A lithophysa (plural lithophysae, from Greek ''lithos'' "stone" + ''physan'' "to blow")
/ref> is a

felsic In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, wh ...

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates ...

rock with a spherulitic structure and interior cavity with concentric chambers. Its outer shape is spherical or lenticular. They vary in size from very small up to twelve feet in diameter depending on the age of the magma chamber. These rocks are usually found within obsidian or

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained ( aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The min ...

lava flows. Lavas low in

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feld ...

minerals may produce a version known as

snowflake obsidian Obsidian () is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Obsidian is produced from felsic lava, rich in the lighter elements such as silicon, ...

. These cavities are believed to be caused by expanding gases in

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

s and rhyolitic lavas before solidification. If the cavity becomes lined with

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...

s it may be referred to as a

geode A geode (; ) 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 fil ...

or if filled partially or fully with

agate Agate () is a common rock formation, consisting of chalcedony and quartz as its primary components, with a wide variety of colors. Agates are primarily formed within volcanic and metamorphic rocks. The ornamental use of agate was common in ...

, jasper or

opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms ...

it is called a

thunderegg A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little mor ...

. The term vug is also used for similar cavities although the meaning of vug is usually restricted to cavities in rocks formed by the removal of material such as soluble minerals. These cavities usually contain layers of various colors (red, pink, gray, etc.) composed by crystals of quartz,

chalcedony Chalcedony ( , or ) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monocl ...

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of ...

fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs sca ...

and various colored oxides or other minerals.

Lithophysae are one of the many forms of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...

(SiO₂),Geowiki
/ref> as quartz, agate, opal, chalcedony, etc. They can have an average diameter , but can be much larger (for example, at Silver Cliff in Colorado). Lithophysae are also related to the spherulites found in obsidians on the Italian island of

Lipari Lipari (; scn, Lìpari) is the largest of the Aeolian Islands in the Tyrrhenian Sea off the northern coast of Sicily, southern Italy; it is also the name of the island's main town and ''comune'', which is administratively part of the Metropol ...

, in

Yellowstone National Park Yellowstone National Park is an American national park located in the western United States, largely in the northwest corner of Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is border ...

, and other places.Geological Survey Bulletin, Ed. 136
/ref> File:Spheroid-01.jpg, Spheroids form in high silica magmas File:1.3-2019Spheroids 003-c1.jpg, Spheroid "trains" in obsidian File:Snowflake obsidian - Utah.jpg, Snowflake obsidian File:1.2-2014-02-29MarDaveGeodes 001cb.jpg, A partially "blown" spheroid pair in obsidian File:01LithTrains.jpg, "Trains" of "blown" spheroids transformed to lithophysae File:02inside1.jpg, "Blown" spheroid transformed to a lithophysa File:02inside2.jpg, "Blown" spheroid transformed to a lithophysa File:1.1-6Jun13-Drum-Oil 008.jpg, "Blown" spheroids transformed to lithophysae

References

Igneous petrology Mineralogy Volcanic rocks Volcanology {{petrology-stub

See also

References