Lava Viscosity Research Articles

Formation of channelized lava flows on an unconfined slope

The formation of channelized lava flows on a wide uniform slope is investigated both theoretically and experimentally. When a lava is released at a constant flow rate from a point source, we predict that it flows both down and across the slope at the same rate in a early time regime before undergoing a transition to a long‐time regime where down‐slope flow is faster than lateral flow. Eventually, the lateral flow is stopped by the strength of the growing surface crust, and the flow then travels down slope in a channel of constant width. Using scaling analysis, we derive expressions for the final channel width in both flow regimes, as a function of the flow rate, the slope, the density difference driving the flow, the lava viscosity, the thermal diffusivity, and the yield strength of the crust. We also find a dimensionless flow morphology parameter that controls whether the subsequent channel flow occurs in a “mobile crust” regime or in a “tube” regime. These theoretical predictions are in good agreement with laboratory experiments in which polyethylene glycol wax flows down a wide uniform slope under cold water. The theory is also applied to the understanding of the formation of a basaltic sheet flow lobe in Hawaii, which had an estimated crust yield strength of order 6 × 104 Pa.

A combined spectrophotometric and morphometric study of the lunar mare dome fields near Cauchy, Arago, Hortensius, and Milichius

In this study we examine the spectral and morphometric properties of the four important lunar mare dome fields near Cauchy, Arago, Hortensius, and Milichius. We utilize Clementine UV–vis multispectral data to examine the soil composition of the mare domes while employing telescopic CCD imagery to compute digital elevation maps in order to determine their morphometric properties, especially flank slope, height, and edifice volume. After reviewing previous attempts to determine topographic data for lunar domes, we propose an image-based 3D reconstruction approach which is based on a combination of photoclinometry and shape from shading. Accordingly, we devise a classification scheme for lunar mare domes which is based on a principal component analysis of the determined spectral and morphometric features. For the effusive mare domes of the examined fields we establish four classes, two of which are further divided into two subclasses, respectively, where each class represents distinct combinations of spectral and morphometric dome properties. As a general trend, shallow and steep domes formed out of low-TiO 2 basalts are observed in the Hortensius and Milichius dome fields, while the domes near Cauchy and Arago that consist of high-TiO 2 basalts are all very shallow. The intrusive domes of our data set cover a wide continuous range of spectral and morphometric quantities, generally characterized by larger diameters and shallower flank slopes than effusive domes. A comparison to effusive and intrusive mare domes in other lunar regions, highland domes, and lunar cones has shown that the examined four mare dome fields display such a richness in spectral properties and 3D dome shape that the established representation remains valid in a more global context. Furthermore, we estimate the physical parameters of dome formation for the examined domes based on a rheologic model. Each class of effusive domes defined in terms of spectral and morphometric properties is characterized by its specific range of values for lava viscosity, effusion rate, and duration of the effusion process. For our data set we report lava viscosities between about 10 2 and 10 8 Pa s , effusion rates between 25 and 600 m 3 s −1 , and durations of the effusion process between three weeks and 18 years. Lava viscosity decreases with increasing R 415 / R 750 spectral ratio and thus TiO 2 content; however, the correlation is not strong, implying an important influence of further parameters like effusion temperature on lava viscosity.

On the effect of lava viscosity on the magnetic fabric intensity in alkaline volcanic rocks

The degree of the anisotropy of magnetic susceptibility (AMS) of basaltic rocks, as is known from the large AMS database of these rocks, is generally very low, while in more acidic volcanic rocks such as andesites, trachytes and phonolites, which have been investigated much less frequently, it is in general much higher. In the present study, the AMS of various volcanic rocks including trachytic and phonolitic rocks was investigated in the Tertiary volcanic region of the Ceske středohoři Mts. Viscosities of the respective lavas were calculated from the chemical composition using the KWARE program. A rough correlation was found between the degree of AMS and lava viscosities, probably resulting from different mechanisms orienting the magnetic minerals. In basaltic lava flows this mechanism is traditionally considered to be of a hydrodynamic nature, in trachytic and phonolitic bodies it can also be represented by quasi-intrusive flows resembling, at least partially, ductile flow deformation. This is in agreement with the AMS data predicted by the viscous (liquid flow) and line/plane (ductile flow) models.

Shield volcano topography and the rheology of lava flows on Io

Stereo and photoclinometry derived topography of shield-like volcanoes on Io indicate little relief (<3 km) and very low slopes (0.2° to 0.6°). Several shield volcanoes appear to be associated with broad rises of 1 to 3 km, but only 5 shield volcanoes have been identified with steep flank slopes (between 4° and 10°). These steep slopes are restricted to within 20–30 km of the summit, but where discernable, most of the lava flows observed on these edifices occur on the outer flanks where slopes are less than a degree. Despite their abundance, ionian shield volcanoes are among the flattest in the Solar System. The steepest volcanoes on Io are most comparable to large venusian shield volcanoes. Using simplistic Bingham rheologies we estimate the viscosity and yield strengths of ionian lavas. Yield strengths are estimated at 10 1–10 2 Pa, lower than most basaltic lavas. Viscosity estimates range from 10 3 to 10 5 Pa s, although these are probably upper limits. Actual values may have been as low as 10 0 Pa s. Viscosity is sensitive to flow velocity, which is poorly known on Io. The best constraint on flow velocity comes from observations of the 1997 Pillan eruption, which bracket the eruptive phase to 132 day maximum, and more probably less than 50 days. Low slopes, long run-out distances and our estimated rheologic properties are consistent with (but not proof of) a low silica, low viscosity, high temperature composition for ionian lavas, supporting arguments for low-silica lava compositions such as basalt or komatiite. We cannot eliminate sulfur on rheologic grounds, however.

Effect of the Galápagos hotspot on seafloor volcanism along the Galápagos Spreading Center (90.9–97.6°W)

Studies along the Mid-Atlantic Ridge (MAR) and East Pacific Rise (EPR) show that seamount abundance is a strong function of spreading rate. At the MAR, small axial seamounts are a dominant morphologic feature of the inner valley floor, while at the EPR seamounts are rarely observed within the neovolcanic zone. The Galápagos Spreading Center (GSC) provides an excellent location to test the influence of hotspot-related magma supply variations on seamount formation at a relatively constant spreading rate. In this study, we use multibeam bathymetry data to examine the distribution of axial seamounts with distance from the hotspot along the GSC. A numerical algorithm is developed to identify isolated volcanic edifices by searching bathymetry for closed, concentric contours protruding above the surrounding seafloor. Seamount populations are fit with a maximum likelihood model to estimate the total number of seamounts per unit area and the characteristic seamount height. The number of seamounts in the axial zone decreases significantly as the Galápagos hotspot is approached, varying from MAR-like abundances west of 95.5°W to EPR-like abundances east of 92.7°W. The along-axis variation in seamount density corresponds to a change in lava flow morphology from dominantly pillow flows in the west to a combination of pillows and sheet/lobate flows in the east. These changes in volcanic style suggest a transition from point-source to fissure-fed eruptions as magma supply increases. We find no evidence for along-axis variations in lava viscosity and, therefore, interpret the differences in seamount abundance and flow morphology to indicate an increase in effusion rates approaching the hotspot. Comparing seamount abundance with axial morphology, crustal thickness, and the presence and depth of an axial magma chamber (AMC), we find that the transition from point-source to fissure-fed eruptions is most sensitive to the presence of a steady-state AMC. In summary, the correlation between seamount abundance, lava morphology, and magma supply at the GSC suggests that effusion rate is directly related to the presence of a steady-state crustal magma chamber.

Effects of regional slope on viscous flows: a preliminary study of lava terrace emplacement at submarine volcanic rift zones

To understand how large submarine lava terraces form and why they are not commonly observed on land, we developed an isoviscous gravity flow model on an inclined surface to simulate the evolution and emplacement of lava flows under submarine conditions. By solving this preliminary model using a finite difference method, we are able to quantify how lava viscosity, pre-existing topographic slope, effusion rate, and lava volume affect meso-scale lava morphology. Our simulations show that, in general, high lava viscosity, gentle regional slope, and low effusion rate favor the formation of large terraces, but environmental conditions also play an important role. A gravity flow spreads more slowly underwater than subaerially. We also conclude that for low viscosity basaltic lava, the cooling of the lava body is one of the most critical factors that affect its shape. This study shows that the isoviscous model, though oversimplified, provides a quantitative tool to relate lava morphology to eruption characteristics. To gain a better understanding of the controls on submarine lava terrace formation, future models must take into account the temporal and spatial variation of lava viscosity, especially the effects of a brittle outer shell.

Viscosity of hydrous Etna basalt: implications for Plinian-style basaltic eruptions

Water dissolved in a silicate melt can strongly influence its physical properties and thus magma behavior during crystallization, degassing, foaming and fragmentation. Etna is a basaltic volcano whose activity is dominated by effusive eruptions which have long represented a threat to the densely populated, surrounding area. Recently, recognition of the products of a Plinian eruption (122 B.C.) has raised further issues for hazard assessment at Etna and other basaltic volcanoes. Constraining the behavior of Etna magma under conditions relevant to both effusive and explosive hazards requires viscosity data under conditions near the glass transition. Here we have investigated the viscosity of hydrous Etna lava in order to better understand eruptive processes which characterize this volcano. The experimental methods which have been used include piston cylinder synthesis of the hydrated melts, micropenetration viscometry for low-temperature viscosity measurements, and near-infrared spectroscopy for the evaluation of sample homogeneity and measurements of water content. Additionally, scanning calorimetric determinations were performed to check whether incipient crystallization had occurred. Sample compositions were determined using electron microprobe analysis and 57Fe Mossbauer spectroscopy. Results from this study are compared with previous reports of trachytic, phonolitic and model calc-alkaline rhyolite (HPG8) compositions. The viscosity of the basaltic melt (dry and wet) has been parameterized in terms of temperature and water content via the non-Arrhenian equation: log10 η=–4.643+(5,812.44–427.04×H2O)/(T(K)–499.31+28.74×ln(H2O)) where η is the viscosity in Pa s, H2O is the water content in wt%, and T is the temperature in Kelvin. We observe that the viscosity of alkali basalt (at more than 0.5 wt% H2O) is similar to that of an alkaline trachyte (Agnano–Monte Spina eruption, Phlegrean Fields) and much higher than that of a peralkaline phonolite (Teide, Tenerife) at similar silica contents and NBO/T. For water contents above 1.5 wt%, the viscosity of the basalt is similar to that of rhyolitic melts with similar water contents. At temperatures ranging from 1,050 to 1,150 °C and with water contents between 0.5 and 2.3 wt% (eruptive conditions), the viscosities calculated using the equation defined in this study are (1) in reasonable agreement with those calculated using Shaw's model, and (2) much lower than those experimentally determined in a previous study. However, outside these temperature and water content ranges, the agreement with Shaw's model (1972) breaks down.

Melt viscosity, temperature and transport processes, Troodos ophiolite, Cyprus

The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO 2 content and a deeper lava sequence with high TiO 2 content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava–sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO 2 content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#.

Valley networks on Venus

Valley networks on Venus are classified as rectangular, labyrinthic and pitted, or irregular. The venusian valley networks are structurally controlled, as indicated by the morphological patterns of valley branches, consistency between valley and fracture orientations, and associations with the deformed terrains. The morphologies resemble those of terrestrial and martian sapping valleys. Valley networks on Venus probably formed initially from fracture systems and became enlarged by low viscosity lava sapping processes. Subsurface flow of lava may locally have been assisted by surface flows. The lavas probably moved through permeable media and fractures. Venusian valley networks have a higher degree of network integration than do lunar sinuous rilles, but they are less integrated than martian and terrestrial sapping valleys. The viscosity of valley-forming lavas must have been very low, but was not low enough to exploit the permeable media so extensively as to attain a high degree of network integration. The compositions of these lavas may have been mafic to ultramafic or mafic alkaline. Alternatively, the lavas could have had more exotic compositions, such as carbonatite and sulfur. Valley networks are often associated with corona and corona-like features, which are hypothesized to be the surface expressions of mantle plumes. A plume association could mean that the lavas came from the mantle.

Constraints on the formation of submarine lava flows from numerical model calculations

To constrain life time and flow rates in a submarine lava tube (pillow), 3D finite-element calculations were carried out for a simplified fixed geometry model tube. The principal parameters included into the model are: a temperature-dependent Newtonian viscosity of the lava, whereby viscosity is also dependent on composition, temperature and crystallinity of the lava, cooling by seawater, and the release of latent heat of crystallization. Time-dependent model runs suggest the longevity of the tube flow is constrained by the competition of advective heat flow along the tube axis and the conduction of heat towards the margins of the flow. In order to keep the lava flowing, a minimum critical pressure gradient along the tube is required. Otherwise, the amount of thermal energy brought in by the influx of hot lava is less than the amount lost at the outer boundaries, and the tube will solidify. Providing an exponentially decaying driving pressure at the tube inlet, this critical pressure gradient has been calculated for different tube lengths and diameters and is compared to critical pressure values resulting from a more simplistic solution, assuming constant viscosity and pure conductive or convective cooling. An analysis of the difference between pressure gradients obtained through the numerical and simplified model is used to derive an appropriate power law for the critical pressure gradient in bodies cooled by convection and conduction including the effect of temperature-dependent viscosity. That gradient scales with the tube radius R as R 3.85. This law is valid for tube-style lava flows and lava viscosities in the basaltic range.

Viscous Newtonian laminar flow in a rectangular channel: application to Etna lava flows

We introduce a 3D model for near-vent channelized lava flows. We assume the lava to be an isothermal Newtonian liquid flowing in a rectangular channel down a constant slope. The flow velocity is calculated with an analytical steady-state solution of the Navier-Stokes equation. The surface velocity and the flow rate are calculated as functions of the flow thickness for different flow widths, and the results are compared with those of a 2D model. For typical Etna lava flow parameters, the influence of levees on the flow dynamics is significant when the flow width is less than 25 m. The model predicts the volume flow rate corresponding to the surface velocity, taking into account that both depend on flow thickness. The effusion rate is a critical parameter to evaluate lava flow hazard. We propose a model to calculate the effusion rate given the lava flow width, the topograhic slope, the lava density, the surface flow velocity, and either the lava viscosity or the flow thickness.

Eruption constraints on tube‐fed planetary lava flows

We examine the role of pressure and gravity as driving forces in planetary lava tubes for Newtonian and power law rheologies. The tubes are assumed to have been filled with lava that was emplaced in constant diameter circular tubes in the laminar flow regime and had constant density, heat capacity, thermal conductivity, and viscosity. Our model provides relationships between tube dimensions, driving forces, and effusion rates and rheology parameters. In general, the pressure term in the driving force dominates for very small slopes, but the gravity term eclipses the pressure term as the slope increases. Applying the model to Alba Patera tube flows suggests effusion rates somewhere between 2 and 105 m3/s and viscosities between 102 and 106 Pa s, with tighter constraints (2 to 4 orders of magnitude) for specific tube sizes and travel times. These effusion rate results are lower and the viscosity ranges are higher than those found in previous studies. This allows eruptions that are closer in style to terrestrial basaltic eruptions, although the flows are still considerably larger in scale than the Hawaiian tube flows. We find that very low lava viscosities are not essential for Alba Patera lava tubes and that tube formation may be a better indicator of the steadiness of the eruption and the presence of low slopes than it is of low viscosities (e.g., 102 Pa s). In addition, this analysis suggests that the tube systems on the steep flanks of Olympus Mons are fundamentally different from those at Alba Patera. The Olympus Mons flows could not have roofed over, been continuously full, or maintained a continuous lava tube transport system in the assumed full, steady, and fully developed conditions.

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.

The effect of crystallization on the rheology and dynamics of lava flows

The dynamics of a lava flow is studied by a two-dimensional model describing a viscous fluid with Bingham rheology, flowing down a slope. The temperature in the flow is calculated assuming that heat is transferred through the plug by conduction and is lost by radiation to the atmosphere at the top of the flow. Taken into account is that the increasing crystallization takes place in the flow as a consequence of cooling. The lava viscosity and yield stress are expressed as a function of crystallization degree as well as of temperature: in particular it is assumed that yield stress reaches a maximum value above the solidus temperature, according to experimental data. Dynamical variables, such as velocity and thickness of the flow, are calculated for different values of the maximum crystallization degree and the flow rate. The model shows how the lava flow dynamics is affected by cooling and crystallization. The cooling of the flow is controlled by the increase of yield stress, which produces a thicker plug and makes the heat loss slower. The increasing crystallization has two opposing effects on viscosity: it produces an increase of viscosity, but at the same time produces an increase of yield stress and hence reduces the heat loss and keeps the internal temperature high. As a consequence, lava flows are significantly affected by the dependence of yield stress on temperature and scarcely by the maximum crystallization degree.

On the variations of flow rate in non-explosive lava eruptions

A physical model is developed to determine factors which influence the dynamics of non-explosive lava eruptions. In the model, magma rises in a laminar regime from an overpressured chamber surrounded by elastic rock and erupts at the surface, where the accumulation of lava may alter the vent pressure. In a flow rate versus volume diagram, an eruption follows a path which is sensitive to relative changes in flow variables such as viscosity, conduit dimensions and thickness of lava over the vent. The path does not depend on the unknown elastic properties of the country rock or the chamber dimensions. With appropriate volcanological data, an observed flow rate versus volume path can be interpreted. The approach is applied to three well-documented eruptions. In each case the eruption rate was partially influenced by decreasing chamber pressure. The 1988–1990 eruption of Lonquimay (Chile) exhibits an early phase of conduit shrinkage which lasted about 100 days, probably due to magma solidification against country rock. The 1979 eruption of La Soufrière de Saint Vincent (West Indies) was initially affected by lava dome growth and later passed through a phase possibly caused by constriction of the conduit. The 1943–1952 eruption of Paricutín (Mexico) was affected by changes of lava level in the cinder cone and magma properties. In these three eruptions, the pre-eruptive chamber overpressures were similar (10–20 MPa) even though the erupted volumes differed by as much as two orders of magnitude.

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.

Extended cooling and viscous flow of large, hot rhyolite lavas: implications of numerical modeling results

Abstract A common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities. Although a high-viscosity lava will indeed flow quite slowly, it may advance a great distance if the lava efficiently retains heat and thus mobility, and if the available magma supply is large. To evaluate this possibility, a finite-difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas. Cooling of such 100–300 m-thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat. Thermal history alone suggests that large, thick lava flows could remain active for several decades. To constrain the duration of flow advance of these units, their inferred apparent viscosities are compared with those of active lava flows. Mean flow velocity of some large rhyolites in southwestern Idaho, USA, was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization, indicating perhaps a 10- to 18-year flow duration, based on studies of Obsidian Dome, California. Mean flow velocities would have been roughly 0.59 to 2.5 km/yr, and calculated average apparent viscosities of the flows ranged from 1.8 to 3.6 orders of magnitude greater than the viscosity of the lava itself. This relationship also holds for active andesite and basaltic andesite block lava flows, which have apparent viscosities 2.7 to 4 orders of magnitude greater than lava viscosities. This implies that large rhyolites may indeed have been emplaced in the same manner as less-silicic block lavas, and that long distance viscous flow is not unrealistic for rhyolite lavas. A good example of a large-volume lava is the 15-km 3 Rhyolite of the Badlands (SW Idaho), where vent relations show that lava effused from a fissure and advanced up to 9 km. More extensive rhyolites could also be emplaced solely by effusion. Extended flow of these lavas may lead to the remelting and removal of basal crumble breccias, and to the formation of welded shard textures in their margins, both processes tending to obscure the lava flow origin of the units. The great dimensions of lava-like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites.

Morphometric study of pillow-size spectrum among pillow lavas

Measurements of H and V (dimensions in the horizontal and vertical directions of pillows exposed in vertical cross-section) were made on 19 pillow lavas from the Azores, Cyprus, Iceland, New Zealand, Tasmania, the western USA and Wales. The median values of H and V plot on a straight line that defines a spectrum of pillow sizes, having linear dimensions five times greater at one end than at the other, basaltic toward the small-size end and andesitic toward the large-size end. The pillow median size is interpreted to reflect a control exercised by lava viscosity. Pillows erupted on a steep flow-foot slope in lava deltas can, however, have a significantly smaller size than pillows in tabular pillowed flows (inferred to have been erupted on a small depositonal slope), indicating that the slope angle also exercised a control. Pipe vesicles, generally abundant in the tabular pillowed flows and absent from the flow-foot pillows, have potential as a paleoslope indicator. Pillows toward the small-size end of the spectrum are smooth-surfaced and grew mainly by stretching of their skin, whereas disruption of the skin and spreading were important toward the large-size end. Disruption involved increasing skin thicknesses with increasing pillow size, and pillows toward the large-size end are more analogous with toothpaste lava than with pahoehoe and are inferred from their thick multiple selvages to have taken hours to grow. Pseudo-pillow structure is also locally developed. An example of endogenous pillow-lava growth, that formed intrusive pillows between ‘normal’ pillows, is described from Sicily. Isolated pillow-like bodies in certain andesitic breccias described from Iceland were previously interpreted to be pillows but have anomalously small sizes for their compositions; it is now proposed that they may lack an essential attribute of pillows, namely, the development of bulbous forms by the inflation of a chilled skin, and are hence not true pillows. Para-pillow lava is a common lava type in the flow-foot breccias. It forms irregular flow-sheets that are locally less than 5 cm thick, and failed to be inflated to pillows perhaps because of an inadequate lava-supply rate or too high a flow velocity.

Toothpaste lava: Characteristics and origin of a lava structural type transitional between pahoehoe and aa

Toothpaste lava, an important basalt structural type which illustrates the transition from pahoehoe to aa, is particularly well displayed on the 1960 Kapoho lava of Kilauea Volcano. Its transitional features stem from a viscosity higher than that of pahoehoe and a rate of flow slower than that of aa. Viscosity can be quantified by the limited settling of olivine phenocrysts and rate of flow by field observations related to the low-angle slope on which the lava flowed. Much can be learned about the viscosity, rheologic condition, and flow velocity of lavas long after solidification by analyses of their structural characteristics, and it is possible to make at least a semiquantitative assessment of the numerical values of these parameters.

Time‐dependent profiles of lava flows

This work investigates the combined effects of a time‐dependent effusion rate and a spatially varying viscosity on the thickness profile of a flowing lava. We formulate relatively simple governing equations as a primitive dynamical model for these two influences on lava flow morphology. Free boundary solutions for the profile of the flow and its extent are presented and analyzed for different boundary conditions at the source of the flow and several models for the spatial dependence of the viscosity. The results presented here suggest that time dependence in the flow depth at the source and the form of the viscosity variation are significant influences on the morphology and dimensions of lava flows. Moreover, this analysis implies that time‐dependent source conditions may contribute to the scatter in plots of eruption rate versus flow length and disparities between field and laboratory estimates of lava viscosity.