Richard V. Fisher

Flow transformations in sediment gravity flows

Sediment gravity flows (subaerial or subaqueous) are those in which movement is driven by gravity and the sediment motion moves the interstitial fluid (gas or liquid). Such flows exhibit flow transformations , a term introduced here, referring to changes between laminar and turbulent flow, in turn related chiefly to particle concentration, thickness of the flow, and flow velocity (slope).

PROPOSED CLASSIFICATION OF VOLCANICLASTIC SEDIMENTS AND ROCKS

Volcaniclastic sediments and rocks are divided here into autoclastic, pyroclastic, and epiclastic types with grain-size limits the same as non-volcanic epiclastic rocks. Autoclastic rocks contain fragments that are produced within (but not usually extruded from) volcanic vents, during movement of lava flows, or by gas explosions within flows that have ceased to flow. Pyroclastic rocks contain fragments produced by volcanic explosion and extruded as discrete particles from volcanic vents. Epiclastic volcanic rocks contain fragments produced by weathering and erosion of solidified or lithified volcanic rocks of any type. Volcaniclastic types may be mixed in all proportions with each other or with nonvolcanic fragments, although these mixtures are not designated within this classification. A non-genetic category, based only upon particle size and the presence of volcanic material, is included for rocks with clasts of unknown origin.

Mobility of a large-volume pyroclastic flow - emplacement of the Campanian ignimbrite, Italy

Abstract The trachytic Campanian Ignimbrite, originally exposed over a 30,000-km 2 area around Naples, Italy, is the product of a highly energetic, gas-rich eruption. The deposit lies in valleys and isolated watersheds, and in its medial and distal extent to the south, north and east of Naples, the Campanian pyroclastic current encountered mountains exceeding 1000 meters Anisotropy of magnetic susceptibility (AMS) measurements indicate that the Campanian pyroclastic current (the transport system) traveled radially outward from the Phlegrean Fields area, but the ignimbrite-forming flow (the deposition system that developed from the base of the transport system) moved downslope from mountainsides to valleys, including slopes facing the eruption source, and flowed down drainage systems from intermontane basins. The Campanian pyroclastic current flowed ∼ 35 km over the water of the Bay of Naples to deposit > 43 m of ignimbrite on the south shore and also overtopped a 685–1000-m-high ridge of the Sorrento Peninsula to deposit more on the other side. The distribution of the ignimbrite and the measured flow directions suggest that the Campanian pyroclastic current moved across the landscape as an expanded (and therefore turbulent) decompressing flow rather than as a high-density, nonturbulent sheet-like current moving over mountains by momentum acquired by eruption column collapse. Strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80% by volume of the original magma.

Geochemical zoning, mingling, eruptive dynamics and depositional processes - The Campanian Ignimbrite, Campi Flegrei caldera, Italy

Abstract The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted at 37 ka from the Campi Flegrei and composed of a fallout deposit overlain by ignimbrite. The ignimbrite was spread over an area of about 30,000 km2 including the Campanian Plain and the Apennine Mountains, with ridges over 1000 m a.s.l. The pumice fragments of the CI range in composition from trachyte to phonolitic-trachyte (DI = 75–90). They do not show any systematic compositional variation with stratigraphic height, but the analyzed sections can be divided into three groups on the basis of chemical composition of pumices. Least-evolved pumices (DI = 75–83) occur in the ignimbrite in the central sector of the Campanian Plain up to 30 km from the vent, while the most-differentiated pumices (DI = 88–90) characterize the cogenetic fallout deposit and the ignimbrite in the western sector of the Campanian Plain, on the Tyrrhenian scarp of the Apennines between Caserta and Mt. Maggiore, on Roccamonfina volcano, and on the Sorrento Peninsula, up to 50 km from the source. Pumice fragments of intermediate composition (DI = 84–87) occur in the ignimbrite on the Apennine Mountains and Roccamonfina volcano, up to 65 km from the vent. In one exposure at Mondragone, at the base of a calcareous ridge, an ignimbrite with pumices of most-evolved composition is overlain by an ignimbrite with pumices of intermediate composition. The observed compositional variation between most- and least-evolved ignimbrite was generated in part by crystal-liquid fractionation, although other magmatic processes such as syn-eruptive mingling between most- and least-evolved magmas accounts for the mineralogical disequilibria and for the bimodality of the glass compositions in the intermediate-composition rocks. Pumice Sr-isotope ratios are positively correlated with degree of differentiation. Feldspar crystals separated from pumices of different compositions have a homogeneous Sr-isotope composition similar to that of the least-evolved pumices. Interaction between fluids and strongly fractionated Sr-poor less-dense magma can account for these isotopic features. Geochemical, mineralogic, stratigraphic and volcanologic data, together with the stratigraphic relations between most-, intermediate- and least-evolved ignimbrite as detected at Mondragone and from bore-hole drillings suggest that: (1) the CI magmatic system was composed of two distinct magma layers — the upper layer was more differentiated and homogeneous in composition, while the deeper was less evolved and slightly zoned; and (2) the CI was mostly emplaced in three main pulses of pyroclastic flows that tapped the chamber at variable levels and with distinct withdrawal dynamics. The eruption began with emission of the most differentiated magma, which gave rise to the fallout deposit. It continued with generation of expanded, turbulent pyroclastic flows that reached the Sorrento Peninsula in the southeast and Roccamonfma volcano in the northwest. These flows, whose thickness was greater than the overtopped relief, were able to travel over the water of the bay of Naples. Subsequently an intermediate-composition magma resulting from mingling of different portions of the magma chamber generated similar flows that spread radially and traveled not less than 65 km from the vent. During the last pulse the least-evolved magma was tapped and generated flows that spread within the Campanian Plain. Variation in eruptive dynamics and composition of magma during the course of the eruption likely reflected variations of both geometry of vent and plumbing system, and efficiency of water/magma interaction, which in turns affected the dynamics of the magma chamber and the withdrawal mechanism, and resulted from the dynamics of the caldera collapse.

Models for pyroclastic surges and pyroclastic flows

Pyroclastic surges are low-concentration turbulent flows that form in at least three ways: (1) eruption column collapse (ground surge, base surge), (2) elutriation from the top of a moving pyroclastic flow (ash cloud), and (3) directly from a crater without an accompanying vertical eruption column.∗ Ground surge deposits occur at the base and ash cloud deposits occur at the top of pyroclastic flow sequences. Ground surges and ash clouds may move independently of their associated pyroclastic flow, and, like flows directly from a crater, their deposits may occur alone or within any part of pyroclastic flow sequence. The presence or absence of surge layers, their position with respect to pyroclastic flow deposits and their physical characteristics have significance with respect to the characteristics or eruption columns and flowage mechanisms of pyroclastic flows.

Features of Coarse-Grained, High-Concentration Fluids and Their Deposits

ABSTRACT Field, laboratory, and theoretical investigations of fluids with very high concentrations of solid particles in water (up to 90%) show that they exhibit the property of strength, have high apparent viscosities which vary with velocity, have high bulk densities, and flow in laminar fashion or with greatly reduced turbulence. Debris flows, observed during movement and emplacement, deposit debris which is poorly sorted, has an unsupported framework, may contain elongate fragments which are roughly aligned parallel to bedding, and may show inverse grading. On low slopes, debris flow deposits commonly overlie easily eroded materials with little or no features of erosion. Subaqueous deposits in the geologic record which show these or somewhat similar features were probably emplaced as highl concentrated dispersions.

The Agnano-Monte Spina eruption 4100 years BP in the restless / Campi Flegrei caldera Italy

Abstract The Agnano–Monte Spina tephra (AMST), dated at 4100 years BP by 40 Ar / 39 Ar and 14 C AMS techniques, is the product of the highest-magnitude eruption in the Campi Flegrei caldera (CFc) during its last epoch of activity (4800–3800 years BP). The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two main pumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the vent area. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flows overtopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit is subdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological characteristics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun during the course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, and generated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali–trachytic composition, whereas later-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that two isotopically distinct magmas, one alkali–trachytic and the other trachytic, were tapped and partially mixed during the eruption. The small volume (1.2 km3 DRE) of erupted magma and the structural position of the vent suggest that the eruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.

Submarine volcaniclastic rocks

Summary The type, relative abundance and stratigraphical relationships of volcanic rocks that comprise island volcanoes are a function of (i) depth of extrusion beneath water, (ii) magma composition, and (iii) lava-water interactions. The water depth at which explosions can occur is called the pressure compensation level (PCL) and is variable. Explosive eruptions that occur above the PCL and below sealevel can give rise to abundant hydroclastic and pyroclastic debris. Below the PCL, clastic material cannot form explosively; it forms from lava by thermal shock. The volcaniclastic products are widely dispersed in basins adjacent to extrusion sources by three principal kinds of marine transport processes. These are slides, sediment gravity flows and suspension fallout. Volcaniclastic debris can be derived in subaqueous and subaerial-to-subaqueous environments (i) directly from eruptions, (ii) from remobilization of juvenile volcaniclastics, or (iii) from epiclastic material which initially develops above sealevel. Sediment gravity flows (fluids driven by sediment motion) exhibit the phenomenon of flow transformation. This term is used here for the process by which (i) sediment gravity flow behaviour changes from turbulent to laminar, or vice versa, within the body of a flow, (ii) flows separate into laminar and turbulent parts by gravity, and (iii) flows separate by turbulent mixing with ambient fluid into turbulent and laminar parts. Dominant kinds of subaqueous volcaniclastic sediment gravity flows are debris flows, hot or cold pyroclastic flows and turbidites. Fine grained material can be thrown into suspension locally during flow transformations or underwater eruptions, but thin, regionally distributed subaqueous fallout tephra is mostly derived from siliceous Plinian eruptions.

The 1982 eruptions of El Chichon volcano, Mexico (3): Physical properties of pyroclastic surges

Two major pyroclastic surges generated during the 4 April 1982 eruption of El Chichon devastated an area of 153 km2 with a quasi-radial distribution around the volcano. The hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation. Field evidence indicates voidage fraction of 0.99 in the surge cloud with extensive entrainment of air. Thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud. The dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10–20, characterized by stoss-side erosion and lee-side deposition of 1–10 cm reversely graded laminae. A systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude. Modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals, resulting in loss of at least 10%–25% of the cloud mass due to winnowing out of fines during surge emplacement. Greatest fractionation from the −1.0–0.0−∅ grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud. Extensive pumice rounding indicates abrasion during bed-load transport. Flow of pyroclastic debris in the turbulent surge cloud was by combination of bed-load and suspended-load transport. The surges are viewed as expanding pyroclastic gravity flows, which entrain and mix with air during transport. The balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence, which persisted to the distal edge of the surge zone.

Erosion by volcanic base-surge density currents: U-shaped channels

Small U-shaped channels eroded by volcanic base surges occur on the steep outer slopes of some tuff cones but rarely on the gentle outer slopes of tuff rings. This suggests that their development is a function of velocity. Corroborating evidence from the 1952 phreatomagmatic eruption of Barcena Volcano is as follows: U-shaped furrows were cut by volcanic density currents on the steep slopes of the volcano, but at the base of the volcano, where the velocity decreased, dunes were deposited with long axes perpendicular to the furrows. These dunes were similar to those deposited by base surges on the low slopes surrounding Taal Volcano, Philippines, during its 1965 eruption. To be preserved, U-shaped base-surge channels must be quickly buried by deposits from penecontemporaneous eruptions; otherwise, as ready-made avenues for run-off, stream action soon destroys them. Even if preserved on the steep sides of a volcano, however, exposures in cross section are rare because (1) the channels are filled with pyroclastic deposits or (2) later stream dissection parallels them without revealing the cross-sectional profile or else completely destroys them. An explanation of the origin of U-shaped channels stems from (1) the parameters of base-surge flow deduced from descriptions of historic base surges and their deposits, (2) descriptions of prehistoric base-surge deposits, (3) the development of U-shaped furrows by base surges at Barcena Volcano, Mexico, and (4) descriptions herein of prehistoric U-shaped channels and their depositional fill at Koko Crater, Hawaii. Fortunate circumstances of erosion on the side of Koko Crater provide excellent cross-sectional exposures along a short stretch of the shoreline. U-shaped channels eroded by base surges superficially resemble equilibrium semicircular channels cut by streams and mud-flows, but the profiles develop by a different mechanism. The fronts of advancing volcanic base surges develop a cleft and lobe pattern; the lobes appear to be individual turbulent cells that splay outward from the source. To carve a smooth U-shaped profile, the concentration of particles must increase gradually (perhaps exponentially) from the edges of the lobes to their central part, where the boundary effects are least and forward velocity is greatest. Small channels that are cut into erodible material by turbidity currents also have rounded cross-sectional profiles and may be cut by a mechanism similar to that ascribed here to base-surge flow.