This is Throwback Thursday #233. In these, we look back into the past at ESCONI specifically and Earth Science in general. If you have any contributions, (science, pictures, stories, etc ...), please sent them to [email protected]. Thanks!
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ESCONI held a geode field trip to Vickers Geodes in Hamilton, IL in August 2024. The area around Hamilton, IL, and Keokuk, IA, is home to several sites where geode collectiing is popular. Throwback Thursday #14 looked back at past ESCONI geode field trips to the area. But what exactly is a geode, and how do they form? Below is an excerpt from the Wikipedia page explaining their formation.
A geode (/ˈdʒiː.oʊd/; from Ancient Greek γεώδης (geṓdēs) 'earthlike') is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter (which may include crystals) are secluded. The crystals are formed by the filling of vesicles in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same or other minerals precipitated from water, groundwater, or hydrothermal fluids.
Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lava; or, as in the American Midwest, in rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.[1]
When cut in half, visible bands corresponding to varied stages of precipitation may at times show patterns that reveal points of fluid entry into the cavity and/or varied colors corresponding to changes in chemistry.
ORIGIN OF GEODESGeodes are usually globular although they may also be irregular, discoid, or sometimes shaped very much like fossils. They are usually found in limestone, but they may also form in shaly rocks. Most of them are hollow, but many have become filled with minerals growing from the walls inward.A typical geode sawed or broken in two will disclose a sequence of layers from the out-side-in as follows: (1) a thin layer of clay: (2) a layer of noncrystalline chalcedony; (3) crystals (usually quartz) projecting into the hollow interior. Less commonly calcite or dolomite crystals will form next to the outer chalcedony layer instead of quartz, and sometimes the inside of a geode will be nothing but chalcedony; and (4) a deposit of olin, aragonite, millerite, chalcopyrite, sphalerite, limonite, smithsonite, malachite gypsum, fluorite, barite, marcasite, geothite, pyrolusite, and possibly tenorite and chalcocite. Perhaps the most thought provoking and rarest of geodes are those which contain petroleum or some thicker bituminous material.By what processes and under what conditions did these interesting features originate? There are many theories, none of which are completely adequate. The following discussion is an attempt to compile some of them into a brief summary.First of all, it is generally agreed that geodes are cavity fillings. The agreement ends here, for the stumbling block is the origin of the initial cavity. One idea is that the cavities are "vugs" caused by gas pockets or by shrinkage of the rock. However, vugs are integral parts of the rock in which they are contained, whereas geodes are complete entities which can be broken out of the rock formation with comparative ease. Some geologists have suggested that they are merely special types of concretions, but geodes grow from the outer shell inward, whereas concretions build up from a central core. Bassler (1908, pp.133-154) has shown that some geodes originate in fossil cavities and upon growth of the geode, the fossil bursts. Upon further growth, the fragments of the fossil are dissolved or absorbed by the growing geode and are lost. Van Tuyl (1916, pp.34-42) believes that the original cavity is the space which was occupied by a concretion. Concretions are easily removed from the rock by percolating waters and would thus leave a likely cavity in which a geode could grow. The fact that some geodes contain calcareous clay concretions lends support to this theory.Pettijohn (1957,pp.204-205) gives a rather complex process by which geodes grow after the formation of an initial cavity. This process may be summarized in the following steps: (1) a cavity is formed in the rock by some means; (2) a salty solution fills the cavity and pore spaces in the rock; (3) a layer of gelatinous silica is then deposited, isolating the salt solution in the cavity; (4) later the water in the surrounding pore spaces becomes fresh. This sets up what is known as an osmotic cell. This particular osmotic cell consists of two different types of solutions separated by a membrane of gelatinous silica which will allow the fresh solution to pass into the geode cavity, but will not allow the salt to pass out of it; (5) the fresh water flowing into the cavity by osmosis builds up internal pressure which pushes on the walls of the geode; (6) this pressure, exerted outward against the surrounding limestone, dissolves the limestone, leaving an insoluble residue which becomes the thin clay layer on the outer surface of the geode; (7) the above process continues until the salt solution is so diluted by the incoming fresh water that the osmotic cell no longer operates. The geode has reached maturity; (8) gradually the silica gel. dehydrates and crystallizes; (9) shrinking and cracking then follows: (10) finally, mineral-bearing waters flowing through the cracks deposit the innermost layer of minerals. These cracks may eventually seal, leaving a completely closed geode.The process by which some of the geodes of the Warsaw beds come to contain petroleum is also very much a mystery. Frank Fleener (1961) gives an interesting account of the problem. He envisions the petroleum having migrated up into the Warsaw Formation from the oil-bearing rocks to the south. There partially formed geodes were found with loose quartz crystals (some doubly terminated) adrift in the thick bitumen. The influx of the bituminous material stopped the growth of geodes, but the mechanism by which the bitumen was enclosed and hermetically sealed remains a matter of conjecture. We believe that many of these geodes are hermetically sealed because. the bituminous material will sometimes squirt out with force when the geode is punctured. This phenomenon is presumably due to the sudden expansion of the material when the pressure under which it was formed is relieved.A more plausible explanation for the petroleum is that it was derived from the enclosing shale and shaly limestone. The weight of overlying sedimentary rocks could easily have freed hydrocarbons from the organic matter in the shale and shaly limestone. The hydrocarbons then migrated to zones of lowest pressure, and these most likely would be cavities inside the geodes. It appears logical that such a pressure difference would exist because the hard shell of the geode could withstand a great amount of lithostatic pressure.The above discussion of the origin of geodes is incomplete and generalized, but we hope that it will stimulate interest in these remarkable geatures. Perhaps as you break them open in search of beautiful crystals, you will reflect upon their history and feel a greater appreciation for the intricate processes by which nature is continuously altering the crust of the earth.REFERENCESBassler, R.S., The Formation of Geodes, with remarks on the Silicification of Fossils: National Museum Proc. vol.35, 1908, pp. 133-154.Van Tuyl, F. M., The Origin of the Geodes of the Keokuk Beds: Proceedings of the Iowa Academy of Science, vol 19, 1912, pp. 169-172.via Gem City Rock News- reprinted from Illinois State Geological Survey Miscellaneous Pamphlet