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Porphyry

A piece of porphyry Rhyolite porphyry. Scale bar in lower left is 1 cm. Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich, generally aphanitic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance. The term porphyry is from Greek (πορφύρα porphúra) and means "purple". Purple was the color of royalty, and the "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase. Some authors claimed the rock was the hardest known in antiquity. "Imperial" grade porphyry was thus prized for monuments and building projects in Imperial Rome and later. Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition. Its chief characteristic is a large difference in size between the tiny matrix crystals and the much larger phenocrysts. Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals as in basalt, or crystals easily distinguishable with the eye, as in granite. Most types of igneous rocks display some degree of porphyritic texture. More details Android, Windows

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Pillow lava

Recently formed pillow lava, off Hawaii Cross-section of pillow lava near Oamaru, New Zealand Weathered Archean pillow lava in the Temagami Greenstone Belt of the Canadian Shield A pillow lava from an ophiolite sequence, Northern Apennines, Italy Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. They form the upper part of 'Layer 2' of normal oceanic crust. More details Android, Windows

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Pyroclastic rock

USGS scientist examines pumice blocks at the edge of a pyroclastic flow from Mount St. Helens Rocks from the Bishop Tuff, uncompressed with pumice on left; compressed with fiamme on right. Play media Flight through a µCT-image stack of a Lapilli of the volcano Katla in Iceland. Find spot: Beach near Vik at the end of road 215. Acquisition done using "CT Alpha" by "Procon X-Ray GmbH", Garbsen, Germany. Resolution 11,2µm/Voxel, width approx. 24 mm. Play media 3D-Rendering of the above image stack, in parts transparent. Heavy particles in red. Pyroclastic rocks or pyroclastics (derived from the Greek: πῦρ, meaning fire; and κλαστός, meaning broken) are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with unsieved volcanic activity—such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions—pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock. Pyroclastic rocks may be a range of clast sizes, from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli, and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas-and-ash mix of a pyroclastic flow event. More details Android, Windows

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Perlite

For the two-phased structure in steel, see pearlite. Expanded perlite Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing. More details Android, Windows

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Pumice

Not to be confused with pomace. Specimen of highly porous pumice from Teide volcano on Tenerife, Canary Islands. Density of specimen approximately 0.25 g/cm3; scale in centimeters. Pumice (pronunciation: /ˈpʌmᵻs/), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough textured volcanic glass, which may or may not contain crystals. It is typically light colored. Scoria is another vesicular volcanic rock that differs from pumice in having larger vesicles, thicker vesicle walls and being dark colored and denser. Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases (including water and CO2) that are dissolved in the lava, causing the gases to rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink is opened). The simultaneous cooling and depressurization freezes the bubbles in a matrix. Eruptions under water are rapidly cooled and the large volume of pumice created can be a shipping hazard for cargo ships. More details Android, Windows

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Pegmatite

Proterozoic pegmatite swarm in the headwall of the cirque of a small mountain glacier, northeastern Baffin Island, Nunavut. Pegmatite with blue corundum crystals Pegmatite containing lepidolite, tourmaline, and quartz from the White Elephant Mine in the Black Hills, South Dakota A pegmatite is a holocrystalline, intrusive igneous rock composed of interlocking phaneritic crystals usually larger than 2.5 cm in size (1 inch); such rocks are referred to as pegmatitic. The word pegmatite derives from Homeric Greek, πεγνυμι (pegnymi), which means “to bind together”, in reference to the intertwined crystals of quartz and feldspar in the texture known as graphic granite. Most pegmatites are composed of quartz, feldspar and mica, having a similar basic composition as granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene, feldspathoids and other unusual minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions. Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 metres (33 ft) long have been found, and many of the world's largest crystals were found within pegmatites. These include, spodumene, microcline, beryl, and tourmaline. Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size and perfection. Feldspar within a pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye. More details Android, Windows

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Obsidian

For other uses, see Obsidian (disambiguation). Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which were used in the past in cutting and piercing tools, and it has been used experimentally as surgical scalpel blades. More details Android, Windows

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Lamproite

Photograph of sample of Lamproite. Lamproites are ultrapotassic mantle-derived volcanic and subvolcanic rocks. They have low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements. Lamproites are geographically widespread yet are volumetrically insignificant. Unlike kimberlites which are found exclusively in Archaean cratons, lamproites are found in terrains of varying age, ranging from Archaean in Western Australia, to Palaeozoic and Mesozoic in southern Spain. They also vary widely in age, from Proterozoic to Pleistocene, the youngest known example being 56,000 ± 5,000 years old. Lamproite volcanology is varied, with both diatreme styles and cinder cone or cone edifices known. More details Android, Windows

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Lava

"Lava flow" redirects here. For the programming anti-pattern, see Lava flow (programming). For other uses, see Lava (disambiguation). Further information: Magma 10-metre (33 ft) high fountain of pāhoehoe lava, Hawaii, United States Lava flow during a rift eruption at Krafla, Iceland in 1984. Lava is the molten rock expelled by a volcano during an eruption. The resulting rock after solidification and cooling is also called lava. The molten rock is formed in the interior of some planets, including Earth, and some of their satellites. The source of the heat that melts the rock within the earth is geothermal energy. When first erupted from a volcanic vent, lava is a liquid usually at temperatures from 700 to 1,200 °C (1,292 to 2,192 °F). A lava flow is a moving outpouring of lava, which is created during a non-explosive effusive eruption. When it has stopped moving, lava solidifies to form igneous rock. The term lava flow is commonly shortened to lava. Although lava can be up to 100,000 times more viscous than water, lava can flow great distances before cooling and solidifying because of its thixotropic and shear thinning properties. Explosive eruptions produce a mixture of volcanic ash and other fragments called tephra, rather than lava flows. The word "lava" comes from Italian, and is probably derived from the Latin word labes which means a fall or slide. The first use in connection with extruded magma (molten rock below the Earth's surface) was apparently in a short account written by Francesco Serao on the eruption of Vesuvius between May 14 and June 4, 1737. Serao described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano following heavy rain. More details Android, Windows

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Calc-alkaline magma series

The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it fractionally crystallizes to become a felsic magma, which is low in magnesium and iron and produces rhyolite or granite. Calc-alkaline rocks are rich in alkaline earths (magnesia and calcium oxide) and alkali metals and make up a major part of the crust of the continents. The diverse rock types in the calc-alkaline series include volcanic types such as basalt, andesite, dacite, rhyolite, and also their coarser-grained intrusive equivalents (gabbro, diorite, granodiorite, and granite). They do not include silica-undersaturated, alkalic, or peralkaline rocks. More details Android, Windows

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Subvolcanic rock

A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at medium to shallow depths within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic and plutonic rocks. Subvolcanic rocks include diabase (also known as dolerite) and porphyry. Common examples of subvolcanic rocks are diabase, quartz-dolerite, micro-granite and diorite. More details Android, Windows

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Volcanic glass

A sand grain of volcanic glass under the petrographic microscope. Its amorphous nature makes it go extinct in cross-polarized light (bottom frame). Scale box in millimeters. Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas. Volcanic glass can refer to the interstitial, or matrix, material in an aphanitic (fine grained) volcanic rock or can refer to any of several types of vitreous igneous rocks. Most commonly, it refers to obsidian, a rhyolitic glass with high silica content. Other types of volcanic glass include: Pumice, which is considered a glass because it has no crystal structure. Apache tears, a kind of nodular obsidian. Tachylite (also spelled tachylyte), a basaltic glass with relatively low silica content. Sideromelane, a less common form tachylyte. Palagonite, a basaltic glass with relatively low silica content. Hyaloclastite, a hydrated tuff-like breccia of sideromelane and palagonite. Pele's hair, threads or fibers of volcanic glass, usually basaltic. Pele's tears, tear-like drops of volcanic glass, usually basaltic. Limu o Pele (Pele's seaweed), thin sheets and flakes of brownish-green to near-clear volcanic glass, usually basaltic. ^ Bates and Jackson, 1984, Dictionary of Geological Terms, 3rd ed., Prepared by the American Geological Institute More details Android, Windows

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Lava dome

Image of the rhyolitic lava dome of Chaitén Volcano during its 2008–2010 eruption. One of the Mono Craters, an example of a rhyolite dome. In volcanology, a lava dome or volcanic dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. The geochemistry of lava domes can vary from basalt to rhyolite although most preserved domes tend to have high silica content. The characteristic dome shape is attributed to high viscosity that prevents the lava from flowing very far. This high viscosity can be obtained in two ways: by high levels of silica in the magma, or by degassing of fluid magma. Since viscous basaltic and andesitic domes weather fast and easily break apart by further input of fluid lava, most of the preserved domes have high silica content and consist of rhyolite or dacite. Existence of lava domes has been suggested for some domed structures on the Martian surface in the western part of Arcadia Planitia and within Terra Sirenum. More details Android, Windows

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Volcanic bomb

Fusiform lava bomb. Capelinhos Volcano, Faial Island, Azores. Volcanic bomb found in the Cinder Cones region of the Mojave National Preserve. Volcanic bomb at Vulcania (Puy-de-Dôme) A volcanic bomb is a mass of molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they are extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight. Bombs can be extremely large; the 1935 eruption of Mount Asama in Japan expelled bombs measuring 5–6 m in diameter up to 600 m from the vent. Volcanic bombs are a significant volcanic hazard, and can cause severe injuries and death to people in an eruption zone. One such incident occurred at Galeras volcano in Colombia in 1993; six people near the summit were killed and several seriously injured by lava bombs when the volcano erupted unexpectedly. Volcanic bombs are known to occasionally explode from internal gas pressure as they cool, but contrary to some claims in popular culture (specifically, the 1997 film Volcano), explosions are rare; in most cases most of the damage they cause is from impact, or subsequent fire damage. Bomb explosions are most often observed in "bread-crust" type bombs. More details Android, Windows

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Variolite

Variolitic pillow lava (varolite) from the Olympic Peninsula, Washington state Variolites are mafic, igneous, and typically volcanic 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 cm. In 1648, Aldrovandi created the term variolite for aphanitic or fine-grained igneous rocks containing varioles. 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. Blatterstein is a local German name for a type of variolite. Gamaicu is a local South American name for a type of variolite. 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 lavas, subaerial lava flows, 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. 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 diabases. 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. Phillips (1973) 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. 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. More details Android, Windows

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Aplite

Glacial erratic with the dykes of the aplite Aplite (pronounced /ˈæplaɪt/) is an intrusive igneous rock in which the mineral composition is the same as granite, but in which the grains are much finer, under 1 mm across. Quartz and feldspar are the dominant minerals. The term 'aplite' or 'aplitic' is often used as a textural term to describe veins of quartz and feldspar with a fine to medium-grain "sugary" texture. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens. Dykes and veins of aplite are commonly observed traversing granitic bodies; they occur also, though less frequently, in syenites, diorites, quartz-diabases and gabbros. Aplites usually have a genetic affinity to the rocks they intrude. The aplites of granite areas, for example, are the last part of the magma to crystallize, and correspond in composition to the quartzo-feldspathic aggregates that fill up the spaces between the early-formed minerals in the main body of the rock. They bear a considerable resemblance to the eutectic mixtures which are formed on the cooling of solutions of mineral salts, and remain liquid till the excess of either of the components has separated out, finally solidifying en masse when the proper proportions of the constituents and a suitable temperature are reached. Aplite, showing fine-grained texture The essential components of aplites are quartz and alkali feldspar (the latter usually orthoclase or microperthite), microcline and albite. Crystallization has been apparently rapid (as the rocks are so fine-grained), and the ingredients have solidified almost at the same time. Hence their crystals are rather imperfect and fit closely to one another in a sort of fine mosaic of nearly equi-dimensional grains. Phenocrysts of feldspar occur occasionally and of quartz more seldom; but the relation of aplite to quartz-porphyry, granophyre and felsite is very close, as all these rocks have nearly the same chemical composition. The aplites associated with diorites and quartz-diabases differ in minor respects from the common aplites which accompany granites. The accessory minerals of these rocks are principally oligoclase, muscovite, apatite and zircon. Biotite and all ferromagnesian minerals rarely appear in them, and never in considerable amounts. Riebeckite-granites have close affinities to aplites, shown especially in the prevalence of alkali feldspars. Tourmaline also occurs in some aplites. The rocks of this group are very frequent in all areas where masses of granite are known. They form dykes and irregular veins, which may be only a few inches or many feet in diameter. Less frequently, aplite forms stocks or bosses, or occupies the edges or irregular portions of the interior of outcrops of granite. Syenite-aplites consist mainly of alkali feldspar; the diorite-aplites of plagioclase; there are nepheline-bearing aplites which intersect some elaeolite-syenites. In all cases, they bear the same relation to the parent masses. By increase of quartz, aplites pass gradually, in a few localities, through highly quartzose modifications into quartz veins. More details Android, Windows

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Jasper

For other uses, see Jasper (disambiguation). Jasper outcrop, Bucegi Mountains, Romania Jasper, an aggregate of microgranular quartz and/or 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 iron(III) inclusions. The mineral aggregate breaks with a smooth surface and is used for ornamentation or as a gemstone. It can be highly polished and is used for vases, seals, and snuff boxes. The specific gravity of jasper is typically 2.5 to 2.9. Along with heliotrope (bloodstone), jasper (green with red spots) is one of the traditional birthstones for March. Jaspilite is a banded iron formation rock that often has distinctive bands of jasper. More details Android, Windows