Jonathan H. Fink

Quantification of submarine lava-flow morphology through analog experiments

We developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows, using observations of flow morphology and estimates of eruption volume, eruption temperature, lava viscosity, and preflow topography. Using laboratory simulations, and correlating these results with sea-floor observations, four submarine lava-flow morphologies are considered to be diagnostic of specific effusion rates: jumbled, folded, and lineated sheets, and striated pillows. We applied this approach to the CoAxial flow, emplaced in less than 14 days on the Juan de Fuca Ridge in early summer, 1993, and we calculated an effusion rate of ∼100 m3/s, giving an emplacement time of ∼10 days.

Solidification and Morphology of Submarine Lavas: A Dependence on Extrusion Rate

The results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity. The experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter, the ratio of the time taken for the surface to solidify, and a time scale for lateral flow. For submarine basalts a solution of the cooling problem (which is dominated by conduction in the lava but convective heat transfer in the water) and estimates of lava viscosities place this parameter within the empirically determined “pillowing” regime over a wide range of extrusion rates. This result is consistent with the observation that pillow basalts are the most common products of submarine eruptions. Smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma. Still higher eruption rates in regions of low topographic relief may produce submarine lava lakes. Minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type.

Radial spreading of viscous-gravity currents with solidifying crust

We have investigated the effect of a solidifying crust on the dynamics and surface morphology of radial viscous-gravity currents. Liquid polyethylene glycol was admitted into the base of a tank filled with cold sucrose solution maintained at a temperature below the wax freezing point. As the radial current advanced away from the inlet, its surface solidified and deformed through a combination of folding and fracturing. For the warmest experiments, during which solidification did not occur, the radius of the current increased in proportion to the square root of time, as demonstrated both experimentally and theoretically for isothermal viscous fluids by Huppert (1982). When cooling was sufficiently rapid, solid crust formed and caused the spreading rate to decrease. A cooling model combining conduction in the wax with convection in the sucrose solution predicts the distance from the source at which the solid crust first appeared Progressively colder experiments revealed a sequence of surface morphologies which resembled features observed on cooling lava flows and lava lakes. Flows in which crust formed very slowly developed marginal levees which contained and channelled the main portion of the current. Colder flows with more rapid crust growth formed regularly spaced surface folds, multi-armed rift structures complete with shear offsets, and bulbous lobate forms similar to pillow lavas seen under the ocean. The same transitions between modes of surface deformation were also generated by keeping the ambient water temperature constant and decreasing the extrusion rate. This demonstration that surfaces can exhibit a well-defined sequence of morphologies which depend on the underlying flow conditions offers the prospect of more successful interpretation of natural lava flows.

Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain, Medicine Lake Highland, northern California

Many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows. The stratigraphy of the Holocene Little Glass Mountain rhyolitic obsidian flow consists of (from bottom to top): air-fall tephra deposits, basal breccia, coarsely vesicular pumice, obsidian, finely vesicular pumice, and surface breccia. Slightly crystalline rhyolite occurs near the vent areas. A model of obsidian-flow emplacement based on these textural relations is presented. This stratigraphy may reflect the distribution of volatiles within the magma source region, with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body. The distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows. This complex structure, as mapped on the Little Glass Mountain flow, reflects both the initial flow stratigraphy and subsequent deformational processes. During emplacement of the lava, three processes disrupt the initial flow configuration: the rise of coarse pumice diapirs from the base of the flow, the inward propagation of fractures in areas of extension, and surface folding in sites of flow-parallel compression. Subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava; most of the observed upper surface of a dome originates as a fracture plane. The structure of domes that form over vent areas may reflect the orientation of dike-like conduits as well as the local state of stress during extrusion.

Morphology, eruption rates, and rheology of lava domes: Insights from laboratory models

The growth of lava domes can be either quiescent or violent, with transitions between styles of behavior commonly occurring with little warning. Here we propose that the behavior depends on the eruption rate, the magma rheology, and the thickness of the cooling surface. We present a model, based on laboratory simulations, field measurements, and photographic analysis, that relates the morphology and texture of a dome to the thickness of its cooled carapace, and thence to eruption conditions. A sequence of four main types of dome (spiny, lobate, platy, and axisymmetric) is identified in laboratory analog experiments with a Bingham plastic. These regimes are associated with progressively higher effusion rates, lower cooling rates, lower yield strengths, and (in real lava flows) decreasing tendency for explosive decompression during flow front collapse and are ordered according to the value of a single dimensionless number. The model allows an estimate of the yield strengths of the magma forming active domes based on data for the effusion rate and composition. It also permits the eruption rates of prehistoric or extraterrestrial lava domes and flows to be appraised from their morphology, if their compositions can be estimated. A comparison with the laboratory results suggests that the Venusian “pancake domes” are likely to have basaltic to basaltic andesitic composition.

Surface folding and viscosity of rhyolite flows

Regularly spaced ridges on rhyolite flows are analyzed through the use of a surface-folding model that was first applied to ropy structures on pahoehoe basalt flows. The requirement that there be a strong folding instability to produce regularly spaced ridges places constraints on three dimensionless parameters related to the properties of the lava and the geometry of the channel: R > 35, S 28. R is the ratio of surface to interior viscosities, S is a ratio between the stress due to the weight of the lava and the compressive stress due to folding, and L dγ is a dimensionless form of the ridge spacing. Estimates of strain rates and measurements of ridge spacings and thicknesses of thermal boundary layers of flows allow these three parameters to be calculated independently for a given flow lobe. For the Big Glass Mountain rhyolitic obsidian flow in northern California, R ≅ 104, S 44. This compatibility between theory and observation supports the folding interpretation for ridges. Furthermore, the model allows calculation of the minimum viscosity of many flows for which such data are otherwise unavailable. The viscosities of a dacite flow in Chile and of a. possible lava flow on Mars are calculated as examples.

Ropy pahoehoe: Surface folding of a viscous fluid

Abstract The regularly spaced surface structure observed on ropy pahoehoe basalt flows may be interpreted as folds which develop at the surface of a fluid whose viscosity decreases with depth. Folds form by the selective amplification of an irregular waviness in surface shape during shortening of the flow surface. The development of a regular fold arc length, predicted by folding theory, is reflected in the length scale of pahoehoe ropes. Pahoehoe fold arc lengths and the strength of the folding instability are determined by: (1) the ratio of the surface viscosity to the interior viscosity; (2) the thickness of the thermal boundary layer across which the viscosity changes; and (3) the ratio of the surface compressive stress to a stress related to the weight of the lava. The braided appearance of many ropy pahoehoe flows can be explained by a superposition of two episodes of folding.

Rapid emplacement of a mid-ocean ridge lava flow on the East Pacific Rise at 9° 46′–51′N

In April, 1991, during a submersible diving expedition to the East Pacific Rise (EPR) crest at 9°46′–51′N, a new volcanic eruption on the sea floor was discovered. Here, we report results from numerical modeling of that eruption, which indicate that ∼ 4 × 106 −6 × 106 m3 of lava was emplaced in ∼ 1–2 h, with an average eruption rate of ∼ 103–106 m3 s−1 — comparable to rates observed in Hawaii at Kilaueas East Rift Zone. If the rapid emplacement of the 1991 EPR lava and its short eruption duration are typical of volcanic events at fast-spreading mid-ocean ridge crests, these characteristics have broad implications for our ability to detect, monitor and understand the evolution of magmatic and volcanic processes within the axial zone.

Effects of surface cooling on the spreading of lava flows and domes

Scaling analyses describe the evolution of an extrusion of viscous or plastic fluid in the presence of surface cooling and solidification, under the assumption that the flows consist of two components: an isothermal interior and a surface crust. The ‘crust’ is a complex thermal and rheological boundary layer which we model using a viscous, plastic or brittle rheology. These models are thought to be relevant to some types of lava flows and address the effects of cooling on their morphology and rate of advance of the flow front. They show that effects of crust strength will dominate over both viscous and yield stresses in the interior when the ratio of crust thickness to flow length exceeds the ratio of effective yield stress of the crust to basal shear stress exerted on the bulk of the flow, a condition that appears likely to be met by many lava flows and small outgrowths on large flows. Similarity solutions are compared with measurements on the spreading of extrusions of wax beneath cold water in the laboratory, where the extruded liquid is viscous but develops a solid crust. Crust strength provides the dominant retarding force for the wax flows in cases where surface solidification occurs rapidly compared with lateral advection. These conditions give flows topped by sheets of solid that buckle or rift apart, or extrusions that enlarge by small bulbous outgrowths (analogous to ‘pillows’ on submarine lavas and ‘toes’ on some sub-aerial basalt flows). A comparison of the models with data for the growth of lava domes in the craters of Mount St Helens and Soufriere of St Vincent volcanoes reveals that spreading of those domes was not controlled by stresses in the flow interior. Instead the data are consistent with a balance between gravity and yield stresses in a thin crustal layer over most of the growth period.

Internal textures of rhyolite flows as revealed by research drilling

Recent drilling of 550- and 140,000-yr-old rhyolite lava flows in the Inyo Dome chain, California, and Valles Caldera, New Mexico, and field inspection of 8 Ma flows in western Arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized. The upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian, coarsely vesicular pumice, and a second obsidian layer. Beneath these layers is the crystalline center of the flow, which in turn is underlain by a basal obsidian layer (lower vitrophyre). Differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence, devitrification, lava rheology, and migration of water vapor.