**5. Discussion**

## **5.1 Distinguishing silicic lavas from welded tuffs and rheomorphic ignimbrites**

Several scholars have attempted to compare and define diagnostic textures and structures to explain how silicic lava flows differ from high-temperature pyroclastic density currents (welded and/or rheomorphic ignimbrites) (e.g., [11, 15, 71, 99]). In this section, we discuss the geological and volcanological features (field physical features and rock's textures and structures) that support the interpretation of trachydacite sheet-like silicic flows of Monte Amiata as lavas and reject the attribution as welded ignimbrites and rheoignimbrites deriving from the emplacement of pyroclastic density currents emitted during a single large explosive eruption.

In **Table 6,** we summarize (i) the diagnostic depositional characteristics of ignimbrite [1, 118–120] that have not found in Monte Amiata trachydacite SLLFs despite careful search, and (ii) the discriminating internal textures and structures of the Monte Amiata trachydacite SLLFs that help to identify them as silicic lava flows [12, 121–123].

In conclusion, the volcanological observations, the field geological evidence, and the petrographic analyses performed on the trachydacite SLLFs of Monte Amiata are consistent with a genetic interpretation by effusive eruptions, which gave rise to long and extensive silicic lava flows.

#### **5.2 Defining a model of silicic lava flows (SLLFs) for Monte Amiata trachydacite**

Observations and models on silicic lava flows have been mainly carried out on small rhyolite obsidian domes and coulées [9, 87, 89] that have some characteristics that were extended to other silicic lavas. The idealized structural model of rhyolite lava flow [92] consists of three principal zone: (1) a basal pyroclastic deposit emplaced during the initial phase of the eruption, (2) a lava dome or short flow formed by the relatively quiet effusion of magma, and (3) an envelope of carapace breccia that grows around the molten core of the lava body as the chilled and brittle outer crust breaks in response to internal expansion and growth. The lava body is further subdivided in a vertical sequence of: basal part composed of breccia of pumice and obsidian blocks, interior part composed of either coarsely vesicular pumice, coherent even flowfoliated glassy obsidian, and finely vesicular pumice, and surface breccia [89].

More recently, a certain number of studies have been carried out so far it concerns the emplacement mechanisms of very large lava flow sequences in the Brazilian sector of the early Cretaceous Paranà Etendeka Magmatic Province (PEMP) [19–21]. Apparently, these huge effusive silicic sequences (> thousands of km3 ) were not accompanied by explosive activity. The same authors have invoked mechanisms, which explain the emplacement of the investigated lava flows as mainly due to the emission of degassed dacitic and rhyolitic low-viscosity lavas.

The peculiarity of Monte Amiata sheet-like trachydacite lava flows appear to have some point in common with those found in the products reported in the PEMP and, with respect to other described silicic flows, their main features, without considering the greater length of the SLLF, are as follows:

a. Absence of tephra or air-fall pumice deposits testifying explosive activity associated with the lava emplacement. Commonly, sequence of explosive



## **Table 6.**

*Characteristics of Monte Amiata SLLFs supporting the interpretation as lava flows.*

eruptions of tephra preceding (and/or following) the extrusion of silicic lava flows and domes is observed [89, 124]. In similar cases, the early-stage explosive activity was considered functional to gas escape from magma and to the subsequent effusive activity.


The recognition of large-volume, extensive silicic lava flow units at Monte Amiata introduces questions regarding the mechanisms of emplacement. Into the literature, the uncommon sheet-like silicic lava flows are related to the effect of large erupted volume and the effectively lava heat retained due to a thick solidified crust, whereas the dome-forming eruptions of silicic lavas are related to lowmagma production rates [125].

Thanks to the direct observation of the events of silicic lava flow emplacement occurred in historical time [2–7], we have understood that the emplacement of large volume and great thickness silicic lava flows with high proportions of suspended crystals should not surprise us.

In fact, it has demonstrated by [5, 6, 126] and, more recently, [19, 20], that the large thickness of a lava flow constitutes the most efficient thermal barrier, which reduces heat loss and allows to maintain the lavas at high temperature well above their glass transition temperatures. This will favor, for years, or even for decades, in the cases of the largest fluxes, the flow. To favor flow will favor other important processes, such as the crystallization of the spherulitae and the contribution of the volatile resorption [117, 127].

In conclusion, the unusual length and the main characteristics described above (absence of related tephra deposits, absence of talus debris, presence of outflow lobes, high porphyricity, glassy fresh groundmass, and absence of quenched or highly vesicular crust) of the SLLFs of Monte Amiata may be referred to the high temperature, high thicknesses, and relatively large volume of each one of these lava flows. In this picture, morphological factors and volatile content seem to have played a minor role.
