On magma fragmentation by conduit shear stress: Evidence from the Kos Plateau Tuff,Aegean Volcanic Arc

Large silicic explosive eruptions are the most catastrophic volcanic events. Yet, the intratelluric mechanisms underlying are not fully understood. Here we report a field and laboratory study of the Kos Plateau Tuff (KPT, 161 ka, Aegean Volcanic Arc), which provides an excellent geological example of conduit processes that control magma vesiculation and fragmentation during intermediate- to large-scale caldera-forming eruptions. A prominent feature of the KPT is the occurrence of quite unusual platy-shaped tube pumice clasts in pyroclastic fall and current deposits from the early eruption phases preceding caldera collapse. On macroscopic and SEM observations, flat clast faces are elongated parallel to tube vesicles, while transverse surfaces often occur at ~ 45° to vesicle elongation. This peculiar pumice texture provides evidence of high shear stresses related to strong velocity gradients normal to conduit walls, which induced vesiculation and fragmentation of the ascending magma. Either an increasing mass discharge rate without adequate enlargement of a narrow central feeder conduit or a developing fissure-like feeder system related to incipient caldera collapse provided suitable conditions for the generation of plate tube pumice within magma volumes under high shear during the pre-climactic KPT eruption phases. This mechanism implies that the closer to the conduit walls (where the stronger are the velocity gradients) the larger was the proportion of plate vs. conventional (lensoid) juvenile fragments in the ascending gas–pyroclast mixture. Consequently, plate pumice clasts were mainly entrained in the outer portions of the jet and convecting regions of a sustained, Plinian-type, eruption column, as well as in occasional lateral blast currents generated at the vent. As a whole, plate pumice clasts in the peripheral portions of the column were transported at lower altitudes and deposited by fallout or partial collapse closer to the vent relative to lensoid ones that dominated in the inner column portions. The plate tube pumice proportion decreased abruptly up to disappearance during the emplacement of the main pyroclastic currents and lithic-rich breccias related to extensive caldera collapse at the eruption climax, as a consequence of an overall widening of the magma feeder system through the opening of multiple conduits and eruptive vents, along with fissure erosion, concomitant to the disruption of the collapsing block.     

Rhyolitic fallout and pyroclastic density current deposits from a phreatoplinian eruption in the eastern Aegean Sea, Greece

The Kos Plateau Tuff consists of pyroclastic deposits from a major Quaternary explosive rhyolitic eruption, centred about 10 km south of the island of Kos in the eastern Aegean, Greece. Five main units are present, the first two (units A and B) were the product of a phreatoplinian eruption. The eruption style then changed to `dry' explosive style as the eruption intensity increased forming a sequence of ignimbrites and initiating caldera collapse. The final waning phase returned to phreatomagmatic eruptive conditions (unit F). The phreatomagmatic units are fine grained, poorly sorted, and dominated by blocky vitric ash, thickly ash-coated lapilli and accretionary lapilli. They are non-welded and were probably deposited at temperatures below 100°C. All existing exposures occur at distances between 10 km and 40 km from the inferred source. Unit A is a widespread (>42 km from source), thin (upwind on Kos) to very thick (downwind), internally laminated, dominantly ash bed with mantling, sheet-like form. Upwind unit A and the lower and middle part of downwind unit A are ash-rich (ash-rich facies) whereas the upper part of downwind unit A includes thin beds of well sorted fine pumice lapilli (pumice-rich facies). Unit A is interpreted to be a phreatoplinian fall deposit. Although locally the bedforms were influenced by wind, surface water and topography. The nature and position of the pumice-rich facies suggests that the eruption style alternated between `wet' phreatoplinian and `dry' plinian during the final stages of unit A deposition.Unit B is exposed 10–19 km north of the inferred source on Kos, overlying unit A. It is a thick to very thick, internally stratified bed, dominated by ash-coated, medium and fine pumice lapilli in an ash matrix. Unit B shows a decrease in thickness and grain size and variations in bedforms downcurrent that allow definition of several different facies and laterally equivalent facies associations. Unit B ranges from being very thick, coarse and massive or wavy bedded in the closest outcrops to source, to being partly massive and partly diffusely stratified or cross-bedded in medial locations. Pinch and swell, clast-supported pumice layers are also present in medial locations. In the most distal sections, unit B is stratified or massive, and thinner and finer grained than elsewhere and dominated by thickly armoured lapilli. Unit B is interpreted to have been deposited from an unsteady, density stratified, pyroclastic density current which decelerated and progressively decreased its particle load with distance from source. Condensation of steam during outflow of the current promoted the early deposition of ash and resulted in the coarser pyroclasts being thickly ash-coated. The distribution, texture and stratigraphic position of unit B suggest that the pyroclastic density current was generated from collapse of the phreatoplinian column following a period of fluctuating discharge when the eruptive activity alternated between `wet' and `dry'. The pyroclastic density current was transitional in particle concentration between a dilute pyroclastic surge and a high particle concentration pyroclastic flow. Unidirectional bedforms in unit B suggest that the depositional boundary was commonly turbulent and in this respect did not resemble conventional pyroclastic flows. However, unit B is relatively thick and poorly sorted, and was deposited more than 19 km from source, implying that the current comprised a relatively high particle concentration and in this respect, did not resemble a typical pyroclastic surge.     

Lateral variations within coarse co-ignimbrite lithic breccias of the Kos Plateau Tuff, Greece

 Coarse, co-ignimbrite lithic breccia, Ebx, occurs at the base of ignimbrite E, the most voluminous and widespread unit of the Kos Plateau Tuff (KPT) in Greece. Similar but generally less coarse-grained basal lithic breccias (Dbx) are also associated with the ignimbrites in the underlying D unit. Ebx shows considerable lateral variations in texture, geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing (F2: 10 wt.%) to fines poor (F2: 0.1 wt.%). Lithic clasts are predominantly vent-derived andesite, although clasts derived locally from the underlying sedimentary formations are also present. There are no proximal exposures of KPT. There is a highly irregular lower erosional contact at the base of ignimbrite E at the closest exposures to the inferred vent, 10–14 km from the centre of the inferred source, but no Ebx was deposited. From 14 to <20 km from source, Ebx is present over a planar erosional contact. At 16 km Ebx is a 3-m-thick, coarse, fines-poor lithic breccia separated from the overlying fines-bearing, pumiceous ignimbrite by a sharp contact. This grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts, pumice and ash, or into a thinner one-clast-thick lithic breccia that grades upward into relatively lithic-poor, pumiceous ignimbrite. Distally, 27 to <36 km from source Ebx is a finer one-clast-thick lithic breccia that overlies a non-erosional base. A downcurrent change from strongly erosional to depositional basal contacts of Ebx dominantly reflects a depletive pyroclastic density current. Initially, the front of the flow was highly energetic and scoured tens of metres into the underlying deposits. Once deposition of the lithic clasts began, local topography influenced the geometry and distribution of Ebx, and in some cases Ebx was deposited only on topographic crests and slopes on the lee-side of ridges. The KPT ignimbrites also contain discontinuous lithic-rich layers within texturally uniform pumiceous ignimbrite. These intra-ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non-erosional basal contacts. The proportion of fine ash within the KPT lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow, turbulence induced locally by interaction with topography, flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud, and to self-fluidisation accompanying the settling of coarse, dense lithic clasts. There are problems in interpreting the KPT lithic breccias as conventional co-ignimbrite lithic breccias. These problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated, non-turbulent systems that deposit en masse. The KPT coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified, waning pyroclastic density currents and from variations in lithic clast supply from source. Received: 21 April 1997 / Accepted: 4 October 1997     

Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption

Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit, close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy” pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits, (2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore, record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular, crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity, tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the centre of the conduit and not necessarily requiring a thick fragmentation zone.     

Sumisu volcano,Izu-Bonin arc,Japan: site of a silicic caldera-forming eruption from a small open-ocean island

Sumisu volcano was the site of an eruption during 30–60 ka that introduced ∼48–50 km³ of rhyolite tephra into the open-ocean environment at the front of the Izu-Bonin arc. The resulting caldera is 8 × 10 km in diameter, has steep inner walls 550–780 m high, and a floor averaging 900 m below sea level. In the course of five research cruises to the Sumisu area, a manned submersible, two ROVs, a Deep-Tow camera sled, and dredge samples were used to study the caldera and surrounding areas. These studies were augmented by newly acquired single-channel seismic profiles and multi-beam seafloor swath-mapping. Caldera-wall traverses show that pre-caldera eruptions built a complex of overlapping dacitic and basaltic edifices, that eventually grew above sea level to form an island about 200 m high. The caldera-forming eruption began on the island and probably produced a large eruption column. We interpret that prodigious rates of tephra fallback overwhelmed the Sumisu area, forming huge rafts of floating pumice, choking the nearby water column with hyperconcentrations of slowly settling tephra, and generating pyroclastic gravity currents of water-saturated pumice that traveled downslope along the sea floor. Thick, compositionally similar pumice deposits encountered in ODP Leg 126 cores 70 km to the south could have been deposited by these gravity currents. The caldera-rim, presently at ocean depths of 100–400 m, is mantled by an extensive layer of coarse dense lithic clasts, but syn-caldera pumice deposits are only thin and locally preserved. The paucity of syn-caldera pumice could be due to the combined effects of proximal non-deposition and later erosion by strong ocean currents. Post-caldera edifice instability resulted in the collapse of a 15° sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano’s surface.     

Mamaku Ignimbrite: a caldera-forming ignimbrite erupted from a compositionally zoned magma chamber in Taupo Volcanic Zone, New Zealand

Mamaku Ignimbrite was deposited during the formation of Rotorua Caldera, Taupo Volcanic Zone, New Zealand, 220–230 ka. Its outflow sheet forms a fan north, northwest and southwest of Rotorua, capping the Mamaku–Kaimai Plateau. Mamaku Ignimbrite can be divided into a partly phreatomagmatic basal sequence, and a main sequence which comprises lower, middle, and upper ignimbrite. The internal stratigraphy indicates that it was emplaced progressively from a pyroclastic density current of varying energy that became less particulate away from source. Gradational contacts between lower, middle, and upper ignimbrite are consistent with it being deposited during one eruptive event from the same source. Variations in lithic clast content and coexistence of different pumice types through the ignimbrite sequence indicate that caldera collapse occurred throughout the eruption, but particularly when middle Mamaku Ignimbrite was deposited and in the final stages of deposition of upper Mamaku Ignimbrite. Maximum lithic data and the location of lithic lag breccias in upper Mamaku Ignimbrite confirm Rotorua Caldera as the source. At least 120 m of geothermally altered intra-caldera Mamaku Ignimbrite occurs inside Rotorua Caldera. Pumice clasts in the Mamaku Ignimbrite are dacite to high-silica rhyolite and can be chemically divided into three types: high–silica rhyolite (type 1), rhyolite (type 2), and dacite (type 3). All are petrogenetically related and types 1 and 2 may be derived by up to 20% crystal fractionation from the type 3 dacite. All three types probably resided in a single, gradationally zoned magma chamber. Andesitic juvenile fragments are found only in upper Mamaku Ignimbrite and inferred to represent a discrete magma that was injected into the silicic chamber and is considered to have accumulated as a sill at the base of the magma chamber. The contrast in density between the andesitic and silicic magmas did not allow eruption of the andesitic fragments during the deposition of lower and middle Mamaku Ignimbrite. The advanced stage of caldera collapse, late in the main eruptive phase, created withdrawal dynamics that allowed andesitic magma to reach the surface as fragments within upper Mamaku Ignimbrite.     

U–Pb zircon chronostratigraphy of early-Pliocene ignimbrites from La Pacana, north Chile: implications for the formation of stratified magma chambers

High spatial resolution U–Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent ²³⁸U/²⁰⁶Pb ages of 4.11±0.20 Ma and 4.65±0.13 Ma (2σ), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U–Pb ages closely agree with the eruption age of Atana previously determined by K–Ar dating (4.0±0.1 Ma) and do not support long (>1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent ²³⁸U/²⁰⁶Pb zircon ages of 13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano–Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented.     

Geomorphic characteristics of the Minjiang drainage basin (eastern Tibetan Plateau) and its tectonic implications: New insights from a digital elevation model study

Abstract   The Minshan Mountain and adjacent region are the major continental escarpments along the eastern Tibetan Plateau. The Minjiang drainage basin is located within the plateau margin adjacent to the Sichuan Basin. Based on the analysis of the digital elevation model (DEM) acquired by the Shuttle Radar Topography Mission (SRTM), we know that the Minjiang drainage basin has distinct geomorphic characteristics. The regular increasing of local topographic relief from north to south is a result of the Quaternary sediment deposition within the plateau and the holistic uplift of the eastern margin of the Tibetan Plateau versus the Sichuan Basin. Results from DEM-determined Minjiang drainage sub-basins and channel profiles show that the tributaries on the opposite sides are asymmetric. Lower perimeter and area of drainage sub-basins, total channel length and bifurcation ratio within eastern flank along the Minjiang mainstream are the result of the Quaternary differential uplift of the Minshan Mountain region. Shorter stream lengths and lower bifurcation ratio might be the indications of the undergrowth and newborn features of these eastern streams, which are also representative for the eastern uplift of the Minshan Mountain.