Introduction: Maar-diatreme volcanism Martin, U., Németh, K., Lorenz, V., White, J.D.L. 2007 Journal of Volcanology and Geothermal Research 159 (1-3), pp. 1-3
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Introduction
This Special Volume on Maar-diatreme volcanism and associated processes follows from the Second International Maar Conference held in Lajosmizse-Kecskemet, Hungary. It was supported by the International Association of Volcanology and Chemistry of the Earth Interior (IAVCEI), the IAVCEI Commission on Volcanogenic Sediments and the International Association of Sedimentologists, and was organized by an international committee from Hungary, the Slovak Republic and Germany. A laboratory workshop in Wuerzburg, Germany and two parallel field trips to maar-diatreme volcanic fields in the Mio-Pliocene Pannonian Basin in Hungary and Slovak Republic accompanied the meeting.
Maar-diatreme volcanoes are commonly considered the phreatomagmatic equivalent of scoria cones. Whereas tuff cones form in shallow water bodies and in groundwater-rich environment maars, many surrounded by tuff rings, form in rather "normal" groundwater environments. Maars occur in volcanic fields, on ring plains surrounding composite cones and inside the calderas of polygenetic volcanoes. Only a few maars have erupted in historic times. Maars usually form when magma rises within a fissure and interacts with groundwater. If groundwater ceases to interact with magma at times during the eruption, continued magma ascent to the surface gives rise to a wide range of volcanic forms and products associated closely with maar-diatreme volcanoes. Maar-diatreme volcanoes are considered to be the second most common volcanic landforms in subaerial environments in many different geological settings. They are complex volcanic features despite their commonly small size and the small volumes of magma involved in their formation, and produce complex deposits both within and surrounding their craters. The records of maar eruptions commonly indicate switching between magmatic and phreatomagmatic processes in the conduit, varied transport processes in vertical plumes and lateral currents, and diverse depositional processes.
Diatremes are the substructures of maar volcanoes, and consist of inverted-cone-shaped volcanic structures, up to 2.5 km deep and up to 2 km in upper diameter, that are cut into pre-eruptive rocks. The structures are filled with clastic debris derived from the structure walls, juvenile fragments and subsided larger blocks, and they are typically cut by intrusive rocks. The volume of the diatreme fill is about the same as that of the thinly bedded tephra ring and distal ash deposits, so a diatreme is an important part of a maar-diatreme volcano in terms of deposit volume as well as of eruptive processes. The rather regularly cone-shaped diatremes extend at depth into root zones. The root zone of a maar-diatreme is irregular in shape and overlies the magmatic feeder dyke of the volcano. Maar-diatreme volcanoes may form from any magma type involved in volcanism, though they seem less commonly associated with evolved compositions. Depending on magma type and other geological variables, diatremes may contain diamonds or other commodities, they may be quarried for road metal, and may represent aquifers.
The below groundlevel deposits of maar-diatreme volcanoes also show complex textural features in their deposits, many of which are characteristic of products of magma–water interaction. Such features as peperite, which results from the interaction and mingling of magma and wet sediment, commonly exhibit a range of complex textures in association with many maar-diatreme volcanoes. The occurrence of peperite demonstrates contemporaneous volcanism and sedimentation; various peperites can provide insights into aspects of subsurface magma transport, magma fragmentation, host-sediment properties and the "pre-mixing" mechanisms of FCI (fuel–coolant interaction) explosions. Outpourings of lava may produce lava lakes in the maar depressions, and recent studies demonstrate that peperite commonly occurs at the contact of lava lakes with the tephra rings of maar and tuff ring/cone volcanoes, as well as in the subsurface surrounding these edifices. When the phreatomagmatic explosions of a maar-diatreme volcano finally come to an end, the crater typically fills up with water. Lavas erupted later, into fully subaqueous environments in lakes impounded within the maars themselves, are characterized by pillow lavas, hyaloclastite breccias and/or peperite breccias.
Maar lakes are deep in relation to their diameter and often isolated from the surroundings by the ejected material (ejecta-rim wall, or tuff ring). This characteristic architecture affects the lakes and their sediments, which often capture the material of an extremely small catchment area. So long as the ring of enclosing ejecta is not breached by erosion or overtopped by aggrading sediment in the surrounding area, allochthonous clastic material reaches the crater floor mainly as turbidity currents originating from the crater rim (ejected material). The autochthonous sediment in maar lakes is often dominated by algal material. Algal-bloom deposits alternate with background clastic sediment layers to create well-laminated deposits.
Tephra deposits of maar-diatreme volcanoes are commonly very similar texturally to subaerial tephras deposited from emergent Surtseyan-style eruptions, which are characterized by interaction of a fluid erupting magma with abundant external water. Surtseyan deposits, however, typically consist almost entirely of glassy fragments formed by fragmentation of the erupting magma and lack the significant country-rock component characteristic of deposits from maar-forming eruptions. This an important difference, and it indicates that in Surtseyan eruptions fragmentation occurs at very shallow levels in the edifice or/and as the magma emerges from it. If or when the erupting magma no longer encounters water (for instance if tephra encloses an emergent vent, or groundwater supplies to a maar-diatreme vent are depleted), both Surtseyan and maar-forming eruptions may transform to Strombolian or Hawaiian ones.
Volcanic activity in terrestrial settings does not always result in formation of a single volcanic edifice, and volcanic fields, especially basaltic ones, are common volcanic systems on Earth. Monogenetic volcanic fields are those in which individual volcanoes (mainly basaltic) commonly form during single episodes of volcanic activity, without subsequent eruptions; the volcanic field as a whole, however, may be active for millions of years. Fundamental aspects of volcanic fields that are the focus of current research include (1) the number, type and eruption history of individual vents; (2) the timing and recurrence rates of the volcanic eruptions in a given volcanic field; (3) the distribution of vents and volcanic complexes; and (4) the relationship of volcanic fields and the volcanoes within them to tectonic features such as basins, faults and rift zones.
In general, there are four major considerations in analysis of the ascent and emplacement of magma either on Earth or other planets, which are: (1) magma generation, (2) magma buoyancy, (3) rheological boundaries in the lithosphere and (4) density boundaries in the lithosphere. In addition to these factors, the stress field (local and regional) plays an important role in controlling magma ascent, with rising magmas generally exploiting favorable pathways related to the structural features of the lithosphere, such as fractures, thinned lithospheric domains, or zones and orientations of least compression.
In this volume, 18 papers are presented that demonstrate the variety of research on maar-diatreme volcanism today. The volume has been organised to first introduce the reader to the key concept of root zones as the "engine rooms" for maar-diatreme volcanoes, and their potential role in development of diamondiferous kimberlites. In this respect, it is among the first scientific works to present on equal footing papers from the two major research groups, one arguing for phreatomagmatism and the other a magmatic eruption style for diamond-bearing kimberlite maar-diatreme volcanism. Subsequent contributions successively address the origin of magmas involved in genesis of volcanic fields with maar-diatreme volcanoes, diatreme deposits and evolution of the diatreme structures, peperite formed by magma interacting with tephra and sediment in diatreme and crater environments, and features of tephra rings of maar ejecta and their depositional setting. Overall, the progression is from the subsurface to the surface, from an eruption's "engine room" to the surficial expression of activity there and its aftermath.
Specific contributions in the volume include field-based papers that analyze dyke and sill complexes as potential representatives of the shallow subsurface architecture of a maar-diatreme volcano, results from geophysical analysis that takes advantage of the low-density volcaniclastic infilling of diatremes to determine their scale and characteristics in the subsurface. New research into complex maar-diatreme volcanoes from Hungary, Israel, Mexico, Spain and Turkey is reported that reminds us that maar volcanoes can develop in many different geological and geographical settings. The link between maar-diatreme volcanism and shallow subaqueous eruptions is illustrated by a contribution concerning a Surtseyan eruption in Iceland that shows in its formation, depositional processes and resulting sedimentary record similarities with products of terrestrial maar-diatreme eruptions. Geochemical studies of boron concentration, and xenolith studies of maar-diatreme deposits help link the themes in this volume with those of other, more geochemically oriented, works. The volume closes with a review paper addressing the potential volcanic hazards posed by maar-diatreme volcanic fields and their constituent volcanoes.
We as Guest Editors wish to dedicate this volume to Alexander McBirney and Grant Heiken, each of whom has had a significant impact on our understanding of the evolution of volcanic fields, many of which host large numbers of maar-diatreme volcanoes. Each was also an early and influential proponent of phreatomagmatic eruptive processes in the origin of tuff rings, cones and maar-diatreme volcanoes.
28th February 2006
Wuerzburg (Germany), Palmerston North (New Zealand) and Budapest (Hungary), Dunedin (New Zealand)
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