This article does not cite any sources . (December 2009) (Learn how and when to remove this template message)
Alkali basalt or alkali olivine basalt is a fine-grained, dark-coloured, volcanic rock characterized by phenocrysts of olivine, titanium-rich augite, plagioclase feldspar and iron oxides. For similar SiO2 concentrations, alkali basalts have a higher content of the alkalis, Na2O and K2O, than other basalt types such as tholeiites. They are also characterized by the development of modal nepheline in their groundmass (visible at highest magnification on a petrographic microscope) and normative nepheline in their CIPW norms. Alkali basalts are typically found on updomed and rifted continental crust, and on oceanic islands such as Hawaii, Madeira and Ascension Island.
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.
A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them. Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than .5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts.
The mineral olivine is a magnesium iron silicate with the formula (Mg2+, Fe2+)2SiO4. Thus it is a type of nesosilicate or orthosilicate. It is a common mineral in Earth's subsurface but weathers quickly on the surface.
Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.
Tephrite is an igneous, volcanic (extrusive) rock, with aphanitic to porphyritic texture. Mineral content is usually abundant feldspathoids, plagioclase, and lesser alkali feldspar. Pyroxenes (clinopyroxenes) are common accessory minerals. Quartz and olivine are absent. Occurrences include leucite nepheline tephrite from Hamberg bei Neckarelz near Heidelberg, Germany, tephritic phonolite to phonolite in lapilli of Mount Vesuvius, Italy, phonolite-tephrite at Monte Vulture, Basilicata, Italy and basanite–tephrite intrusions in Namibia.
Basanite is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture.
Nepheline, also called nephelite (from Greek: νεφέλη, "cloud"), is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites.
Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene. If olivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble basalt in hand specimen. However, basalt consists mostly of clinopyroxene (augite) and calcic plagioclase.
Nepheline syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent.
Essexite, also called nepheline monzogabbro, is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, Massachusetts, in the United States.
Theralite is, in petrology, the name given to calcic foidal gabbro, a plutonic hylocrystalline rock consisting of augite, olivine, calcic plagioclase (labradorite), and nepheline, along with accessories including biotite, magnetite, ilmenite and analcime.
In petrology, limburgite is a dark-colored volcanic rock resembling basalt in appearance, but containing normally no feldspar. The name derives from the type locality the Limberg or the Limburg, close to Sasbach am Kaiserstuhl in Baden-Württemberg, where they occur in the well-known rock of the Kaiserstuhl. They consist essentially of olivine and augite with a brownish glassy groundmass. The augite may be green, but more commonly is brown or violet; the olivine is usually pale green or colourless, but is sometimes yellow. Within the groundmass a second generation of small euhedral augites frequently occurs; more rarely olivine is present also as an ingredient of the matrix. The principal accessory minerals are ilmenite and apatite. Feldspar though sometimes present, is never abundant, and nepheline also is unusual. In some limburgites large phenocrysts of dark brown hornblende and biotite are found, mostly with irregular borders blackened by resorption; in others there are large crystals of anorthoclase. Hauyne is an ingredient of some of the limburgites of the Cape Verde Islands.- Rocks of this group occur in considerable numbers in Germany and in Bohemia, also in Scotland, Auvergne, Spain, Africa (Kilimanjaro) and Brazil. They are associated principally with basalts, nepheline and leucite basalts and monchiquites. From the last-named rocks the limburgites are not easily separated as the two classes bear a very close resemblance in structure and in mineral composition, though many authorities believe that the ground mass of the monchiquites is not a glass but crystalline analcite. Limburgites may occur as flows, as sills or dykes, and are sometimes highly vesicular. Closely allied to them are the augitites, which are distinguished only by the absence of olivine; examples are known from Bohemia, Auvergne, the Canary Islands and Ireland.
Picrite basalt, picrobasalt is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts and black to dark brown pyroxene, mostly augite.
The tholeiitic magma series, named after the German municipality of Tholey, is one of two main magma series in 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 into a more evolved, silica rich end member. The International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to the term "tholeiite".
Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals, olivine and low-calcium (Ca) pyroxene (enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium-rich spinel as an accessory mineral. Garnet-bearing harzburgite is much less common, found most commonly as xenoliths in kimberlite.
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 and alkali metals and make up a major part of the crust of the continents.
Leucitite or leucite rock is an igneous rock containing leucite. It is scarce, many countries such as England being entirely without them. However, they are of wide distribution, occurring in every quarter of the globe. Taken collectively, they exhibit a considerable variety of types and are of great interest petrographically. For the presence of this mineral it is necessary that the silica percentage of the rock should be low, since leucite is incompatible with free quartz and reacts with it to form potassium feldspar. Because it weathers rapidly, leucite is most common in lavas of recent and Tertiary age, which have a fair amount of potassium, or at any rate have potassium equal to or greater than sodium; if sodium is abundant nepheline occurs rather than leucite.
Hawaiite is an olivine basalt with a composition between alkali basalt and mugearite. It was first used as a name for some lavas found on the island of Hawaii.
Trachybasalt is a volcanic rock with a composition between trachyte and basalt. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and likely very small amounts of leucite or analcime. Trachybasalt is a basalt with high alkali content (5 to 7% Na2O + K2O, see TAS diagram).
The Heldburger Gangschar is a Cenozoic volcanic system in the Franconian parts of southern Thuringia and northern Bavaria. The term Gangschar refers to the fact that few of the volcanoes have retained their characteristic topographical shape, rather their former activity can be detected by filled fissures known as Gänge. These veins are mostly oriented in south-southwest direction, their cross-section is often less than one metre wide. The Heldburger Gangschar' is named after the small settlement of Heldburg, part of the borough of Bad Colberg-Heldburg. The surrounding area, the Heldburger Land, belongs entirely to the northern part of the volcanic zone. The most impressive of the surviving volcanic cones by far are the twin peaks of the Gleichberge, 641 metres and 679 metres high, in nearby Heldburger Land.
Dorrite is a silicate mineral that is isostructural to the aenigmatite group. Although it is most chemically similar to the mineral rhönite [Ca2Mg5Ti(Al2Si4)O20], the lack of titanium (Ti) and presence of Fe3+ influenced dorrite's independence. Dorrite is named for Dr. John (Jack) A. Dorr, a late professor at the University of Michigan that researched in outcrops where dorrite was found in 1982. This mineral is sub-metallic resembling colors of brownish-black, dark brown, to reddish brown.
São Tomé and Príncipe both formed within the past 30 million years due to volcanic activity in deep water along the Cameroon line. Long-running interactions with seawater and different eruption periods have generated a wide variety of different igneous and volcanic rocks on the islands with complex mineral assemblages.
|This volcanology article is a stub. You can help Wikipedia by expanding it.|