How Polygenetic are Monogenetic Volcanoes: Case Studies of Some Complex Maar‐Diatreme Volcanoes

The increasing number of field investigations and various controlled benchtop and large‐scale experiments have permitted the evaluation of a large number of processes involved in the formation of maar‐diatreme volcanoes, the second most common type of small‐volume subaerial volcanoes on Earth. A maar‐diatreme volcano is recognized by a volcanic crater that is cut into country rocks and surrounded by a low‐height ejecta rim composed of pyroclastic deposits of few meters to up to 200 m thick above the syn‐eruptive surface level. The craters vary from 0.1 km to up to 5 km wide and vary in depth from a few dozen meters to up to 300 m deep. Their irregular morphology reflects the simple or complex volcanic and cratering processes involved in their formation. The simplicity or complexity of the crater or the entire maar itself is usually observed in the stratigraphy of the surrounding ejecta rings. The latter are composed of sequences of successive alternating and contrastingly bedded phreatomagmatic‐derived dilute pyroclastic density currents (PDC) and fallout depositions, with occasional interbedded Strombolian‐derived spatter materials or scoria fall units, exemplifying the changes in the eruptive styles during the formation of the volcano. The entire stratigraphic sequence might be preserved as a single eruptive package (small or very thick) in which there is no stratigraphic gap or significant discordance indicative of a potential break during the eruption. A maar with a single eruptive deposit is quantified as monogenetic maar, meaning that it was formed by a single eruptive vent from which only a small and ephemeral magma erupted over a short period of time. The stratigraphy may also display several packages of deposits separated either by contrasting discordance surfaces or paleosoils, which reflect multiple phases or episodes of eruptions within the same maar. Such maars are characterized as complex polycyclic maars if the length of time between the eruptive events is relatively short (days to years). For greater length of time (thousands to millions of years), the complex maar will be quantified as polygenetic. These common depositional breaks interpreted as signs of temporal interruption of the eruptions for various timescales also indicate deep magma system processes; hence magmas of different types might erupt during the formation of both simple and complex maars. The feeding dikes can interact with groundwater and form closely distributed small craters. The latter can coalesce to form a final crater with various shapes depending on the distance between them. This observation indicates the significant role of the magmatic plumbing system on the formation and growth of complex and polygenetic maar‐diatreme volcanoes

Modern analogues for Miocene to Pleistocene alkali basaltic phreatomagmatic fields in the Pannonian Basin: "Soft-substrate" to "combined" aquifer controlled phreatomagmatism in intraplate volcanic fields

The Pannonian Basin (Central Europe) hosts numerous alkali basaltic volcanic fields in an area similar to 200 000 km2. These volcanic fields were formed in an approximate time span of 8 million years producing smallvolume volcanoes typically considered to be monogenetic. Polycyclic monogenetic volcanic complexes are also common in each field however. The original morphology of volcanic landforms, especially phreatomagmatic volcanoes, is commonly modified. by erosion, commonly aided by tectonic uplift. The phreatomagmatic volcanoes eroded to the level of their sub-surface architecture expose crater to conduit filling as well as diatreme facies of pyroclastic rock assemblages. Uncertainties due to the strong erosion influenced by tectonic uplifts, fast and broad climatic changes, vegetation cover variations, and rapidly changing fluvio-lacustrine events in the past 8 million years in the Pannonian Basin have created a need to reconstruct and visualise the paleoenvironment into which the monogenetic volcanoes erupted. Here phreatomagmatic volcanic fields of the Miocene to Pleistocene western Hungarian alkali basaltic province have been selected and compared with modern phreatomagmatic fields. It has been concluded that the Auckland Volcanic Field (AVF) in New Zealand could be viewed as a prime modern analogue for the western Hungarian phreatomagmatic fields by sharing similarities in their pyroclastic successions textures such as pyroclast morphology, type, juvenile particle ratio to accidental lithics. Beside the AVF two other, morphologically more modified volcanic fields (Pali Aike, Argentina and Jeju, Korea) show similar features to the western Hungarian examples, highlighting issues such as preservation potential of pyroclastic successions of phreatomagmatic volcanoes.Fil: Németh, Karoly. Massey University; Nueva ZelandaFil: Cronin, Shane. Massey University; Nueva ZelandaFil: Haller, Miguel Jorge F.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; ArgentinaFil: Brenna, Marco. Massey University; Nueva ZelandaFil: Csillag, Gábor. Geological Institute of Hungary; Hungrí

El Hierro Island Global Geopark

This open access book explores El Hierro Island, which is geologically the youngest of the Canary Islands (Spain). Having registered its latest volcanic eruption in 2011-2012, it is an oceanic subtropical island with low population pressure and a largely unchanged natural landscape. Accordingly, a great geodiversity of volcanic morphologies and erosion processes has been preserved. In addition, half of the land is protected as a Biosphere Reserve and as a UNESCO Global Geopark, and the island is pursuing energy self-sufficiency. Local tourism is a sustainable activity, as the main attractions are either diving or hiking through the island’s various volcanic landscapes. Covering these and other aspects, and using accessible language, the book will appeal to scientists specialized in geotourism, active leisure entrepreneurs, and members of the general public interested in volcanic geoheritage and geotourism

The Basaltic Monogenetic Volcanic Field of the Bakony–Balaton UNESCO Global Geopark, Hungary: From Science to Geoeducation and Geotourism

As a part of the long-standing volcanism of the Carpathian–Pannonian Region, a basaltic monogenetic volcanic field developed here from 8–2.3 Ma. This is a specific type of volcanism, when mostly a small volume of magma erupts intermittently and always in a new place. The Bakony–Balaton Uplands area is an excellent natural laboratory, where several unique volcanological features can be observed and which provides an insight into how such volcanism is taking place. This volcanic field consists of more than 50 volcanic centers and almost all volcanic eruption types characterizing basalt volcanism can be recognized here, such as hydrovolcanic (phreatic to phreatomagmatic) eruptions and magmatic (Strombolian and Hawaiian) explosive eruptions with proximal and distal pyroclastic deposits, clastogenetic lava, valley-channeled lava flow, lava lake and vent-filling basalts. Since significant uplift and erosion occurred after the volcanism, the original volcanic edifices have been variously eroded, enabling the unique exposure even of the vent and conduit sections. The lava lake and valley-filled basalts were resistant to erosion that resulted in an inverted morphology landscape. Building on scientific results gained from petrological and volcanological studies for more than a century, the Bakony–Balaton UNESCO Global Geopark makes a great effort to transfer this knowledge to geoeducation and geotourism development. This includes volcanological nature trails over 40 km in length and visitor centers with exhibitions designed not only to unravel the nature of volcanic processes, but also to serve as entertainment and recreation. This is accomplished by regular guided outdoor activities led by certified local partners, who successfully passed the geopark geotour-guide training courses

Metabasites of Middle Transangaria, Yenisei ridge: e-morb relicts of neoproterozoic lithosphere

Приведены первые данные по 40Ar/39Ar датированию методом ступенчатого нагрева, геохимии редких рассеянных элементов (ICP-MS) и изотопному (Nd, Sr) составу метабазитов в бассейнах малых притоков рр. Вельмо и Большой Пит заангарской части Енисейского кряжа. Изученные породы образуют небольшие ареалы субсогласных будинированных пластинообразных тел амфиболитов среди мраморов, кальцифиров и кристаллических сланцев позднего архея и относятся к производными метапикрит-базальтового комплекса. Они имеют сланцеватое полосчатое строение и состоят из роговой обманки в ассоциации с андезином, биотитом, цоизитом, карбонатом, кварцем и акцессорным апатитом, сфеном, ильменитом. Минеральный парагенезис соответствует условиям низкотемпературного метаморфизма амфиболитовой фации. По химическому составу (SiO2 - 44-49, Na2O + K2O - 2-4, TiO2 - 1.1-1.8, Fe2O3 - 12-17, CaO - 8-11, MgO - 7-11 мас. %; FeO(t)/MgO 1-2) породы соответствуют базальтам и трахибазальтам толеитовой серии океанического дна. Установленный позднедокембрийский (≈700 млн лет) возраст породообразующего амфибола сопоставим с начальными стадиями развития Палеоазиатского океана. По характеру распределения LILE (Ba ≈ 20-1000, Sr ≈ 100-635 г/т) и HFSE (REE - 46-83, Nb - 4-10, Ta - 0.3-0.7, Zr - 30-90, Th 0.6-1.1, U ≈ 0.2 г/т) мафитовые породы соответствуют толеитовым E-MORB, которые формировались в условиях задугового спрединга и имели обогащенный астеносферный источник. Изотопные особенности (εNd(t) - +3.6...-5.2; T NdDM ≈ 1.4-2.2 млрд лет; 87Sr/86Sr(t) - 0.7046-0.7154) свидетельствуют о том, что мантийный диапиризм мог сопровождаться смешением материала плюмовой, субдукционной и коровой природы (DMM + PREMA + EM). Повышенные концентрации HREE (La N /Yb N - 1-3, LREE/HREE - 2.2-3.2) и Y позволяют предполагать отсутствие реститового граната и экстракцию исходной толеитовой магмы E-MORB в условиях ≈ 4-20 % равновесного плавления шпинелевого лерцолита верхней мантии

History of scoria-cone eruptions on the eastern shoulder of the Kenya–Tanzania Rift revealed in the 250-ka sediment record of Lake Chala near Mount Kilimanjaro

Reconstructions of the timing and frequency of past eruptions are important to assess the propensity for future volcanic activity, yet in volcanic areas such as the East African Rift only piecemeal eruption historiesexist. Understanding thevolcanichistory of scoria‐conefields, whereeruptionsare often infrequentand deposits strongly weathered, is particularly challenging. Here we reconstruct a history of volcanism from scoria cones situated along the eastern shoulders of the Kenya–Tanzania Rift, using a sequence of tephra (volcanic ash) layers preserved in the ~250‐ka sediment record of Lake Chala near Mount Kilimanjaro. Seven visible and two non‐visible (crypto‐) tephra layers in the Lake Chala sequence are attributed to activity from the Mt Kilimanjaro (northern Tanzania) and the Chyulu Hills (southern Kenya) volcanic fields, on the basis of their glass chemistry, textural characteristics and known eruption chronology. The Lake Chala record of eruptions from scoria cones in the Chyulu Hills volcanic field confirms geological and historical evidence of its recent activity, and provides first‐order age estimates for seven previously unknown eruptions. Long and well‐resolved sedimentary records such as that of Lake Chala have significant potential for resolving regional eruption chronologies spanning hundreds of thousands of years.NERC (NE/ P011969/1