation of a geode is quite simple in the grand scheme of the universe, yet the product of the process is anything but. Where the largest quantity of geodes can be found are either in a) stratified volcanic deposits like basalts of tuffs, or b) stratified sedimentary carbonate deposits like limestones and dolomites. Many other types of environments may yield geodes, but in smaller quantities. Although geodes can be grouped into two main types, they can be formed in a multitude of ways.

Volcanic Geodes 

The most popular and in-demand geodes are those that are formed in areas of volcanic activity. These geodes are created in one of two ways: 1) when gasses become trapped in hardened lava or 2) when the outer layer of lava cools more quickly than the inside (when hot lava touches water), in which case the gas or molten lava later seeps out of its “casing” – elongated lava flow causes the formation meters-long geodes. Both instances leave a pocket inside the hardened rock into which groundwater can enter, carrying minerals which then grow on top of each other to form crystals. Volcanic voids are usually infilled with quartz, agate, or both and in some rare cases, even opal! 

Sedimentary Geodes 

Perhaps less popular than volcanic geodes, but nonetheless spectacular are geodes existing inside sedimentary rocks. These are typically found in limestones, dolomites and specific types of shale. The voids in these rocks inside which crystals form are often made by gasses and different types of fossils such as shells, tree debris, roots, and other organic materials which disintegrate over time, leaving the pocket behind. These are quite smaller than volcanic geodes and host quartz, opal, agate, and carbonate materials. The outer layer of sedimentary geodes are often made of quartz or chalcedony, which are left exposed after the host rocks have weathered away, making them much easier to find than other types of geodes. 

A geode’s color comes from the same minerals which form the crystals inside, but additional elements often enter to add their magnificence to the mix. Iron presents rich red or purple hues, whereas titanium creates blues and chromium makes greens. Pink hues typically come from manganese. Of course, these same elements are responsible for the type of crystal which will grow within. This is where the names come in. 

Geode names are often given based on the type of crystals which make up the interior, with the word “geode” in front. For example, Amethyst geode, or Agate geode. Others may also be given a geographic or stratigraphic name such as “Keoukuk geode” or “Brazilian geode”. 

Surprisingly enough, not all geodes contain crystals, but rather some will be broken open to reveal other interesting landscapes. One of the most common alternatives to crystal-lined geodes are geodes lined with chalcedony, a microcrystalline variety of quartz which is difficult to see with the naked eye. 

Often confused with geodes are other objects formed under similar conditions in the earth, yet hold a variety of different properties which can help to differentiate among them. One of the most common rock occurrences often confused with geodes is that of the Nodule, a solid rock composed of precipitated mineral material. They may have been hollow at one point, and then filled completely with said mineral material. Another nodule formation method is by mineral growth on a sedimentary surface, within a cavity or replacement of their host rock. Most similar to geodes are Vugs, which are cavities that may contain crystals, but do not have a strong enough lining to sustain the composition of a geode once it is removed from its host. Most vugs will not stay intact after its host rock has disintegrated. Another type of solid rock which (which is quite opposite to the geode) are Concretions, aggregates made up of sedimentary grains and a cementing material. When a chemical precipitation begins a nucleus in the sediment such as a fossil or grain, material accumulates around the nucleus to fill pore spaces or replace mineral grains. This is the formation of concretions, starting in the center and then growing outwards without a cavity. Perhaps the most similar to geodes are Thundereggs, spherical or subspherical masses of rhyolite that form out of disintegrating volcanic strata. Unlike concretions, they contain an internal cavity inside which agate, opal, and other mineral material can grow. 

The World’s Largest Geode has been met with speculation and debate as to whether it is indeed the largest or even classified as a geode, but most agree that it is at least one of the largest. Discovered in 1999, it was intersected by a passage of the Mina Rica silver mine. The “Geode of Pulpi” resides near Almeria, Spain and has a volume of approximately 11 cubic meters with with a width of about 12 meters. Its host is a Triassic dolostone and its mineral sequence begins with iron carbonates and barite, followed by celestine, and concluding with its cavity of gian gypsum (selenite). These crystals are colorless and transparent, often compared to large blocks of ice.