Editing Variolite

{{short description|Igneous rocks which contain varioles}}
[[File:Sample of Variolite, Olympic Peninsula, Washington USA.jpg|thumb|Beach pebble of variolitic pillow lava (''varolite'') from the [[Olympic Peninsula]], Washington state]]

'''Variolites''' are [[mafic]], [[igneous]], and typically [[volcanic]] [[Rock (geology)|rocks]], e.g. [[tholeiite]], [[basalt]] or [[komatiite]], that contain centimeter-scale spherical or globular structures, called ''varioles'', in a fine-grained matrix. These structures are lighter colored than the host rock and typically range in diameter from 0.05mm to over 5&nbsp;cm.<ref name="NeuendorfOthers2005a">Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. {{ISBN|0-922152-76-4}}</ref><ref name="MaitreOthers2005a">Le Maitre, R.W., A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, and P. Bateman, eds., (2005) ''Igneous Rocks: A Classification and Glossary of Terms Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks.'' United Kingdom, Cambridge University Press Cambridge. 256 pp. {{ISBN|9780521619486}}</ref><ref name="Condie1981a">Condie, K.C. (1981) ''Archean Greenstone Belts. Developments in Precambrian Geology, vol. 3.'' New york, New York, Elservier. 434 p. {{ISBN|0-444-41854-7}}</ref> In 1648, Aldrovandi created the term ''variolite'' for [[aphanitic]] or fine-grained igneous rocks containing varioles.<ref name="Aldrovandi1648">Aldrovandi, U. (1648) ''Bononiensis Musæi Metallici.'' Bologna, Ferronii. 992 pp.</ref><ref name="Johannsen1938">Johannsen, A. (1938) ''A Descriptive Petrography of the Igneous Rocks. Volume 4, Part 1, The Feldspathoid Rocks and Part II, The Peridotites and Perknites''. The University of Chicago Press, Chicago, Illinois. 523 pp.</ref> The [[weathering]] of varioles often cause variolites to have a pock-marked appearance. In allusion to the pock-marked appearance of weathered surfaces of variolite, this term is derived from the Latin word, ''variola'', for [[smallpox]].<ref name="MaitreOthers2005a"/><ref name="Condie1981a"/>

'''Varioles''' are millimeter- to centimeter-scale, light-colored, globular to spherical structures, that are conspicuously observable within aphanitic, mafic igneous rocks, such as basalt, komatiite, and [[tachylite]], that comprise either [[pillow lava]]s, subaerial [[lava flow]]s, or volcanic [[Dike (geology)|dyke]]s. Typically, they are less resistant to weathering than the enclosing aphanitic rock and, as a result, form pock-marks on the weathered surfaces of mafic rocks.<ref name="ArndtOthers2004a">Arndt, N., and Fowler A.D. (2004) ''Textures in komatiites and variolitic basalts.'' In: Erikson, K. ed., ''The Precambrian Earth: Tempos and Events.'' Elsevier, Amsterdam, pp 298–311. {{ISBN|978-0444515063}}</ref><ref name="SandstaOthers2011a">Sandsta, N.R., B. Robins, H. Furnes, and M. de Wit (2011) ''The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africa.'' Contributions to Mineralogy and Petrology. vol. 162, no. 2, pp. 365–377.</ref>

In the geologic literature, the usage of the term ''variole'' has been inconsistent and confusing. Initially, they were defined as spherical masses, which may or may not be spherulites, that are observed on the weathering surfaces of some basalts and [[diabase]]s. In some modern literature, the term ''variole'' is defined as a type of [[spherulite]] that occurs in a mafic rock. However, because several different mechanisms can produce these small-scale, light-colored, globular to spherical structures, a specific set of varioles may or may not be spherulites that are composed of radiating crystals of either [[plagioclase]] or [[pyroxene]]. As a result, it is recommended that the term ''variole'' should be retained as originally defined. This definition is useful, not only because varioles may arise through several different mechanisms, but also because the alteration, specifically mineralization, and deformation associated with many [[Precambrian]] volcanic rocks, particularly [[Archean]] volcanic rocks, makes the determination of their origin difficult, if not impossible, without further laboratory analyses.<ref name="ArndtOthers2004a"/><ref name="FowlerOthers1987a">Fowler A.D., L.S. Jensen, and S.A. Peloquin (1987) ''Varioles in Archean basalts; products of spherulitic crystallization.'' Canadian Mineralogist. vol. 25, pp. 275–289</ref> Phillips (1973)<ref name="Phillips1973a">Phillips, W.J. (1973). Interpretation of crystalline spheroidal structures in igneous rocks. ''[[Lithos (journal)|Lithos]]''. vol. 6, pp. 235-244.</ref> provides a detailed review of the nomenclature of different types of varioles that have been proposed. Confusingly, a few Earth scientists use ''variolite'' as if it is synonymous with ''variole''.<ref name="Cas1987a">Cas, R.A.F., and J.V. Wright (1987) ''Volcanic Successions, Modern and Ancient: A Geological Approach to Processes, Products, and Succession'' United Kingdom, London, Allen & Unwin. 528 p. {{ISBN|978-0-412-44640-5}}</ref>

Petrographic and geochemical analyses of varioles demonstrates that they can be the result of one of three possible processes. They are the blotchy alteration of a fine-grained igneous rock; the mingling of magma from two distinctly different sources; and the alteration  and degradation of plagioclase spherulites. These analyses also found that their internal organization and geochemistry is incompatible with the hypothesis that they are quenched immiscible liquids, as has been suggested in the past by various authors.<ref name="ArndtOthers2004a"/>

==Varieties and occurrence==
The variety of mafic igneous rocks that contain varioles are, with rare exceptions, no longer classified as ''variolites'', which is not recommended for usage. Instead, they are designated using the modifier '''variolitic''' in conjunction with the major [[lithology]]. The major varieties of ''variolites'' are variolitic basalts, variolitic pillow lavas and variolitic komatiites.<ref name="NeuendorfOthers2005a"/><ref name="MaitreOthers2005a"/> Variolitic pillow lavas, that have been previously identified as ''variolites'' and also classified as [[spilite]]s, are found in the Durance, [[France]]; on Mont Genvre, France; in [[Devonian]] rocks of [[Germany]]; and as cobbles on the beaches of the [[Strait of Juan de Fuca]] along the northern edge of the [[Olympic Peninsula]].<ref name="Lewis1914a">Lewis, J.V.  (1914) ''Origin of Pillow Lava.'' Bulletin of the Geological Society of America. vol. 25, pp. 15-25.</ref><ref name="SnavelyOthers1973a">Snavely, P.D., Maceod, N.S., and Holly, C.W. (1973) ''Miocene tholeiitic basalts of coastal Oregon and Washington and their relations to coeval basalts of the Columbia Plateau.'' Geological Society of America Bulletin. vol. 84, pp. 387–424</ref> Variolitic basalts and variolitic komatiites occur commonly as [[Archean]] lava flows in the [[greenstone belt]]s of South Africa and the [[Canadian Shield]].<ref name="SandstaOthers2011a"/><ref name="FowlerOthers1987a"/> Finally, there is a group of spotted volcanic rocks formerly known to [[France|French]] petrographers as the ''variolites du Drac'' from the locality in which they are found in [[Parc National des Ecrins]], France.<ref name="EB1911">{{Cite EB1911|wstitle=Variolites|volume=27|page=921}}</ref>  Additional research has found them to be [[hydrothermal]]ly altered basalt volcanic rocks that contain [[amygdule]]s filled with white [[calcite]] and other secondary minerals.<ref name="Buffet-Croix-Blanche1989a">Buffet-Croix-Blanche, G. (1989) ''Volcans fossiles dans la region du parc national des Ecrins; sept excursions pour une initiation volcanologique sur des lambeaux de volcans dates de 200 millions d'années.'' Parc National des Ecrins, France. 73 p.</ref>

==References==
{{Reflist}}
{{Rock type}}
[[Category:Igneous rocks]]