Stephen Self

Tarawera 1886, New Zealand — A basaltic plinian fissure eruption

Abstract New Zealands biggest and most destructive volcanic eruption of historical times was that of Tarawera in 1886. The resulting scoria fall has a dispersal very similar in extent to that of the Vesuvius A.D. 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type. A new estimate of the volume (2 km 3 ) is significantly greater than previous estimates. The basalt came mainly from a 7-km length of fissure, and emission and exit velocity were fairly uniform along at least 4 km of it, this is one of the few documented examples of a plinian eruption from a fissure vent. Primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min −1 . A model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1.5–3%. Violent phreatic explosions occurred in the southwestern extension of the fissure across the Rotomahana geothermal field, and it is thought that some of the water responsible for the power of the plinian eruption came from this source, though its amount was not sufficient to turn the eruption into a phreatoplinian one.

Extension and rotation of crustal blocks in northern Central America and effect on the volcanic arc

Sinistral displacements across the North American–Caribbean plate boundary in northern Central America are distributed among major arcuate faults that have been active in Neogene time. South of the Jocotan and Motagua faults in Guatemala, extensional tectonics has accompanied rotation of the trailing edge of the Caribbean plate around these faults. Segmentation of the volcanic arc in northern Central America, hitherto attributed to transverse breaks in the subducting Cocos plate, may instead be a result of this block rotation. Accompanying changes along the arc are observed in seismicity, gravity anomaly patterns, volume and composition of volcanic products, and topography. Therefore, the complex volcano-tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding Caribbean plate above the magma production zone along the downgoing Cocos slab.

Widespread, lavalike silicic volcanic rocks of Trans-Pecos Texas

Several silicic units of the Trans-Pecos volcanic field have outcrop and thin-section scale features of lava flows but areal extents and aspect ratios of ignimbrites. These voluminous rocks (up to hundreds of cubic kilometres per unit) are quartz trachytes to low-silica rhyolites (68% to 72%SiO 2 ). Lava flow features include flow banding and folding, elongated vesicles, and autobreccias and vitrophyres at the base and top of units. Pyroclastic flow features include sheetlike geometry, lateral extents up to 70 km, aspect ratios as low as 1:700, and areal extents up to 3000 km 2 . A few of these units are clearly rheomorphic ignimbrites, but others show no unambiguous evidence of a primary pyroclastic origin. Although no adequate explanation currently exists for the origin of the latter, we evaluate two end-member hypotheses: (1) they are ignimbrites in which extreme rheomorphism has obliterated primary internal features, and (2) they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas. Either origin requires a rock type and eruptive mechanism not commonly recognized.

The relationship between volcanic eruptions and climate change: Still a conundrum?

What is the relationship between volcanic eruptions and climate change? More than 200 years after the connection was first proposed, it remains a thorny question. This article provides a brief historical overview of the problem and a review of the various data bases used in evaluating volcanic events and associated climatic change. We use the term “climate” to describe changes in the atmosphere over wide regions for periods of several months and longer. We use “weather” to describe shorter-term, variable atmospheric fluctuations experienced over more restricted areas. We appraise the present state of knowledge and highlight some pitfalls involved in using available information. Cautiously, we suggest future avenues for study, including the possibility of “volcanic winters,” or severe eruption-induced coolings.

Quaternary silicic pyroclastic deposits of Atitlán Caldera, Guatemala

Abstract Atitlan caldera has been the site of several silicic eruptions within the last 150,000 years, following a period of basalt/andesite volcanism. The silicic volcanism began with 5–10 km3 of rhyodacites, erupted as plinian fall and pyroclastic flows, about 126,000 yr. B.P. At 85,000 yr. B.P. 270–280 km3 of compositionally distinct rhyolite was erupted in the Los Chocoyos event which produced widely dispersed, plinian fall deposits and widespread, mobile pyroclastic flows. In the latter parts of this eruption rhyodacite and minor dacite were erupted which compositionally resembled the earliest silicic magmas of the Atitlan center. As a result of this major eruption, the modern Atitlan (III) caldera formed. Following this event, rhyodacites were again erupted in smaller (5–13 km3) volumes, partly through the lake, and mafic volcanism resumed, forming three composite volcanoes within the caldera. The bimodal mafic/silicic Atitlan volcanism is similar to that which has occurred elsewhere in the Guatemalan Highlands, but is significantly more voluminous. Mafic lavas are thought to originate in the mantle, but rise, intrude and underplate the lower crust and partly escape to the surface. Eventually, silicic melts form in the crust, possibly partly derived from underplated basaltic material, rise, crystallize and erupt. The renewed mafic volcanism could reflect either regional magmato-tectonic adjustment after the large silicic eruption or the onset of a new cycle.

Timing of events during the late Proterozoic Beardmore Orogeny, Antarctica: Geological evidence from the La Gorce Mountains

The Beardmore Orogeny previously has been designated for deformational and magmatic activity that occurred during the late Proterozoic in the central Transantarctic Mountains. It is recognized in folding of Beardmore Group turbidites, unconformably overlain by Cambrian limestones, and by silicic magmatism dated ∼650 Ma. Until this study, the relationship between the deformation and the magmatism had not been known. In the La Gorce Mountains, La Gorce Formation (Beardmore Group) was tightly folded during a regional metamorphic event producing biotite and limited axial-plane cleavage. The late Precambrian Wyatt Formation is a silicic porphyry with both volcanic and hypabyssal phases, possibly representing the eroded roots of a caldera complex. Wyatt Formation is conformably overlain by Ackerman Formation, a sequence of interbedded volcanics and shallow-marine sedimentary rocks. At a newly discovered locality, Wyatt Formation intrudes folded La Gorce Formation. This relationship demonstrates that the episode of folding and low-grade regional metamorphism was completed prior to the silicic magmatism of the Wyatt and Ackerman Formations. A contact between Ackerman and La Gorce Formations, which previously had been interpreted as conformable, has been shown by this study to be a fault. We advocate that the usage of the term “Beardmore Orogeny” be restricted to the deformational and metamorphic event and that the late Proterozoic magmatism be viewed as the initial stage in an episode of widespread silicic volcanism that continued during the Early and Middle Cambrian in the central Transantarctic Mountains.

Remote sensing of volcanos and volcanic terrains

In recent years, much progress has been made in the use of both satellite and aircraft remote sensing techniques to collect data on the dynamics of volcanic eruptions and on the interactions between volcanos and the atmosphere and ecosphere. Measurements made in the ultraviolet provide estimates of the mass of SO 2 released, while the hemispheric dispersal of eruption plumes can be tracked via weather satellites. Infrared images can be processed to produce temperature maps of lava flows and volcanic craters, and volumes of volcanic flows and cones can be measured via radar interferometry. Because the study of volcanos crosses many interdisciplinary boundaries, from geology and geophysics to atmospheric chemistry, climatology and ecology, the global perspective provided by satellite remote sensing techniques will become another valuable tool in the analysis of volcanos and their deposits.

Large wave forms from the Fish Canyon Tuff, Colorado

Wave-like bedforms of large amplitude and wavelength (6 m, 60 m) occur in an exposure at the base of the Fish Canyon Tuff, Colorado, an ignimbrite (ash-flow tuff) of great volume (> 3,000 km 3 ). The bedforms are located some 30–40 km from the center of the source caldera. They are interpreted to be deposits from a series of large pyroclastic surges, precursors to the main pyroclastic flows. The pyroclastic surge beds have bigger wave forms than those previously recognized. Such megawaves were perhaps produced by violent explosions related to major unroofing of a magma chamber. The depositional sequence of this great ignimbrite-forming event is similar to that of much smaller eruptions and implies a similar eruptive sequence.

Lava-dome growth and explosive volcanism in the Jemez Mountains, New Mexico: Evidence from the plio-pleistocene puye alluvial fan

Abstract The Plio-Pleistocene Puye Formation, north-central New Mexico, is a 200-km2 volcanogenic alluvial fan shed eastward from the Tschicoma volcanic center, part of the Jemez Mountains volcanic field. The fan contains > 15km3 of volcaniclastic material derived from closely spaced lava domes in the northeastern portion of the Tschicoma center. Interbedded in the fan sediments are at least 25 primary pyroclastic units from explosive eruptions of dacitic and rhyolitic lava domes. Tephra occur mainly as pumice falls but include several pumiceous ignimbrites and two thick proximal block-and-ash pyroclastic flows. The upper part of the fan also contains rhyolitic plinian deposits erupted from sources in the central portion of the Jemez field and basaltic ash derived from the central Rio Grande rift. Fanglomeratic (pyroclastic and epiclastic) facies exhibit considerable lateral variation. Primary tephra deposits, however, provide a stratigraphic framework for reconstruction of the growth of individual lava domes. While only volcanic domes and lava flows are exposed in the Tschicoma center, Puye tephra layers show that vulcanian, subplinian and plinian activity, and block-and-ash pyroclastic flows accompanied many dome-forming events. Degradation of these lava domes produced coarse-grained debris flows dominated by lava and dome carapace clasts. From these epiclasts it is possible to identify lithologies of lavas that are either currently buried or have been obliterated by erosion or by collapse of the Valles calderas. The preservation of Puye deposits reflects very high rates of aggradation and gradual down-faulting in the Espanola basin of the central Rio Grande rift. This record of pyroclastic and epiclastic deposition allows detailed interpretation of the evolution of a volcanic center from its pyroclastic and erosion products, and is the first example of a volcanogenic alluvial fan in an intracontinental rift setting to be described in detail. This study emphasizes that interpretation of any ancient volcanic terrain based solely upon data derived from mapping, radiometric dating, and chemical analysis of lavas or existing denuded edifices is likely to greatly underestimate both the role of explosive volcanism in the build-up of volcanic centers and subsequent estimates of the volumes of material produced.

Volcanic Winter? Climatic Effects of the Largest Volcanic Eruptions

Calculations suggest that the largest volcanic eruptions could have significant effects on global climate. We estimate the amount of sulfur volatiles that could have been released in very large eruptions by scaling up from smaller historical eruptions. The greatest well-known Late Quaternary explosive eruption, Toba (Indonesia, 75,000 years B.P.) erupted at least 1000 km3 of magma, and may have released enough sulfur volatiles to have formed 9 × 1014 to 5 × 1015 g of H2SO4 stratospheric aerosols. Basaltic fissure eruptions release even greater amounts of sulfur volatiles, which can be lofted into the stratosphere in convective plumes rising above fire fountains. The Roza flow eruption (about 700 km3 of magma) of the Miocene Columbia River Basalt Group could have produced up to 6 × 1015 g of aerosols. Distributed worldwide, these aerosol mass loadings would lead to effects ranging from a noticeable dimming of the sun to conditions similar to those described in some models of nuclear winter. Unless self-limiting mechanisms of stratospheric aerosol formation and removal are important, very large eruptions may lead to widespread darkness, cold weather, and acid precipitation. Even the minimum estimated effects of these great eruptions would represent significant perturbations of the global atmosphere.