Misplaced Pages

#617382

79-411: The Kleiner Gleichberg (641 m) and the neighbouring Großer Gleichberg (679 m) form a pair of "geological twins". Both mountains are basalt cones which are volcanic in origin. The name Steinsburg (867: Steinberg) ("stone castle") probably derives from the large stone fields that surround the mountain summit plateau. Johann Wolfgang von Goethe recognised the volcanic origin of these stone fields:

158-423: A mantle wedge above the descending slab. The slab releases water vapor and other volatiles as it descends, which further lowers the melting point, further increasing the amount of decompression melting. Each tectonic setting produces basalt with its own distinctive characteristics. The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene . Olivine can also be

237-411: A sill -like pattern, affecting magma propagation beneath the surface. Other unique features of Venus's surface are novae (radial networks of dikes or grabens ) and arachnoids . A nova is formed when large quantities of magma are extruded onto the surface to form radiating ridges and trenches which are highly reflective to radar. These dikes form a symmetrical network around the central point where

316-493: A basalt is diagnostic of how and where it erupted—for example, whether into the sea, in an explosive cinder eruption or as creeping pāhoehoe lava flows, the classic image of Hawaiian basalt 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

395-439: A deep mantle plume system which contains enough volatiles to produce explosive volcanism. In 2010, Suzanne E. Smrekar et al. published that Venus Express observed three volcanoes that have had eruptions about 250,000 years ago or less, which suggests that Venus is periodically resurfaced by lava flows. She has proposed two missions to Venus to elucidate the planet: Venus Origins Explorer (VOX), and VERITAS . Meanwhile,

474-442: 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 metres. When pāhoehoe lava enters

553-427: 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 . These structures, or basalt prisms , are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on

632-401: A gummier lava that restricts the exit of the gases, and for that reason, composite volcanoes tend to erupt more violently. On Venus, where there are no tectonic plates or seawater , volcanoes are mostly of the shield type. Nevertheless, the morphology of volcanoes on Venus is different: on Earth, shield volcanoes can be a few tens of kilometres wide and up to 10 km (6.2 mi) high in

711-465: A high content of plagioclase; these are sometimes described as leucobasalts . It can be difficult to distinguish between lighter-colored basalt and andesite , so field researchers commonly use a rule of thumb for this purpose, classifying it as basalt if it has a color index of 35 or greater. The physical properties of basalt result from its relatively low silica content and typically high iron and magnesium content. The average density of basalt

790-621: 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 less common than basaltic lava flows. Usually basalt

869-623: A lower total content of alkali oxides than trachybasalt and most basanites and tephrites. Basalt generally has a composition of 45–52 wt% SiO 2 , 2–5 wt% total alkalis, 0.5–2.0 wt% TiO 2 , 5–14 wt% FeO and 14 wt% or more Al 2 O 3 . 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% Al 2 O 3 ; boninites have magnesium (MgO) contents of up to 15 percent. Rare feldspathoid -rich mafic rocks, akin to alkali basalts, may have Na 2 O + K 2 O contents of 12% or more. The abundances of

SECTION 10

#1732764724618

948-550: A major role in shaping its surface. There are more than 1,000 volcanic structures and possible periodic resurfacing of Venus by floods of lava. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Venus has an atmosphere rich in carbon dioxide , with a pressure that is 90 times that of Earth's atmosphere. There are over 80,000 volcanoes on Venus detected through radar mapping. For many years scientists debated on whether Venus

1027-484: A perspective view of the elevation of the surface of Venus, which assists in the identification of volcanoes. Volcanic features discovered include flood lavas, edifice clusters, shield volcanoes, volcanic cones, and volcanic domes. Since the Magellan mission, more than 1,660 volcanic landforms have been identified on the surface of Venus. Further analysis of the Magellan data revealed more than 85,000 volcanoes. After

1106-507: 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 . Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are labradorite to andesine in composition. Augite

1185-523: 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

1264-498: A study by University of Maryland supported by Swiss National Science Foundation and NASA discovered that 37 of Venus's coronae show signs of ongoing activity. Maryland professor Laurent Montesi said, "we are able to point to specific structures and say 'Look, this is not an ancient volcano but one that is active today, dormant perhaps, but not dead...'" The active coronae are clustered near each other, so positioning geologic survey instruments would now be easier. In March 2023, at

1343-427: Is 2.9 g/cm , compared, for example, to granite ’s typical density of 2.7 g/cm . The viscosity of basaltic magma is relatively low—around 10 to 10 cP —similar to the viscosity of ketchup , but that is still several orders of magnitude higher than the viscosity of water, which is about 1 cP). Basalt is often porphyritic , containing larger crystals ( phenocrysts ) that formed before the extrusion event that brought

1422-507: Is also a common rock on the surface of Mars , as determined by data sent back from the planet's surface, and by Martian meteorites . Volcanism on Venus The surface of Venus is dominated by volcanic features and has more volcanoes than any other planet in the Solar System . It has a surface that is 90% basalt , and about 65% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played

1501-401: Is also produced by some subglacial volcanic eruptions. Basalt is the most common volcanic rock type on Earth, making up over 90% of all volcanic rock on the planet. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below the ocean ridges . Basalt is also the principal volcanic rock in many oceanic islands , including

1580-417: Is an 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 surface of a rocky planet or moon . More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro . The eruption of basalt lava

1659-761: 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. 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

SECTION 20

#1732764724618

1738-481: Is defined as volcanic rock with a content of between 45% and 52% silica and no more than 5% alkali metal oxides. This places basalt in the B field of the TAS diagram. Such a composition is described as mafic . Basalt is usually dark grey to black in colour, due to a high content of augite or other dark-coloured pyroxene minerals, but can exhibit a wide range of shading. Some basalts are quite light-coloured due to

1817-457: Is derived from phosphide (P ) through the following interaction with sulfuric acid in Venus' atmosphere: Phosphide comes from metals such as iron and magnesium, which should exist in great quantities in Venus' mantle . The phosphines were detected at a height of 70 km, which implies a volcanic eruption on the explosive scale of Krakatau or Yellowstone on Earth. The implication of this

1896-564: Is difficult to determine whether they occurred or not. In 2014, the first direct evidence for ongoing volcanism was located, in the form of infrared "flashes" over the edges of rift zone Ganis Chasma , near the shield volcano Sapas Mons . These flashes were detectable during two or three consecutive Earth days in 2008 and 2009 and are thought to be caused either by hot gases or lava released from volcanic eruptions. Scientists suspect that there are four volcanoes that may be active: Maat Mons , Ozza Mons , Sapas Mons and Idunn Mons . In 2020,

1975-431: Is not only that Venus has experienced recent volcanism, but that it is capable of explosive eruptions despite the lack of hydrated melts similar to those created at subduction zones on Earth. It is thought that Venus may have primordial water in the mantle that could be concentrated through fractionation . Biological activity has been suggested as an alternate explanation for the phosphines in Venus' atmosphere, but this

2054-693: Is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System . For example, the bulk of the plains of Venus , which cover ~80% of the surface, are basaltic; the lunar maria are plains of flood-basaltic lava flows ; and basalt is a common rock on the surface of Mars . Molten basalt lava has a low viscosity due to its relatively low silica content (between 45% and 52%), resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying. Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometres and constitute

2133-583: 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

2212-402: Is suppressed. Above this depth, submarine eruptions are often explosive, tending to produce pyroclastic rock rather than basalt flows. These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts of pumice . When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms

2291-452: Is three kilometres long and encloses an area of 66 hectares. Only a few short sections of these walls have been partially preserved, but the collapsed sections enable the old structure to be made out. In clearing large parts of the stone field for roadbuilding (from 1838), metal objects and other artefacts were discovered that were later recognised as of Celtic origin. The increasing amount of basalt quarrying from 1858 destroyed parts of

2370-440: 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

2449-455: 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 "). The modern petrological term basalt , describing a particular composition of lava -derived rock, became standard because of its use by Georgius Agricola in 1546, in his work De Natura Fossilium . Agricola applied

Oppidum Steinsburg - Misplaced Pages Continue

2528-422: Is unlikely due to the absence of any other biosignatures. Another hypothesis states that the phosphine could be produced in Venus' clouds, but this process requires water which is generally unavailable on Venus. Some scientists question that the phosphine levels found are truly as high as indicated. If the phosphine is present in amounts of 1–5 ppb and can be determined to originate in the mantle, it will imply

2607-539: The Clementine mission demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant. 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

2686-1228: The Paraná Traps in Brazil, the Siberian Traps in Russia , the Karoo flood basalt province in South Africa, and the Columbia River Plateau of Washington and Oregon . Basalt is also prevalent across extensive regions of the Eastern Galilee , Golan , and Bashan in Israel and Syria . Basalt also is common around volcanic arcs, specially those on thin crust . 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. As well as forming large parts of

2765-461: The basalt of the former volcanic pipe crumbled as a result of erosion to form the blockfields visible today that are a typical indicator of collapsed volcano structures. These basalt blocks were used in Celtic times for the construction of dry stone walls that probably protected the oppidum . A total of three rings of walls were built surround the mountain like city walls. The outermost wall

2844-524: The hypabyssal and plutonic equivalents of basalt. During the Hadean , Archean , and early Proterozoic eons of Earth's history, the chemistry of erupted magmas was significantly different from what it is today, due to immature crustal and asthenosphere differentiation. The resulting ultramafic volcanic rocks, with silica (SiO 2 ) contents below 45% and high magnesium oxide (MgO) content, are usually classified as komatiites . The word "basalt"

2923-524: 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 Eu can substitute for Ca in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form cations . Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are

3002-439: The 54th Lunar Planetary Science Conference, a team revealed the first images of volcanic activity on the surface of Venus. The announcement consisted of two radar images from different cycles of Magellan data (8 months apart) that displayed a volcanic vent that had expanded by almost 2 square kilometers. This data was over 30 years old at the time of this discovery. The scientists checked that this expansion could not be explained by

3081-463: The Earth's crust, basalt also occurs in other parts of the Solar System. Basalt commonly erupts on Io (the third largest moon of Jupiter ), and has also formed on the Moon , Mars , Venus , and the asteroid Vesta . The dark areas visible on Earth's moon , the lunar maria , are plains of flood basaltic lava flows. These rocks were sampled both by the crewed American Apollo program and

3160-598: The Japanese spacecraft Akatsuki has been orbiting Venus since December 2015 and one of its goals is to scan for active volcanism using its infrared cameras, although the infrared detector that was supposed to do this failed in December 2016 after a relatively short period of observations. Three missions are expected to launch in the 2030s, VERITAS, DAVINCI, and EnVision, all of which will help detect volcanism. Both VERITAS and EnVision will use radar remote sensing to map

3239-1701: The Maat Mons region taken by the Magellan orbiter. Using computer simulations they determined that the topography had changed during an 8-month interval, and have concluded that active volcanism was the cause. Until 2023, there had only been hints of active volcanism. In March 2023, Herrick et al. announced that they had imaged a vent expanding in Magellan images, indicating active volcanism on Venus. Venus has shield volcanoes , widespread lava flows and some unusual volcanoes called pancake domes and "tick-like" structures which are not present on Earth . Pancake dome volcanoes are up to 15 km (9.3 mi) in diameter and less than 1 km (0.62 mi) in height and are 100 times larger than lava domes formed on Earth. They are usually associated with coronae and tesserae (large regions of highly deformed terrain, folded and fractured in two or three dimensions, which are unique to Venus). The pancakes are thought to be formed by highly viscous, silica -rich lava erupting under Venus's high atmospheric pressure. The "tick-like" structures are called scalloped margin domes . They are commonly called ticks because they appear as domes with numerous legs . They are thought to have undergone mass wasting events such as landslides on their margins. Sometimes deposits of debris can be seen scattered around them. On Earth, volcanoes are mainly of two types: shield volcanoes and composite or stratovolcanoes . The shield volcanoes, for example those in Hawaii , eject magma from

Oppidum Steinsburg - Misplaced Pages Continue

3318-437: The amount of sulphur dioxide gas in the upper atmosphere. Sulphur dioxide is an important component of volcanic outgassing . However, the sulphur dioxide in the lower atmosphere remains stable. This could mean that a change in the global atmosphere caused the sulphur dioxide concentration to increase above the clouds. Even though the change in the atmosphere may be evidence that there have been volcanoes that erupted in Venus, it

3397-423: The angle at which the images were taken through computer simulations, which revealed that the change must be structural. Lightning on Venus may serve as a diagnostic of volcanism or atmospheric convection, so some effort has been devoted to detecting possible lightning on Venus. No lightning has been directly observed, but the most compelling evidence is the very low frequency (VLF) radio emissions recorded beneath

3476-712: The atmosphere. The analysis of SO 2 and other gasses will help to learn about out gassing from recent volcanoes. DAVINCI will have a probe that descends into the atmosphere collecting data along the way. Atmospheric analysis will provide important information to pair with the recent discovery of active volcanism. VERITAS will also have the Venus Emissivity Mapper (VEM) and a radar imager VISAR (Venus Interferometric Synthetic Aperture Radar). These will map lava fields and volcanoes on Venus' surface. Originally set to launch in 2027, this mission has been delayed until 2030. If VERITAS resumes its original launch date,

3555-480: The basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of under 52%. It is often not practical to determine the mineral composition of volcanic rocks, due to their very small grain size, in which case geologists instead classify the rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica ( TAS ); in that context, basalt

3634-508: The case of Mauna Kea , measured from the sea floor. On Venus, these volcanoes can cover hundreds of kilometres in area, but they are relatively flat, with an average height of 1.5 km (0.93 mi). Large volcanoes cause the Venusian lithosphere to flex downward because of their enormous vertical loads, producing flexural moats or ring fractures around the edifices. Large volcano edifice loading also causes magma chambers to fracture in

3713-478: 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 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

3792-460: The clouds by all four of the Venera landers . The Japanese orbiter Akatsuki is currently searching for visible lightning on Venus, among other science objectives. In 2020, Greaves et al. detected phosphine levels of 1–5  parts per billion in Venus' atmosphere using ALMA and JCMT . Historic data from Pioneer Venus also shows the possible detection of phosphine. Phosphine (PH 3 )

3871-447: 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. During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While

3950-476: The data between VERTIAS and EnVision will pair together similarly to the various cycles of Magellan data. They would then have the opportunity to view volcanic changes across a set of years. Locating volcanoes on Venus became possible during the Magellan mission in 1990, which mapped over 95% of the surface of Venus. The surface of Venus is hidden by clouds but surface features can be mapped using synthetic aperture radar. Some images created by this mapping can give

4029-405: The depths of the Earth in zones called hot spots . The lava from these volcanoes is relatively fluid and permits the escape of gases. Composite volcanoes, such as Mount St. Helens and Mount Pinatubo , are associated with tectonic plates. In this type of volcano, the oceanic crust of one plate is sliding underneath the other in a subduction zone , together with an inflow of seawater, producing

SECTION 50

#1732764724618

4108-545: The evolution of the Earth's mantle . Isotopic ratios of noble gases , such as He / He, 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 that produce basaltic magma probably include both peridotite and pyroxenite . The shape, structure and texture of

4187-598: The flanking slopes of a volcano do not have wrinkles ridges, then they would be considered young. The Sif Mons volcano is 350 km in diameter, 2 km high, and is in the Western Eistla Regio Rise. Based on the mapping of the volcano, the area around the central caldera is mostly flat with many chain pits surrounding the area. On eastern parts of the volcano, lava has flooded from the main caldera to smaller calderas nearby. Evidence suggests that there were many flank eruptions at this volcano. Most of

4266-440: The flow fields around this volcano are sheet flow fields. The Gula Mons volcano is 460 km in diameter, 3.2 km high and is in the Western Eistla Regio Rise. Gula Mons is considered to be a shield volcano. This volcano has a central edifice that is surrounded by the peaks of the volcano. Mapping suggests that there are multiple caldera pits in this volcano that are partially filled with lava. The Kunapipi Mons volcano has

4345-523: The hillfort, especially the easily accessible lower wall rings and brought a huge quantity of archaeological finds to the surface, of which mainly the metal objects have survived in various collections. On the initiative of Alfred Götze, the quarrying was stopped between 1902 and 1927. In 1929 the Steinsburg Museum was built on the saddle between the Großer and Kleiner Gleichberg and this is where

4424-740: The islands of Hawaiʻi , the Faroe Islands , and Réunion . The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is the rock most typical of large igneous provinces . These include continental flood basalts , the most voluminous basalts found on land. Examples of continental flood basalts included the Deccan Traps in India , the Chilcotin Group in British Columbia , Canada ,

4503-403: The last 2.5 million years; however, until recently there had been no absolute proof that any volcano on Venus has erupted recently. Recent radar imagery shows more than 1,000 volcanic structures and evidence of possible periodic resurfacing of the planet by floods of lava . In addition to the radar images, there is supporting evidence that volcanism has taken place, including an unusual change in

4582-480: The lava emerged, where there may also be a depression caused by the collapse of the magma chamber. Arachnoids are so named because they resemble a spider's web, featuring several concentric ovals surrounded by a complex network of radial fractures similar to those of a nova. It is not known whether the 250 or so features identified as arachnoids actually share a common origin, or are the result of different geological processes. Volcanism on Venus has taken place within

4661-776: The location and connexion of the finds was not noted. In addition, the older finds include very few items of pottery, because pieces of old ceramic vessels were not looked after in the 19th century. Excavations were carried out as early as 1874 by a Romhild doctor. In the early 20th century systematic excavations were carried out by prehistorian , Alfred Götze . In recent times several of the surviving fortification remains have been conserved, but in some cases had to be dismantled and re-assembled. 50°24′44″N 10°35′34″E  /  50.412091°N 10.592884°E  / 50.412091; 10.592884 Basalt Basalt ( UK : / ˈ b æ s ɔː l t , - əl t / ; US : / b ə ˈ s ɔː l t , ˈ b eɪ s ɔː l t / )

4740-604: The magma as it decompresses during its approach to the surface; the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described as scoria . 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 more properly referred to either as diabase (also called dolerite) or—when they are more coarse-grained (having crystals over 2 mm across)—as gabbro . Diabase and gabbro are thus

4819-416: The magma to the surface, embedded in a finer-grained matrix . These phenocrysts are usually made of augite, olivine , or a calcium-rich plagioclase, which have the highest melting temperatures of any of the minerals that can typically crystallize from the melt, and which are therefore the first to form solid crystals. Basalt often contains vesicles ; they are formed when dissolved gases bubble out of

SECTION 60

#1732764724618

4898-518: The majority of finds are still on display today. The Gleichberge are thought by several researchers to the same place mentioned in the Geography of Ptolemy as the settlement of Bicurgium . In the specialist and local history literature, the names Kleiner Gleichberg and Steinsburg are both used to refer to the hillfort. The widespread destruction has led to the uncovering of an unusually high number of finds, especially metal objects. However, often

4977-710: The mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200 km. 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. Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO 2 and the alkali oxides, i.e., Na 2 O + K 2 O , consistent with their TAS classification . Basalt contains more silica than picrobasalt and most basanites and tephrites but less than basaltic andesite . Basalt has

5056-406: The most voluminous of all volcanic formations. Basaltic magmas within Earth are thought to originate from the upper mantle . The chemistry of basalts thus provides clues to processes deep in Earth's interior . Basalt is composed mostly of oxides of silicon, iron, magnesium, potassium, aluminum, titanium, and calcium. Geologists classify igneous rock by its mineral content whenever possible;

5135-410: 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. From 1972 to 1985, five Venera and two VEGA landers successfully reached the surface of Venus and carried out geochemical measurements using X-ray fluorescence and gamma-ray analysis. These returned results consistent with

5214-475: 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. The character of submarine basalt eruptions is largely determined by depth of water, since increased pressure restricts the release of volatile gases and results in effusive eruptions. It has been estimated that at depths greater than 500 metres (1,600 ft), explosive activity associated with basaltic magma

5293-411: The relative volume percentages of quartz (crystalline silica (SiO 2 )), alkali feldspar , plagioclase , and feldspathoid ( QAPF ) are particularly important. An aphanitic (fine-grained) igneous rock is classified as basalt when its QAPF fraction is composed of less than 10% feldspathoid and less than 20% quartz, and plagioclase makes up at least 65% of its feldspar content. This places basalt in

5372-595: 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 ), ranging from less than 1 wt% TiO 2 , 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

5451-403: The rock at the landing sites being basalts, including both tholeiitic and highly alkaline basalts. The landers are thought to have landed on plains whose radar signature is that of basaltic lava flows. These constitute about 80% of the surface of Venus. Some locations show high reflectivity consistent with unweathered basalt, indicating basaltic volcanism within the last 2.5 million years. Basalt

5530-462: 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

5609-404: The summit and on the northern flank. Although many lines of evidence such as this suggest that volcanoes on Venus have been recently active, present-day eruptions at Maat Mons have not been confirmed. Nevertheless, other more recent studies, in 2020, suggest that Venus, though not Maat Mons specifically, is indeed currently volcanically active. In 2023, scientists reexamined topographical images of

5688-405: The surface of Venus and areas considered to be volcanoes are reshaped into a vector and processed through a series of equations. This algorithm has been used to identify multiple volcanoes in different mapping images from Venus. Scientists are also able to determine the age of volcanoes on Venus using images from the Magellan mission, for example by examining wrinkle ridges on regional plains; if

5767-478: The surface of Venus at resolution 10 times better than that of Magellan. These missions will allow mapping over different time periods that could display more, higher resolution, evidence of current day volcanism. EnVision has the VenSAR (Venus Synthetic Aperture Radar) instrument that will map down to 30 m resolution and even down to 1 m in select areas. The SRS (Subsurface Radar Sounder) will penetrate

5846-488: The surface of Venus was mapped, the California Institute of Technology created an algorithm for automatically identifying volcanoes from the mapping images. Certainty that all the identified features are volcanoes is not possible but a system of categories was developed that label the confidence of whether a surface feature is a volcano or not. The algorithm examines images of a 30 km × 30 km area of

5925-401: The surface up to a kilometer and receive signals back that can be used to describe the internal structures of the planet. This will help to learn about internal workings of volcanic structures. The Venus Emisivity Mapper (VEM) will map the surface in infrared wavelengths which when added to radar can describe the topography of the surface. DAVINCI will not be mapping the surface, but analyzing

6004-670: The term "basalt" to the volcanic black rock beneath the Bishop of Meissen's Stolpen castle , believing it to be the same as the "basaniten" described by Pliny the Elder in AD ;77 in Naturalis Historiae . On Earth, most basalt is formed by decompression melting of the mantle . The high pressure in the upper mantle (due to the weight of the overlying rock ) raises the melting point of mantle rock, so that almost all of

6083-580: The upper mantle is solid. However, mantle rock is ductile (the solid rock slowly deforms under high stress). When tectonic forces cause hot mantle rock to creep upwards, pressure on the ascending rock decreases, and this can lower its melting point enough for the rock to partially melt , producing basaltic magma. Decompression melting can occur in a variety of tectonic settings, including in continental rift zones, at mid-ocean ridges , above geological hotspots , and in back-arc basins . Basalt also forms in subduction zones , where mantle rock rises into

6162-473: Was currently active or if the volcanic structures were remnants from the past. There are few impact craters on Venus' surface which pointed to relatively recent resurfacing. The most likely resurfacing event would have been volcanic flows. Radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus 's highest volcano Maat Mons , in the form of ash flows near

6241-400: 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

#617382