*4.2.2 Internal structures and textures*

Internal structures and textures, such as vesiculation and gas cavities, flow banding, cooling joints, and deformations, have been observed in Monte Amiata SLLFs.

Vesicles and gas cavities are segregation structures of fluids and show different shape, orientation, and dimension in function of their temporal and genetic relationships with the flowing molten lava [72]. In Monte Amiata SLLFs, the dominant coherent lithofacies are poorly or evenly microvesiculated. Vesicles, ranging from less than a millimeter across to cavities more than 10 cm large, are concentrated in trains that form discrete interlayers defining flow layering (**Figure 4e**) and the local flow directions. In coarsely vesicular beds, tubular gas cavities occur together in zones that form planar lenses and layers (**Figure 4e**). The tridimensional geometry of these cavities is complex and unusual (**Figure 5e**). They are apparently continuous for several tens of centimeters in length, horizontally elongated with the main axis parallel to the flow foliation and flow direction. They show an irregular ellipsoidal section, up to 1 cm across and 10 cm along the vertical axis, with scalloped bubble walls (**Figure 5e**). These peculiar gas cavities show some differences as compared with the pipe vesicle typically occurring in basaltic lava [73] that is characterized by individual, subvertical cylindrical tubes with a subcircular section whose formation is attributed to the upward migration of a single bubble of magmatic gas. In the case of Monte Amiata SLLFs, tubular vesicles are formed probably by both the upward coalescence of smaller bubble and lateral migration along the flow direction under the influence of a strong volatile segregation in not-quenched lava. The bubbles confinement in discrete levels may suggest gas entrapment in discrete superimposed domains impermeable among them to the further upward volatile migration. This molten lava partitioning resulted from laminar flow processes, possibly enhanced by the mingling



### **Table 2.**

*Summary of the macroscopic lithofacies of SLLFs of Monte Amiata trachydacite.*

of different lithofacies. Another characteristic of the Monte Amiata SLLFs is the presence of cavities with irregular shape formed around meta-sedimentary xenoliths and megacrysts of plagioclase and sanidine (**Figure 5f**). In this case, gas segregation may result directly from the inclusions.

A layered banded aspect, often flat-lying, is well developed in the Monte Amiata SLLSs, defined both by the interlayering of different lithofacies and by variation in vesicularity, crystallinity, grain size, color, and groundmass texture. Flow bands are laterally continuous, parallel, and dominantly quite planar.

Two types of flow banding must be distinguished in the Monte Amiata SLLFs. A pervasive fine-scale flow banding structure (flow lamination) is produced by interlayering of mm- to cm-sized dark and light laminae of different coherent lithofacies (**Figure 4h**). In addition, sheeting joints, generally attributed to shear partings developed during laminar flow, form well-developed bedding (**Figure 4g**). The stratified structure (flow bedding) comprises massive beds (cm- to m-thick) of porphyritic and vitrophyric coherent lithofacies enclosed at top and bottom by interlayers (mm- to cm-thick) of microvesicular or obsidian lithofacies (**Figure 4a** and **b**). Several processes have been taken into account for the formation of bedding in silicic lavas [74]: (i) mingling of different parts of the magma [75–77]; (ii) welding and rheomorphism [78]; (iii) repeated brecciation followed by reannealing into the conduit [79, 80]; and (iv) laminar flowage inherited during flow in the conduit in response to shear stresses along the conduit walls that continue and propagate upward during the lava advance for the shear stresses at the lava flow base [9].

The flow bands define local sinuosity, steep dips, folding, and convolution in frontal and margin zones of SLLFs (see Section 4.2.3).

Some of the Monte Amiata SLLF trachydacite (Sorgente del Fiora and Piancastagnaio) shows parallel columnar cooling joints that post-date the flow bedding and flow lineation. They are roughly defined, with spacing of 1.5–2.5 m, vary from subvertical to dipping at moderate angles toward the flow front.
