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91-491: In the Miocene – Pliocene (10-1 mya ), calderas erupted felsic ignimbrites in four distinct pulses separated by periods of low levels of activity. At least three volcanic centres ( Guacha caldera , La Pacana , Pastos Grandes , Vilama ) had eruptions of a Volcanic Exposivity Index (VEI) of 8 , as well as smaller scale eruptive centres. Activity waned after 2 mya , but present-day geothermal activity and volcanoes dated to

182-526: A broader aridification trend. The EMCI ended 18 million years ago, giving way to the Middle Miocene Warm Interval (MMWI), the warmest part of which was the MMCO that began 16 million years ago. As the world transitioned into the MMCO, carbon dioxide concentrations varied between 300 and 500 ppm. Global annual mean surface temperature during the MMCO was about 18.4 °C. MMCO warmth was driven by

273-519: A clade of large terrestrial predatory crocodyliformes distantly related to modern crocodilians, from which they likely diverged over 180 million years ago, are known from the Miocene of South America. The last Desmostylians thrived during this period before becoming the only extinct marine mammal order. The pinnipeds , which appeared near the end of the Oligocene, became more aquatic. A prominent genus

364-561: A cooler, drier climate. C 4 grasses, which are able to assimilate carbon dioxide and water more efficiently than C 3 grasses, expanded to become ecologically significant near the end of the Miocene between 6 and 7 million years ago, although they did not expand northward during the Late Miocene. The expansion of grasslands and radiations among terrestrial herbivores correlates to fluctuations in CO 2 . One study, however, has attributed

455-513: A density barrier for mafic melts which subsequently ponded below the melt generating zone. Dacitic melts escaped from this zone, forming diapirs and the magma chambers that generated APVC ignimbrite volcanism. Magma generation in the APVC is periodical, with pulses recognized 10, 8, 6, and 4 mya. The first stage included the Artola, Granada, Lower Rio San Pedro and Mucar ignimbrites. The second pulse involved

546-413: A depth of 15–30 kilometres (9.3–18.6 mi). The total amount of water has been estimated to be c. 14,000,000,000,000,000 kilograms (3.1 × 10 lb), comparable to large lakes on Earth . Seismic tomography is a technique that uses seismic waves produced by earthquakes to gather information on the composition of the crust and mantle below a volcanic system. Different layers and structures in

637-473: A good analogue for future warmer climates caused by anthropogenic global warming , with this being especially true of the global climate during the Middle Miocene Climatic Optimum (MMCO), because the last time carbon dioxide levels were comparable to projected future atmospheric carbon dioxide levels resulting from anthropogenic climate change was during the MMCO. The Ross Sea margin of

728-435: A good model for a "living fossil". Eucalyptus fossil leaves occur in the Miocene of New Zealand , where the genus is not native today, but have been introduced from Australia . Both marine and continental fauna were fairly modern, although marine mammals were less numerous. Only in isolated South America and Australia did widely divergent fauna exist. In Eurasia, genus richness shifted southward to lower latitudes from

819-579: A major expansion of Antarctic glaciers. This severed the connection between the Indian Ocean and the Mediterranean Sea and formed the present land connection between Afro-Arabia and Eurasia. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis ) near

910-480: A major expansion of grass-grazer ecosystems . Herds of large, swift grazers were hunted by predators across broad sweeps of open grasslands , displacing desert, woodland, and browsers . The higher organic content and water retention of the deeper and richer grassland soils , with long-term burial of carbon in sediments, produced a carbon and water vapor sink. This, combined with higher surface albedo and lower evapotranspiration of grassland, contributed to

1001-502: A partial delamination of the crust under the Puna, resulting in increased volcanic activity and terrain height. Download coordinates as: Miocene The Miocene ( / ˈ m aɪ . ə s iː n , - oʊ -/ MY -ə-seen, -⁠oh- ) is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene

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1092-568: A prelude to caldera formation and large scale eruptive activity. Other active centres include the El Tatio and Sol de Mañana geothermal fields and the fields within Cerro Guacha and Pastos Grandes calderas. The latter also contains <10 ka rhyolitic flows and domes. The implications of recent lava domes for future activity in the APVC are controversial, but the presence of mafic components in recently erupted volcanic rocks may indicate that

1183-493: A result, the deposits are massive and homogeneous and show few size segregation or fluidization features. Such eruptions have been argued to require external triggers to occur. There is a volume-dependent relationship between homogeneity of the eruption products and their volume; large volume ignimbrites have uniform mineralogical and compositional heterogeneity. Small volume ignimbrites often show gradation in composition. This pattern has been observed in other volcanic centres such as

1274-460: A role in caldera formation. The area's calderas are poorly understood and some may yet be undiscovered. Some calderas were subject to comprehensive research. Research in this area is physically and logistically difficult. Neodym , lead and boron isotope analysis has been used to determine the origin of eruption products. The dry climate and high altitude of the Atacama Desert has protected

1365-578: A significant drop in atmospheric carbon dioxide levels. Both continental and oceanic thermal gradients in mid-latitudes during the Early Miocene were very similar to those in the present. Global cooling caused the East Asian Summer Monsoon (EASM) to begin to take on its modern form during the Early Miocene. From 22.1 to 19.7 Ma, the Xining Basin experienced relative warmth and humidity amidst

1456-695: A significant local decline along the northeastern coast of Australia during the Tortonian, most likely due to warming seawater. Cetaceans attained their greatest diversity during the Miocene, with over 20 recognized genera of baleen whales in comparison to only six living genera. This diversification correlates with emergence of gigantic macro-predators such as megatoothed sharks and raptorial sperm whales . Prominent examples are O. megalodon and L. melvillei . Other notable large sharks were O. chubutensis , Isurus hastalis , and Hemipristis serra . Crocodilians also showed signs of diversification during

1547-401: A sudden appearance of rhyodacitic and dacitic ignimbrites occurred. During this flare-up it erupted primarily dacites with subordinate amounts of rhyolites and andesites. The area was uplifted during the flare-up and the crust thickened to 60–70 kilometres (37–43 mi). This triggered the formation of evaporite basins containing halite , boron and sulfate and may have generated

1638-627: A zone of low rainfall in the Late Miocene. The Indian Plate continued to collide with the Eurasian Plate , creating new mountain ranges and uplifting the Tibetan Plateau , resulting in the rain shadowing and aridification of the Asian interior. The Tian Shan experienced significant uplift in the Late Miocene, blocking westerlies from coming into the Tarim Basin and drying it as a result. At

1729-520: Is not contemporaneous in the entire APVC area; north of 21°S the Alto de Pica and Oxaya Formations formed 15–17 and 18–23 mya respectively, whereas south of 21°S large scale ignimbrite activity didn't begin until 10.6 mya. Activity waned after 2 mya , and after 1 mya and during the Holocene , activity was mostly andesitic in nature with large ignimbrites absent. Activity with composition similar to ignimbrites

1820-879: The Fish Canyon Tuff in the United States and the Toba ignimbrites in Indonesia . Petrologically, ignimbrites are derived from dacitic – rhyodacitic magmas. Phenocrysts include biotite , Fe – Ti -oxides, plagioclase and quartz with minor apatite and titanite . Northern Puna ignimbrites also contain amphibole , and clinopyroxene and orthopyroxene occur in low- Si magmas, while higher Si magmas also contain sanidine . These magmas have temperatures of 700–850 °C (1,292–1,562 °F) and originate in depths of 4–8 kilometres (2.5–5.0 mi). The ignimbrites are collectively referred to as San Bartolo and Silapeti Groups. Since

1911-598: The Holocene , as well as recent ground deformation at Uturunku volcano indicate still-extant present-day activity of the system. The Andes mountain chain originated from the subduction of the Nazca Plate below the South American Plate and was accompanied by extensive volcanism. Between 14° S and 28° S lies one volcanic area with over fifty recently active systems, the Central Volcanic Zone (CVZ). Since

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2002-539: The Middle Miocene Climate Transition (MMCT). Abrupt increases in opal deposition indicate this cooling was driven by enhanced drawdown of carbon dioxide via silicate weathering. The MMCT caused a sea surface temperature (SST) drop of approximately 6 °C in the North Atlantic. The drop in benthic foraminiferal δ O values was most noticeable in the waters around Antarctica, suggesting cooling

2093-550: The carbon cycle occurred approximately 6 Ma, causing continental carbon reservoirs to no longer expand during cold spells, as they had done during cold periods in the Oligocene and most of the Miocene. At the end of the Miocene, global temperatures rose again as the amplitude of Earth's obliquity increased, which caused increased aridity in Central Asia. Around 5.5 Ma, the EAWM underwent a period of rapid intensification. Life during

2184-483: The crust and generate a zone of mafic volcanism. Increases in the melt flux and thus heat and volatile input causes partial melting of the crust, forming a layer containing melts reaching down to the Moho that inhibits the ascent of mafic magmas because of its higher buoyancy . Instead, melts generated in this zone eventually reach the surface, generating felsic volcanism. Some mafic magmas escape sideward after stalling in

2275-574: The geology of the Himalaya occurred during that epoch, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere. The Miocene faunal stages from youngest to oldest are typically named according to the International Commission on Stratigraphy : Regionally, other systems are used, based on characteristic land mammals; some of them overlap with

2366-525: The nitrate deposits of the Atacama Desert . The sudden increase is explained by a sudden steepening of the subducting plate, similar to the Mid-Tertiary ignimbrite flare-up . In the northern Puna, ignimbrite activity began 10 mya, with large-scale activity occurring 5 to 3.8 Ma in the arc front and 8.4 to 6.4 Ma in the back arc. In the southern Puna, backarc activity set in 14–12 Ma and the largest eruptions occurred after 4 Ma. The start of ignimbritic activity

2457-523: The " Zanclean flood ". Also during the early Miocene (specifically the Aquitanian and Burdigalian Stages), the apes first evolved, began diversifying, and became widespread throughout the Old World . Around the end of this epoch, the ancestors of humans had split away from the ancestors of the chimpanzees and had begun following their own evolutionary path during the final Messinian Stage (7.5–5.3 Ma) of

2548-504: The 65 by 35 kilometres (40 mi × 22 mi) caldera. Magma generation rates during the pulses are about 0.001 cubic kilometres per year (0.032 m/s), based on the assumption that for each 50–100 cubic kilometres (12–24 cu mi) of arc there is one caldera. These rates are substantially higher than the average for the Central Volcanic Zone, 0.00015–0.0003 cubic kilometres per year (0.0048–0.0095 m/s). During

2639-673: The Central Paratethys, cut off from sources of freshwater input by its separation from the Eastern Paratethys. From 13.36 to 12.65 Ma, the Central Paratethys was characterised by open marine conditions, before the reopening of the Bârlad Strait resulted in a shift to brackish-marine conditions in the Central Paratethys, causing the Badenian-Sarmatian Extinction Event. As a result of the Bârlad Strait's reopening,

2730-826: The Early to the Middle Miocene. Europe's large mammal diversity significantly declined during the Late Miocene. In the Early Miocene, several Oligocene groups were still diverse, including nimravids , entelodonts , and three-toed equids. As in the previous Oligocene Epoch, oreodonts were still diverse, only to disappear in the earliest Pliocene. During the later Miocene mammals were more modern, with easily recognizable canids , bears , red pandas , procyonids , equids , beavers , deer , camelids , and whales , along with now-extinct groups like borophagine canids , certain gomphotheres , three-toed horses , and hornless rhinos like Teleoceras and Aphelos . The late Miocene also marks

2821-488: The Earth have different propagation speeds of seismic waves and attenuate them differently, resulting in different arrival times and strengths of waves travelling in a certain direction. From various measurements 3D models of the geological structures can be inferred. Results of such research indicate that a highly hydrated slab derived from the Nazca Plate – a major source of melts in a collisional volcanism system – underlies

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2912-677: The East Antarctic Ice Sheet (EAIS) was highly dynamic during the Early Miocene. The Miocene began with the Early Miocene Cool Event (Mi-1) around 23 million years ago, which marked the start of the Early Miocene Cool Interval (EMCI). This cool event occurred immediately after the Oligocene-Miocene Transition (OMT) during a major expansion of Antarctica's ice sheets, but was not associated with

3003-577: The East Asian Winter Monsoon (EAWM) became stronger synchronously with a southward shift of the subarctic front. Greenland may have begun to have large glaciers as early as 8 to 7 Ma, although the climate for the most part remained warm enough to support forests there well into the Pliocene. Zhejiang, China was noticeably more humid than today. In the Great Rift Valley of Kenya , there

3094-457: The LMC; extratropical sea surface temperatures dropped substantially by approximately 7–9 °C. 41 kyr obliquity cycles became the dominant orbital climatic control 7.7 Ma and this dominance strengthened 6.4 Ma. Benthic δ O values show significant glaciation occurred from 6.26 to 5.50 Ma, during which glacial-interglacial cycles were governed by the 41 kyr obliquity cycle. A major reorganisation of

3185-563: The Late Cretaceous, are known from the Miocene of Patagonia, represented by the mole-like Necrolestes . The youngest known representatives of metatherians (the broader grouping to which marsupials belong) in Europe, Asia and Africa are known from the Miocene, including the European herpetotheriid Amphiperatherium , the peradectids Siamoperadectes and Sinoperadectes from Asia, and

3276-646: The Late Miocene, the Earth's climate began to display a high degree of similarity to that of the present day . The 173 kyr obliquity modulation cycle governed by Earth's interactions with Saturn became detectable in the Late Miocene. By 12 Ma, Oregon was a savanna akin to that of the western margins of the Sierra Nevada of northern California . Central Australia became progressively drier, although southwestern Australia experienced significant wettening from around 12 to 8 Ma. The South Asian Winter Monsoon (SAWM) underwent strengthening ~9.2–8.5 Ma. From 7.9 to 5.8 Ma,

3367-567: The Miocene Epoch was mostly supported by the two newly formed biomes , kelp forests and grasslands . Grasslands allow for more grazers, such as horses , rhinoceroses , and hippos . Ninety-five percent of modern plants existed by the end of this epoch . Modern bony fish genera were established. A modern-style latitudinal biodiversity gradient appeared ~15 Ma. The coevolution of gritty , fibrous, fire-tolerant grasses and long-legged gregarious ungulates with high-crowned teeth , led to

3458-500: The Miocene, less silicic magmas containing olivine , plagioclase and  clinopyroxene have been erupted by the APVC as well. These "mafic" magmas form various monogenetic volcanoes , inclusions in more silicic magmas and lava flows which sometimes occur in isolation and sometimes are linked to stratovolcanoes . Eruptions are affected by the local conditions, resulting in high altitude eruption columns that are sorted by westerly stratospheric winds. Coarse deposits are deposited close to

3549-484: The Miocene. As in the Oligocene before it, grasslands continued to expand, and forests to dwindle. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems. The plants and animals of the Miocene were recognizably modern. Mammals and birds were well established. Whales , pinnipeds , and kelp spread. The Miocene is of particular interest to geologists and palaeoclimatologists because major phases of

3640-449: The Miocene. The largest form among them was a gigantic caiman Purussaurus which inhabited South America. Another gigantic form was a false gharial Rhamphosuchus , which inhabited modern age India . A strange form, Mourasuchus also thrived alongside Purussaurus . This species developed a specialized filter-feeding mechanism, and it likely preyed upon small fauna despite its gigantic size. The youngest members of Sebecidae ,

3731-534: The North American Great Plains and in Argentina . The global trend was towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture, particularly after 7 to 8 million years ago. Uplift of East Africa in the late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered

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3822-648: The Oligocene–Miocene transgression. As the southern Andes rose in the Middle Miocene (14–12 million years ago) the resulting rain shadow originated the Patagonian Desert to the east. Far northern Australia was monsoonal during the Miocene. Although northern Australia is often believed to have been much wetter during the Miocene, this interpretation may be an artefact of preservation bias of riparian and lacustrine plants; this finding has itself been challenged by other papers. Western Australia, like today,

3913-533: The Panizos, Sifon and Vilama ignimbrites and the third was the largest, with a number of ignimbrites. The fourth pulse was weaker than the preceding ones and involved the Patao and Talabre ignimbrites among others. The magmas beneath the APVC are noticeably rich in water derived from the subduction of water-rich rocks. A volume ratio of about 10-20% of water has been invoked to explain the pattern of electrical conductivity at

4004-598: The Qiongdongnan Basin in the northern South China Sea indicates the Pearl River was a major source of sediment flux into the sea during the Early Miocene and was a major fluvial system as in the present. During the Oligocene and Early Miocene, the coast of northern Brazil, Colombia, south-central Peru , central Chile and large swathes of inland Patagonia were subject to a marine transgression . The transgressions in

4095-578: The Western Cordillera. Below the Altiplano, low-velocity zones indicate the presence of large amounts of partial melts that correlate with volcanic zones south of 21° S, whereas north of 21° S thicker lithospheric layers may prevent the formation of melts. Next to the Eastern Cordillera, low-velocity zones extend farther north to 18.5° S. A thermally weakened zone, evidenced by strong attenuation, in

4186-487: The activity of the Columbia River Basalts and enhanced by decreased albedo from the reduction of deserts and expansion of forests. Climate modelling suggests additional, currently unknown, factors also worked to create the warm conditions of the MMCO. The MMCO saw the expansion of the tropical climatic zone to much larger than its current size. The July ITCZ, the zone of maximal monsoonal rainfall, moved to

4277-472: The aridity of the former. Unequivocally-recognizable dabbling ducks , plovers , typical owls , cockatoos and crows appear during the Miocene. By the epoch's end, all or almost all modern bird groups are believed to have been present; the few post-Miocene bird fossils which cannot be placed in the evolutionary tree with full confidence are simply too badly preserved, rather than too equivocal in character. Marine birds reached their highest diversity ever in

4368-619: The beginning of the Miocene, the northern margin of the Arabian plate, then part of the African landmass, collided with Eurasia; as a result, the Tethys seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish – Arabian region. The first step of this closure occurred 20 Ma, reducing water mass exchange by 90%, while the second step occurred around 13.8 Ma, coincident with

4459-481: The climate slowly cooled towards a series of ice ages . The Miocene boundaries are not marked by distinct global events but by regionally defined transitions from the warmer Oligocene to the cooler Pliocene Epoch. During the Early Miocene, Afro-Arabia collided with Eurasia, severing the connection between the Mediterranean and Indian Oceans, and allowing the interchange of fauna between Eurasia and Africa, including

4550-566: The course of this epoch . The youngest representatives of Choristodera , an extinct order of aquatic reptiles that first appeared in the Middle Jurassic , are known from the Miocene of Europe, belonging to the genus Lazarussuchus , which had been the only known surviving genus of the group since the beginning of the Eocene. The last known representatives of the archaic primitive mammal order Meridiolestida , which dominated South America during

4641-440: The crust is associated with the APVC. This indicates the presence of melts in the crust. A layer of low velocity (shear speed of 1 kilometre per second (0.62 mi/s)) 17–19 kilometres (11–12 mi) thick is assumed to host the APVC magma body. This body has a volume of about 480,000–530,000 cubic kilometres (120,000–130,000 cu mi) and a temperature of about 1,000 °C (1,830 °F). Other seismological data indicate

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4732-446: The deposits of APVC volcanism from erosion , but limited erosion also reduces the exposure of buried layers and structures. Evidence of volcanic activity and cyclic variation has been obtained from remote fallout deposits as well. The APVC area before the upper Miocene was largely formed from sedimentary layers of Ordovician to Miocene age and deformed during previous stages of Andean orogeny, with low volume volcanics. Activity until

4823-570: The dispersal of proboscideans and hominoids into Eurasia. During the late Miocene, the connections between the Atlantic and Mediterranean closed, causing the Mediterranean Sea to almost completely evaporate. This event is referred to as the " Messinian salinity crisis ". Then, at the Miocene–Pliocene boundary, the Strait of Gibraltar opened, and the Mediterranean refilled. That event is referred to as

4914-580: The early to mid Miocene (23–15 Ma). Oceans cooled partly due to the formation of the Antarctic Circumpolar Current , and about 15 million years ago the ice cap in the southern hemisphere started to grow to its present form. The Greenland ice cap developed later, in the Middle Pliocene time, about 3 million years ago. Fish Canyon Tuff The Fish Canyon Tuff is the large volcanic ash flow or ignimbrite deposit resulting from one of

5005-564: The end of the Miocene due to increased habitat uniformity. The expansion of grasslands in North America also led to an explosive radiation among snakes. Previously, snakes were a minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically with the first appearances of vipers and elapids in North America and the significant diversification of Colubridae (including

5096-422: The end of the Miocene. The Paratethys underwent a significant transgression during the early Middle Miocene. Around 13.8 Ma, during a global sea level drop, the Eastern Paratethys was cut off from the global ocean by the closure of the Bârlad Strait, effectively turning it into a saltwater lake. From 13.8 to 13.36 Ma, an evaporite period similar to the later Messinian salinity crisis in the Mediterranean ensued in

5187-612: The enigmatic Saint Bathans Mammal . Microbial life in the igneous crust of the Fennoscandian Shield shifted from being dominated by methanogens to being primarily composed of sulphate-reducing prokaryotes . The change resulted from fracture reactivation during the Pyrenean-Alpine orogeny, enabling sulphate-reducing microbes to permeate into the Fennoscandian Shield via descending surficial waters. Diatom diversity

5278-461: The evolution of both groups into modern representatives. The early Miocene Saint Bathans Fauna is the only Cenozoic terrestrial fossil record of the landmass, showcasing a wide variety of not only bird species, including early representatives of clades such as moa , kiwi and adzebills , but also a diverse herpetofauna of sphenodontians , crocodiles and turtles as well as a rich terrestrial mammal fauna composed of various species of bats and

5369-499: The expansion of grasslands not to a CO 2 drop but to the increasing seasonality and aridity, coupled with a monsoon climate, which made wildfires highly prevalent compared to before. The Late Miocene expansion of grasslands had cascading effects on the global carbon cycle, evidenced by the imprint it left in carbon isotope records. Cycads between 11.5 and 5 million years ago began to rediversify after previous declines in variety due to climatic changes, and thus modern cycads are not

5460-711: The extinction of the last-surviving members of the hyaenodonts . Islands began to form between South and North America in the Late Miocene, allowing ground sloths like Thinobadistes to island-hop to North America. The expansion of silica-rich C 4 grasses led to worldwide extinctions of herbivorous species without high-crowned teeth . Mustelids diversified into their largest forms as terrestrial predators like Ekorus , Eomellivora , and Megalictis and bunodont otters like Enhydriodon and Sivaonyx appeared. Eulipotyphlans were widespread in Europe, being less diverse in Southern Europe than farther north due to

5551-604: The findings of marine invertebrate fossils of both Atlantic and Pacific affinity in La Cascada Formation . Connection would have occurred through narrow epicontinental seaways that formed channels in a dissected topography . The Antarctic Plate started to subduct beneath South America 14 million years ago in the Miocene, forming the Chile Triple Junction . At first the Antarctic Plate subducted only in

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5642-412: The formation of hybrid magmas. Partial melting of the crust and of hydrous basalt generates andesitic – dacitic melts that escape upwards. A residual forms composed from garnet pyroxenite at a depth of 50 kilometres (31 mi). This residual is denser than the mantle peridotite and can cause delamination of the lower crust containing the residual. Between 18 and 12 mya the Puna-Altiplano region

5733-411: The human lineage) appeared in Africa at the very end of the Miocene, including Sahelanthropus , Orrorin , and an early form of Ardipithecus ( A. kadabba ). The chimpanzee–human divergence is thought to have occurred at this time. The evolution of bipedalism in apes at the end of the Miocene instigated an increased rate of faunal turnover in Africa. In contrast, European apes met their end at

5824-422: The lake levels of the Eastern Paratethys dropped as it once again became a sea. The Fram Strait opened during the Miocene and acted as the only throughflow for Atlantic Water into the Arctic Ocean until the Quaternary period. Due to regional uplift of the continental shelf, this water could not move through the Barents Seaway in the Miocene. The modern day Mekong Delta took shape after 8 Ma. Geochemistry of

5915-681: The largest known explosive eruptions on Earth, estimated at 1,200 cu mi (5,000 km ). (see List of largest volcanic eruptions ). The Fish Canyon Tuff eruption was centred at the La Garita Caldera in southwest Colorado ; the caldera itself would have formed by collapse, as a result of the eruption. Studies of the tuff show that it all belongs to one eruption due to its uniform bulk-chemical composition (SiO 2 =bulk 67.5–68.5% ( dacite ), matrix 75–76% ( rhyolite ) and consistent phenocryst content (35–50%) and mineralogical composition ( plagioclase , sanidine , quartz , biotite , hornblende , sphene , apatite , zircon , Fe-Ti oxides are

6006-558: The late Miocene between 21° S and 24° S a major ignimbrite province formed over 70 kilometres (43 mi) thick crust, the Altiplano–Puna volcanic complex, between the Atacama and the Altiplano . The Toba volcanic system in Indonesia and Taupō in New Zealand are analogous to the province. The APVC is located in the southern Altiplano-Puna plateau, a surface plateau 300 kilometres (190 mi) wide and 2,000 kilometres (1,200 mi) long at an altitude of 4,000 metres (13,000 ft), and lies 50–150 kilometres (31–93 mi) east of

6097-437: The late Miocene was effusive with andesite as the major product. After a volcanic pause related to flat slab subduction , starting from 27 mya volcanism increased suddenly. Ignimbrites range in age from 25 mya to 1 mya. In the late Miocene , more evolved andesite magmas were erupted and the crustal components increased. In the late Tertiary until the Quaternary , a sudden decrease of mafic volcanism coupled with

6188-447: The least evolved magmas being the 6.7 mya Cerro Morado and 8–7 m Rachaite complex flows. Basaltic over shoshonitic (both 25 and 21 m) to andesitic (post- Miocene ) lavas are found in the southern Altiplano. Ignimbrites deposited during eruptions of APVC volcanoes are formed by "boiling over" eruptions, where magma chambers containing viscous crystal-rich volatile-poor magmas partially empty in tranquil, non-explosive fashion. As

6279-408: The magma system is being recharged. The APVC erupted over an area of 70,000 square kilometres (27,000 sq mi) from ten major systems, some active over millions of years and comparable to Yellowstone Caldera and Long Valley Caldera in the United States. The APVC is the largest ignimbrite province of the Neogene with a volume of at least 15,000 cubic kilometres (3,600 cu mi), and

6370-435: The melt containing zone; these generate more mafic volcanic systems at the edge of the felsic volcanism, such as Cerro Bitiche . The magmas are mixtures of crust derived and mafic mantle-derived melts with a consistent petrological and chemical signature. The melt generation process may involve several different layers in the crust. Another model requires the intrusion of basaltic melts into an amphibole crust, resulting in

6461-496: The north, increasing precipitation over southern China whilst simultaneously decreasing it over Indochina during the EASM. Western Australia was at this time characterised by exceptional aridity. In Antarctica, average summer temperatures on land reached 10 °C. In the oceans, the lysocline shoaled by approximately half of a kilometre during warm phases that corresponded to orbital eccentricity maxima. The MMCO ended around 14 million years ago, when global temperatures fell in

6552-641: The northern Puna is of Ordovician to Eocene age. The APVC is generated by the subduction of the Nazca Plate beneath the South American Plate at an angle of nearly 30°. Delamination of the crust has occurred beneath the northern Puna and southern Altiplano. Below 20 kilometres (12 mi) depth, seismic data indicate the presence of melts in a layer called the Altiplano–Puna low velocity zone or Altiplano Puna magma body . Regional variations of activity north and south of 24°S have been attributed to

6643-523: The origin of many modern genera such as Nerodia , Lampropeltis , Pituophis and Pantherophis ). Arthropods were abundant, including in areas such as Tibet where they have traditionally been thought to be undiverse. Neoisopterans diversified and expanded into areas they previously were absent from, such as Madagascar and Australia. In the oceans, brown algae , called kelp , proliferated, supporting new species of sea life, including otters , fish and various invertebrates . Corals suffered

6734-463: The possible herpetotheriid Morotodon from the late Early Miocene of Uganda. Approximately 100 species of apes lived during this time , ranging throughout Africa, Asia and Europe and varying widely in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominid clade, but molecular evidence indicates this ape lived between 18 and 13 million years ago. The first hominins ( bipedal apes of

6825-828: The preceding Oligocene and following Pliocene Epochs: Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean , causing both the rise of the Andes and a southward extension of the Meso-American peninsula. Mountain building took place in western North America , Europe , and East Asia . Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in

6916-628: The primary phenocrysts). This tuff and eruption is part of the larger San Juan volcanic field and the Oligocene Southern Rocky Mountain ignimbrite flare-up . The Fish Canyon Tuff eruption occurred around 28 Million years ago. Sanidine crystals from the Fish Canyon Tuff (FCTs) are used as a reference mineral in Ar- Ar dating , and the current 'astronomically calibrated' age for the FCTs

7007-695: The southernmost tip of Patagonia, meaning that the Chile Triple Junction lay near the Strait of Magellan . As the southern part of Nazca Plate and the Chile Rise became consumed by subduction the more northerly regions of the Antarctic Plate begun to subduct beneath Patagonia so that the Chile Triple Junction advanced to the north over time. The asthenospheric window associated to the triple junction disturbed previous patterns of mantle convection beneath Patagonia inducing an uplift of ca. 1 km that reversed

7098-542: The southwards moving subduction of the Juan Fernández Ridge . This southwards migration results in a steepening of the subducting plate behind the ridge, causing decompression melting . Between 1:4 to 1:6 of the generated melts are erupted to the surface as ignimbrites . Mafic rocks are associated with strike-slip faults and normal faults and are found in the southern Puna and Altiplano. The southern Puna has calc-alkaline andesites erupted after 7 mya , with

7189-412: The three strong pulses, extrusion was even higher at 0.004–0.012 cubic kilometres per year (0.13–0.38 m/s). Intrusion rates range from 0.003–0.005 cubic kilometres per year (0.095–0.158 m/s) and resulted in plutons of 30,000–50,000 cubic kilometres (7,200–12,000 cu mi) volume beneath the calderas. Modelling indicates a system where andesitic melts coming from the mantle rise through

7280-483: The underlying magmatic body is considered to be the largest continental melt zone, forming a batholith . Alternatively, the body revealed by seismic studies is the remnant mush of the magma accumulation zone. Deposits from the volcanoes cover a surface area of more than 500,000 square kilometres (190,000 sq mi). La Pacana is the largest single complex in the APVC with dimensions 100 by 70 square kilometres (39 sq mi × 27 sq mi), including

7371-465: The vents, while fine ash is carried to the Chaco and eastern cordillera. The highest volcanoes in the world are located here, including 6,887 metres (22,595 ft) high Ojos del Salado and 6,723 metres (22,057 ft) high Llullaillaco . Some volcanoes have undergone flank collapses covering as much as 200 square kilometres (77 sq mi). Most calderas are associated with fault systems that may play

7462-572: The volcanic front of the Andes. Deformational belts limit it in the east. The Altiplano itself forms a block that has been geologically stable since the Eocene ; below the Atacama area conversely recent extensional dynamics and a weakened crust exist. The Puna has a higher average elevation than the Altiplano, and some individual volcanic centres reach altitudes of more than 6,000 metres (20,000 ft). The basement of

7553-542: The west coast of South America are thought to be caused by a regional phenomenon while the steadily rising central segment of the Andes represents an exception. While there are numerous registers of Oligocene–Miocene transgressions around the world it is doubtful that these correlate. It is thought that the Oligocene–Miocene transgression in Patagonia could have temporarily linked the Pacific and Atlantic Oceans, as inferred from

7644-423: Was Allodesmus . A ferocious walrus , Pelagiarctos may have preyed upon other species of pinnipeds including Allodesmus . Furthermore, South American waters witnessed the arrival of Megapiranha paranensis , which were considerably larger than modern age piranhas . New Zealand 's Miocene fossil record is particularly rich. Marine deposits showcase a variety of cetaceans and penguins , illustrating

7735-610: Was a gradual and progressive trend of increasing aridification, though it was not unidirectional, and wet humid episodes continued to occur. Between 7 and 5.3 Ma, temperatures dropped sharply again in the Late Miocene Cooling (LMC), most likely as a result of a decline in atmospheric carbon dioxide and a drop in the amplitude of Earth's obliquity, and the Antarctic ice sheet was approaching its present-day size and thickness. Ocean temperatures plummeted to near-modern values during

7826-458: Was arid, particularly so during the Middle Miocene. Climates remained moderately warm, although the slow global cooling that eventually led to the Pleistocene glaciations continued. Although a long-term cooling trend was well underway, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene . The climate of the Miocene has been suggested as

7917-623: Was inversely correlated with carbon dioxide levels and global temperatures during the Miocene. Most modern lineages of diatoms appeared by the Late Miocene. There is evidence from oxygen isotopes at Deep Sea Drilling Program sites that ice began to build up in Antarctica about 36 Ma during the Eocene . Further marked decreases in temperature during the Middle Miocene at 15 Ma probably reflect increased ice growth in Antarctica. It can therefore be assumed that East Antarctica had some glaciers during

8008-450: Was limited to the eruption of lava domes and flows, interpreted as escaping from a regional sill 1–4 kilometres (0.62–2.49 mi) high at 14–17 kilometres (8.7–10.6 mi) depth. The APVC is still active, with recent unrest and ground inflation detected by InSAR at Uturuncu volcano starting in 1996. Research indicates that this unrest results from the intrusion of dacitic magma at 17 kilometres (11 mi) or more depth and may be

8099-627: Was most intense there. Around this time the Mi3b glacial event (a massive expansion of Antarctic glaciers) occurred. The East Antarctic Ice Sheet (EAIS) markedly stabilised following the MMCT. The intensification of glaciation caused a decoherence of sediment deposition from the 405 kyr eccentricity cycle. The MMWI ended about 11 Ma, when the Late Miocene Cool Interval (LMCI) started. A major but transient warming occurred around 10.8-10.7 Ma. During

8190-500: Was named by Scottish geologist Charles Lyell ; the name comes from the Greek words μείων ( meíōn , "less") and καινός ( kainós , "new") and means "less recent" because it has 18% fewer modern marine invertebrates than the Pliocene has. The Miocene followed the Oligocene and preceded the Pliocene. As Earth went from the Oligocene through the Miocene and into the Pliocene,

8281-401: Was subject to an episode of flat subduction of the Nazca Plate . A steepening of the subduction after 12 mya resulted in the influx of hot asthenosphere. Until that point, differentiation and crystallization of rising mafic magmas had mostly produced andesitic magmas. The change in plate movements and increased melt generation caused an overturn and anatexis of the melt generating zone, forming

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