Volcanic centers are complex, dynamic landforms. The stunning morphological variety of volcanic landforms is due to a combination of tectonic setting, eruption style, magma composition and volume, surface environment, and age, i.e., the time the landform has existed and evolved. The considerable variety of volcanic landforms reflects the large variability of these parameters. In turn, volcanoes affect their surrounding landscapes. The influence that volcanism exerts on a regional landscape is the result of many different factors. These include the nature and pattern of various fissures and vents, the length of time that volcanism is active, the relative age of volcanism, the composition and physical characteristics of extruded materials, the volume of erupted material, and the amount and extent of subsequent erosion. In some cases, voluminous lava flows and thick blanket tephras that accumulate over large areas may partially or entirely bury the preexisting landscape, whereas, in others, a focused distribution of lavas and tephras may produce a distinctive assemblage of lava-capped hills and mesas.

Volcanic hazards are direct and indirect, and they encompass diverse phenomena such as pyroclastic and lava flows, poisonous-gas emissions, landslides, lahars, tsunamis, jökulhlaups, and climate change. The geomorphology of volcanic landforms and their deposits provide essential insights into the past and future volcanic hazards. Volcanic disaster management and mitigation require understanding the geologic and geomorphic context, history, and processes behind them, monitoring volcanic centers, and disaster preparedness. Instrument measurements fall into four major categories, subdivided by instrument type, and include seismic, deformation, thermal emission, and gas emission.

在内蒙古乌拉特前旗大佘太镇附近的什那干群"下岩组"中下部,第一次发现了层凝灰岩等火山沉积夹层,并对其中的层凝灰岩(NM-1191)开展了SHRIMP锆石U-Pb年代学研究,精确标定其喷发时间为1614±8 Ma.由此可以确认,什那干群整体上应属于国际古元古代固结纪(Statherian Period,1800～1600 Ma)末期-中元古代盖层纪(Ca-lymmian Period,1600～1400 Ma)初期(相当于中国中元古代长城纪(1800～1600 Ma)末期-蓟县纪(1600～1400 Ma)初期)沉积.这一新的年代地层学归属的确定,显示什那干群与毗邻的渣尔泰山群及白云鄂博群(化德群)的相应层位,应属于"三群并立"、"同时异相"的沉积古地理格局;同时这也表明,该群基本可与华北克拉通北缘-中部的大红峪组-高于庄组对比,并与南缘洛峪口组-龙家园组大体相当.结合相关资料可进一步推知,位于鄂尔多斯西缘贺兰山-千里山一带的黄旗口组-王全口组及阿拉善南缘龙首山地区墩子沟群中下部(即第一、第二岩组),也应与什那干群的层位基本一致.这一广泛存在的对比关系很可能也说明,至少到中元古代盖层纪早期,阿拉善(阴山)地块仍隶属于华北克拉通的范畴,并与鄂尔多斯西缘、燕辽盆地-华北中部带及熊耳裂谷区等,共同拥有一个统一的"泛华北"陆表海.什那干群新的年代学约束及相关地层单元年代学等时框架的建立,为重新认知该阶段华北克拉通北缘沉积-构造古地理及其演化,探讨华北克拉通与哥伦比亚超大陆关系等重要命题,提供了关键的年代地层学约束.

Cretaceous strata preserved in Wyoming contain numerous large bentonite deposits formed from the felsic ash of volcanic eruptions, mainly derived from Idaho batholith magmatism. These bentonites preserve a near-continuous 40 m.y. chronology of volcanism and their whole-rock and mineral chemistry has been used to document igneous processes and reconstruct the history of Idaho magmatism as emplacement migrated across the Laurentian margin. Using LA-ICP-MS, we analyzed the U-Pb ages and Hf isotopic compositions of nearly 700 zircon grains from 44 bentonite beds from the Bighorn Basin, Wyoming. Zircon populations contain magmatic autocrysts and antecrysts which can be linked to the main pulses of the Idaho batholith and xenocrysts ranging from approx. 250 Ma to 1.84 Ga from country rocks and basement source terranes. Initial εHf compositions of Phanerozoic zircons are diverse, with compositions ranging from −26 to nearly +12. Based on temporal trends in zircon ages and geochemistry, four distinct periods of plutonic emplacement are recognized during the Mid- to Late Cretaceous that follow plutonic emplacement across the Laurentian suture zone in western Idaho and into western Montana with the onset of Farallon slab shallowing. Our data demonstrate the utility of using zircons in preserved tephra to track the regional-scale evolution of convergent margins related to terrane accretion and the spatial migration of magmatism related to changes in subduction dynamics.

Bentonite beds, which are clay depos- of its produced by the submarine alteration of volcanic tephra, preserve millions of years of volcanic products linked to magmatic systems for which records are otherwise lost through erosion and alteration. Cretaceous strata from the Bighorn Basin, Wyoming, and southwestern South Dakota contain bentonites that originated from arc magmatism produced by subduction of the Farallon plate. We analyzed the bulk major- and trace-element geochemistry, and the Sr-87/Sr-86 (rt = 87) and Nd-143/Nd-144 (rt = 26) isotopic compositions of individual bentonite beds from these areas spanning 40 m.y. of volcanism to recover signals of magmatic processes and to attempt to trace bentonite geochemical and isotopic signatures to contemporaneous Cordilleran plutonic rocks. Using multiple immobile elements (e.g., Zr, TiO2, Nb, Ta, and rare earth elements), distinct temporal trends show variations in the effects of mineral fractionation and changes in crustal thickness. Bentonite Sr and Nd isotopic compositions allow ash beds to be correlated with specific batholithic complexes in Idaho and western Montana. With this data set, we observed the following: (1) The volcanic arc migrated across the 0.706 iso-pleth between 115 and 105 Ma; (2) between 105 and 95 Ma, magmatism stalled in central Idaho and was supported through significant MASH (mixing-assimilation-storagehomogenization) processing; (3) by 85 Ma, a shallowing subduction angle resulted in the eastward migration of the volcanic front into western Montana while volcanism in Idaho diminished; and (4) around 75 Ma, evidence of Idaho volcanism is lost. Montana plutonism continued with significant assimilation of radiogenic basement and regional centers local magma emplacement (i.e., Pioneer batholith).

Bentonites serve as an important stratigraphic tool for facies correlation due to their instantaneous deposition and preservation across multiple depositional environments. Correlating one bed to another can be a complex issue limited by the 3-dimensional availability of outcrops and core. This process is reliant on the assumption that the geochemical signatures of ash beds do not vary enough to statistically alter the correlation coefficients that tie one ash-fall to another. This assumption may be faulty, however, and the problem compounds as bentonite bed thickness increases because the chemical composition varies through an eruption due to stratification of the magma chamber and through physical transport. Linear discriminant analysis (LDA) can significantly aid with this issue, allowing for the utilization of a multivariate dataset to statistically separate chemically-similar groups. Here we present a statistical approach to assessing the primary discriminatory variables on a suite of 87 Cretaceous bentonites deposited in the Bighorn Basin, Wyoming, USA, that span five geologic stages and seven formations, from the Cloverly Formation (110 Ma) to the Meeteetse Formation (70 Ma), and includes nine named ash beds. Using LDA, bentonites were sorted into groups based on geologic stage, formation, and unique beds using key elemental concentrations and radiogenic isotopes (e.g., Sr-87/Sr-86, TiO2, Sc, Na, and Th). Additionally, distinct similarities in the discrimination pattern between formations reveal two periods of magmatism, the development of the Idaho batholith and the migration and development of the Boulder batholith and surrounding related plutons. With well-preserved, temporally-constrained ash, LDA can correlate ancient tephra deposits while also shedding light on the cyclical nature of geochemical trends in a migrating subduction zone. (C) 2019 Elsevier B.V. All rights reserved.

The first age constraints for the Jixian System at the well-known, carbonate-dominated Mesoproterozoic Jixian Section, North China Craton (NCC), were derived from U-Pb age date in 2014. One of these dating results was SHRIMP zircon U-Pb ages from a K-bentonite bed in the basal part of the second member of the Tiding Formation at the Dayushan Section, northwestern Jixian County, Tianjin. Although it has been accepted as one of the critical "anchor points" for the chronostratigraphic calibration and correlation of the Meso- and Neoproterozoic standard section for the NCC, new highway construction around the Jixian urban area has destroyed almost all of the Tiding Formation at the Dayushan Section, meaning that most of the section is not accessible to those who need it. Recently, on the northern slope of Mount Fujunshan, northeastern Jixian County, a new continuous outcrop section of the Tiding Formation has been found at the Dawujian limestone quarry (mining-banned now), and several K-bentonite beds occur in basal parts of the second member of the formation. LA-ICPMS zircon U-Pb ages(1445 +/- 12Ma and 1442 +/- 10Ma) have been obtained from two of the K-bentonite beds, while another LA-ICPMS zircon U-Pb age of 1439 +/- 11Ma was also obtained from the same K-bentonite bed in the Tiding Formation at the former Dayushan Section. Within the range of error, not only the LA-ICPMS zircon U-Pb ages of all three new samples are consistent with each other, but they are also consistent with the two earlier reported SHRIMP zircon U-Pb ages (1437 +/- 21Ma and 1439 +/- 14Ma) of K-bentonites at the same horizon of the Tiding Formation, which are from the Liujiagou Section (Pingquan, Hebei, NCC) and the former Dayushan Section, respectively. In addition, a series of Lu-Hf isotope of the dated zircons of the three new K-bentonite samples was analyzed. They yielded Hf-176/(177) Hf ratios between 0.281703 and 0.281880, with epsilon(Hf) (t) = -6.7 similar to -0.2, t(DM2) = 2200 similar to 2603 Ma and 2360Ma Gaussian distribution peak age. These values indicate that the parent magmas of the current K-bentonites from the Tiding Formation were largely derived from crustal materials of Neoarchean to Early Paleoproterozoic. In summary, these new data provide additional and reliable age constraints for the Tiding Formation, both at the Jixian Section and the entire Yanshan Mountains in the northern NCC, with possible information about the parent magmas of the K-bentonite beds. In addition, the section in the Dawujian quarry is in a rural area far from the planned development zone of the Jixian urban area, in which mining has been officially prohibited, meaning that it will be available to preserve longer. Consequently, this research has laid a more solid foundation for the chronostratigraphy of the Jixian Section as the conventional Standard Section for the Meso- and Neoproterozoic strata in the NCC.

Bentonite, a claystone formed from the devitrification of volcanic ash, demonstrates varying amounts of alteration in elemental concentrations through the transition from glass to clay. Many studies have sought to characterize and correlate bentonites using various elemental signatures, but the information garnered from this can be misleading since 1) volcanoes produce highly variable compositions during single events, making tephra classifications reliant on the position within the bed and the distance from the source, and 2) the elemental concentrations may vary significantly due to devitrification. Strontium and neodymium isotopic signatures offer a powerful dataset to supplement these efforts, but the extent to which the isotopic ratio is preserved during devitrification and post-depositional diagenesis is not well documented. Here we perform a detailed isotopic, geochemical, and mineralogical examination of sixteen bentonites preserved in Cenomanian and Campanian strata (Late Cretaceous) from South Dakota in order to characterize the diagenetic factors that may influence the ash geochemistry, while checking for the fidelity of the isotopic signatures to the magmatic source. A mixing model between the isotopic composition of bentonite and seawater can be generated to determine the original endmember composition of each deposit, since Sr was likely derived from both sources during devitrification. Initial 87Sr/86Sr ratios from Cenomanian samples ranged higher than the Campanian, demonstrating a negative correlation with εNd values that point towards magmatic signatures with the influence of crustal assimilation. X-Ray diffraction was utilized to determine the speciation of the clay mineral and sedimentary input, displaying a complete lack of smectite illitization that would indicate the presence of a hydrothermal influence. Bentonite isotopic chemistry, therefore, is reliably traced to the magmatic source under the appropriate diagenetic conditions. With careful consideration for diagenesis, the isotopic composition of the Cretaceous bentonites can be used for provenance identification and points towards magmatic emplacement within an evolved crustal source on the Laurentian craton. This isolates the most likely provenance of Cenomanian bentonites as being the Idaho batholith, while Campanian bentonites were likely derived from the Elkhorn Volcanic complex.

Changes in the global atmospheric budget of platinum reportedly correspond to explosive volcanic eruptions. Using inductively coupled plasma mass spectrometry (ICP-MS) elemental analysis we examined eight widely separated stratified sites to evaluate the geographic extent of three late Holocene high magnitude volcanic events. We found characteristic Pt anomalies across the Western Hemisphere dating to the Laki, Iceland (CE 1783–1784), Kuwae, Vanuatu (CE 1452–1453), and Eldgjá, Iceland (CE 934) explosive volcanic eruptions. Pt anomalies in sediments over a broad geographic area indicate distinctive time-correlative atmospheric deposition rates of platinum-rich volcanic ash. These anomalies provide new chronostratigraphic markers for these late Holocene high magnitude volcanic eruptions, which are especially valuable in the Western Hemisphere in strata with limited chronometric control. Pt anomalies provide an important tracer for the age of these volcanic events and ultimately a new chronostratigraphic marker in archaeological, geological, palynological, and paleontological sediments.

Permian-Triassic (P-Tr) altered volcanic ashes (tuffs) are widely distributed within the P-Tr boundary successions in South China. Volcanic altered ashes from terrestrial section-Chahe (CH) and marine section-Shangsi (SS) are selected to further understand the influence of sedimentary environments and volcanic sources on diagenetic alterarion on volcanic tuffs. The zircon 206Pb/238U ages of the corresponding beds between two sections are almost synchronous. Sedimentary environment of the altered tuffs was characterized by a low pH and did not experience a hydrothermal process. The dominant clay minerals of all the tuff beds are illite-smectite (I-S) minerals, with minor chlorite and kaolinite. I-S minerals of CH (R3) are more ordered than SS (R1), suggesting that CH also shows a higher diagenetic grade and more intensive chemical weathering. Besides, the nature of the volcanism of the tuff beds studied is derived from different magma sources. The clay mineral compositions of tuffs have little relation with the types of source volcanism and the depositional environments. Instead, the degree of the mixed-layer clay minerals and the REE distribution are mainly dependent upon the sedimentary environments. Thus, the mixed-layer clay minerals ratio and their geochemical index can be used as the paleoenvironmental indicator.

The Doushantuo negative carbon isotope excursion (DOUNCE) is the largest known marine inorganic carbon isotope anomaly. The origin of this pronounced negative excursion is still an enigmatic issue that attracts geologists. Time constraints on the excursion are the critical information that would provide insight into its genesis. In previous decades, the timing of its termination has been constrained by the widely cited zircon U-Pb age of 550.5 ± 0.8 Ma for the tuff at the top of the Miaohe Member at the Jiuqunao section in the Yangtze Gorges area, South China. However, results of recent studies indicate that the reliability of this time constraint needs to be re-evaluated. Here, a geochronological study was carried out using two K-bentonites from Fanglong in South China. A K-bentonite in the lower Dengying Formation yielded a U-Pb age of 557 ± 3 Ma, while a K-bentonite in the basal Liuchapo Formation yielded an age of 550 ± 3 Ma. Based on regional correlations between the Ediacaran successions in South China, the age (557 ± 3 Ma) for the K-bentonite in the lower Dengying Formation may serve as a second critical timing constraint for the ending of the DOUNCE. Combined with available estimates of the DOUNCE duration, our new data indicate that the DOUNCE has a maximum onset age ∼570 Ma.