![Sample of Variolite, Olympic Peninsula, Washington USA](https://upload.wikimedia.org/wikipedia/commons/d/d1/Sample_of_Variolite%2C_Olympic_Peninsula%2C_Washington_USA.jpg)
Variolites are
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
,
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
, and typically
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 a ...
rocks
In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
, e.g.
tholeiite
The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma i ...
,
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
or
komatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite wa ...
, 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 cm.
[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. ][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. ][Condie, K.C. (1981) ''Archean Greenstone Belts. Developments in Precambrian Geology, vol. 3.'' New york, New York, Elservier. 434 p. ] In 1648, Aldrovandi created the term ''variolite'' for
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geologic ...
or fine-grained igneous rocks containing varioles.
[Aldrovandi, U. (1648) ''Bononiensis Musæi Metallici.'' Bologna, Ferronii. 992 pp.][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.] The
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...
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
Smallpox was an infectious disease caused by variola virus (often called smallpox virus) which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) c ...
.
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
Tachylite ( ; also spelled tachylyte) is a form of basaltic volcanic glass. This glass is formed naturally by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is chemical decomposition, decomposable by acids and readi ...
, that comprise either
pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of disconti ...
s, subaerial
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s, or volcanic
dykes. 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.
[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. ][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.]
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
Diabase (), also called dolerite () or microgabbro,
is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-graine ...
s. In some modern literature, the term ''variole'' is defined as a type of
spherulite
In petrology, spherulites () are small, rounded bodies that commonly occur in volcanic glass, vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone, and rhyolite as globules about the size of millet seed or rice g ...
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
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
or
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
. 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
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
volcanic rocks, particularly
Archean
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
The Earth
Earth ...
volcanic rocks, makes the determination of their origin difficult, if not impossible, without further laboratory analyses.
[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] Phillips (1973)
[Phillips, W.J. (1973). Interpretation of crystalline spheroidal structures in igneous rocks. '']Lithos
Lithos is a glyphic sans-serif typeface designed by Carol Twombly in 1989 for Adobe Systems. Lithos is inspired by the unadorned, geometric letterforms of the engravings found on Ancient Greek public buildings. The typeface consists of only cap ...
''. vol. 6, pp. 235-244. 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''.
[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. ]
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.
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 lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
. The major varieties of ''variolites'' are variolitic basalts, variolitic pillow lavas and variolitic komatiites.
Variolitic pillow lavas, that have been previously identified as ''variolites'' and also classified as
spilite
Spilite (from el, σπιλάς) is a fine-grained igneous rock, resulting particularly from alteration of oceanic basalt.
The term was introduced into the geological literature by Alexandre Brongniart in 1827.Manfred Schidlowski: ''Spilite and ...
s, are found in the Durance,
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
; on Mont Genvre, France; in
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
rocks of
Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
; and as cobbles on the beaches of the
Strait of Juan de Fuca
The Strait of Juan de Fuca (officially named Juan de Fuca Strait in Canada) is a body of water about long that is the Salish Sea's outlet to the Pacific Ocean. The international boundary between Canada and the United States runs down the centre ...
along the northern edge of the
Olympic Peninsula
The Olympic Peninsula is a large arm of land in western Washington that lies across Puget Sound from Seattle, and contains Olympic National Park. It is bounded on the west by the Pacific Ocean, the north by the Strait of Juan de Fuca, and the ...
.
[Lewis, J.V. (1914) ''Origin of Pillow Lava.'' Bulletin of the Geological Society of America. vol. 25, pp. 15-25.][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] Variolitic basalts and variolitic komatiites occur commonly as
Archean
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
The Earth
Earth ...
lava flows in the
greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies.
The name comes from the green ...
s of South Africa and the
Canadian Shield
The Canadian Shield (french: Bouclier canadien ), also called the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), the anc ...
.
Finally, there is a group of spotted volcanic rocks formerly known to
French
French (french: français(e), link=no) may refer to:
* Something of, from, or related to France
** French language, which originated in France, and its various dialects and accents
** French people, a nation and ethnic group identified with Franc ...
petrographers as the ''variolites du Drac'' from the locality in which they are found in
Parc National des Ecrins, France.
Additional research has found them to be
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
ly altered basalt volcanic rocks that contain
amygdule
Amygdules or amygdales () form when the vesicles (pores from gas bubbles in lava) of a volcanic rock or other extrusive igneous rock are infilled with a secondary mineral, such as calcite, quartz, chlorite, or one of the zeolite
Zeolites are mi ...
s filled with white
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
and other secondary minerals.
[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.]
References
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Igneous rocks