Peter Kokelaar

A reappraisal of ignimbrite emplacement : progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrite

We propose a mechanism by which massive ignimbrite and layered ignimbrite sequences — the latter liable to have been previously interpreted as multiple flow units-form by progressive aggradation during sustained passage of a single particulate flow. In the case of high-temperature eruptive products the mechanism simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variations in texture, including between non-welded, eutaxitic, rheomorphic (lineated) and lava-like. Agglutination can occur within the basal part of a hot density-stratified flow. During initial incursion of the flow, agglutinate chills and freezes against the ground. During sustained passage of the flow, agglutination continues so that the non-particulate (agglutinate) layer thickens (aggrades) and becomes mobile, susceptible to both gravity-induced motion and traction-shear imparted by the overriding particulate part of the flow. The particulate to non-particulate (P-NP) transition occurs in and just beneath a depositional boundary layer, where disruptive collisions of hot viscous droplets give way, via sticky grain interactions, to fluidal behavior following adhesion. Because they have different rheologies, the particulate and non-particulate flow components travel at different velocities and respond to topography in different ways. This may cause detachment and formation of two independent flows. The P-NP transition is controlled by factors that influence the rheological properties of individual erupted particles (strain rate, temperature, and composition including volatiles), by cooling and volatile exsolution during transport, and by the particle-size population and concentration characteristics of the depositional boundary layer. At any one location along the flow path one or more of these can change through time (unsteady flow). Thus the P-NP transition can develop momentarily or repeatedly during the passage of an unsteady flow, or it can occur continuously during the passage of a quasi-steady flow supplied by a sustained explosive eruption. Vertical facies successions developed in the deposit (high-grade ignimbrite) reflect temporal changes in flow steadiness and in material supplied at source. The P-NP transition is also influenced by factors that affect flow behaviour, such as topography. It may occur at any location laterally between a proximal site of deflation (e.g. a fountain-fed lava) and a flows distal limit, but it most commonly occurs throughout a considerable length of the flow path. Up-sequence variations in welding-deformation fabric (between oblate uniaxial to triaxial and prolate) reflect evolving characteristics of the depositional boundary layer (e.g. fluctuations from direct suspension-sedimentation to deposition via traction carpets or traction plugs), as well as possible modifications resulting from subsequent, post-depositional hot loading and slumping. Similar processes can also account for lateral lithofacies gradations in conduits and vents filled with welded tuff. Our consideration of high-grade ignimbrites has implications for ignimbrite emplacement in general, and draws attention to the limitations of the widely accepted models of emplacement involving mainly high-concentration non-turbulent transport and en masse ‘freezing’ of high-yield-strength plug flows.

Sector collapse, sedimentation and clast population evolution at an active island-arc volcano: Stromboli, Italy

Sciara del Fuoco is the subaerial part of a partially filled sector-collapse scar that extends to 700 m below sea level on Stromboli volcano. The collapse occurred <5000 years ago, involved ≤1.81 km3 of rock and is the latest of a series of major collapses on the north-west flank of Stromboli. A north-east trending arc-axial fault system channels magmas into the volcano and has caused tilting and/or downthrow to the north-west. The slope of the partial cone constructed between the lateral walls of the collapse scar acts as a channelway to the sea for most eruptive products. From 700 m below sea level and extending to >2200 m and >10 km from the shore to the NNW, a fan-shaped mounded feature comprises debris avalanche deposits (>4 km3) from two or more sector collapses. Volcaniclastic density currents originating from Sciara del Fuoco follow the topographic margin of the debris avalanche deposits, although overbank currents and other unconfined currents widely cover the mounded feature with turbidites. Historical (recorded) eruptive activity in Sciara del Fuoco is considerably less than that which occurred earlier, and much of the partial fill may have formed from eruptions soon after the sector collapse. It is possible that a mass of eruptive products similar to that in the collapse scar is dispersed as volcanogenic sediment in deep water of the Tyrhennian basin. Evidence that the early post-collapse eruptive discharge was greater than the apparent recent flux (≈2kg/s) counters suggestions that a substantial part of Strombolis growth has been endogenous. The partial fill of Sciaria del Fuoco is dominated by lava and spatter layers, rather than by the scoria and ash layers classically regarded as main constituents of Strombolian (‘cinder’) cones. Much of the volcanic slope beneath the vents is steeper than the angle of repose of loose tephra, which is therefore rapidly transported to the sea. Delicate pyroclasts that record the magmatic explosivity are selectively destroyed and diluted during sedimentary transport, mainly in avalanches and by shoreline wave reworking, and thus the submarine deposits do not record well the extent and diversity of explosive activity and associated clast-forming processes. Considerable amounts of dense (non-vesicular) fine sand and silt grains are produced by breakage and rounding of fragments of lava and agglutinate. The submarine extension of the collapse scar, and the continuing topographic depression to >2200 m below sea level, are zones of considerable by-passing of fine sand and silt, which are transported in turbidity currents. Evidently, volcanogenic sediments dispersed around island volcanoes by density currents are unlikely to record well the true spectrum and relative importance of clast-forming processes that occurred during an eruption. Marine sedimentary evidence of magmatic explosivity is particularly susceptible to partial or complete obliteration, unless there is a high rate of discharge of pyroclastic material into the sea.

Tectonically controlled piecemeal caldera collapse: A case study of Glencoe volcano, Scotland

Caldera collapse at Glencoe, Scotland, was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned. Intracaldera depocenters had the form of grabens or half grabens, locally with downsag and bounding monoclinal flexures, and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history. This piecemeal caldera collapse is in contrast to previous interpretations, in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault. Collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast. A dominant northwest-trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through Glencoe. The main graben was transected orthogonally by two cross grabens. Each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity, which built tuff cones, followed by magmatic fountaining that produced lava-like silicic ignimbrites. The three lava-like ignimbrites total more than 300 m thick. Two later eruptions produced eutaxitic silicic ignimbrites, together more than 300 m thick, and contemporaneous progressive downsag was associated with the formation of extensional fractures (crevasses) hundreds of meters deep. Sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence. Unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and/or lacustrine deposition between each ignimbrite eruption. The changes in drainage and sedimentation, and the development of coarse debris-avalanche breccias, reflect tectonic faulting during the periods between major eruptions. The duration of the period including the five caldera-forming eruptions was probably ∼0.5 m.y., and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments (>0.5 km/m.y. normal displacement) is similar to that of the most actively subsiding sedimentary basins. Glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems, which, with knowledge that caldera volcanoes commonly overlie faults, suggests that piecemeal calderas may be more common than previously recognized. The research also shows that the emplacement of granites (s.l.) was episodic rather than synchronous throughout the magmatic province, and it suggests that numerous plutons in the province, e.g., in Donegal, Ireland, may have formed at sites of central volcanoes that are no longer preserved.

Subaqueous Explosive Eruption and Welding of Pyroclastic Deposits

Silicic tuffs infilling an ancient submarine caldera, at Mineral King in California, show microscopic fabrics indicative of welding of glass shards and pumice at temperatures >500�C. The occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water, which would cause chilling. Intracaldera progressive aggradation of pumice and ash from a thick, fast-moving pyroclastic flow occurred during a short-lived explosive eruption of ∼26 cubic kilometers of magma in water ≥150 meters deep. The thickness, high velocity, and abundant fine material of the erupted gas-solids mixture prevented substantial incorporation of ambient water into the flow. Stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general, both above the vent and along any flow path, may be the main process giving rise to buoyant-convective subaqueous eruption columns and attendant fallout deposits.

Giant bed from a sustained catastrophic density current flowing over topography: Acatlán ignimbrite, Mexico

A giant bed of ash and pumice (≤ 80 m thick and covering >300 km 2 ) in central Mexico demonstrates that thick clastic beds lacking sedimentary structures can aggrade incrementally from the base of sustained density currents. Like similar giant beds with matrix-supported clasts elsewhere, it had been interpreted as having formed en masse from a giant flow with high yield strength. However, intergradational compositional zones within the bed record changes in the material supplied at the source over time; the zones show that the density current was initially topographically restricted and that valleys progressively filled with deposit so that later parts of same density current were able to pass over and bury successively higher ground. This example provides a key to understanding the origin of massive parts of ignimbrites, megaturbidites, lahar deposits, and some debris-flow deposits. It shows that (1) an absence of sedimentary structures cannot be used to infer near-instantaneous deposition by en masse “freezing” or “collapse” of a giant flow, and (2) the thickness and vertical organization of a giant bed tell us little about thickness, vertical organization, and rheology of the current that produced it.

Marine emplacement of welded ignimbrite: the Ordovician Pitts Head Tuff, North Wales

The Pitts Head welded ignimbrite records a subaerial, large‐volume eruption and associated pyroclastic current that flowed into the sea. The current 15 km from source was steadily sustained with high mass flux and high particle concentration towards its base, and it entered the sea without substantial mixing with water and thus without large‐scale hydroexplosivity and without general cooling. It continued to aggrade ignimbrite at >580°C for at least 3–4 km from the original shoreline, in water initially ≥50 m deep. Entirely subaqueous, hot‐state, progressive aggradation and welding of the ignimbrite occurred where the water could not be wholly displaced by the current, although eventually the deposit displaced the shore >4 km seawards. Wet sea‐floor sediments buried by the ignimbrite were heated and locally fluidized by steam, and several square kilometres of hot ignimbrite with variable thicknesses of sedimentary substrate detached and slid downslope. Directions of sliding and the order of piling‐up of slide sheets are shown by hot‐state (rheomorphic) deformation fabrics and the geometric relations of detachment surfaces. Extensional disruption of the ignimbrite is marked by breaks in the sheet via which fluidized sediments were mobilized, locally to form rootless vents. Both the initial incursion of the pyroclastic current into the sea and the subsequent submarine sliding of the ignimbrite are likely to have caused tsunamis. Similar occurrences at modern coastlines presently susceptible to incursion of high mass flux pyroclastic currents (e.g. Taupo Volcanic Zone, New Zealand; Neapolitan region, Italy; vicinity of Manila (Taal), Philippines) would, according to tsunami propagation behaviour, cause significant near‐and far‐field coastal hazard.

Tectonic influences in piecemeal caldera collapse at Glencoe Volcano, Scotland

Glencoe Volcano is renowned as the archetype for caldera collapse that results from piston-like subsidence of a coherent crustal block on a ring fault. However, the caldera-floor rocks, and the geometry of seven intracaldera units that record large volume explosive eruptions, show that the collapse involved incremental and haphazard subsidence of numerous crustal blocks before development of the ring fault. Some subsidence involved flexure (downsag) with associated development of extensional crevasses hundreds of metres deep. A system of orthogonal faults and related grabens records the influence of two intersecting basement discontinuities, one of which is inferred to have linked with the active Great Glen fault. Substantial extensional and/or transtensional faulting, with distinct related sedimentary responses, occurred without eruptions. The tectonic framework substantially controlled the magmatic plumbing and the locations of vents, caldera depocentres, and through-going rivers. The related sediments influenced contrasting eruption styles (e.g. transitions between phreatomagmatic and magmatic) and the shallow-level emplacement of sills.

Ordovician marine volcanic and sedimentary record of rifting and volcanotectonism: Snowdon, Wales, United Kingdom

Wales was part of a continental convergent plate margin for ∼50 m.y. during Ordovician time. The crustal stress regime was one of sinistral transtension, due largely to oblique subduction. Early uplift and subsequent growth of subaerial arc volcanoes were followed by development of a broad, fault-bound, marine basin (>150 km wide) in an infra-arc or back-arc setting. Within the basin, relatively narrow grabens became sites of pronounced subsidence and bimodal (basalt-rhyolite) volcanism. The grabens developed in fault-splays above steep, crust-penetrating discontinuities that focused extension and plumbing of magmas at these sites. At Snowdon, the Bedded Pyroclastic Formation records submarine and island basaltic volcanism with associated sedimentation at the site of an active rhyolitic caldera volcano. The volcanoes formed on the axis of the Snowdon Graben, and the basaltic succession, which contains several unconformities due to marine abrasion at sea level, shows a complex history of alternate uplift and subsidence. The uplift is attributable to resurgence of rhyolitic magma and was >336 m in total. Subsidence, due mainly to crustal extension, was >500 m at a rate of ≥500 m/m.y. The Bedded Pyroclastic Formation shows clear evidence of tectonic influence in the timing, location, and style of basaltic magmatism. Repeated reactivation of magmatic plumbing systems demonstrates fault- or fault-intersection controlled channeling of magmas in the upper crust. Episodes of relatively vigorous subsidence preceded copious magmatism, and both intrusive and extrusive basalt magmas exploited contemporaneously active normal faults. Pronounced topographic ponding of extrusive products, and highly variable trends of fault-block subsidence and rotation evident from the volcanogenic sedimentary deposits, testify to chaotic (piecemeal) displacements on faults spaced commonly at

Friction melting, catastrophic dilation and breccia formation along caldera superfaults

At Glencoe caldera volcano, friction melts and magmas transformed explosively to froth or spray where they encountered rapid decompression in dilatant sections of superfaults. The friction melts were blasted upwards, plastering free surfaces, and were rapidly followed by fragmented magma and then liquid magma that formed intrusions. Irregular contacts of fault intrusions record explosive disruption of hydrothermal systems cut by the dilational faults, with lithic breccias removed from the rapidly formed cavities before the arrival of melt spray. Layers of lithic breccia are common in ignimbrites around calderas and usually show incorporation of hot and hydrothermally altered rocks. Such layers may specifically reflect initial superfault dilation during caldera collapse.

The timing of Ordovician magmatism in the English Lake District and Cross Fell inliers

Volcanic and hypabyssal intrusive rocks of the Lower Palaeozoic English Lake District and Cross Fell inliers are elements of the Ordovician destructive plate margin system of microcontinental Avalonia. Two igneous sheets within the marine sedimentary Skiddaw Group of these inliers, previously described as lavas, are reinterpreted as sills. Sedimentary rocks enclosing these sills are of late Tremadoc-early Arenig (c . 493 Ma) and early Llanvirn (> 476 Ma) age, and breccias along the upper contacts of both were produced by steam explosivity and fluidization ahead of theadvancing tips of the intrusions. Previous interpretation of the breccias on the older sheet, as sediment deposited on the eroded top of a lava flow, implied an early Ordovician onset of arc magmatism. Such early magmatism would have been virtually coincident with the latest Tremadoc initiation of arc magmatism in Wales, but evidence for such a near synchronous response tothe putative onset of subduction is lacking. Respective onsets of magmatismwere probably separated by at least 17 m.y., and possibly by as much as 29 m.y. The apparent contemporaneity of mid and late Ordovician volcanic episodes in England and Wales, and similarities in extensional tectonic style, suggest that the two areas then were part of the same subduction system responding similarly to plate-scale magma-generating and tectonic processes. The early Ordovician situation is uncertain, but the absence of arc volcanic rocks of this age in the English Lake District suggests that this area and Wales are not tectonically juxtaposed elements of a former simple linear arc.