Abstract Small-scale volcanic systems are the most widespread type of volcanism on Earth and occur in all of the main tectonic settings. Most commonly, these systems erupt basaltic magmas within a wide compositional range from strongly silica undersaturated to saturated and oversaturated; less commonly, the spectrum includes more siliceous compositions. Small-scale volcanic systems are commonly monogenetic in the sense that they are represented at the Earth’s surface by fields of small volcanoes, each the product of a temporally restricted eruption of a compositionally distinct batch of magma, and this is in contrast to polygenetic systems characterized by relatively large edifices built by multiple eruptions over longer periods of time involving magmas with diverse origins. Eruption styles of small-scale volcanoes range from pyroclastic to effusive, and are strongly controlled by the relative influence of the characteristics of the magmatic system and the surface environment.

Abstract We report here a growth model for phreatomagmatic maar-diatreme volcanoes with respect to the number of eruptions documented in the tephra beds of maar tephra rings and the upper bedded diatreme facies. We show that the number of tephra beds in large diatremes is larger than that in maar tephra rings. Base surges that lack sufficient momentum to scale high maar crater walls deposit their tephra only inside the crater. Thus the total number of eruptions at large maar-diatreme volcanoes will be larger than the number recorded in maar tephra rings. As many maar-diatreme volcanoes erupt dominantly accidental clasts, an incremental mathematical model was applied to study the growth of diatremes. The model is based only on the ejection of distinct amounts of accidental clasts per unit eruption and the chosen number of eruptions is assumed to be identical. The incremental growth of cone-shaped diatremes follows cube-root functions with respect to diameter and depth and slows down with ongoing eruptions. In nature, small and large maar-diatreme volcanoes are formed and filled syn-eruptively, mostly by tephra, depending on the duration and quantity of magma involved in phreatomagmatic eruptions. In our opinion, this mathematical model is the only current method able to model the growth of diatremes.

Abstract Geometric analysis was used to define seven morphometric parameters for the description of the shape of scoria cones, including volume, orientation and five shape parameters (flat-topped-ness, steep-sided-ness, relative crater depth, crater slope and elongation). The measurement and analysis of these shape factors for five distinct scoria cone populations facilitated an exploration of the overlapping effects of growth and erosion processes on shape. Flat-topped-ness seemed to be insensitive to either growth or erosion, suggesting a dependence on eruption energetics. Most of the variation in steep-sided-ness originated from eruptions ending during variable (early to mature) stages of development (growth) and from erosional processes, although steep-sided-ness was also sensitive to eruption energetics (through fragmentation and its impact on the angle of repose). Steep-sided-ness was therefore a poor indicator of geomorphic age. Distinct types of scoria cone shape (ideal, gully, horseshoe, tilted, crater row, parasitic and amorphous) captured a variety of deviations in shape from the purely conical, yet the conical shape parameters differed little in average value between the shape types. The conical shape parameters captured the growth and erosion processes (e.g. ballistic delivery, tephra redistribution via avalanching and erosional transport) common to all scoria cones rather than the particular processes that made the shape types unique (e.g. breaching, rafting and wind effects). Supplementary material: Locations of examples of the shape types are available at https://doi.org/10.6084/m9.figshare.c.3661907

Abstract Most kimberlite maar-diatreme volcanoes erupted during the Tertiary or earlier and therefore their tephra rings and, less often, their near-surface diatreme-filling deposits have usually been eliminated by erosion. Poorly eroded Quaternary non-kimberlite maar-diatreme volcanoes, especially those of mafic and ultramafic magma types, have the same diatreme size range (diameter and depth) as kimberlite pipes and show similar internal volcaniclastic diatreme lithofacies. In addition, these young volcanoes often have a more or less preserved tephra ring consisting of hundreds to perhaps a few thousand thin tephra beds. Volcanological analyses of the xenolith-rich primary volcaniclastic deposits both within these diatremes and in the tephra ring beds reflect phases of explosive pipe growth and are of convincingly phreatomagmatic origin. The similarities between non-kimberlite pipes and kimberlite pipes suggest to some researchers that phreatomagmatic processes were also responsible for pipe excavation processes in kimberlite maar-diatreme volcanoes. In contrast, other researchers have suggested that kimberlite maar-diatreme volcanoes were emplaced largely by magmatic processes as a consequence of exsolution and the explosive expansion of juvenile volatiles. We therefore analysed and compared some key geological features of kimberlite and ultrabasic to basic ‘basaltic’ maar-diatreme volcanoes to determine similarities and differences with respect to their emplacement behaviour. The following problems were addressed – the layout of the abstract; an amendment to the caption of Fig. 1 ; and some changes to Zimanowski’s references in the reference list.

Abstract The Newer Volcanics Province of SE Australia is a very large continental basaltic province, with an area of >23 000 km 2 , a dense rock equivalent volume of <900 km 3 and >400 monogenetic volcanoes; it has been active since c. 8 Ma. Lava fields, shields, scoria cones are common, and there are >40 maars and volcanic complexes. Maars occur dominantly in the south where magmas erupted through Tertiary sedimentary aquifers, whereas in the north, over Palaeozoic crust, there are few. Complex interactions of the magma volatile content, magma ascent rates, conduit characteristics and the availability and depth of aquifers caused diverse eruption styles. Volcanoes commonly occur close to major crustal faults, which acted as magma conduits. There is no simple age pattern of volcanism across the province. Volcanism was probably triggered by transtensional decompression in the crust where fault sets intersect, affecting hot, hydrated mantle that had welled up through edge-driven convection where the base of the lithosphere thins abruptly at the edge of the continent. Rock compositions range from picritic to basaltic andesitic. Some volcanoes are polymagmatic. Regional geophysical datasets have clarified the regional characteristics of the province, whereas detailed ground magnetic and gravity surveys resulted in new insights into the subsurface structure of maar-diatremes.

Abstract Harrat Rahat (<10 Ma) is one of the largest volcanic fields on western Arabia. In the north of the field, some of the youngest volcanic centres evolved through either point-like, complex or multiple aligned vents (i.e. along fissures), and have pyroclastic cones, lapilli fall deposits and/or lava flows associated with them. The products reflect dominantly Hawaiian eruptions, and only one centre experienced phreatomagmatism. Results from new 3 He surface-exposure dating provide constraints on stratigraphy of the youngest (<0.3 Ma) products. The rocks are compositionally alkali-basalt and hawaiite, with intra-plate basalt (prevalent mantle (PREMA)) affinity. Each eruption displays a distinct whole-rock composition in an overall linear trend. We suggest that the magma source for each centre is similar, and that composition of the products is different due to different degrees of fractionation. In a single eruption, the magma that reaches the surface first is the least evolved, with the most evolved magma erupting last. We also found that the most primitive magmas erupt less explosively. We think that the degree of magma evolution might correlate with ascent times, assuming that the more evolved magma spent more time en route. We suggest that magma ascent time is likely to be longer than that of other more primitive intra-plate basalts. Supplementary material: Whole-rock chemistry results, mineral chemistry results and fractional crystallization modeling data are available at https://doi.org/10.6084/m9.figshare.c.3488988

Abstract Extensive drilling of the Tango Extension kimberlite pipe resulted in the construction of an emplacement model that revealed the complex architecture of two amalgamated pipes: an older pipe, the Tango Extension Deep, which is cut along its northern margin by the smaller Tango Extension pipe. The resulting volcano forms a complex pipe-in-pipe structure called the Tango Extension Super Structure. The emplacement of the Tango Extension Super Structure sequence indicates prolonged hiatuses, which, similar to other volcanoes classified as monogenetic, puts the classical monogenetic and polygenetic definitions of maar-diatreme volcanoes to the test. Although the Tango Extension and Tango Extension Deep volcanoes could be characterized individually as monogenetic volcanoes, the Tango Extension Super Structure shows evidence of the occurrence of the significant hiatuses typical of polygenetic volcanoes. We suggest that hiatuses that are long enough to consolidate earlier tephra unambiguously differentiate polygenetic from monogenetic maar-diatreme volcanoes. The original version was incorrect. This was due to the Acknowledgements and Funding section being omitted, a missing citation, and a change needed to the caption of Fig. 8 .

Abstract Rincón de Parangueo is a Quaternary maar that has been recently desiccated. The crater was partially occupied by a soda lake, and near the shoreline microbialites have formed. Evaporites (mainly trona and halite) precipitated as the water level dropped. Active subsidence of the lake floor ( c. 24 m since 1980) produced countless structures close to the lakeshore, where deformation is extensional. Closer to the depocentre, in the western half of the basin, gliding/spreading produced folds and mud-injection domes. The most remarkable structure throughout the basin is a monocline that forms a ring-like, nearly continuous scarp, approximately 15 m high, which in the eastern half of the basin was produced as a fault-propagation fold developed above the buried diatreme–country rock boundary. A more diffuse (wider) monocline, locally associated with compressive structures, occurs in the western half of the basin. These structures are interpreted as having developed above a gently inclined, irregular lake sediment–country rock (andesite) interphase. The monocline was modified by high-angle extensional faults/fractures with large heaves/apertures. In the eastern half of the basin, there is a second (outer) scarp, approximately 13 m high, formed by a high-angle, listric, normal fault. Rollover antiforms occur in the hanging wall of this structure. Rincón is an example of centripetal gravitational gliding/spreading.

Abstract The Joya Honda maar (JHm) is located in central Mexico, 35 km NNE of the city of San Luis Potosí. It lies in the Plio-Quaternary alkaline Ventura-Espíritu Santo Volcanic Field located in the eastern part of Mesa Central province. The JHm eruption occurred at 311±19 ka ( 40 Ar/ 39 Ar) along a fissure that formed an elliptical crater ( c. 1.3 × 0.9 km wide and c. 270 m deep) with a major axis oriented to the ENE–WSW. The eruption generated pyroclastic surge deposits that preferentially extended up to a distance of 7 km to the NW–NE of the crater, with a very limited distribution to the south. At the crater rim, the sequence is 60–80 m thick on the NE–NW wall and 1–15 m thick on the south–SW rim. The JHm sequence is divided into five units with different structures, textures, granulometry and components. The juvenile basanite clasts of these units display differences in vesicularity, density and morphology under scanning electron microscopy. These units correspond to the same number of eruptive phases as follows: Phase 1 occurred as a series of alternating strombolian and phreatomagmatic explosions that dispersed fall deposits and base surges; Phase 2 began with strombolian activity that emplaced basanite scoria with low contents of mantle xenoliths; Phase 3 continued with phreatomagmatic explosions that emplaced wet and dry pyroclastic surges; Phase 4 generated strombolian explosions rich in mantle xenoliths; and Phase 5 produced a violent strombolian phase that dispersed fallouts rich in mantle xenoliths and intermixed with discrete phreatomagmatic explosions that emplaced pyroclastic surges. These eruptive fluctuations during the genesis of JHm are a response to the relative proportions of magma–water interaction through time and complex faulting of the calcareous rocks underneath the volcano. The distribution and textural characteristics of the deposits suggest that simultaneous or alternating vents were active during the eruption, possibly following a fissure. These variations may have been subordinated to factors such as the availability of groundwater, the velocity of magma ascent, the discharge rate and degassing.

Abstract On the continental platform of western Mexico, there is a young island of volcanic origin: Isla Isabel. It is the only volcanic manifestation in the region, contrasting with Islas Tres Marías, which are not volcanic. We aim at characterizing the source of the volcanic activity present in this particular location. Under Isla Isabel there is a bathymetric bulge that rises 60–80 m above the platform, and extends 20 km in the NW–SE direction and 17 km in the perpendicular direction. Isla Isabel is the only emerged portion of the bulge, extending only 1.8 km in the NW–SE direction. The island shows Plio-Pleistocene volcanic activity, including the formation of maars and the presence of mantle xenoliths. Using independent 2D modelling and 3D inversion methods for the gravity and magnetic fields, we analyse the nature of the bulge and its surroundings. A magnetotelluric station yields information about the electrical resistivity under the island, with penetration depths of approximately 20 km. The models are consistent with the presence of dense bodies of varying magnetizations that are interpreted as intrusive bodies. Results support the presence of an intrusion that locally has raised the ocean-floor topography. Volcanic activity projected from the bulge created Isla Isabel; the existence of additional, submerged volcanic centres in the area is most probable. We are inclined to identify the Isabel Bulge as a laccolith.

Abstract Isla Isabel is a small island located on top of a 20 km-long bathymetric bulge, part of the continental shelf of Nayarit in the vicinity of the mouth of the Gulf of California; it is of volcanic origin with tuff cones, basaltic flows and Surtseyan-type explosion craters exposed. Geophysical surveys were carried out in order to model the geological bodies associated with the local gravity and magnetic fields. 2D and 3D models are presented, and a 1D inversion model is derived from magnetotelluric data. Analysis of the density models of the upper 1400 m suggests two growth stages for Isla Isabel in this depth range: the first one reaching 800 m in depth and the second one projecting from that depth to the surface. The corresponding magnetic susceptibility models concur with this observation. The bodies projected to the surface appear to correspond to diatremes. The 1D magnetotelluric inversion shows two conductive anomalies within the crust between 3–5 and 8–17 km, followed by a resistive substratum that coincides with the seismically derived limit of the Earth’s crust at the island’s position.

Abstract Despite a number of published papers focusing on the geodynamic implications of the recent Southern Puna mafic magmatism, there have been fewer studies of the volcanology and stratigraphy of this outstanding volcanism. This paper presents a detailed map of two well-preserved Quaternary scoria cones showing their complex stratigraphy. Complementary morphometric, morpho-structural, petrographic and geochemical data were used to reconstruct the evolution of both volcanoes. The occurrence of more than one eruption at each volcano was inferred by the recognition of temporal hiatuses using morpho-stratigraphic criteria. The polycyclic nature of both scoria cones could be related to a combination of a high input magma in response to lithospheric delamination, a favourable regional stress field and the interaction of rising magma with pre-existing faults. The youngest eruptions in both volcanoes were complex, with shifts in the eruptive style from violent strombolian to hawaiian/strombolian phases, and probably lasted for a few years. The explosive activity was accompanied by the emission of lava flows from lateral vents. Phreatomagmatic activity was triggered during the waning stages of the eruptions. The occurrence of more than one eruption in a single scoria cone and the changes in the eruptive style during long-lasting eruptions are important topics for volcanic hazard assessment in the Southern Puna.

Abstract The Neogene mafic volcanism of the Northern Puna region in the Central Andes is represented by scoria cones and lava flows dispersed over a wide region ( c. 9150 km 2 ) as isolated or poorly clustered centres. Although all the products are basaltic andesites to andesites, the behaviour of these magmatic systems resembles that seen in basaltic monogenetic fields. These centres were studied with the aim of defining the main volcanic lithofacies and evaluating the eruptive styles. The results suggest that the eruptions developed under a dry strombolian dynamic, with brief periods of lava fountaining and hydrovolcanism, the latter usually restricted to the early stages of cone construction. Changes in eruptive style are thought to be caused by variations in both the internal (e.g. magma ascent) and external (e.g. surficial water availability) conditions. The transitions do not reflect compositional changes, as evidenced by the small chemical differences observed among the products of the studied eruptive centres. Stratigraphic analysis, in addition to a few pre-existing radiometric dates, suggests that this volcanic activity occurred during the Late Miocene to Early Pliocene. This information supports the inference that these eruptions occurred before the peak of Southern Puna mafic volcanism and that they were coeval with eruptions of some of the most important silicic calderas of the Altiplano-Puna Volcanic Complex. The good preservation of volcanic edifices reveals that erosion rates were extremely low, in agreement with the high aridity conditions that have prevailed in the Puna region since the Mid- to Late Miocene.

Abstract The San Diego maar is a volcano located at the middle of the Colombian Central Cordillera that forms part of the dispersed and isolated Samaná Volcanic Field (with at least three volcanoes). San Diego is excavated on Triassic metamorphic rocks and Neogene sub-volcanic andesitic units, and forms approximately 15 m of mostly unconsolidated eruption-related deposits. These deposits were divided into four stratigraphic units (U1–U4) that together define the evolution of the eruption: (1) an unsteady vent-clearing stage; (2) drier to wet pyroclastic base surges; (3) dry base surges; and (4) a final wet stage. At the end of the activity, the crater was enlarged due to a complex set of rock falls and slides in the crater walls. Finally, the volcanic activity switched to a magmatic phase, creating a dacitic dome. The northern dome flank was affected by a gravitational collapse that produced a small-volume debris avalanche. The eruption style of the San Diego maar was controlled by the stratigraphic, structural and hydrogeological characteristics of the substrate, as well as the surficial environmental condition. In addition, the mountainous terrain around the maar crater controlled the deposit extension. These constraints provide additional information for a hazard assessment related to this type of eruption in the region.

The nature and origin of the small-scale volcanic systems, generally referred to as “monogenetic”, have enjoyed an elevated level of interest during the past decade. There has been recognition that their ostensibly simple volcano types are a window into the nature of explosive volcanism, landscape evolution and the processes of magma generation in the Earth’s upper mantle. In the past few years, major conferences have offered specialized technical sessions dealing with monogenetic volcanism and there have been thematic conferences, such as the IAVCEI International Maar Conference series, which have provided a focus for discussion of volcanological and geochemical aspects of small-scale basaltic volcanism. Many new aspects of monogenetic volcanism have emerged and have clearly demonstrated that this volcanism can be very complex on a fine scale. This book is a collection of papers arising from two recent Maar Conferences (the fifth in Queretaro Mexico and the sixth in Changchun, China) and serves as a snapshot of current research on monogenetic volcanism.

Abstract The eastern Trans-Mexican Volcanic Belt is characterized by a diversity of volcanoes that are related to different processes and eruptive styles. The spectacular exposures of late Pleistocene and Holocene volcanism provide a unique opportunity to explore a variety of volcanic features and deposits that may be relevant for volcanic hazard assessments within the area. This three-day field guide describes selected representative examples of the regional volcanism showing volcanic features including thick pyroclastic successions derived from the explosive activity of Los Humeros caldera volcano, caldera-rim effusions, alternating explosive and effusive activity of a vitrophyric rhyolite dome (Cerro Pizarro), and the eruptive activity of two composi-tionally contrasting maar volcanoes: Atexcac, a classic basaltic maar and Cerro Pinto, a rhyolitic tuff ring—dome complex.