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Basalte

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Hidden : 07/11/2020
Difficulty:
2 out of 5
Terrain:
3.5 out of 5

Size: Size:   other (other)

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Cette cache est placée dans le cadre de séries de Montgenevre au Collet Vert revenant via le domaine skiable de Claviere. C'est 17km et 800m +/- mais vous pouvez aller aussi loin que vous le souhaitez; 2 courtes sections sur les pistes de ski et le reste de beaux sentiers. 

A GZ, vous êtes au pied d'une impressionnante falaise de basalte et pour enregistrer ce cache-terre, vous devez répondre à quelques questions simples. Nous avons trouvé de très bonnes vidéos sur Internet, une très impressionnante de basalte d'oreiller en cours de formation à Hawai et une autre, "Géologie de 2 minutes" sur le basalte en colonne qui pourrait vous aider énormément.

This cache is placed as part of series from Montgenevre to the Collet Vert returning via the Claviere ski area.  It's 17km and 800m+/- but you can go as far as you wish; 2 short sections on ski slopes and the rest lovely paths.  At GZ you are at the foot of an impressive cliff of basalt and to log this earthcache you need to answer a few simple questions. We found some really good videos on the internet, one very impressive one of pillow basalt being formed in Hawai and another, "2 minute geology" feature on columnar basalt which could help you enormously.

Questa cache viene inserita come parte della serie da Montgenevre al Collet Vert che ritorna attraverso l'area sciistica di Claviere. Sono 17 km e 800 m +/- ma puoi andare fino in fondo; 2 brevi tratti sulle piste da sci e gli altri splendidi percorsi. 

A GZ sei ai piedi di un'imponente scogliera di basalto e per registrare questa mal di terra devi rispondere ad alcune semplici domande. Abbiamo trovato alcuni video davvero buoni su Internet, uno molto impressionante di cuscino di basalto che si sta formando in Hawai e un altro, una funzione di "geologia di 2 minuti" su basalto colonnare che potrebbe aiutarti enormemente.

French

Basalte

Basalte (Hérault, France)
 
Lave en coussins de basalte vu au fond de l'océan, au niveau de Hawaï.

Le basalte est une roche magmatique volcanique issue d'un magma refroidi rapidement et caractérisée par sa composition minéralogique : plagioclases (50 %), de pyroxènes (25 à 40 %), d'olivine (10 à 25 %), et de 2 à 3 % de magnétite. Sur Terre, il a une origine volcanique et est un des constituants principaux de la croûte océanique. Sur la Lune, elle constitue la surface des mers lunaires. Ce serait un constituant important des croûtes de Mars1, Vénus2 et Mercure3.

Le mot basalte est emprunté au latin basaltes, terme dont on dit souvent qu'il est dérivé d'un terme éthiopien signifiant « roche noire ». Mais, d'une part ce terme n'évoque rien aux locuteurs amhariques, d'autre part une autre origine est citée, peut-être égyptienne.

Le basalte est une roche mélanocrate à holomélanocrate (sombre à très sombre) à structure microlitique, qui est issue de la fusion partielle du manteau terrestre de composition péridotitique (lherzolite).

Les plus grands épanchements basaltiques connus sont les trapps de Sibérie en Russie, les trapps du Deccan en Inde, le plateau de la Columbia aux États-Unis, Tassili des Ajjers en Algérie ou encore les laves triasiques nord-américaines. La structure basaltique la plus célèbre est sans doute la chaussée des Géants en Irlande, où l'on peut admirer des orgues basaltiques (formations en forme de colonnes, généralement de coupe hexagonale). En France, ils se rencontrent principalement dans le Massif central. Les régions sombres de la Lune (les « mers ») sont formées de basaltes4.

Le basalte est une roche basique. Les roches plutoniques de même composition minéralogique sont les gabbros.

Classification des basaltes

 
Modèle basaltique:
  • plan de saturation de la silice
  • opx : orthopyroxène
  • cpx : clinopyroxène
  • ab : albite
  • Ne : néphéline
  • Qz : quartz
  • ol : olivine

Les basaltes se classent par leur taux de saturation en silice.

Lorsque le basalte n'atteint pas le plan de saturation de la silice, de la néphéline [SiAlO4]Na est exprimée. C'est le domaine des basanites, et, à l'approche du plan de saturation, celui du basalte alcalin à olivines. Au-delà du plan de saturation, c'est le domaine tholéiitique, avec le basalte tholéiitique, si le quartz n'est pas exprimé, et sinon le quartz tholéiite.

Les néphélinites et mélilitites

Les néphélinites et mélilitites sont des roches holofeldspathoïdiques. Elles ne se trouvent que dans les rifts, généralement en fin de vie.

La basanite

La basanite est caractéristique du volcanisme intraplaques ponctuel et de faible volume. En France, on la trouve dans les rifts rhénan, de Limagne, des monts du Cantal et des Causses.

Le basalte alcalin à olivine

Le basalte alcalin à olivine est une roche ubiquiste. On le trouve dans le volcanisme intraplaques océanique et continental lorsque celui-ci est de faible volume.

Le basalte tholéiitique

Le basalte tholéiitique (ou olivine tholéiite, ou tholéiite à olivine) constitue les fonds océaniques. Les MORB (MORB = basalte de dorsale, de l'anglais Mid Ocean Ridge Basalt) – K2O inférieur à 0,2 % et TiO2 inférieur à 2,0 % sont les constituants essentiels de la croûte océanique. Il se trouve également dans le volcanisme intraplaques océanique et continental. Il contient un orthopyroxène normatif (non exprimé).

La tholéiite à quartz

La tholéiite à quartz (ou quartzique) est beaucoup plus rare. Cette dénomination est trompeuse car on ne trouve jamais de quartz dans ces roches. Le quartz apparaît seulement virtuellement dans la normalisation de la composition chimique.

L'origine des basaltes

 
Coulée de lave basaltique refroidie de type pahoehoe (Kilauea, Hawaï).

Le magma à l'origine des basaltes provient de la fusion partielle des roches du manteau terrestre. Le manteau est constitué de péridotite dans sa partie supérieure. Un modèle de péridotite a été avancé : la pyrolite. Le géotherme ne croise le solidus de la pyrolite que si elle est hydratée (0,1 %). Il n'y a pas de fusion en dessous de la LVZ car nous arrivons dans une zone de pyrolite sèche.

Selon la pression à laquelle se fait la fusion partielle, les minéraux affectés par la fusion ne sont pas les mêmes. Pour des taux de fusion faibles, le liquide est riche en eau et en alcalins : on obtient des basanites ou des basaltes alcalins à olivines. Pour des taux de fusion élevés, le liquide est plus riche en calcium, fer et magnésium, et on obtient des olivines tholéiites.

Au niveau des points chauds, le taux de fusion de la pyrolite va de 5 % en périphérie, avec formation de basanite, à 30 % au centre, avec formation d'olivine tholéiite. Lorsque le taux de fusion est de 10 %, il y a formation de basalte alcalin à olivines.

Au niveau des rides médio-océaniques, le taux de fusion est de 30 %, et nous obtenons de l'olivine tholéiite.

Orgues basaltiques

 
Orgues basaltiques dans le Plateau d'Antrim (Irlande du Nord), montrant les fracturations longitudinales et transversales

Les orgues basaltiques ou les colonnes basaltiques sont une formation géologique composée de colonnes régulières. Elle résulte de la solidification et de la contraction thermique d'une coulée basaltique peu de temps après son émission. La partie inférieure, qui se refroidit ou s'assèche plus lentement, se fracture de la surface vers la profondeur sous formes de prismes sub-verticaux à section hexagonale d'ordre décimétrique. Ces colonnes sont surmontées d'une zone de petits prismes moins réguliers (ou « faux prisme ») pouvant s'associer en gerbes.

La genèse des orgues a été mise en équation par Lucas Goehring et ses collègues de l'Université de Toronto. La loi d'échelle mise en évidence relie la largeur entre deux fissures aux propriétés du milieu et au flux de chaleur ou d'humidité. Elle est vérifiée avec un modèle basé sur de la fécule de maïs qui se comporte comme de la lave. La régularité des colonnes basaltiques de la Chaussée des Géants en Irlande du Nord serait ainsi due à une perte de chaleur constante.

Utilisation

 
La Cathédrale Saint-Étienne d'Agde est construite en basalte5

Les basaltes sont utilisés dans la construction et dans la statuaire6. Les basaltes, en raison de leur texture fine et isotrope, sont également valorisés comme granulats de forte compacité et de grande résistance mécanique. Certains peuvent être utilisés pour la réalisation du ballast des voies ferrées7. La laine de roche est obtenue à partir de basalte ou de roche diabase analogue au basalte, via un procédé de fusion (dans un cubilot dans lequel on ajoute des fondants et du coke pour le porter à 1 500 °C) et d'extrusion.

Une méthode de géo-ingénierie, l'altération forcée, envisage l'épandage de basalte finement broyé pour fixer du CO2 atmosphérique dans les sols agricoles.

"Loguez cette cache "Found it" et envoyez-moi vos propositions de réponses soit via mon profil, soit via la messagerie geocaching.com (Message Center), et je vous contacterai en cas de problème."

1) Quelle type de basalte en trouve ici, coussin ou orgues?

2) Comparez le basalte immédiatement au-dessus de la plate-forme avec celui de 5 mètres à droite. Décrivez la forme et estimez la taille du basalte au-dessus de la plate-forme et à droite.

3)Selon vous, quelle pourrait être la raison de la différence?

4) Que vous dit votre réponse à la Q1 sur votre position?

5) Facultatif; prenez une photo de vous ou de votre GPS chez GZ sans montrer le basalte!

 

English

Basalt

From Wikipedia, the free encyclopedia
 
 
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BasaltIgneous rockBasaltUSGOV.jpgCompositionMafic: amphibole and pyroxene, sometimes plagioclase, feldspathoids, and/or olivine.

Basalt (US: /bəˈsɔːlt, ˈbeɪsɒlt/, UK: /ˈbæsɔːlt, ˈbæsəlt/)[1][2][3][4] is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava[5] 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.[6] 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.

Definition

 
Columnar basalt flows in Yellowstone National Park, USA

By definition, basalt is an aphanitic (fine-grained) igneous rock with generally 45–53% silica (SiO2)[7] and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase. This is as per definition of the International Union of Geological Sciences (IUGS) classification scheme.[9][10] It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaiʻi. Basalt commonly features a very fine-grained or glassy matrix interspersed with visible mineral grains. The average density is 3.0 g/cm3.

Basalt is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into hematite and other iron oxides and hydroxides. Although usually characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts can be quite light-coloured, superficially resembling andesite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow; it is often porphyritic, containing larger crystals (phenocrysts) formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. These phenocrysts usually are of olivine or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt.

Basalt with a vesicular texture is called vesicular basalt, when the bulk of the rock is mostly solid; when the vesicles are over half the volume of a specimen, it is called scoria. This texture forms when dissolved gases come out of solution and form bubbles as the magma decompresses as it reaches the surface, yet are trapped as the erupted lava hardens before the gases can escape.

The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are generally referred to as diabase (also called dolerite) or, when more coarse-grained (crystals over 2 mm across), as gabbro. Gabbro is often marketed commercially as "black granite."

 
Columnar basalt at Szent György Hill, Hungary
 
Vesicular basalt at Sunset Crater, Arizona. US quarter for scale.

In the Hadean, Archean, and early Proterozoic eras of Earth's history, the chemistry of erupted magmas was significantly different from today's, due to immature crustal and asthenosphere differentiation. These ultramafic volcanic rocks, with silica (SiO2) contents below 45% are usually classified as komatiites.

Etymology

The word "basalt" is ultimately derived from Late Latin basaltes, a misspelling of Latin basanites "very hard stone", which was imported from Ancient Greek βασανίτης (basanites), from βάσανος (basanos, "touchstone") and perhaps originated in Egyptian bauhun "slate".[11] The modern petrological term basalt describing a particular composition of lava-derived rock originates from its use by Georgius Agricola in 1556 in his famous work of mining and mineralogy De re metallica, libri XII. Agricola applied "basalt" to the volcanic black rock of the Schloßberg (local castle hill) at Stolpen, believing it to be the same as the "very hard stone" described by Pliny the Elder in Naturalis Historiae.[12]

Types

 
Large masses must cool slowly to form a polygonal joint pattern, as here at the Giant's Causeway in Northern Ireland
 
Columns of basalt near Bazaltove, Ukraine

Petrology

 
Photomicrograph of a thin section of basalt from Bazaltove, Ukraine

The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvöspinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt.

In tholeiitic basalt, pyroxene (augite and orthopyroxene or pigeonite) and calcium-rich plagioclase are common phenocryst minerals. Olivine may also be a phenocryst, and when present, may have rims of pigeonite. The groundmass contains interstitial quartz or tridymite or cristobalite. Olivine tholeiitic basalt has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass. Ocean floor basalts, erupted originally at mid-ocean ridges, are known as MORB (mid-ocean ridge basalt) and are characteristically low in incompatible elements.

Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are labradorite to andesine in composition. Augite is rich in titanium compared to augite in tholeiitic basalt. Minerals such as alkali feldspar, leucite, nepheline, sodalite, phlogopite mica, and apatite may be present in the groundmass.

Basalt has high liquidus and solidus temperatures—values at the Earth's surface are near or above 1200 °C (liquidus) and near or below 1000 °C (solidus); these values are higher than those of other common igneous rocks.

The majority of tholeiitic basalts are formed at approximately 50–100 km depth within the mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200 km.[17][18] The origin of high-alumina basalt continues to be controversial, with disagreement over whether it is a primary melt or derived from other basalt types by fractionation.[19]:65

Geochemistry

Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO2 and the alkali oxides, i.e., Na2O + K2O, consistent with the TAS classification.

Basalt generally has a composition of 45–55 wt% SiO2, 2–6 wt% total alkalis, 0.5–2.0 wt% TiO2, 5–14 wt% FeO and 14 wt% or more Al2O3. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.

High-alumina basalts have aluminium contents of 17–19 wt% Al2O3; boninites have magnesium (MgO) contents of up to 15 percent. Rare feldspathoid-rich mafic rocks, akin to alkali basalts, may have Na2O + K2O contents of 12% or more.

The abundances of the lanthanide or rare-earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled. In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly. It arises because Eu2+ can substitute for Ca2+ in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form 3+ cations.

Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat rare-earth element (REE) patterns normalized to mantle or chondrite values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt is considered a key to understanding plate tectonics, its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the Mid-Atlantic Ridge, and the compositions also define different ranges in different ocean basins.[20] Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB).

Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much studied to learn about the evolution of the Earth's mantle. Isotopic ratios of noble gases, such as 3He/4He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from mantle plumes.

Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007).

Morphology and textures

 
An active basalt lava flow

The shape, structure and texture of a basalt is diagnostic of how and where it erupted—whether into the sea, in an explosive cinder eruption or as creeping pāhoehoe lava flows, the classic image of Hawaiian basalt eruptions.

Subaerial eruptions

Basalt that erupts under open air (that is, subaerially) forms three distinct types of lava or volcanic deposits: scoria; ash or cinder (breccia); and lava flows.

Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidized iron from weathered iron-rich minerals such as pyroxene.

ʻAʻā types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawaiʻi. Pāhoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms lava lakes. Lava tubes are common features of pāhoehoe eruptions.

Basaltic tuff or pyroclastic rocks are rare but not unknown. Usually basalt is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and buildup of volcanic gases. Hawaiʻi's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera, New Zealand in its violent 1886 eruption. Maar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano.

Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

Columnar basalt
 
The Giant's Causeway in Northern Ireland
 
Columnar jointed basalt in Turkey
 
Columnar basalt at Cape Stolbchaty, Russia

During the cooling of a thick lava flow, contractional joints or fractures form.[21] If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed.[22] The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns.

Submarine eruptions

 
Pillow basalts on the south Pacific seafloor
Pillow basalts

When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow. This "pillow" texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. The size of individual pillows varies from 10 cm up to several meters.

When pāhoehoe lava enters the sea it usually forms pillow basalts. However, when ʻaʻā enters the ocean it forms a littoral cone, a small cone-shaped accumulation of tuffaceous debris formed when the blocky ʻaʻā lava enters the water and explodes from built-up steam.

The island of Surtsey in the Atlantic Ocean is a basalt volcano which breached the ocean surface in 1963. The initial phase of Surtsey's eruption was highly explosive, as the magma was quite fluid, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone. This has subsequently moved to a typical pāhoehoe-type behaviour.

Volcanic glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions.

Pillow basalt is also produced by some subglacial volcanic eruptions.

Life on basaltic rocks

The common corrosion features of underwater volcanic basalt suggest that microbial activity may play a significant role in the chemical exchange between basaltic rocks and seawater. The significant amounts of reduced iron, Fe(II), and manganese, Mn(II), present in basaltic rocks provide potential energy sources for bacteria. Some Fe(II)-oxidizing bacteria cultured from iron-sulfide surfaces are also able to grow with basaltic rock as a source of Fe(II).[23] Fe- and Mn- oxidizing bacteria have been cultured from weathered submarine basalts of Loihi Seamount.[24] The impact of bacteria on altering the chemical composition of basaltic glass (and thus, the oceanic crust) and seawater suggest that these interactions may lead to an application of hydrothermal vents to the origin of life.

Distribution

On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt commonly erupts on Io (the third largest moon of Jupiter),[25] and has also formed on the Moon, Mars, Venus, and the asteroid Vesta.

The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below, the ocean ridges.

 

Basalt is one of the most common rock types in the world. Basalt is the rock most typical of large igneous provinces. The largest occurrences of basalt are in the ocean floor that is almost completely made up by basalt. Above sea level basalt is common in hotspot islands and around volcanic arcs, specially those on thin crust. However, the largest volumes of basalt on land correspond to continental flood basalts. Continental flood basalts are known to exist in the Deccan Traps in India, the Chilcotin Group in British Columbia, Canada, the Paraná Traps in Brazil, the Siberian Traps in Russia, the Karoo flood basalt province in South Africa, the Columbia River Plateau of Washington and Oregon.

Many archipelagoes and island nations have an overwhelming majority of their exposed bedrock made up of basalt due to being above hotspots, for example, Iceland and Hawaiʻi.

Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts, because low-grade metamorphism of basalt produces chlorite, actinolite, epidote and other green minerals.

Lunar and Martian basalt

 
Lunar olivine basalt collected by Apollo 15.

The dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic lava flows. These rocks were sampled by the manned American Apollo program, the robotic Russian Luna program, and are represented among the lunar meteorites.

Lunar basalts differ from their Earth counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO. They also possess a wide range of titanium concentrations (present in the mineral ilmenite),[26] ranging from less than 1 wt% TiO2, to about 13 wt.%. Traditionally, lunar basalts have been classified according to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti. Nevertheless, global geochemical maps of titanium obtained from the Clementine mission demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant.[27]

Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism, lack of the oxidation typical of terrestrial basalts, and a complete lack of hydration. Most of the Moon's basalts erupted between about 3 and 3.5 billion years ago, but the oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method of crater counting, are estimated to have erupted only 1.2 billion years ago.

Basalt is also a common rock on the surface of Mars, as determined by data sent back from the planet's surface,[28] and by Martian meteorites.

Alteration of basalt

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Metamorphism

 
Metamorphosed basalt from an Archean greenstone belt in Michigan, US. The minerals that gave the original basalt its black colour have been metamorphosed into green minerals.

Basalts are important rocks within metamorphic belts, as they can provide vital information on the conditions of metamorphism within the belt.

Metamorphosed basalts are important hosts for a variety of hydrothermal ore deposits, including gold deposits, copper deposits, volcanogenic massive sulfide ore deposits and others.[29]

Weathering

Compared to other rocks found on Earth's surface, basalts weather relatively fast. The typically iron-rich minerals oxidise rapidly in water and air, staining the rock a brown to red colour due to iron oxide (rust). Chemical weathering also releases readily water-soluble cations such as calcium, sodium and magnesium, which give basaltic areas a strong buffer capacity against acidification. Calcium released by basalts binds up CO2 from the atmosphere forming CaCO3 acting thus as a CO2 trap. To this it must be added that the eruption of basalt itself is often associated with the release of large quantities of CO2 into the atmosphere from volcanic gases.

Uses

Basalt is used in construction (e.g. as building blocks or in the groundwork), making cobblestones (from columnar basalt) and in making statues. Heating and extruding basalt yields stone wool, said to be an excellent thermal insulator.

Carbon sequestration in basalt has been studied as a means of removing carbon dioxide, produced by human industrialization, from the atmosphere. Underwater basalt deposits, scattered in seas around the globe, have the added benefit of the water serving as a barrier to the re-release of CO2 into the atmosphere.[30]

See also

  • Basalt fan structure – Rock formation composed of columnar jointed basalt columns that have slumped into a fan shape
  • Basalt fiber – Strucural fibres spun from melted basalt
  • Flood basalt – The result of a very large volume eruption of basalt lava
  • Igneous rock – Rock formed through the cooling and solidification of magma or lava
  • Mafic – Silicate mineral or igneous rock that is rich in magnesium and iron
  • Polybaric melting
  • Spilite – A fine-grained igneous rock, resulting from alteration of oceanic basalt
  • Sideromelane – A vitreous basaltic volcanic glass
  • Volcano – rupture in the crust of a planetary-mass object that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface

References

Wikipedia

Logging requirements:

To log this cache as a find, email or message me the answers to the following questions.  You can log a 'find' and I will contact you if I need clarification.

1) What type of basalt do you see here, eg pillow or columnar?

2) Compare the basalt immediately above the platform with that 5m to the right.  Describe the shape and estimate the size of the basalt above the platform and to the right.

3)What do you think might be the reason for the difference?

4)What does your answer to Q1 tell you about where you are standing?

5)Optional;take a photo of you or your gps at GZ without showing the basalt!

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