**4.6 Rheological properties**

In order to quantify the effect of rheology on Monte Amiata eruptive style, in this study we combine two empirical models. The first model (GRD) [115] allows to calculate the residual liquid viscosity as a function of the temperature (T) and the composition (X). The second model (CM) [116] allows to estimate the rheological effects due to the presence of crystals in strained magmas, through the calculation of the relative viscosity (i.e., the ratio between the viscosity of the mixture and the viscosity of the pure liquid). The input data are constituted by the textural data providing the crystal fraction (Section 4.4), the temperature estimation, and dissolved fluid content (Section 4.5).

**Figure 13** reports the pure liquid viscosity vs. the inverse of the temperature. The calculations are performed on the entire temperature range, but we put in evidence the interval 900–1070°C, representative of an early-stage-crystallization (PES) magma and the interval 800–900°C, representative of the late-stage-crystallization magma and erupted products (ES). Considering the total content of fluids present inside this magma, which is strongly influenced by the presence of CO2—see Section 4.5—and considering that the latter is completely separated from the magma during the ascent and the emplacement process, a quantitative of 0.3 wt% of residual water appears acceptable as an input parameter into the proposed models. The crystal fraction of the suspended crystals presents in each SLLF is based on the average values proposed by [41]. These values are reported in

#### **Figure 13.**

*Diagram of the calculated liquid + crystal suspension viscosity values for the investigated Monte Amiata SLLFs. Here, the suspending liquid is taken as the residual glass matrix (GM) or as the total rock (TR), whereas the crystal content (ϕ) is taken as the value proposed by [45] of 40 volume % (ϕ=0.4). Water content (H2O) is considered to be 0 or 0.3 wt% as potentially presence as dissolved at the basis of the most tick lava flows. The calculations provided here do not account for the effect of suspended vesicles in the multiphase magmatic and volcanic mixture. For that reason, we could assume that the calculated values are an upper limit of the real volcanic mixture viscosity (vesicles in fact have an important effect in reducing magmatic and volcanic mixture viscosity), which may be representative of the most glassy crystal rich (vesicle free) lavas as those found at Monte Amiata. The figure also reports (at 1012Pa s, in dash and dot thick gray) the line which marks the glass transition, that is, at first approximation, representative of the limit of viscous flow of lava flows [70]. In all cases, the calculated volcanic mixture viscosities estimated in the temperature interval (900– 1070°C) are well below the flow limit as above defined. Both GM and TR, at any crystal and water content conditions in the (800–900°C) temperature interval, mostly fall below Tg and, only at the very lowest T, they overpass it.*

volume percent (the number in the parenthesis followed by the symbol %). All the calculations are carried out by assuming the lowest strain rate value (10–<sup>5</sup> s 1 ), which provides the highest viscosity value.

In **Figure 13,** we also report the value of the glass transition temperature (Tg) as taken at a viscosity of 10<sup>12</sup> Pa s [70]. Tg constitutes that barrier below which most of processes (e.g., diffusion, crystallization, vesiculation, flow, and welding) are significantly inhibited, if not halted. On the other hand, above the glass transition, the above-mentioned processes are still active and potentially rapid enough to still influence somehow the magmatic and volcanic processes. Ref. [117] published a comprehensive review of the ways how the various magmas and volcanic bodies may cross the glass transition temperature (e.g., conventional thermal cooling; cooling along a retrograde solubility curve; effective raising of Tg as due to degassing) and enhance the welding of pyroclastic materials or the flow of silicic lavas, other than affecting the depth of the fragmentation depth.

*Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*
