V31A-01 INVITED
What is a volcano?
In a volcano, magma, generated at a source in a planetary interior, flows upward with varying amounts of physicochemical evolution, intruding the encasing rocks. Once near the top of the lithosphere, that is at a major rigid-fluid, high-low-density interface, the magma erupts, piercing this interface. While gas, vapors and thinnest particles mix up with the atmosphere and stratosphere, larger drops and particles will eventually accumulate on top of the interface to form volcanic deposits, giving rise in the area around the crater to a volcanic edifice. In turn, these deposits may be intruded or modified by magma, eruptions, geothermal fluids, tectonics, erosion, landsliding and all other kinds of geologic processes. In this view, volcanism is a self-similar process that ranges many orders of magnitude in space and time scales from small cinder cones to large ocean ridges. For instance, at Amiata Volcano, Italy, many of the above mentioned processes have interacted. The volcano is deeply dissected by volcanic spreading, to the point of loosing its original cone-like shape; large diapirs and thrust-related structure have formed in the clay- and gypsum-rich substratum all around the volcano generating dismembered lava flows. In addition, the spreading have created the conditions for the existence of mercury mineralization and geothermal reservoirs. All this complexity that, in our opinion must be considered volcanic is not easy described by commonly used definitions of volcanoes. In fact, former definitions of "volcano", for instance that from the Glossary of Geology (1997) "a vent in the surface of the Earth through which magma and associated gases and ash erupt" or "the form or structure, usually conical, that is produced by the ejected material" are clearly insufficient and cannot capture the geologic complexity of a real volcanic environment. All definitions, that we encountered, tend to consider volcanoes from the point of view of a single discipline, each of them neglecting relevant aspects belonging to other disciplines. For the two cases mentioned above a volcano is seen only from the point of view of eruptive activity or of morphology. We attempt to look at "volcano" holistically to provide a more comprehensive definition. We define a volcano as a geologic environment that, at any scale, is characterized by three elements: magma, eruption and edifice. It is sufficient that only one of these elements is proven, as long as the others can be inferred to exist, to have existed, or that will exist.
V31A-02
From Definition of „Volcano" to Conceptual Volcanology
Volcanology textbooks either use the classical hole-in-the-ground definition of a volcano or simply avoid a definition. Here I consider a possible systemic redefinition of the term volcano. Starting from the classical requirements of a definition, it is shown that only a definition which is part of a hierachically organised system of definitions can be accepted, because Nature is self-organised in hierachical entities, thus conceptual constructs should reflect the same type of organisation. Such a line of reasoning implies that a volcano should be defined by making an explicit mention of the hierarchy of systems to which it belongs. Therefore, volcano can be defined as either a subsystem (i.e. the eruptive subsystem) of the broader igneous system or as a particular type of igneous system (i.e. one reaching the surface of Earth). A volcano, viewed as a volcanic system, is composed of a magma-generation subsystem, a magma transport subsystem, magma storage subsystem(s), and an eruptive subsystem. The accurate definition and identification of each subsystem should allow distinction between individual volcanoes in both space and time. Minimal conventional requirements need to be agreed upon by volcanologists to identify and recognise a particular volcano from other volcanoes (including those partially occupying the same space but separated in time, or those partially overlapping in both space and time). For example, two volcanoes can be distinguished from each other if at least two of their respective subsystems are not shared, one of them being the shallowest magma-storage subsystem. Accurate definition of volcano using the systemic approach involves definition of other basic terms and concepts of volcanology in a similar way, eventually resulting in a hierarchical system of definitions that would lend volcanology a solid, consistent and coherent conceptual core increasing its scientific maturity. Conceptual volcanology can be envisaged as addressing the issue of accurate definition of basic terms and concepts in volcanology aiming to reach the conceptualisation level of more basic sciences.
V31A-03 INVITED
Beyond Earth: How extra-terrestrial volcanism stretches our definition of a volcano
Volcanism is a fundamental geologic process that has affected every solid body in the solar system and, presumably, in other solar systems as well. As we explore other worlds, we come across signs of active and past volcanism, some in unexpected places. Volcanism in extraterrestrial worlds can be much different from the examples we see on Earth, but the similarities are also striking. Understanding the eruption mechanisms on other planets is important for better constraining how eruptions behave on Earth under present and past conditions. The discovery of numerous extra-terrestrial volcanoes, including active ones, has stretched our traditional definition of what is a volcano. Prior to the Voyager 1 and 2 spacecraft observations during the late 1970s and early 1980s, the Earth was the only planet known to have active volcanism. When Voyager 1 found active volcanism on Jupiter's moon Io, our understanding of active volcanism, and what causes it, dramatically changed. Io's volcanism is driven by tidal dissipation, fundamentally different from what causes volcanism on Earth. To date, no planet outside the Earth shows evidence of plate tectonics. Despite these differences, the eruption styles and products on other planets show great similarity to Earth's. Voyager went on to observe geysers on Neptune's moon Triton, showing the first evidence of cryovolcanism, a process that has no terrestrial analogue but which appears to be widespread in the outer solar system. The Galileo spacecraft showed that relatively recent cryovolcanic activity may have occurred on Europa and Ganymede. In the last few years, the Cassini spacecraft showed dramatic active plumes on Saturn's moon Enceladus. Features thought to be volcanic have been shown to exist on Titan's very young surface, raising the possibility that active or recently active cryovolcanism may have been present there. As we continue our exploration of the solar system, we need to redefine the fundamental geologic processes using a planetary rather than terrestrial context.
V31A-04
Gigantic self-confined pahoehoe inflated lava flows in Argentina
The largest lava flows on Earth are pahoehoe basalts emplaced by inflation, a process which can change lava lobes initially a few decimetres thick into large lava sheets several metres thick. Inflation involves the initial formation of a thin, solidified, viscoelastic crust, under which liquid lava is continually added. This thermally efficient endogenous growth process explains the spread of huge volumes of lava over large, almost flat areas, as in the sheet flows which characterise the distal portions of Hawaiian volcanoes or some continental flood basalt provinces. Long, narrow, inflated pahoehoe flows have occasionally been described, either emplaced along pre-existing river channels or confined within topographic barriers. In this contribution we present previously unknown inflated pahoehoe lava flows following very long, narrow pathways over an almost flat surface, with no topographic confinement. Lava, which erupted in Late Quaternary times from the eastern tip of a 60 km long volcanic fissure in Argentina, formed several discrete flows extending as far as 180 km from the source. This fissure was characterized by a long-lasting and complex activity. Alkali-basaltic lava flows were emitted at the two extremities of the fissure system. In the intermediate section of the fissure, the Payun Matru, a great trachitic composite volcano, developed, giving rise to a large caldera which produced large pyroclastic flows. Alkali-basalts predate and postdate the trachitic activity, in fact at the end of the trachitic activity, new basaltic lava flows (mainly aa) were emitted from both ends of the fissure. We studied in details the youngest of the gigantic flows (Pampas Onduladas lava flow), which progressively develops through differing thermally-efficient flow mechanisms. The flow created a large shield volcanic structure at the eastern tip of the E-W fissure and spread to the E forming a very large and thick inflated pahoehoe sheet flow. Leaving the flanks of the volcano, the flow spreads all over a large tectonic depression, forming a large inflated pahoehoe sheet flow. The flow continues downstream, always showing typical inflation features, forming a very long and narrow tongue, developed over the nearly flat Pampa plain (gradient 0.5%) with an average width of 3 km and a length of 120 km. A peculiar feature of this portion of the flow, apart from its exceptional length, is the very low width-to-length ratio. This is even more surprising if we consider that no pre-existing topographic feature (e.g. river channel, etc.) is responsible for this behaviour, which appears to be only the result of some kind of self-confinement mechanism. The structural, morphological and eruptive complexities of this volcanic structure are exceptional by themselves since there are no similar features both in the Andes calcalkaline volcanism or in the Patagonian basaltic plateaus and they pose problems even in the nomenclatural definition of the Payun Matru as an individual volcanic construct. Moreover, understanding the mechanisms responsible for the exceptional behaviour of this lava flow may provide new constraints on the physics of inflated pahoehoe flow emplacement. Results in this direction may also offer useful proxies for interpreting volcanic processes on terrestrial planets such as Mars and Venus, on which individual lava flows of similar shape and dimensions have been observed.
V31A-05 INVITED
Megatsunamis from Flank Failures in Hawaii: Determining Recurrence and Mapping Paleorunup from Tsunami Deposits
McMurtry et al. (2004a) proposed that calcareous conglomerate deposits on Kohala volcano on Hawaii were tsunami deposits. Since Hawaii is known to be subsiding, other mechanisms for emplacement of these deposits are unlikely. Similar deposits on Lanai, Maui and Molokai have also been postulated to be tsunami deposits, but this idea is not totally accepted. If, as we have already demonstrated for Kohala, these deposits are demonstrated to be from tsunamis, we shall then have the information necessary to hindcast the mechanics of specific megatsunamis that have impacted the southeastern Hawaiian islands. Since our initial work we have collected more samples of in situ coral clasts from Kohala as well as from southern Lanai, Molokai, and West Maui. Our additional Kohala samples display two age extremes. One least-altered sample, with an apparent age of 112±2 ka, extends our initial linear trend of calcite abundance versus apparent age for Kohala. The other sample, with an apparent age of 170±3 ka, appears to date a much older reef that was brought up to the Kohala deposit site. We are convinced that southern Lanai experienced megatsunamis at ~120 ka and ~240 ka, thus reconciling Moore and Moore (1984) with Rubin et al. (2000). These tsunamis entrained calcareous material from the respective stage 6 and 8 apex aged reefs and left evidence for multiple waves; two for the 120 ka event and three for the 240 ka event. Runups on southern Lanai were at least to 180 m; using the 1.9 mm/yr subsidence rate of Campbell (1986) suggests a minimum runup to 408 m, or to 554 m using the 326-m site of Stearns (1978). These runups exceed even our largest simulation results for Lanai (McMurtry et al., 2004b). Another unexpected result of the new U-series dating is apparent ages of 111±1.2 ka and 107±0.9 ka for two generations of coral from a composite in situ clast on West Maui that are stratigraphically reversed. This result is not inconsistent with the relationship of decreasing apparent age with calcite abundance found for Kohala, however. One sample from Lanai, with an apparent age of only 100±2 ka and no detectible calcite, is harder to explain, although this sample had a distinct reddish color from fine included iron oxides that may have affected the apparent age. Strontium isotope analyses were performed on the Kohala samples to test if this method could delineate the origin of the carbonates in Units 2 and 3. The samples were subjected to a series of mild acetic acid leaches. We also included a sample of obvious caliche from a vein in a nearby Hawi andesite flow. The results show the relative seawater origin for the marine fossiliferous Unit 2, the rock origin for the caliche, and a mixture of seawater and terrestrial origins for the Sr in the carbonate matrix of afossiliferous Unit 3. Unit 3 is clearly a stratigraphically-older, carbonate- cemented basaltic conglomerate with palesol features in many places. The apparent complete lack of marine fossils suggests alteration, consistent with its greater age and with the interpretation of this deposit as a mixed seawater-terrestrial origin, or, more likely, an original seawater origin with subsequent terrestrial overprinting (soil formation). A working hypothesis is this massive, enigmatic deposit on Kohala represents the 240 ka megatsunami event found on Lanai and apparently exclusively on Molokai.
V31A-06
Causal link between Quaternary paleoclimatic changes and flank collapses on volcanic islands
Giant landslides and resulting tsunamis represent the main geologic hazards linked to volcanic island evolution. From bathymetric data and on-land geological studies, flank failures have been identified around numerous volcanic islands, in most geodynamic contexts. However, the enabling and triggering conditions are still poorly understood and several internal and external causes may act simultaneously to reach a critical threshold. We here present a compilation of well-dated flank destabilization events within the last 1 Myr from Tahiti, Hawaii, Canary Islands, Guadeloupe and Martinique (Lesser Antilles), and examine their relationships with global paleoclimatic changes evidenced by a global stack of benthic ?18O records. We show that a causal relationship between flank collapse of volcanic islands and global climatic changes has existed at least since 900 kyr. Moreover, high precision ages reported here favor the hypothesis that major flank collapse events occurred during the onset of glacial to interglacial transitions when a sudden influx of melt water from polar ice caps causes rapid sea level rise. We propose that following a sub aerial erosion interval during low sea level stands, rapid sea level rise induces enhanced coastal erosion and sudden changes of pore pressure conditions within basal layers, which favor edifice failure.
V31A-07 INVITED
Collapse calderas
A collapse caldera is a volcanic explosive structure that forms during the collapse of crustal blocks on top of a shallow magma chamber. During this collapse, a large volume of magma is evacuated, first explosively, in the form of pyroclastic fallouts and pyroclastic flows, and then effusively, as lava domes or flows after collapse. The result is a catastrophic explosive volcanic collapse that forms a depression that could end with different shapes, circular, oval, rectangular, or irregular. Three main types of collapse calderas can be defined, 1) summit caldera, 2) classic caldera, and 3) graben caldera. Summit calderas are those formed at the top of large volcanoes and are related to relatively small-volume pyroclastic products that include plinian fallouts and ignimbrites, such as Crater Lake, Las Cañadas, and Somma-Vesuvio. Classic calderas are semi-circular to irregular-shaped large structures, several km in diameter that are related to relatively large-volume pyroclastic products including pumice fallouts and widespread ignimbrites, such as Long-Valley, Campi Flegrei, and Los Humeros. Graben calderas are explosive volcano-tectonic collapse structures from which large-volume, ignimbrite-forming eruptions occurred through several vents along the graben walls and the intra-graben block faults causing the collapse of the graben or of a sector of the graben. The main products of graben calderas are surge-deposits and large-volume widespread ignimbrite sheets. Pumice fallouts are practically absent. Examples include the Sierra Madre Occidental in Mexico, La Pacana (Andes), Catalan Pyrenees, and perhaps Scafell (United Kingdom). Any of the three caldera types mentioned above could have collapsed in three different ways, 1) piston, when the collapse occurs as a single crustal block; 2) trap-door, when collapse occurs unevenly along one side while the opposite side remains with no collapse; 3) piece-meal, when collapse occurs as broken pieces of the crust on top of the magma chamber.
V31A-08
How Deep can be a Dyke?
Magma transport through dykes is a fundamental component of volcanic activity. Due to limited exposure in the field, however, the depth to the magma source that fed a given dyke might remain undeterminable. Based on the width to length ratio determined from horizontal exposures, one could conclude that some dykes might have extended from magma sources 250 km deep to the surface, therefore providing a temporal hydraulic connection between both extremes. On the other hand, dykes might be conceived as buoyant cavities of more or less fixed dimensions that ascend through the lithosphere. In this case, dykes are predicted to have a much more limited vertical extent. Both of these conceptual models serve to explain equally well the existence of deep crust or upper mantle xenoliths in erupted products, as such xenoliths only require fast enough magma ascent rates for their preservation, and this can be achieved in both models. In this work the assumptions made in both conceptual models of a dyke are examined with detail. Although physically plausible ranges for dyke heights are obtained, it is shown that an answer to the question of how deep can be a dyke ultimately resides in a subjective choice of the conceptual model.