Harry Pinkerton

Methods of determining the rheological properties of magmas at sub-liquidus temperatures

Non-vesicular basaltic melts behave as Newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by Shaw (1972) and Bottinga and Weill (1972). Because many igneous processes involve the flow of silicates at sub-liquidus temperatures, numerous attempts have been made to calculate the interactive effects of the crystal phase. However, current methods are appropriate only for relatively low crystal concentrations, and they assume Newtonian behaviour; we argue that this assumption is invalid when bubble or crystal concentrations exceed 30%. At higher concentrations, factors other than concentration become increasingly important; these include particle shape and size distribution. A method which takes these factors into account is tested on a range of suspensions, including magmas at sub-liquidus temperatures, and rheological properties calculated using this method agree closely with the measured values. We also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of Mount St. Helens dacite (Murase et al., 1985), and which is currently used to calculate the rheological properties of crystallising lavas, generates viscosities which may be in error by several orders of magnitude. This difference is argued to be caused by a combination of factors, including the ten orders of magnitude range in the strain rates utilised during the Mount St. Helens measurements causing orders of magnitude difference in the resulting apparent viscosities. Rheological data on crystallising silicic melts are reinterpreted taking into account the non-Newtonian rheology of the magmas and changes in activation energy of flow during crystallisation.

Repeated fracture and healing of silicic magma generate flow banding and earthquakes

Textures in an exceptionally preserved effusive rhyolite conduit at Torfajokull, Iceland, indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit (RFH process). Anastomosing tuffisite veins filled by fine-grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval. Welding of the particulate material during subsequent deformation led to thorough healing of veins, allowing repeated fracture of the same body of magma. We propose that the RFH process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions. The RFH process may also form the flow banding that is nearly ubiquitous in obsidian.

Formation of lava tubes and extensive flow field during the 1991–1993 eruption of Mount Etna

Detailed mapping during the 1991–1993 eruption of Mount Etna has shown that there is a relationship between tumuli, ephemeral vents, lava tubes, and their parent lava flows. During this eruption, many tubes formed in stationary, inflated ‘a’a lava flows. Ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows, many of which subsequently developed new tubes. The resulting complex network of tubes, ephemeral vents, and secondary flows was responsible for most of the widening, thickening, and lengthening of the 1991–1993 Etna lava flow field. The supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel-fed lava flow. Our observations suggest that lava tubes play a more important role in the formation of extensive ‘a’a flow fields on Etna than has previously been recognized.

The evolution of lava flow-fields: observations of the 1981 and 1983 eruptions of Mount Etna, Sicily

The eruptions of Mount Etna in 1981 on the north flank and 1983 on the south flank of the volcano were of strikingly different character. The former was a short duration, high effusion rate eruption producing for the most part a simple flow-field; the latter was of relatively long duration and low effusion rate, producing a compound flow-field of overlapping flows.Despite the differences between the eruptive behaviour of these two events and the way in which the flow-field developed, both the flow-fields achieved about the same maximum length. This is considered fortuitous. The evidence suggests that the main 1981 flow stopped because the lava supply ceased and was thus volume controlled. The 1983 flow-field had a more complex history of branching, but in this case it appears that, for the longest individual flow, cooling played an important role in controlling the maximum extent of the flows.

Effusion rate estimations during the 1999 summit eruption on Mount Etna, and growth of two distinct lava flow fields

Detailed studies of the evolution of two major flow fields during the 1999 eruption on Mount Etna provide useful insights into the development of different types of flow fields. During this eruption, two large lava flow fields were emplaced. The Eastern flow field, which formed between February and November, was erupted from three primary vents at the base of the Southeast Cone, one of four eruptive centres in the summit region of Mount Etna. This compound flow field was characterised by a complex tube network, skylights, ephemeral vents and tumuli. Between mid-October and early November, while the Eastern flow field was still active, another flow field was erupted from the western rim of the Bocca Nuova, one of the other eruptive centres. This Western flow field was emplaced during one month of discontinuous activity and is composed of discrete, channel-fed a′a flow units that formed a fan-shaped flow field. Major periods of flow advance within this flow field took place during phases of relatively high flow rate that lasted a few hours to days. The discontinuous supply prevented the formation of lava tubes within this flow field. The Eastern and Western lava flow fields from the Southeast Cone and Bocca Nuova have distinctive morphologies that reflect their emplacement mechanisms. Many of these morphological features are large enough to be seen on aerial photographs. This has implications for assessing the emplacement conditions of older flow fields on Earth and on other planets.

The 1975 sub-terminal lavas, mount etna: a case history of the formation of a compound lava field

The 1975 sub-terminal activity was characterised by low effusion rates (0.3–0.5 m3 s−1) and the formation of a compound lava field composed of many thousands of flow units. Several boccas were active simultaneously and effusion rates from individual boccas varied from about 10−4 to 0.25 m3s−1. The morphology of lava flows was determined by effusion rate (E): aa flows with well-developed channels and levees formed when E > 2 × 10−3 m3 s−1, small pahoehoe flows formed when 2 × 10−3 m3 s−1 >E > 5 > 10−4 m3 s−1 and pahoehoe toes formed when E < 5 × 10−4 m3 s−1. There was very little variation with time in the effusion temperature, composition or phenocryst content of the lava. New boccas were commonly formed at the fronts of mature lava flows which had either ceased to flow or were moving slowly. These secondary boccas developed when fluid lava in the interior of mature aa flows either found a weakness in the flow front or was exposed by avalanching of the moving flow front. The resulting release of fluid lava was accompanied by either partial drainage of the mature flow or by the formation of a lava tube in the parent flow. The temperature of the lava forming the new bocca decreased with increasing distance from the source bocca (0.035°C m−1). It is demonstrated from the rate of temperature decrease and from theoretical considerations that many of the Etna lavas still contained a substantial proportion of uncooled material in their interior as they came to rest. The formation of secondary boccas is postulated to be one reason why direct measurements of effusion rates tend, in general, to overestimate the total effusion rates of sub-terminal Etna lava fields.

Rheological properties of basaltic lavas at sub-liquidus temperatures: laboratory and field measurements on lavas from Mount Etna

Models of many magmatic processes require accurate data on the rheological properties of lava at sub — liquidus temperatures. Laboratory measurements of the rheological properties of basalts erupted on Mount Etna in 1983 were made at various crystal concentrations in a specially designed furnace using a Haake Rotovisco viscometer attached to a spindle designed to eliminate slippage at the melt-spindle interface. Measurements were made at strain rates between 0.3 and 5 s−1 over the range of eruption temperatures on Mount Etna (1084–1125 °C). At temperatures above 1120 °C, the 1983 lava behaves as a Newtonian fluid. At lower temperatures, the lava is a thixotropic, pseudoplastic fluid with a maximum yield strength of 78 Pa at a temperature of 1087 °C. In view of its low yield strength over this temperature range, the Theological behaviour of the lava approximates to that of a power law fluid. Apparent viscosities at unit shear strain rates increase from 150 Pa s at 1125 °C to 3000 Pa s at 1084 °C. Unit strain rate apparent viscosities measured in the field using a rotating vane viscometer range from 1385 Pa s to 1630 Pa s. These values are in close agreement with those measured at the same temperature in the laboratory and with those calculated theoretically from the physico-chemical properties of the lava.

Classification and formation of lava levees on Mount Etna, Sicily

Observations of the 1975 subterminal lava flows and sections through the larger flank lavas on Mount Etna show that there are four principal types of levees formed in Etnean lavas: initial, accretionary, rubble, and overflow. Initial levees are formed because of the yield strength of these non-Newtonian lavas and are thought to determine channel width. The other types of levees are formed subsequently and are built up over the rarely preserved initial levees. Mechanisms of formation of each levee type vary; most levees are hybrids of two or more of the four types. We also discuss the prospects of deducing lava rheology from morphology and the application of this technique to surfaces of other planets.

Lava tube morphology on Etna and evidence for lava flow emplacement mechanisms.

Lava tubes play a pivotal role in the formation of many lava flow fields. A detailed examination of several compound ‘a‘a lava flow fields on Etna confirmed that a complex network of tubes forms at successively higher levels within the flow field, and that tubes generally advance by processes that include flow inflation and tube coalescence. Flow inflation is commonly followed by the formation of major, first-order ephemeral vents which, in turn, form an arterial tube network. Tube coalescence occurs when lava breaks through the roof or wall of an older lava tube; this can result in the unexpected appearance of vents several kilometers downstream. A close examination of underground features allowed us to distinguish between ephemeral vent formation and tube coalescence, both of which are responsible for abrupt changes in level or flow direction of lava within tubes on Etna. Ephemeral vent formation on the surface is frequently recorded underground by a marked increase in size of the tube immediately upstream of these vents. When the lining of an inflated tube has collapsed, ‘a‘a clinker is commonly seen in the roof and walls of the tube, and this is used to infer that inflation has taken place in the distal part of an ‘a‘a lava flow. Tube coalescence is recognised either from the compound shape of tube sections, or from breached levees, lava falls, inclined grooves or other structures on the walls and roof. Our observations confirm the importance of lava tubes in the evolution of extensive pahoehoe and ‘a‘a flow fields on Etna.

Physicochemical properties of alkali carbonatite lavas:Data from the 1988 eruption of Oldoinyo Lengai, Tanzania

Alkali carbonatite lavas extruded from Oldoinyo Lengai, Tanzania, in November 1988 are similar in composition to lavas extruded in 1960. Extrusion temperatures are 585 ±10 °C. Apparent viscosities in this temperature range are between 0.3 and 120 Pa⋅s, the highest values coming from very frothy and phenocryst-rich magma. The viscosities and temperatures are the lowest known for terrestrial magmas.