Maar-forming Eruptions Research Articles

Lateral migration of explosive hazards during maar eruptions constrained from crater shapes

Maar volcanoes are produced by subsurface phreatomagmatic explosions that can move vertically and laterally during an eruption. Constraining the distances that maar-forming explosions move laterally, and the number of relocations common to these eruptions, is vital for informing hazard scenarios and numerical simulations. This study uses 241 intact Quaternary maar crater shapes to establish global trends in size and spacing of explosion position relocations. Maar craters are sorted into shape classes based on the presence of uniquely identifiable combinations of overlapping circular components in their geometry. These components are used to recognize the minimum number of explosion locations responsible for observed crater shapes. Craters with unique solutions are then used to measure the size and spacing of the explosion footprints, the circular area of the largest crater produced by a single explosion of a given energy, that produce the crater shape. Thus, even in the absence of abundant observations of maar-type eruptions, the typical range, size and spacing of explosion positions are derived from maar crater shapes. This analysis indicates that most Quaternary maar eruptions involved at least three different explosion locations spanning distances of 200–600 m that did not always follow the trend of the dike feeding the eruption. Additional evaluation of larger maars, consistent with stratigraphic studies, indicates that centers of explosive activity, and thus the origin of ballistic and density current hazards, can move as many as twenty times during a maar-forming eruption. These results provide the first quantitative constraints on the scale and frequency of lateral migration in maar eruptions and these values can directly contribute to hazard models and eruption event trees in advance of future maar-type eruptions.

10Be exposure ages for the Late Pleistocene Gour de Tazenat maar (Chaîne des Puys volcanic field, Auvergne, France)

Abstract Gour de Tazenat is a maar volcano on the crystalline Plateau des Domes, the uplifted margin of the Limagne Rift of the Massif Central of France, and forms one of the northernmost volcanic centers of the Chaine des Puys volcanic field. Although long recognized as a relatively young volcanic center in the area, age determinations have been elusive, with the only one quantitative estimate based upon radiocarbon ages of bulk sediments in the crater lake and extrapolation of sedimentation rates to account for the full thickness of the sediments; that age is ∼35 cal ka. Here we test application of cosmogenic 10Be exposure dating to quartz-bearing subvolcanic rocks exposed in the wall of the maar crater. Considering erosion rates (∼5 m/Myr) that we determined from 10Be data on a nearby tor (located outside the crater), and corroborated by inverted relief of older, dated lavas in the region, the 10Be data yield an age of 29–34 ka for the maar-forming eruption of Gour de Tazenat. This is consistent with the one previous quantitative estimate, and provides confidence in the age range of the maar. This age corresponds to a pulse of volcanic activity around 30 ka in other parts of the Chaine des Puys.

Compound maar crater and co-eruptive scoria cone in the Lunar Crater Volcanic Field (Nevada, USA)

Bea's Crater (Lunar Crater Volcanic Field, Nevada, USA) consists of two coalesced maar craters with diameters of ~440m and ~1050m, combined with a co-eruptive scoria cone that straddles the northeast rim of the larger crater. The two craters and the cone form an alignment that parallels many local and regional structures such as normal faults, and is interpreted to represent the orientation of the feeder dyke near the surface. The maar formed among a dense cluster of scoria cones; the cone-cluster topography resulted in crater rim that has a variable elevation. These older cones are composed of variably welded agglomerate and scoria with differing competence that subsequently affected the shape of Bea's Crater. Tephra ring deposits associated with phreatomagmatic maar-forming eruptions are rich in basaltic lithics derived from <250m depth, with variable contents of deeper-seated ignimbrite lithic clasts, consistent with ejection from relatively shallow explosions although a diatreme might extend to deeper levels beneath the maar. Interbedding of deposits on the northeastern cone and in the tephra ring record variations in the magmatic volatile driven and phreatomagmatic eruption styles in both space and time along a feeder dike.

Late Pleistocene Alberca de Guadalupe maar volcano (Zacapu basin, Michoacán): Stratigraphy, tectonic setting, and paleo-hydrogeological environment

The Late Pleistocene (~21,000yrBP) Alberca de Guadalupe maar is one of the few phreatomagmatic volcanoes occurring within the scoria-cone dominated Plio-Quaternary Michoacán-Guanajuato Volcanic Field. The scarcity of this type of volcano implies that conditions favoring their formation are rarely met in this region. We identify these factors by implementing current methods of investigation with emphasis on hydrogeological conditions. We present the stratigraphy (including a set of 40Ar/39Ar and 14C dates) of the SE margin of the Zacapu intermontane tectonic basin, where the maar just forms a hole (~1km in diameter, ~150m deep, bearing a ~9m deep lake) in the otherwise planar topography of the underlying early Pleistocene lava flows of the Cerro Pelón scoria cone. The maar is composed of typical phreatomagmatic surge deposits that are poorly sorted and rich in accidental lithics (>60wt.% of the deposit) with few juveniles (basaltic andesite, SiO2=54–58wt.%). The entire structure is cut by an ENE–WSW trending normal fault and is underlain by andesite lavas and silicic ignimbrites (partly inferred from xenoliths encountered in the maar deposits) that are Miocene to Early Pleistocene in age.The crater is at the center of a N–S elongated drainage basin surrounded by topographic highlands that channel water with high hydraulic pressure from most directions towards the location of the maar. This geometric configuration was already in existence at the time of the maar-forming eruption, but the climate was different. Colder and more humid conditions during the Last Glacial Maximum (Cw2-climate type, annual precipitation of >1000mm) favored the saturation of the fractured shallow aquifer system (hydraulic conductivity of 10−8 to 10−7m/s) that supplied sufficient groundwater at a high flow rate directed towards the center of the basin. Upon contact near surface (<200m) with the rising dike of basaltic andesite magma, the continuous supply of both dike-fed basaltic andesite magma and groundwater at an optimal ratio of 0.2 allowed explosive interaction for the entire duration of the eruption (17–57days) creating a >150m deep crater with steep inner walls and an underlying shallow diatreme (<300m). Since then, partial collapse of the walls along listric ring-fractures has reduced crater steepness. The present climate is much drier (Cw1 climate conditions with annual precipitation of 697.5–874.8mm) and it seems unlikely that a maar would form if these conditions persist. Nonetheless, precipitation is still high enough to sustain a ~9-m-deep lake in the crater with insignificant seasonal water table fluctuations. These findings lend further support to the notion that maars can be used as paleoclimate indicators in central Mexico.

Towards the reconstruction of the shallow plumbing system of the Barombi Mbo Maar (Cameroon) Implications for diatreme growth processes of a polygenetic maar volcano

Understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention. While there is no direct evidence of the presence of diatremes beneath most of the young maars, their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters. Properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars. As an alternative, this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar-forming eruptions. Based on field investigations, pyroclast distribution, componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the Barombi Mbo Maar (BMM), a polygenetic maar volcano in Cameroon. The BMM consists of three diatremes that formed during distinct eruptive events and coalesced to produce an “amalgamated maar–diatreme”. Two end-member types of eruption styles from the “dry” magmatic to the “wet” phreatomagmatic explosions governed the formation of the maar. In total, a minimum of ~0.0658km3 of magma (Dense Rock Equivalent corrected) was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from SRTM30m data corrected by rock textural data collected from the field. The distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process, which originated probably within the uppermost part of the diatreme. From the distribution and morphology of juvenile clasts in the deposits, it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions, sometimes with lateral and vertical variations in the position of the explosion loci. A prospective juvenile-based conceptual model is proposed for the formation of the BMM. The model suggests that, during maar-forming eruptions, explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic-dominated upper crater infill to deposit juvenile-poor (<10vol.%) tephra beds. Layers with a juvenile content of 10–60vol.%, for example, might result from deep to shallow-seated explosions, with a common entrainment of lithics from the crater infill region, and with much of the remobilized tephra being transported to the ejecta ring sequence. In contrast, explosions occurring at shallower positions will produce mainly juvenile-rich beds (juvenile>90vol.%).

Exploring the influence of vent location and eruption style on tephra fall hazard from the Okataina Volcanic Centre, New Zealand

Uncertainties in modelling volcanic hazards are often amplified in geographically large systems which have a diverse eruption history that comprises variable eruption styles from many different vent locations. The ~700 km2 Okataina Volcanic Centre (OVC) is a caldera complex in New Zealand which has displayed a range of eruption styles and compositions over its current phase of activity (26 ka–present), including one basaltic maar-forming eruption, one basaltic Plinian eruption and nine rhyolitic Plinian eruptions. All three of these eruption styles occurred within the past 3.5 ky, and any of these styles could occur in the event of a future eruption. The location of a future eruption is also unknown. Future vents could potentially open in one of three different areas which have been activated in the past 26 ky at the OVC: the Tarawera linear vent zone (LVZ) (five eruptions), the Haroharo LVZ (five eruptions) or outside of these LVZs (one eruption). A future rhyolitic or basaltic Plinian eruption from the OVC is likely to generate widespread tephra fall in loads that will cause significant disruption and have severe socio-economic impacts. Past OVC tephra hazard studies have focused on evaluating hazard from a rhyolitic Plinian eruption at select vent locations in the OVC’s Tarawera LVZ. Here, we expand upon past studies by evaluating tephra hazard for all possible OVC eruption vent areas and for both rhyolitic and basaltic Plinian eruption styles, and explore how these parameters influence tephra hazard forecasts. Probabilistic volcanic hazard model BET_VH and advection–diffusion model TEPHRA2 were used to assess the hazard of accumulating ≥10 kg m−2 of tephra from both basaltic Plinian and rhyolitic Plinian eruption styles, occurring from within the Tarawera LVZ, the Haroharo LVZ or other potential vent areas within the caldera. Our results highlight the importance of considering all the potential vent locations of a volcanic system, in order to capture the full eruption catalogue in analyses (e.g. 11 eruptions over 26 ky for the OVC versus only five eruptions over 26 ky for the Tarawera LVZ), as well as the full spatial distribution of tephra hazard. Although the Tarawera LVZ has been prominently discussed in studies of OVC hazard because of its recent activity (1886 and ~1315 ad), we find that in the event of a future eruption, the estimated likelihood of a vent opening within the Haroharo LVZ (last eruption 5.6 ka) is equivalent (<1 % difference) to that for the Tarawera LVZ (31.8 compared to 32.5 %). Including both the Haroharo LVZ and the Tarawera LVZ as possible source areas in the hazard analysis allows us to assess the full spatial extent of OVC tephra fall hazard. By considering both basaltic Plinian and rhyolitic Plinian eruption styles, as well as multiple vent location areas, we present a hazard assessment which aims to reduce bias through incorporating a greater range of eruption variables.

Temporal Evolution of the Barombi Mbo Maar, a Polygenetic Maar-Diatreme Volcano of the Cameroon Volcanic Line

The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 m-thick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was thought to have been active a million years ago. However, layers stratigraphically separated by the identified paleosoils have been dated to shed lights on its age and to reconstruct the chronology of its past activity. The results showed that the BMM formed through three eruptive cycles: the first ~0.51 Ma ago, the second at ~0.2 Ma and the third ~0.08 Ma B.P. The ages indicate that the BMM maar-forming eruptions were younger than a million years. The findings also suggested that the maar is polygenetic. At a regional scale, the eruptive events would have occurred during some volcanic manifestations at Mt Manengouba and Mt Cameroon. Therefore, with the decrease in the recurrence time of eruptions from ~0.3 Ma to 0.1 Ma, and given the possible relation between its eruptive events and those of its neighboring polygenetic volcanoes, the BMM is expected to erupt within the next 20 ka.

Shallow plumbing systems for small-volume basaltic volcanoes, 2: Evidence from crustal xenoliths at scoria cones and maars

Monogenetic basaltic volcanoes record complex eruption processes and the relationships between those processes and shallow plumbing are poorly understood. This paper explores these relationships, building upon earlier studies of exposed shallow plumbing (in the upper hundreds of meters of crust) beneath volcanoes produced by magmatic eruption processes and those produced by phreatomagmatic processes, such as scoria cones and maars, respectively. Eruptive facies and xenolith abundances are described at three scoria cones and at tephra rings around two maars in the San Francisco Volcanic Field (Arizona, USA). Well-constrained subvolcanic sedimentary stratigraphy provides constraints on the depths of origin of xenoliths of different types. Sedimentary xenolith contents at scoria cones are <10-3 and commonly <10-4 (volume fraction of xenoliths) and are composed almost entirely of fragments from the uppermost sedimentary formation (Kaibab Formation, depth interval ~50-200m). These xenolith contents are consistent with conduits or dikes that widen mainly in the uppermost tens of meters of the crust, as observed at exposed plumbing systems of eroded scoria cones. Sedimentary xenolith contents in tephra ring deposits at one of the studied maar volcanoes also are typically <10-3 (volume fraction) and the remaining fraction is dominated by clasts of pre-maar volcanic rocks that formed an ~50 thick surface layer over the sedimentary formations; the second studied maar has much higher xenolith contents but this also appears to be dominated by the shallowest unit. The maars’ tephra ring deposits contain xenoliths from all of the major sedimentary units beneath the volcanoes (to depths of ~1200m) but the abundances and proportions of xenoliths are not consistent with the volumes of sub-volcanic units that would be disrupted assuming dimensions that are commonly observed in exposed maar plumbing systems (diatremes). These differences illustrate the different mechanisms for conduit/dike widening in magmatic versus phreatomagmatic eruptions. Namely, ascending eruptive mixtures driven by magmatic volatiles widen their relatively shallow conduits by erosion and mechanical failure of the walls and ejection of the resulting xenoliths. Maar-forming eruptions produce wide and deep diatremes mainly by mechanical disruption of country rock during many discrete magma-water explosions at varying depths in the subsurface. Deep explosions cause debris jets that may not erupt, and the disrupted country rock and juvenile material gradually churns and mixes within the diatreme. Only a small fraction of the debris is ejected from the crater by especially strong and/or shallow explosions, to form tephra ring deposits.

Origins and energetics of maar volcanoes: examples from the ultrapotassic Sabatini Volcanic District (Roman Province, Central Italy)

Maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water. Here, we provide information on a number of maars in the ultrapotassic Sabatini Volcanic District (SVD, Roman Province) as young as ∼90 ka. The SVD maars are characterised in terms of crater and ejecta ring morphologies, eruptive successions and magma compositions, in light of the local substrate settings, with the aim of assessing magma–water interaction conditions, eruption energetics and genetic mechanisms. Feeder magmas spanned the whole SVD differentiation trend from trachybasalts–shoshonites to phonolites. From the ejected lithic fragments from aquifer rocks, the range of depth of magma–water explosive interaction is estimated to have been mostly at ∼400–600 m below ground level, with a single occurrence of surficial interaction in palustrine–lacustrine environment. In particular, the interaction with external water may have triggered the explosive behaviour of poorly differentiated magmas, whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for the most differentiated magma batches during low-flux ascent in an incipiently fragmented state. Crater sizes, ejecta volumes and ballistic data allow a reconstruction of the energy budget of SVD maar-forming eruptions. Erupted tephra volumes from either monogenetic or polygenetic maars ranged 0.004–0.07 km3 during individual maar-forming eruptions, with corresponding total magma thermal energies of 8 × 1015–4 × 1017 J. Based on energy partitioning and volume balance of erupted magmas and lithic fractions vs. crater holes, we consider the different contributions of explosive excavation of the substrate vs. subsidence in forming the SVD maar craters. Following available models based on crater sizes, highly variable fractions (5–50%) of the magma thermal energies would have been required for crater excavation. It appears that subsidence may have played a major role in some SVD maars characterised by low lithic contents, whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation.

Deep explosive focal depths during maar forming magmatic-hydrothermal eruption: Baccano Crater, Central Italy

We describe the eruptive activity of the Pleistocene composite Baccano maar crater in the Sabatini Volcanic Complex (Central Italy) combining stratigraphy, grain size/componentry and rare earth element and Yttrium (REY) composition of its eruptive products with the stratigraphy and geothermal data derived from deep wells drilled on the Baccano structural high. The main lithological characteristics of the basal Baccano maar pyroclastic deposit, composed of more than 60% wt of non-thermometamorphosed lithic clasts from the sedimentary basement, show that the first eruption was magmatic-hydrothermal in nature. The lithology of the sedimentary lithic clasts indicates that the fragmentation level was at a depth of −1,000 to −1,200 m, with fragment depth verified by deep well stratigraphy. The 15% wt juvenile non-vesicular glass components suggest that magma played a minor role in powering the eruption. Assuming that the high-salinity hot hydrothermal fluids (365 −1,400 m) by the aggressive hydrothermal fluids enriched in acid components (HF, HCl, and H2SO4) may have contributed to increased CO2 partial pressure that helped to drive the hydrofracturing. This could have caused rapid vapour separation and pressure drop, allowing the almost simultaneous breaking of the aquifer cover and brecciation of the calcareous units down to −1,000 to −1,200 m depth. The relative abundance of calcareous lithics in the basal part of the first Baccano eruptive unit, representing about the upper 200 m of stratigraphy below the top of the Baccano structural high, reveals the descent of the piezometric surface during the eruption. Combining deep well information and maar product stratigraphy, using also REY data from maar pyroclastic fragments and the basement rocks we draw an interpretative model for the Baccano maar-forming eruption, concluding that a) magmatic-hydrothermal eruptions may originate deeper than previously thought, and b) hydrothermal fluids circulating in limestone aquifers may play an important role in triggering such eruptions.

Temporal development of the Atherton Basalt Province, north Queensland

The Atherton Basalt Province is centred on the Atherton Tableland, ∼50 km southwest of Cairns in north Queensland. Forty-eight K – Ar age determinations and four U/Th analyses from these basalts provide information on the distribution of the volcanics over time. Volcanism commenced at 7.1 Ma and continued into the Early Holocene, with volumetric peaks in activity occurring 3.5 – 3 Ma and 2 – 1 Ma. The province shows a change with time from eruptions of voluminous lava flows that built relatively large shield volcanoes, to the production of less voluminous lavas and pyroclastics associated with cinder cones during the last million years. Phreatomagmatic, maar-forming eruptions are also preserved among the most recent eruptions. Previous radiocarbon dating of swamp sediments suggested that volcanic activity may have occurred within the past 10 000 years. No systematic change over time is apparent in the location of the volcanoes of the province. Rather, a source region ∼80 km in diameter for the province as a whole has evolved over time. Extensive partial melting in the upper mantle may have led to the ascent and adiabatic melting of lesser volumes of mantle material from deeper levels. Changes in lithospheric stresses in north Queensland, caused by the docking of the Ontong Java Plateau and subsequent development of northward subduction at the San Cristobal Trench, may have allowed the ascent of magma from this mantle region.

Distribution and eruptive mechanism of maars in the Kamchatka Peninsula

Maars are monogenic volcanic edifices of peculiar morphology with a wide circular crater surrounded by a low gentle rim of erupted pyroclastic material. Craters of maars are deeply incised into underlying rocks (frequently down to the aquifer), which explains their frequent partial or complete water infill. Owing to their morphological peculiarities, maars are easily identifiable in air photos. Maars are widespread in many volcanic areas of the world, where they occur both within volcanic fields and on slopes of stratovolcanoes (craters of subsidiary eruptions). It is established that maars are formed during the highly explosive interaction between ascending magma and groundwaters. Therefore, their presence is indicative of the phreatomagmatic eruptive mechanism [11]. We present the first overview of maars of the Kamchatka Peninsula. The main purpose of this work is to estimate the role of the phreatomagmatic mechanism during eruptions of monogenic volcanoes of Kamchatka and to reveal factors controlling the geographical distribution of maar-forming eruptions. Occurrence frequency and distribution of maars. In total, 14 maars with a crater diameter of >600 m have been identified in Kamchatka using air photos and published materials [3‐8] (table). It is difficult to identify smaller maars. We believe that their number does not exceed 20. Thus, maars constitute less then 1% of the total number of monogenic volcanic edifices in the peninsula. Tuff cones and tuff rings that are composed of products of phreatomagmatic eruptions are even rarer. Other types of monogenic volcanoes dominated by cinder cones are related to eruptions characterized mainly by the magmatic mechanism of magma fragmentation [11]. Hence, eruptions with a prevalent phreatomagmatic eruption mechanism are rare in Kamchatka. The geographic distribution of maars throughout the Kamchatka Peninsula is very irregular (Fig. 1), with their concentration being independent of the total number of monogenic edifices in particular areas. For example, the Klyuchevskaya group of volcanoes hosts numerous monogenic cinder cones, but maars are lacking here. They are also extremely scarce in the Sredinnyi Range (Kamchatka), where intense volcanism produced numerous isolated cinder cones, lava flows, and extrusive domes. Inasmuch as phreatomagmatic eruptions require water, in addition to magma, we attempted to estimate the influence of this factor on the localization of maar-forming eruptions in Kamchatka.

The Lamongan volcanic field, East Java, Indonesia: physical volcanology, historic activity and hazards

The Lamongan volcanic field (LVF) in East Java, Indonesia, consists of up to 61 basaltic cinder or spatter cones, a minimum of 29 prehistoric maars, and a central compound complex comprising three main vents including the historically active Lamongan volcano. The field lies on the Sunda volcanic arc and abuts onto the currently active Tengger caldera–Semeru complex to the west. Between 1799 and 1898, Lamongan was persistently active and discharged up to 15 lava flows of basaltic to basaltic andesite composition, which is probably more than what was erupted by any other Indonesian volcano in that period. Several seismic crises have been recorded in the LVF since Lamongan's last eruption. There are no existing records of cinder cone or maar-forming eruptions in the area. Despite the possible hazards posed by future activity in the LVF, the history and structure of the field have been subject to little scrutiny. This contribution presents the results of a detailed morphometric and volumetric investigation of prehistoric vents and historic lava flows in the LVF, using field data, historic records, satellite imagery and modelling of erupted volumes. A typical LVF maar is characterised as a crater with a diameter of 450 m; generated by an eruption with a model ejecta volume of 0.016 km 3 dense rock equivalent (DRE) and a model juvenile component volume of 0.0096 km 3 DRE. Diameter/depth ratios of some maars display a morphological freshness comparable to recently formed phreatomagmatic eruption craters elsewhere. The median LVF cinder/spatter cone has a basal diameter of 600 m, a height of 75 m, a volume of 0.009 km 3 and a model total erupted volume of 0.014 km 3. Variation in cone basal diameter to height ratios indicates that the age of some prehistoric cones may not exceed a few centuries, and morphological classification identifies the youngest prehistoric vents on the northern flanks of the central edifice. Historic activity from vents on the western flanks of Lamongan produced at least ∼0.05 km 3 of basaltic to basaltic andesite lava. Resurfacing rates at Lamongan in the 19th Century imply an age of ∼900–2100 years for the surface of the volcano. The steady-state eruptive rate from 1843–1898 is evaluated as 0.025 m 3 s −1, which is low in comparison to estimates from many presently active systems. Using eruptive rate and volume data, the age of the LVF is estimated as between 13 and 40 ka, placing it among the youngest dated Indonesian volcanoes. Complex regional tectonics in conjunction with magmatic pressure have probably been the major influences on the distribution of volcanism in the LVF. The occurrence of maar-forming eruptions may have been controlled by a porous substrate, possibly a deposit from the Holocene Iyang–Argapura complex to the east. Volcanic hazards from future eruptions in the LVF include base surges, ash flows, ashfall and ejected blocks associated with phreatomagmatic activity, and ashfall and ejecta from magmatic eruptions.

鹿児島湾北岸におけるマグマ水蒸気噴火とこれに影響を与えた縄文海進

In the volcanic areas to the north of Aira caldera, southern Kyushu, two maars named Sumiyoshi-ike and Yonemaru were formed by phreatomagmatic eruptions in the early stage of Holocene. These maars were located in a lowland composed of coastal deposits. In this paper, we discuss first the characteristics of these eruptions on the basis of distribution, structure of pyroclastic deposits called Yonemaru and Sumiyoshi-ike tephra formations which were produced by these eruptions. And then the paleo-environment of the vents is discussed with special reference to coastal changes associated with Holocene sea level rise, because the nature of such eruptions was possibly affected by water condition around eruption plumes.Yonemaru tephra formation, 0.04km3 in volume, is characterised by many thin consolidated beds containing abundant scoriaceous ash with accidental lapilli and block. These thin beds show plane bedding forms, with intercalating wavy or lenticular beds. Such structural features and distribution highly affected by topography indicate that this eruption was phreatomagmatic and dominantly generated a base surge flowing towards the southeast to a distance of about 3 or 4km from the vent, with minor amount of tephra falls. Sumiyoshi-ike tephra formation, 0.005km3 in volume, also emblaces alternating thin ash and scoriaceous lapilli beds with accidental blocks, generally coarser than Yonemaru tephra. Structural features and distribution of this formation indicate that this eruption was also phreatomagmatic, and produced mainly tephra falls dispersed north of the vent.A chronological study of these two tephra formations and of Holocene coastal terraces and deposits around the maars strongly suggests that changes in water condition around the maars associated with the Holocene sea level rise were responsible for these maar-forming eruptions of phreatomagmatic type. Sumiyoshi-ike and Yonemaru maars were respectively formed around 7, 0006, 500 yBP, when sea level was rapidly rising and the sea invaded inland to the vicinity of the maars. As a results, the condition that rising magma most effectively contacted with water was produced in this area at these ages.This region has been so affected by active uplifts that paleoshores at several stages are found considerably higher than the present sea level as follows : c. 6m at c. 7, 000 yBP, c. 10m at c. 6, 500 yBP and c. 15m at 6, 0006, 300 yBP.