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64-404: Tlatilco is noted in particular for its high quality pottery pieces, many featuring Olmec iconography, and its figurines, including Olmec-style baby-face figurines . Much else, however, seems to be in a native ceramic tradition. These Olmec-style artifacts have led to speculation concerning the nature of Olmec influence on other Mesoamerican cultures . The Tlatilco site was used in modern times as

128-423: 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

192-481: 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 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

256-493: 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

320-648: A cessation of the use of Olmec iconography and forms. Many Tlatilco figurines show deformities or other anomalies, including a "duality" mask and several two-headed female figures . This has led some researchers to wonder whether Tlatilco was perhaps a cluster site for conjoined twins. The name "Tlatilco" comes from the Nahuatl language , in which it means "in the place of mounds". 19°30′N 99°15′W  /  19.500°N 99.250°W  / 19.500; -99.250 Olmec figurine#Baby-face figurines Olmec figurines are archetypical figurines produced by

384-442: 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

448-427: 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

512-465: 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

576-621: 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

640-623: 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

704-553: A number of tiny naturalistic figurines. Another pervasive Olmec figurine type features crouching figurines with thin bodies and over-large oval heads with small noses and receding chins. Some researchers such as Miguel Covarrubias generally characterise these figurines as "dwarfs". many others, also including Covarrubias, see evidence of "what looks like pre-natal posture". In a 1999 article, Carolyn Tate and Gordon Bendersky analysed head-to-body ratios and concluded that these figurines are naturalistic sculptures of fetuses, and discuss

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768-507: 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

832-477: A semicircle in front of six jade celts , perhaps representing stelae or basalt columns. Two of the figurines were made from jade , thirteen from serpentine , and one of reddish granite. This granite figurine one was positioned with its back to the celts, facing the others. All of the figurines had similar classic Olmec features including bald elongated heads. They had small holes for earrings, their legs were slightly bent, and they were undecorated – unusual if

896-523: 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

960-522: A source of clay for brick-making. By the 1930s, many of the ancient artifacts thereby uncovered made their way into the hands of collectors, including Miguel Covarrubias , artist and ethnographer. Covarrubias led the first controlled excavation in 1942. By 1949, over 200 burials were identified at Tlatilco, leading to its categorisation as a necropolis . Two major archaeological excavations followed, with over 500 burials eventually identified, many with intact grave offerings. The last field season also undertook

1024-450: A systematic survey of non-burial structures, leading to the realization that these hundreds of burials were apparently located under ancient houses—although no traces of them remain - as well as among the various trash pits, and that Tlatilco was not a necropolis, but rather a major chiefdom center. Many burials, primarily of high status individuals, show evidence of dental mutilation and artificial cranial deformation , most probably through

1088-427: 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

1152-401: 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

1216-753: 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

1280-478: 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

1344-579: 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

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1408-402: 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

1472-449: Is the jaguar transformation figurine ( see Commons photo ), which show a wide variety of styles, ranging from human-like figurines to those that are almost completely jaguar, and several where the subject appears to be in a stage of transformation. Despite the many stylised figurines, Olmec-period artisans and artist also portrayed humans naturalistically with "a most extraordinary realistic technique". The lead photo for this article shows

1536-513: Is thought, based on wooden busts recovered from the water-logged El Manati site, that figurines were also carved from wood, but, if so, none have survived. More durable and better known by the general public are those figurines carved, usually with a degree of skill, from jade , serpentine , greenstone , basalt , and other minerals and stones. The " baby-face " figurine is a unique marker of Olmec culture, consistently found in sites that show Olmec influence, although they seem to be confined to

1600-440: 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

1664-455: 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

1728-529: 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

1792-655: The Formative Period inhabitants of Mesoamerica . While not all of these figurines were produced in the Olmec heartland , they bear the hallmarks and motifs of Olmec culture. While the extent of Olmec control over the areas beyond their heartland is not yet known, Formative Period figurines with Olmec motifs were widespread in the centuries from 1000 to 500 BCE, showing a consistency of style and subject throughout nearly all of Mesoamerica. These figurines are usually found in household refuse, ancient construction fill, and, outside

1856-455: The La Venta archaeological site, archaeologists found what they subsequently named "Offering 4". These figurines had been ritually buried in a deep, narrow hole, and covered over with three layers of colored clay. At some point after the original burial, someone dug a small hole down just to the level of their heads and then refilled it. Offering 4 consists of sixteen male figurines positioned in

1920-1176: 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

1984-520: 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"

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2048-524: 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

2112-869: The "rain baby", the were-jaguar's body, if shown, is baby- or childlike. Its eyes are almond-shaped – or occasionally slit-like. Its nose is human. Its downturned mouth is open, as if in mid-squall. The upper lip is everted and toothless gums are often visible. Olmec motifs associated with the were-jaguar include a cleft on the head or headdress, a headband, and cross-bars. Most were-jaguar figurines show an inert were-jaguar baby being held by an adult. Many other Olmec figurines combined human and animal features. Although figurines showing such combinations of features are generally termed "transformation figures", some researchers argue that they represent humans in animal masks or animal suits, while others state that they likely represent shamans . At least one transformation figure displays bat-like features. Most common, however,

2176-457: 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

2240-456: The Olmec heartland, graves. However, many Olmec-style figurines, particularly those labelled as Las Bocas - or Xochipala -style, were recovered by looters and are therefore without provenance . The vast majority of figurines are simple in design, often nude or with a minimum of clothing, and made of local terracotta . Most of these recoveries are mere fragments: a head, arm, torso, or a leg. It

2304-476: 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

2368-475: 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

2432-409: The characteristics of Gulf Coast Olmec artifacts. Group 2 figurines are also slimmer than those of Group 1, lacking the jowly face or fleshy body, and their bodies are larger in proportion to their heads. Given the sheer numbers of baby-face figurines unearthed, they undoubtedly fulfilled some special role in the Olmec culture. What they represented, however, is not known. Michael Coe , says "One of

2496-447: 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

2560-432: The detail shown on their faces. Also called "hollow babies", these figurines are generally from 25–35 cm (9.8–13.8 in) high and feature a highly burnished white- or cream- slip . They are only rarely found in archaeological context . Archaeologist Jeffrey Blomster divides baby-face figurines into two groups based on several features. Among the many distinguishing factors, Group 1 figurines more closely mirror

2624-503: The early Olmec period and are largely absent, for example, in La Venta . These ceramic figurines are easily recognized by the chubby body, the baby-like jowly face, downturned mouth, and the puffy slit-like eyes. The head is slightly pear-shaped, likely due to artificial cranial deformation . They often wear a tight-fitting helmet not dissimilar to those worn by the Olmec colossal heads . Baby-face figurines are usually naked, but without genitalia . Their bodies are rarely rendered with

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2688-871: The elongated, flat-topped heads are reflective of the practice of artificial cranial deformation , as found in the Tlatilco burials of the same period or among the Maya of a later era. No direct evidence of this practice has been found in the Olmec heartland , however. The ears often have small holes for ear flares or other ornaments. These figurines may have therefore once worn earrings and even clothes made of perishable materials. It has been proposed that these figurines had multiple outfits for different ritual occasions – as Richard Diehl puts it, "a pre-Columbian version of Barbie 's Ken ". These figurines are usually carved from jade and well under 1 ft (30 cm) in height. For another example, see this Commons photo . At

2752-541: 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

2816-492: The figurines were gods or deities – but were instead covered with cinnabar . Interpretations abound. Perhaps this particular formation represents a council of some sort—the fifteen other figurines seem to be listening to the red granite one, with the celts forming a backdrop. One of the most striking offerings found at La Venta, the celts in Offering Number 4, depict a person with a ceremonial headdress “flying” and also

2880-414: The four Massive Offerings and four mosaics. Why such works would be buried continues to generate much speculation. The so-called were-jaguar motif runs through much of Olmec art, from the smallest jade to some of the largest basalt statues. The motif is found inscribed on celts , votive axes, masks, and on "elongated man" figurines. Also termed, somewhat more neutrally, the "composite anthropomorph" or

2944-487: The great enigmas in Olmec iconography is the nature and meaning of the large, hollow, whiteware babies". Another common figurine style features standing figurines in a stiff artificial pose and characterized by their thin limbs, elongated, bald, flat-topped heads, almond-shaped eyes, and downturned mouths. The figurines' legs are usually separated, often straight, sometimes bent. Toes and fingers, if shown at all, are frequently represented by lines. It has been theorized that

3008-708: 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 ,

3072-604: 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

3136-416: 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

3200-415: The maize deity. There appears to be a definite symbolic link here, but it is unclear whether it is tied to the Olmec rudimentary writing system. To the red granite figurine's right, there seems be a line of three figurines filing past him. Another researcher has suggested that the granite figure is an initiate. As the name implies, Offering 4 is one of many ritual offerings uncovered at La Venta, including

3264-704: 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

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3328-406: 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;

3392-410: 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

3456-407: The possibility of infanticide and infant sacrifice. 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 / ) 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

3520-475: 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

3584-411: 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

3648-595: 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

3712-403: 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

3776-455: 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

3840-480: 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

3904-648: 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

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3968-580: 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

4032-434: The use of cradleboards . The Tlatilcans' agriculture was focused on maize, but also included beans, amaranth , and squash, and chili peppers. These plants were supplemented with various fowl, including migratory birds, wild rabbits and other smaller mammals, and deer and antelope. Tlatilco reached its heyday during the period from 1000 to 700 BCE , during the Olmec horizon . The following Zacatenco phase (700-400 BCE) saw

4096-400: 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

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