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Effusive lava flows are usually rare in high silica volcanic centers showing a predominant explosive character. Due to their high silica content and hence their increased viscosity, evolved magmas tend to fragment explosively as a response to applied stress rather than forming flow-spread deposits. Nevertheless, certain chemical and physical constraints (e.g. high alkali and/or water content) may also enhance the ability of felsic magmas to form locally widespread lava flows. What, other than discharge rates, are the parameters that influence the rheology of magma to allow lava to form widespread individual rhyolitic flow deposits up to 200 km³ (e.g. Snake River Plain volcanic Province, Idaho/USA; sample 1) on the one hand and locally restricted obsidian flows that only extent for a few hundred meters (e.g. Rocche Rosse, Lipari/Italy; sample 2) on the other ?

In this study we are concentrating our investigations on a study of the natural cooling rates of obsidian flows in order to determine what may influence their rheology, and thus their tendency to form either widespread or locally restricted deposits. Scanning calorimetry is being used to model and compare natural cooling rates of the Lipari and Idaho flows. One particular aspect is to find out whether a foamed flow surface could create an insulating carapace and thus prevent the interior of a flow from intensive chilling thereby allowing the lava to flow in a widespread manner. Specific heat capacities (c_p) were measured as a function of temperature by heating the glassy flow samples at 5°/min above its glass transition temperature (T_g, defined as the temperature of the c_p-peak value) to allow thermal equilibrium of the supercooled liquid. This gives us the natural cooling rate c_p-curve (raw curve). The calorimetric trace of the sample in the glass transition region and T_g itself is a function of the thermal history of the glass. By reheating the sample at a constant heating rate of 5°C/min, after applying a set of known cooling rates of 16°C/min, 8°C/min, 4°C/min, three different calorimetric traces for each sample are obtained (Fig. 3.7-1) where the influence of cooling rate on the hysteresis in the glass transition interval is evident. The sample cooled significantly more slowly than 4°C/min. With this full set of 5 cooling curves and the aid of a fitting procedure (with several variable parameters) the natural cooling rate of the samples is being determined. With the modeled cooling rates rheological constraints on the flow behaviour of such silicic lava flows will be constrained.
 
 

Fig. 3.7-1: The calorimetric trace of the Rocche Rosse obsidian flow, Lipari/Italy, derived from its natural volcanic cooling rate (raw curve), compared to the ones produced at set cooling rates. The sample was cooled with the matched cooling rates and indivdually reheated at a constant heating rate of 5°C/min. The value of the glass transition temperature as well as the shape of the reheating trace in the glass transition region is seen to vary as a function of the previously applied cooling rate.