Showing papers on "Phreatomagmatic eruption published in 2015"

Basaltic Volcanic Fields

Abstract: Basaltic volcanic fields consist of one or more volcanoes within a defined area that is separate from other volcanic areas and that has an internally consistent tectonic/structural setting. Most of the volcanoes are monogenetic, each having only a single eruptive episode lasting from weeks to decades before becoming extinct, and are of small volume (typically around 1 km3 or less). These volcanoes share many eruptive processes with larger, polygenetic volcanoes of basaltic composition, including Strombolian, Hawaiian, and phreatomagmatic explosive activity and effusion of lava flows. They form a range of landforms including lava fields, scoria cones, small shields, maars, tuff cones, and tuff rings depending upon the dominance of magmatic volatiles versus phreatomagmatic explosions in driving eruptions, and upon the magma volume flux and processes within the shallow crust. Basaltic volcanic fields typically contain volcano alignments that may be related to crustal structures, and clusters of high spatial vent density that are likely related to variability in the upper mantle magma sources. Hazard assessment is complicated by the many factors that affect the timing and locations of successive eruptions within a volcanic field. Recent research on magma sources and ascent for monogenetic volcanoes suggests an important role for compositional heterogeneity in the upper mantle, and that volatile fluxes during eruptions can be of similar magnitude as at larger polygenetic volcanoes.

Monogenetic volcanism: personal views and discussion

Abstract: Monogenetic volcanism produces small-volume volcanoes with a wide range of eruptive styles, lithological features and geomorphic architectures They are classified as spatter cones, scoria (or cinder) cones, tuff rings, maars (maar–diatremes) and tuff cones based on the magma/water ratio, dominant eruption styles and their typical surface morphotypes The common interplay between internal, such as the physical–chemical characteristics of magma, and external parameters, such as groundwater flow, substrate characteristics or topography, plays an important role in creating small-volume volcanoes with diverse architectures, which can give the impression of complexity and of similarities to large-volume polygenetic volcanoes In spite of this volcanic facies complexity, we defend the term “monogenetic volcano” and highlight the term’s value, especially to express volcano morphotypes This study defines a monogenetic volcano, a volcanic edifice with a small cumulative volume (typically ≤1 km3) that has been built up by one continuous, or many discontinuous, small eruptions fed from one or multiple magma batches This definition provides a reasonable explanation of the recently recognized chemical diversities of this type of volcanism

Chapter 13 – Sizes of Volcanic Eruptions

Abstract: Sizes of volcanic eruptions are quantified in terms of the mass of material erupted (magnitude) and mass eruption rate (intensity), for a spectrum of eruptions, both ancient and modern, and effusive and explosive in style. This analysis allows comparison of events across the full spectrum of styles of volcanic activity.

Plinian and Subplinian Eruptions

Abstract: The term “plinian” encompasses powerful explosive eruptions characterized by the quasi-steady, hours-long, high-speed discharge into the atmosphere of a high-temperature, multiphase mixture (gas, solid, and liquid particles), forming a buoyant vertical column that reaches heights of tens of kilometers and often alternates with phases of column collapse. Subplinian eruptions have lower intensity but dynamics similar to plinian events, mainly distinguishing by the occurrence of high-frequency fluctuations or of temporary breaks in the discharge. In these eruptions, the repeated generation of short-lived convective plumes may alternate with phases of quiescence or of lower intensity, explosive or effusive activity.The complex dynamics of all these events is modulated by the conditions of magma withdrawal from crustal reservoirs, and ascent, fragmentation, and dispersal in the atmosphere. The accurate observation of the sedimentological and compositional features of the deposits is a powerful tool to quantify the main parameters which control these eruptions.

Stratigraphy, structure, and volcano-tectonic evolution of Solfatara maar-diatreme (Campi Flegrei, Italy)

Abstract: This study focuses on the Solfatara volcano within Campi Flegrei, a volcanic field located on the Tyrrhenian coast of southern Italy. Volcanism at the Campi Flegrei caldera has included phreatic to phreatomagmatic explosions and both magmatic (ranging from small scoria-producing events to those with Plinian columns) and effusive eruptions. These eruptions have formed tuff cones, tuff rings, minor scoria cones, and lava domes. A detailed stratigraphic, structural, and geophysical study of the area indicates that the Solfatara volcano is a maar-diatreme structure previously not recognized within the Campi Flegrei caldera. It is characterized by a crater cut into earlier volcanic deposits, a small rim of ejecta, and a deep structure (down to 2–3 km). This maar-diatreme has allowed the gases and fluids to flow up to the surface over a long time. A new geological map and cross sections show a complex architecture of different volcano-tectonic features including scoria cones, lavas, cryptodomes, feeder dikes, pipes, ring and regional faults, and explosive craters. Volcanological data were collected with the main aim of characterizing the eruptive activity in a limited sector of the caldera. Fault and fracture analyses, using the scan line methodology, highlight the role of the main structures that accompanied the volcanic evolution within this sector of the Campi Flegrei caldera. To better constrain the subsurface structure of the Solfatara crater, electrical resistivity tomography investigations were integrated with the volcano-tectonic information. All data suggest that the Solfatara area is dominated by a maar-diatreme evolution. Presently, the Solfatara area shows widespread hydrothermal and fumarolic activity that is localized along the major faults. The results allow us to define a particular type of volcanic activity in the recent past, in what is still considered today an area with a higher probability of opening new vents, particularly for possible phreatic activity.

Experiments with vertically and laterally migrating subsurface explosions with applications to the geology of phreatomagmatic and hydrothermal explosion craters and diatremes

Abstract: We present results of experiments that use small chemical explosive charges buried in layered aggregates to simulate the effects of subsurface hydrothermal and phreatomagmatic explosions at varying depths and lateral locations, extending earlier experimental results that changed explosion locations only along a vertical axis. The focus is on the resulting crater size and shape and subcrater structures. Final crater shapes tend to be roughly circular if subsurface explosion epicenters occur within each other’s footprints (defined as the plan view area of reference crater produced by a single explosion of a given energy, as predicted by an empirical relationship). Craters are elongate if an epicenter lies somewhat beyond the footprint of the previous explosion, such that their footprints overlap, but if epicenters are too far apart, the footprints do not overlap and separate craters result. Explosions beneath crater walls formed by previous blasts tend to produce inclined (laterally directed) ejecta jets, while those beneath crater centers are vertically focused. Lateral shifting of explosion sites results in mixing of subcrater materials by development of multiple subvertical domains of otherwise pure materials, which progressively break down with repeated blasts, and by ejection and fallback of deeper-seated material that had experienced net upward displacement to very shallow levels by previous explosions. A variably developed collar of material that experienced net downward displacement surrounds the subvertical domains. The results demonstrate key processes related to mixing and ejection of materials from different depths during an eruptive episode at a maar-diatreme volcano as well as at other phreatomagmatic and hydrothermal explosion sites.

Magmatic versus phreatomagmatic fragmentation: absence of evidence is not evidence of absence

Abstract: Fragmentation processes in eruptions are commonly contrasted as phreatomagmatic or magmatic; the latter requires only fragmentation of magma without external intervention, but often carries the connotation of disruption by bubbles of magmatic gas. Phreatomagmatic fragmentation involves vaporization and expansion of water as steam with rapid cooling and/or quenching of the magma. It is common to assess whether a pyroclast formed by magmatic or phreatomagmatic fragmentation using particle vesicularity, shape of particles, and degree of quenching. It is widely known that none of these criteria is entirely diagnostic, so deposit features are also considered; welding and/or agglomeration, particle aggregation, lithic fragment abundance, and proportion of fines. Magmatic fragmentation yields from rhyolite pumice to obsidian to basaltic achneliths or carbonatitic globules, making direct argument for magmatic fragmentation difficult, so many have taken an alternative approach. They have tested for phreatomagmatism using the fingerprints listed above, and if the fingerprint is lacking, magmatic fragmentation is considered proven. We argue that this approach is invalid, and that the criteria used are typically incorrect or incorrectly applied. Instead, we must consider the balance of probabilities based on positive evidence only, and accept that for many deposits it may not be possible with present knowledge to make a conclusive determination.

Chapter 26 – Magma–Water Interaction and Phreatomagmatic Fragmentation

Abstract: Extrusive submarine volcanism must be influenced by magma–water interaction. In the continental areas, due to groundwater and hydrous fluids, uprising magma frequently interacts with water. The major effect of magma–water interaction is an increase in thermal energy flux and induction of a special type of magma fragmentation, i.e., phreatomagmatic fragmentation. The products of this process are typical and discriminative: angular, blocky particles with diameters between 30 and 130 μm, and peculiar surface textures. Intensities of magma–water interaction strongly depend on the material and environmental parameters and on the contact dynamics. They range from mild effusion to extremely destructive explosive pulses.

Phreatomagmatic and Related Eruption Styles

Abstract: This chapter describes “wet” explosive eruptions, and their products, resulting from the combination of magmatic heat and “external water,” such as surface water or groundwater. Nine case studies illustrate some of the defining characteristics of wet eruptions, and how they can be recognized through direct observation and in the geological record. Water-influenced eruptions in nature occur well outside the boundaries of simplified definitions such as “phreatomagmatic,” “phreatic,” and “hydrothermal.” The case studies make it clear that we require more rigorous, comprehensive criteria to distinguish phreatomagmatic eruption styles. A second compelling need is comprehensive modeling of the thermodynamics of such eruptions.

Dynamics of the major plinian eruption of Samalas in 1257 A.D. (Lombok, Indonesia)

Abstract: The 1257 A.D. caldera-forming eruption of Samalas (Lombok, Indonesia) was recently associated with the largest sulphate spike of the last 2 ky recorded in polar ice cores. It is suspected to have impacted climate both locally and at a global scale. Extensive fieldwork coupled with sedimentological, geochemical and physical analyses of eruptive products enabled us to provide new constraints on the stratigraphy and eruptive dynamics. This four-phase continuous eruption produced a total of 33–40 km3 dense rock equivalent (DRE) of deposits, consisting of (i) 7–9 km3 DRE of pumiceous plinian fall products, (ii) 16 km3 DRE of pyroclastic density current deposits (PDC) and (iii) 8–9 km3 DRE of co-PDC ash that settled over the surrounding islands and was identified as far as 660 km from the source on the flanks of Merapi volcano (Central Java). Widespread accretionary lapilli-rich deposits provide evidence of the occurrence of a violent phreatomagmatic phase during the eruption. With a peak mass eruption rate of 4.6 × 108 kg/s, a maximum plume height of 43 km and a dispersal index of 110,500 km2, the 1257 A.D. eruption stands as the most powerful eruption of the last millennium. Eruption dynamics are consistent with an efficient dispersal of sulphur-rich aerosols across the globe. Remarkable reproducibility of trace element analysis on a few milligrammes of pumiceous tephra provides unequivocal evidence for the geochemical correlation of 1257 A.D. proximal reference products with distal tephra identified on surrounding islands. Hence, we identify and characterise a new prominent inter-regional chronostratigraphic tephra marker.

The Al-Du’aythah volcanic cones, Al-Madinah City: implications for volcanic hazards in northern Harrat Rahat, Kingdom of Saudi Arabia

Abstract: The basaltic Al-Du’aythah volcanic cones lie in the northern part of the extensive lava field of Harrat Rahat, and only 13 km from the centre of Al-Madinah City, in the Kingdom of Saudi Arabia. Historical records indicate they may have erupted in AD 641. The four cones are formed by deposits that record a transition from phreatomagmatic to magmatic explosions followed by minor lava effusion. Three cones display elongated tuff rings at the base, and two produced late-stage lava flows. The cones themselves are symmetrical and constructed mostly by the accumulation of ballistically ejected pyroclasts. Spherical bombs and lapilli (cannonball bombs/lapilli), occasionally with country-rock fragments inside (both cored and loaded bombs/lapilli) are common within the tuff ring deposits. LiDAR data show a total volume of 1,664 × 10−6 km3 for the four cones (418 × 10−6 km3 DRE). Whole-rock chemical analyses indicate alkali-basalt compositions (SiO2 44.7–45.9 wt%), with little compositional variation and no relationship between chemistry and eruptive styles. Small differences in composition may reflect variations in fractional crystallisation of clinopyroxene and olivine. A magnetotelluric 2D cross-section shows that the cones are located adjacent to a buried sediment-filled alluvial channel along a NNW-SSE fault dipping to the east. The Al-Du’aythah eruption was related to the ascent of magma through this structure, with the first phase of the eruption triggered by the interaction of the magma with water from the northern Harrat Rahat aquifer that exists in the Al-Madinah basin. This initial water source was rapidly exhausted, while the eruption progressed roughly from north to south and from west to east, the latter motion probably along the fault-controlled feeding dyke. Our work draws attention to the existence of recent explosive phreatomagmatic eruptions in the Al-Madinah basin, which, despite the hyperarid climate of the area, must be considered a potential future eruption hazard.

Volcanic Geotopes and Their Geosites Preserved in an Arid Climate Related to Landscape and Climate Changes Since the Neogene in Northern Saudi Arabia: Harrat Hutaymah (Hai’il Region)

Abstract: Maars and tuff rings are some of the most common volcanic landforms on Earth. They are inferred to be the product of the explosive interaction between rising magma (mostly basaltic) and various groundwater sources or surface water bodies. Maar and tuff ring volcanoes are commonly associated with extensive scoria cone fields that are fed by dispersed volcanic vents, providing access to the surface for magma over a long period of time (thousands to millions of years’ timescale). The presence of maar and tuff ring volcanoes, therefore, is an important signifier of the availability of water from sub-surface and/or surface water sources. As environmental conditions change over time, the groundwater table, as well as surface water availability, can change dramatically and this is likely be reflected in the type of volcanoes formed on the surface. Such changes are the most graphic and visible in volcanic fields that are today located in arid environments, where the presence of young volcanoes formed through interactions with water demonstrates how the environment can change over geological timescales. Therefore, these areas have high geoeducational values and can contribute to our understanding of how external (water sources controlled by climatic factors) and internal (magmatic) forces can shape the style of volcanism of a volcanic field. Harrat Hutaymah is one of the excellent locations where there is great abundance of maars and tuff rings. They are located in an area dominated today by various types of deserts. Harrat Hutaymah, therefore, demonstrates the global geological changes that can affect the style of volcanism and hence the resulting volcanic landscape. The richness of the region in archaeological sites and early settlements indicates the importance of this region for the early evolution of civilizations in the Middle East, which is likely to have been enhanced and/or modified by similar environmental changes over a much smaller timescale. Harrat Hutaymah provides a firm basis to demonstrate global changes through its volcanic heritage that are easily accessible and well exposed.

Maars to calderas: end-members on a spectrum of explosive volcanic depressions

Abstract: We discuss maar-diatremes and calderas as end-members on a spectrum of negative volcanic landforms (depressions) produced by explosive eruptions (note – we focus on calderas formed during explosive eruptions, recognizing that some caldera types are not related to such activity). The former are dominated by ejection of material during numerous discrete phreatomagmatic explosions, brecciation, and subsidence of diatreme fill, while the latter are dominated by subsidence over a partly evacuated magma chamber during sustained, magmatic volatile-driven discharge. Many examples share characteristics of both, including landforms that are identified as maars but preserve deposits from non-phreatomagmatic explosive activity, and ambiguous structures that appear to be coalesced maars but that also produced sustained explosive eruptions with likely magma reservoir subsidence. A convergence of research directions on issues related to magma-water interaction and shallow reservoir mechanics is an important avenue toward developing a unified picture of the maar-diatreme-caldera spectrum.

Eruptive history of a low-frequency and low-output rate Pleistocene volcano, Ciomadul, South Harghita Mts., Romania

Abstract: Based on a new set of K–Ar age data and detailed field observations, the eruptive history of the youngest volcano in the whole Carpathian-Pannonian region was reconstructed. Ciomadul volcano is a dacitic dome complex located at the southeastern end of the Calimani-Gurghiu-Harghita Neogene volcanic range in the East Carpathians. It consists of a central group of extrusive domes (the Ciomadul Mare and Haramul Mare dome clusters and the Koves Ponk dome) surrounded by a number of isolated peripheral domes, some of them strongly eroded (Balvanyos, Puturosul), and others topographically well preserved (Haramul Mic, Dealul Mare). One of the domes (Dealul Cetaţii) still preserves part of its original breccia envelope. A large number of bread-crust bombs found mostly along the southern slopes of the volcano suggest that the dome-building activity at Ciomadul was punctuated by short Vulcanian-type explosive events. Two late-stage explosive events that ended the volcanic activity of Ciomadul left behind two topographically well-preserved craters disrupting the central group of domes: the larger-diameter, shallower, and older Mohos phreatomagmatic crater and the smaller, deeper and younger Sf. Ana (sub)Plinian crater. Phreatomagmatic products of the Mohos center, including accretionary lapilli-bearing base-surge deposits and poorly sorted airfall deposits with impact sags, are known close to the eastern crater rim. A key section studied in detail south of Baile Tusnad shows the temporal succession of eruptive episodes related to the Sf. Ana (sub)Plinian event, as well as relationships with the older dome-building stages. The age of this last eruptive event is loosely constrained by radiocarbon dating of charcoal pieces and paleosoil organic matter at ca. 27–35 ka. The age of the Mohos eruption is not constrained, but we suggest that it is closely related to the Sf. Ana eruption. The whole volcanic history of Ciomadul spans over ca. 1 Myr, starting with the building up of peripheral domes and then concentrating in its central part. Ciomadul appears as a small-volume (ca. 8.74 km3) and very low-frequency and low-output rate volcano (ca. 9 km3/Myr) at the terminus of a gradually diminishing and extinguishing volcanic range. A number of geodynamically active features strongly suggest that the magma plumbing system beneath Ciomadul is not completely frozen, so future activity cannot be ruled out.

Submarine Explosive Eruptions

Abstract: No large submarine explosive eruption has ever been witnessed, yet such eruptions are known to be fairly common from deposits of ancient seafloors, from tiny eruptions recently witnessed by remote vehicle, and from when submarine explosions breach the ocean surface. Most of the seafloor comprises basaltic lavas, but on seamounts and even some seafloor spreading centers explosive eruptions have produced primary volcaniclastic deposits up to hundreds of meters thick. Particles are characteristically glassy, dense to highly vesicular, and in some cases comprise tiny folded glass sheets, the remnants of burst bubbles. The eruptions producing all these deposits are seafloor-specific, modulated both by the ocean environment and the extent of pre-eruption volatile exsolution and escape from magma. Surtsey’s birth in 1963 highlighted features of submarine basalt eruptions as they breach the ocean surface, while in Japanese waters Myojinsho’s 1957 eruption had showcased explosive submarine eruption of silicic magmas. The modern seafloor south of Japan, and in many other places in the geological past have hosted large-scale, caldera-forming explosive submarine eruptions of silicic magmas, and in 2012 the sea-surface expression of such a large-scale explosive eruption was identified for the first time ever. Our understanding of submarine explosive volcanism grows as the seafloor becomes better known, but much remains to be learned.

Construction of the North Head (Maungauika) tuff cone: a product of Surtseyan volcanism, rare in the Auckland Volcanic Field, New Zealand

Abstract: The Auckland Volcanic Field (AVF) comprises at least 52 monogenetic eruption centres dispersed over ∼360 km2. Eruptions have occurred sporadically since 250 ka, predominantly when glacio-eustatic sea levels were lower than today. Now that around 35 % of the field is covered by shallow water (up to 30 m depth), any eruption occurring in the present or near future within this area may display Surtseyan dynamics. The North Head tuff cone evidences eruptive dynamics caused by magma interaction with seawater. The first stages of the eruption comprise a phreatomagmatic phase that built a 48-m-high tuff cone. North Head tuff deposits contain few lithic fragments (<10 vol%) and are characterized by deposits from collapsing tephra jets and fall from relatively wet tephra columns. The conditions needed for this eruption existed between 128 and 116 ka, when the sea level in the Auckland area was at least 10–12 m above the pre-eruptive surface. The hazards associated with this type of eruption pose a risk to the densely populated coastal residential zones and the activities of one of the busiest harbours in New Zealand.

Mechanisms of Crater Lake Breaching Eruptions

Abstract: In this chapter we review physical models on phreatomagmatic, phreatic, hydrothermal, and geyser-like eruptions and, for the first time, place them in a crater lake context. Examples of known crater lake systems for the different eruption types are provided. Besides the direct injection of a fresh magma into a crater lake, leading to phreatomagmatic activity, a crater lake is a strong condensing medium, sensitive to sudden pressure changes when injected by gas-vapor batches, which can lead to non-magmatic, though violent eruptions. The implosive nature, the role of the heat pipe and molten sulfur pool at the lake bottom are central in the phreatic eruption model. Contrary to phreatic eruptions, hydrothermal eruptions are instigated by a sudden pressure drop, causing boiling and vapor release, rather than by the input of a gas-vapor phase of magmatic origin. Geyser-like activity beneath or near crater lakes is analog to classic geysering, and becomes more obvious when lake water level is low. Although not explosive, the peculiar lake drainage and refill cycles of two lakes are discussed. The first outcomes of numerical simulation approaches help to better quantify injection pressure and vapor/liquid proportions of the input fluid. We stress that the various manifestations of eruptive activity at crater lakes is not necessarily linked to changes in magmatic activity, which could lead to misleading interpretations regarding volcano monitoring.

The Surtsey Magma Series

Abstract: The volcanic island of Surtsey (Vestmannaeyjar, Iceland) is the product of a 3.5-year-long eruption that began in November 1963. Observations of magma-water interaction during pyroclastic episodes made Surtsey the type example of shallow-to-emergent phreatomagmatic eruptions. Here, in part to mark the 50(th) anniversary of this canonical eruption, we present previously unpublished major-element whole-rock compositions, and new major and trace-element compositions of sideromelane glasses in tephra collected by observers and retrieved from the 1979 drill core. Compositions became progressively more primitive as the eruption progressed, with abrupt changes corresponding to shifts between the eruption's four edifices. Trace-element ratios indicate that the chemical variation is best explained by mixing of different proportions of depleted ridge-like basalt, with ponded, enriched alkalic basalt similar to that of Iceland's Eastern Volcanic Zone; however, the systematic offset of Surtsey compositions to lower Nb/Zr than other Vestmannaeyjar lavas indicates that these mixing end members are as-yet poorly contained by compositions in the literature. As the southwestern-most volcano in the Vestmannaeyjar, the geochemistry of the Surtsey Magma Series exemplifies processes occurring within ephemeral magma bodies on the extreme leading edge of a propagating off-axis rift in the vicinity of the Iceland plume.

Stratigraphic relations of Santorini’s intracaldera fill and implications for the rate of post-caldera volcanism

Abstract: We propose a new volcanological interpretation of Santorini’s intracaldera fill deposits as revealed by seismic reflection profiles. This interpretation, along with supplementary gravity and geophysical studies, reveals three distinct volcaniclastic units (1–3). From top to bottom these units are attributed to (1) modern infilling sediment, (2) shallow marine phreatomagmatic volcanism associated with the relatively recent formation of the Kameni Islands, and (3) downfaulted ‘Minoan’ pyroclastic deposits, which formed during caldera collapse towards the end of the Bronze Age eruption. Estimates of the volumes of the seismic units and Kameni Islands yield a dense rock equivalent magma volume of 4.85 ± 0.7 km 3 . The average rate of volcanism over the past c . 3641 years is estimated at 1.3 ×10 −3 km 3 a −1 , and is similar to the rate since the AD 1707 eruption of 1.2 ×10 −3 km 3 a −1 based on historical lava volume estimates.

Long‐term evolution of an Oligocene/Miocene maar lake from Otago, New Zealand

Abstract: Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits near the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51 ± 0.24 and 23.38 ± 0.24 Ma (2σ). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17 ± 0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have been coeval with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic mineral concentration indicate a decrease in erosional input throughout the depositional period, suggesting long-term (tens of thousands of years) environmental change in New Zealand in the latest Oligocene/earliest Miocene.