A lithophysa (plural lithophysae, from Greek ''lithos'' "stone" + ''phusa'' "bubble")
/ref> is a

felsic In geology, felsic is a grammatical modifier, 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 ...

volcanic A volcano is commonly defined as a vent or fissure 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 fo ...

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 Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...

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

lava flows. Lavas low in

feldspar Feldspar ( ; sometimes spelled felspar) is 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 ''plagiocl ...

minerals may produce a version known as snowflake obsidian. 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 co ...

s and

rhyolitic 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 miner ...

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, macros ...

s it may be referred to as a

geode A geode (; ) is a geology, geological secondary formation within sedimentary rock, sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter (which may include crystals) are secluded. The crys ...

or if filled partially or fully with

agate Agate ( ) is a banded variety of chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and sometimes include macroscopic quartz. They are common in nature and can be found globally in a large number of d ...

jasper Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to ...

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

it is called a thunderegg. 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 monoclinic ...

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 scal ...

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 , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

(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 Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, a Romance ethnic group related to or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance languag ...

island of

Lipari Lipari (; ) is a ''comune'' including six of seven islands of the Aeolian Islands (Lipari, Vulcano, Panarea, Stromboli, Filicudi and Alicudi) and it is located in the Tyrrhenian Sea off the northern coast of Sicily, Southern Italy; it is ...

, in

Yellowstone National Park Yellowstone National Park is a List of national parks of the United States, national park of the United States located in the northwest corner of Wyoming, with small portions extending into Montana and Idaho. It was established by the 42nd U ...

, 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 {{igneous-petrology-stub

See also

References