question: how do geodes form, are they rock that gets hollowed out, or are they formed from the inside first?
comment: They start out as empty pockets in rocks. Dissolved minerals (in water, for example) make their way into the air pocket and crystalize.
passages: Geode
  
  Geodes (derived from the Greek word "γεώδης" meaning "Earth like") are geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely circular 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 sub-volcanic 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.  
  
  Section::::Formation.
   Genesis allows modeling with legacy sectional method using polygons (longitudinal polygons and polygons by benches) following the previously modelized mineralized intervals in drill holes as well as the modern modelling using the creation of meshed envelopes by connecting polygons or using implicit modelling.
   BULLET::::3. the principle of lateral continuity: "At the time when any given stratum was being formed it was either encompassed on its sides by another solid substance, or it covered the entire spherical surface of the earth. Hence it follows that in whatever place the bared sides of the strata are seen, either a continuation of the same strata must be sought, or another solid substance must be found which kept the matter of the strata from dispersion."
   Frequently, erosion has removed the layers of altered rock and other saprolite surrounding corestones that were produced by spheroidal weathering. This leaves many corestones as freestanding boulders on the ground's surface. Often the spheroidal weathering, which created these corestones and the enclosing saprolite occurred in the prehistoric past during periods of humid, even tropical climates. Frequently, the removal of the saprolite by erosion and exposure of corestones as freestanding residual boulders, tors, or other landforms occurs many thousands of years later and during vastly different climatic conditions.
   Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.
  
  Section::::Prevalence.
  
  Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and Utah). They also are common in Brazil, Namibia, and Mexico. Geodes are also abundant in the Mendip Hills in Somerset, England, where they are known locally as "Potato Stones."
  
  Section::::Vugs and crystal caves.
   Occasionally, mineralization is developed in faults and feeder conduits which fed the mineralizing system. For instance, the Sullivan orebody in south-eastern British Columbia was developed within an interformational diatreme, caused by overpressuring of a lower sedimentary unit and eruption of the fluids through another unit en route to the seafloor.
   Section::::Properties.:Geometric structure.
  
  Globus Cassus has the form of a compressed geodesic icosahedron with two diagonal openings. Along the edges of the icosahedron run the skeleton beams, the gaps between the beams contain a shell and, where there are windows, inward-curving domes.
  
  Section::::Properties.:Building material.
  
  Earth's crust, mantle and core are gradually excavated, transported outwards and then transformed to larger strength and reduced density. While the crust is mined from open pits in the continents' centers, magma and the liquid mantle are pumped across transfer hoses. The core is dismantled from the surface.
  
  Section::::Properties.:Planetary scale.
   Section::::Glenburgh Terrane.
  
  In this section the component rock units of the Gascoyne Complex, and the orogenies that shaped the complex, are discussed in order of decreasing age.
  
  Section::::Glenburgh Terrane.:Halfway Gneiss.
   As with the A horizon, the B horizon may be divided into B1, B2, and B3 types under the Australian system. B1 is a transitional horizon of the opposite nature to an A3 – dominated by the properties of the B horizons below it, but containing some A-horizon characteristics. B2 horizons have a high concentration of clay minerals or oxides. B3 horizons are transitional between the overlying B layers and the material beneath it, whether C or D horizon.
   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. 
   Narrow ridges occur in some places on Mars. They may be formed by different means, but some are probably caused by molten rock moving underground, cooling into hard rock, then being exposed by the erosion of softer, surrounding materials. Such a feature is termed a dike. They are common on Earth—some famous ones are Shiprock, New Mexico; around Spanish Peaks, Colorado; and the "Iron Dike" in Rocky Mountain National Park, Colorado.
   Large planation surfaces, like the African Surface, are typically formed diachronously. 
  
  Planation surfaces are often thought to form at distinct base levels with sequences of them representing uplift events. However the disposition of some so-called planation surfaces may better reflect the easiness of erosion into different lithologies or structures and may therefore not fit any uplift scheme.
   Section::::Formation environments.
  
  The environments of mineral formation and growth are highly varied, ranging from slow crystallization at the high temperatures and pressures of igneous melts deep within the Earth's crust to the low temperature precipitation from a saline brine at the Earth's surface.
  
  Various possible methods of formation include:
  
  BULLET::::- sublimation from volcanic gases
  
  BULLET::::- deposition from aqueous solutions and hydrothermal brines
  
  BULLET::::- crystallization from an igneous magma or lava
  
  BULLET::::- recrystallization due to metamorphic processes and metasomatism
  
  BULLET::::- crystallization during diagenesis of sediments
   BULLET::::- 2000-1600 Ma; Basic dykes, related to the Gascoyne Complex and Capricorn Orogeny
  
  BULLET::::- 2700-2600 Ma; br Granite sheets and juxtaposition with the Yilgarn craton.
  
  BULLET::::- 3350-3300 Ma; Amphibolite to granulite facies metamorphism
  
  BULLET::::- ~3350-3400 Ma; deposition of metasedimentary rocks
  
  BULLET::::- 3400 Ma; Intrusion of syenogranites to form Dugel Gneiss, basic and ultramafic dykes
  
  BULLET::::- 3400-3680 Ma; Deformation
  
  BULLET::::- ~3680 Ma; Meeberrie Gneiss
  
  BULLET::::- 3680-3730 Ma; Deformation
  
  BULLET::::- ~3780 Ma; Manfred Complex ultramafic-mafic layered intrusion
  
  BULLET::::- 4100 Ma; Metasedimentary precursor gneiss to host the Manfred Complex
  
  Section::::References.
   Section::::Formation.:Concrete derived micro-gours.
   Generally, a spar will form underwater, either in a phreatic zone, or below the water table, the essential place where most caves form, or in standing pools, as pool spar. If a cave floods and a pool forms, that submerges stalactites, a formation known as bottlebrushes may form. Mineral component ions in the water, mostly calcite or gypsum, but sometimes even halite, quartz, and fluorite, are deposited through the course of thousands of years, building up on each other.
  
  Section::::Formation in the air.
   Fossils are often found flattened along the bedding plane of the rocks in which they occur, though may be found in three dimensions when they are infilled by iron pyrite or some other minerals. They vary in shape, but are most commonly dendritic or branching (such as "Dictyonema"), saw-blade like, or "tuning fork" shaped (such as "Didymograptus murchisoni"). Their remains may be mistaken for fossil plants by the casual observer, as it has been the case for the first graptolite descriptions.
   BULLET::::- Void-fill structure (known in some texts as simply a geopetal structure) is a void or cavity in a rock that has been part-filled with sediments or mineral deposits, that indicates an initial horizontal that can be used as a way up structure and as a precise measure of post-depositional tilting. There are two main examples of this:
  
  BULLET::::- "Fossils" - Any fossil that starts with a significant internal cavity may develop such structures, like molluscs and brachiopods. The initial fill is often of sediments with the remaining part filled by calcite, giving a clear geopetal structure.
   Section::::Summary.
  
  Taking the above information from 1911 with that from other sources, we seem to have a district composed of glacial till, from a period when the Alpine ice sheet extended over the area, probably from the Mindel and Riss glacials. This appears to be mainly clay deposited "in situ" as the ice melted. There appears to be also, sand and gravel on the clay in places. This may date from the latest glacial, the Würm.
   As defined, ganisters can be created by either the cementation of quartzose by surficial soil-forming processes to form silicrete, or by diagenetic cementation within the subsurface. Detailed studies of ganisters, which occur either as seatearths or elsewhere within coal-bearing strata, have found them to be ancient paleosols, which are equivalent in both physical characteristics and origin to modern silica-cemented soils, called silcretes. Modern formation of ganisters has been observed in the Okavango Delta of Botswana.
   Section::::Sedimentary microstructures.:Diagenetic features.
  
  Usually diagenesis results in a weak bedding-plane foliation. Other effects can include flattening of grains, pressure dissolution and sub-grain deformation. Mineralogical changes may include zeolite or other authigenic minerals forming in low-grade metamorphic conditions.
  
  Section::::Sedimentary microstructures.:Diagenetic features.:Sorting.
   Section::::Paragonimus.
   The original definition comes from Bruno Sander in 1936, translated from German to English in 1951, which states: 
  
  "Geopetal Fabrics" - All the widely distributed spatial characters of a fabric that enable us to determine what was the relation of "top" to "bottom" at the time when the rock was formed are termed "geopetal" fabrics. Such fabrics are mechanical and chemical internal deposition; grains on a boundary surface; cross-bedding, etc.
  
  Section::::Examples.
  
  BULLET::::- Unconformities - Clear angular unconformities provide unequivocal evidence of the relative age of two rock sequences.
   Ore-hosting structures are most commonly zolinknes of highly brecciated dolomite.
  
  These structures may be more or less vertical, crossing bedding at high angles, or they may be lensoid in shape extending in the same direction as bedding.
  
  Section::::Mississippi Valley type mineral deposits.:MVT Characteristics.
  
  Mississippi Valley-type deposits can be compared with the Red Sea deposits, which are modern analogues of ancient sedex deposits, some differences can be made:
  
  BULLET::::- MVT deposits are hosted in carbonate rocks, whereas sedex deposits are found within marine shales
   Accumulation occurs as crystals are formed by fractional crystallisation and, if they are dense enough, precipitate out from the magma. In large, hot magma chambers having vigorous convection and settling, pseudo-sedimentary structures such as flow banding, graded bedding, scour channels, and foreset beds, can be created. The Skaergaard intrusion in Greenland is a prime example of these quasi-sedimentary structures.
  
raw_presuppositions: geodes are rocks that are hollowed out,
raw_corrections: Geodes are pockets in rocks that fill with water with dissolved minerals that then crystalizes. 