*4.2.3 Surface and margin structures*

At Monte Amiata, *in situ* paleo-weathering, overlaying by younger volcanic units, and current poorly exposure preclude the detection of SLLF original surface structures, such as creases [81] and fractures [82], typical of large silicic lava flows. At the meso-scale, the only surface structures that were preserved in SLLFs include surface ridges or ogives.

Surface ridges or ogives are arcuate reliefs, transversely oriented to the downslope flow. Their convexity indicates the direction of flow. The arcuate shape is caused by the increase in frictional resistance toward the margins of the flow [38, 83]. At Monte Amiata, the presence of surface ridges or ogives is mainly evident on the Quaranta and Castel del Piano units (**Figure 2**). Both these trachydacite SLLFs are stratigraphically below the surface of saprolite paleo-weathering that marks the boundary between BAS and MAS. The Monte Amiata ogives look as linear accumulations of lava boulders. Boulders are rounded-shaped, stacked in an open framework, and arranged in transversal arcuate ridges that are spacing 50–100 m apart from each other with apparent amplitudes averaging 5–15 m upon the topographic surface. These structures have already been distinguished by [30], which, however, considered ogives as the discriminating structure supporting his interpretation of Monte Amiata rheoignimbrites.

Several interpretations have been proposed to explain the formation of ogives on silicic lava flows. Taking Monte Amiata as a model, Rittmann [30] first explained the formation of ogives as pressure ridges due to the differential viscosity between the hot and fluid core and the rigid surface of flows that is compressed and corrugated in folds. The boulders forming the ridges are considered by [30] of primary volcanic origin, resulting as the remnant of rootless dikes of molten material intruded from below in the fractured core of the anticlinal-like ridges subjected to extensive stresses. These seminal ideas of Alfred Rittmann are the premise for various subsequent interpretations. However, most of the observations and models on surface ridges have been addressed to rhyolite obsidian domes and coulées rather than to the structures present on the surface of long silicic lava flows. Macdonald [84] proposed that the ridges on rhyolitic block lava flows are the surface expression of the upward bending—named ramp structure—of the shear planes of the internal flowing lava. The conception that ogives are folds of the lava crust produced by ductile deformation in compression (pressure ridges) as due to differential movements between two layers (center and outside crust of the flow) having temperature and viscosity contrasts was applied by previous scientists [9, 83, 85] among others. Another interpretation proposes that surface ridges are evidence of an extensional regime due to the inflation and deformation of a rigid crust. The moving fluid-rich melt beneath the insulation crust extrudes through regularly spaced cracks forming diapir-like structures [9, 86, 87]. In a more extreme way, Andrews and colleagues [88] challenged the "fold theory" of Fink and proposed that ogives are tensile fracture-bound structures that record brittle failure and stretching of the crust as the lava advances and spreads.

Based on our field observations (**Figure 6**), we interpret the ogives developed on the surface of Monte Amiata SLLFs as compressional structures [89] produced during downslope flowage of viscous silicic lava. The core of these small antiform folds is fractured, enhancing a stronger spheroidal *in situ* weathering of lava in rounded boulders (corestones) during the paleo-weathering processes (**Figure 6**; [62]). Being prominent on the topographic surface, ogives are also the site of more intense surface erosion that removed the sandy saprolite matrix. Consequently, the residual blocks have collapsed in place on themselves forming the boulder accumulations exposed on flow surface [62].

Previous authors [39, 40, 43] have interpreted the surface ridges and boulder accumulations on Monte Amiata SLLFs surface as the evidence of the emplacement of block lava flows. Block lava flows are defined [84, 90, 91] as lava flows in which a highly irregular surface is completely covered by continuous, open clast-supported debris of dense and solidified lava blocks, up to several meters in size, with a

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

#### **Figure 6.**

*Natural section showing the internal structure of a ridge (ogive) on the surface of the Quaranta SLLF. The flow banding (dashed black lines) defines an antiform fold. Radially distributed fractures (red solid lines) are related to the extension in the anticline hinge. At the intersection between joints defined by the flow banding and extensional fractures,* in situ *saprolite paleo-weathering isolated subrounded corestones of trachydacite. As an extreme effect of this alteration, the ridge resulted as an accumulation of residual lava blocks. Geologist for scale.*

polyhedral shape delimited by smooth, slightly curved faces and angular edges. Normally in the block lava body, a central mass of massive lava is preserved, but the fragmented material remains predominant [84]. Block lavas formed from magmas with silicic to intermediate composition and high viscosity. The mechanism of formation of a blocky breakage of lava is interpreted as dependent on the rapid growth on the flow surface of a thick, more or less glassy crust, which shatters due to the movement of the warmer underlying flow [90]. The extensive deposits of large subrounded lava boulders arranged in arcuate ridges on the surface of the SLLFs of Monte Amiata are not identifiable with the block lava flow as defined above.

The typical structural model of silicic lava flows comprises an internal coherent core enveloped with flow-generated breccia that forms loose deposits on the surface, along the flow margins (levées; [3]) and at the flow front [12]. In Monte Amiata SLLFs, evidence of the presence of lateral levées and accumulation of auto-brecciated debris at the flow front were not observed.

Surface breccia is also absent, but of a localized exception in the Leccio eruptive unit. In the outcrop of Pian di Ballo quarry, located in the medial part of the Leccio lava flow, coherent trachydacite is enveloped with flow-generated breccia (**Figure 7**). Based on texture of the deposit, we distinguished five lithofacies: (1) coherent massive trachydacite with concentric flow foliation forming a monolithic core; (2) coherent stratified trachydacite protruding laterally from the massive core; (3) fractured trachydacite (both massive and stratified) with close fractures and jigsaw cracks; (4)

fragmented and poorly disaggregate trachydacite; and (5) monogenetic, structure-less matrix-supported lithic breccia. These lithofacies undergo lateral transitions with grading boundaries and complex sedimentary architecture (**Figure 7**). Fragmented and poorly disaggregated lava (lithofacies 4) is characterized by polyhedral blocks, up to 1 m large (**Figure 7**) with low intra-clast matrix. Even though most of the blocks are

#### **Figure 7.**

*Quarry escarpment in the locality Pian di Ballo (red triangle in f) showing a complete section transversal to the flow direction of the Leccio SLLF. The structure of the lava flow shows the development of a surface autobreccia. (a) Photo and (b) sketch of the outcrop. A central core of massive trachydacite (cm) with concentric flow foliation is embedded in a monogenetic breccia (br) through progressive fragmentation and dispersion of lava blocks. Although the different lithofacies on the sketch are separated by solid lines, the contacts are gradational. Flow direction is toward the observer. (c) Particular of the gradational transition from coherent trachydacite (cst), to fragmented and poorly dispersed trachydacite (fdp), and to chaotic breccia (br). (d) Close-up of the massive lava core (cm). While the outermost part of the flow was breaking, the center was still molten and moving lava. (e) The flow banded lava (cst) is progressively pervasively fractured by a network of jigsaw cracks but remaining coherent, then is fragmented in a jigsaw-fit breccia that retains the original stratification (fpd), and, finally, is dispersed in the breccia (br). Location of boxes c, d, and e is depicted in a. (f) Map of the Leccio eruptive unit showing the areal distribution of lithofacies and structures. In the inset is the frontal ramp at the edge of the SLLF.*

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

shattered, they retain a recognizable geometry of original bedding and jointing of the primary depositional and cooling structures of lava flows. Bigger blocks are either fractured or shattered, and many of the block interiors exhibit pervasive jigsaw cracks and jigsaw-fit fractures. This lithofacies represents portions of the coherent prefragmentation lavas, slightly disaggregated and displaced. The monolithologic massive and poorly sorted breccia (lithofacies 5) is a chaotic assemblage of matrix-supported clasts from medium to well consolidated. Angular to sub-rounded fragments of trachydacite, ranging from a few centimeters to more than a few decimeters in size, are completely disaggregated and dispersed in the prevailing sandy matrix with the composition of the adjacent clasts. This matrix was likely produced by the disaggregation of the same clasts during transport.

Geologic relationships indicate that fragmentation of a rhyolite lava flow occurred mainly when the flow was spreading [92]. The spatial distribution of the breccia deposit present at the surface of the Leccio eruptive unit (**Figure 7f**) indicates that this unusual (for Monte Amiata SLLFs) volcanic facies probably formed as a function of the change in topography that the lava has encountered during emplacement. From the source area, represented by an eruptive fissure of the summit volcanic rift zone (**Figures 2** and **7f**), the Leccio trachydacite flowed for up to 3 km along a slope of about 15° and then reached a low gradient area (2–3° in slope) without confining walls at the edge of the volcanic edifice. When the flow arrived at the abrupt break-in-slope, it decelerated, and a dynamic block fragmentation occurred at the margins and surface of the flow, forming the monogenetic matrix-supported breccia observed in this part of the Leccio eruptive unit (**Figure 7**). The increase in the degree of fragmentation suggested by the textural characteristics of the breccia (grain size reduction, jigsaw cracks, and jigsaw-fit fragmentation), proceeding from the internal coherent core to external surfaces of flow, is probably the consequence of progressive fracturation, disaggregation, and dilation processes that have occurred during this deceleration phase [93, 94]. Then, the massive coherent trachydacite core (**Figure 7d**) resulted in a thermally insulated internal part of the lava that continued to flow downstream confined through a paleo-valley, attaining a total length of more than 6 km (**Table 1**), and terminated with a frontal ramp structure (**Figure 7f**).

Monte Amiata SLLFs have steep flow front, tens of meters thick, exhibiting ramp structures and monoclinal folds outlined by the orientation of the flow bedding (**Figure 8**). In ramp structures, the flow bedding varies from a flat foliation parallel to the base of the flow to an upward curved, steeply dipping to vertical shape. The stack of layers involved in the upward spoon-shaped deformation overrides along a plane of shearing and discontinuity the underlying gently dipping layers (**Figure 8a**). Contact occurs without the interposition of breccia. Moving upstream away from the ramp, the attitude of the flow bedding becomes para-concordant. These ramp structures have been observed near the frontal portion of several flow units and are probably to be attributed to an increased stress regime owing to an increasing frictional resistance toward the base of the flow ramp. Indeed, they are well developed at the front of flow that was channelized and ponded in paleo-valleys (Leccio and Vivo d'Orcia; **Figure 7f**, **8a** and **b**). The ramp structures observed in Monte Amiata SLLFs are different from both the sheet-like flow ramps described in obsidian block lava flows and attribute to individual flow units having upper and lower surfaces that are composed of *in situ* brecciated, brittle glassy carapaces (i.e., the Rocche Rosse flow at Lipari; [95]) and ramps triggered by shear planes inside the lava interior and related to the formation of surficial ogive structures [96].

#### **Figure 8.**

*Flow front structures in SLLFs of Monte Amiata. (a) Outcrop exposure and (b) interpretative sketch of flow ramp structure at the flow front of Vivo d'Orcia eruptive unit composed of fine laminated vitrophyric trachydacite with perlitic glassy groundmass. Arrow is the flow direction. Note the transition from para-concordant to discordant bedding and the absence of breccia deposits. (c) Outcrop exposure and (d) interpretative sketch of monoclinal fold structure of the flow front of Piancastagnaio eruptive unit. The core of the fold is composed of coherent massive trachydacite (cm), enveloped in flow-laminated (fl) and flow-banded (fb) trachydacite. Along the subvertical limb of the fold, the flowlaminated lava is deformed in small asymmetric parasitic folds and the flow-banded lava is stretched.*

Another type of flow front observed at Monte Amiata consists of layers which, instead of being bent upward, as in the ramps, are bent downward to form a monoclinal fold (**Figure 8c** and **d**). In this case, flow bedding is planar and subhorizontal in the upper part of the flow body, whereas near the front, it curves sharply downward until it becomes subvertical. Flow bedding experienced a stratal stretching and thinning caused by the extension in the fold limb (**Figure 8c** and **d**). The core of the fold is composed of coherent massive trachydacite, enveloped by faintly tiny laminated trachydacite with layers deformed in crumpled minor folds. Locally, a detachment surface has been observed at the base of the frontal fold. Also in this case, the deformation occurs without the presence of breccia. This type of flow front has been observed in the Piancastagnaio and Sorgente del Fiora eruptive units.

Flow front of lavas usually is obscured by a talus apron. At Monte Amiata, the presence of frontal breccia has never been observed. It cannot be excluded *a priori* that breccia has been completely removed by erosion, but the lack of both any deposit remains and its continuation in a basal breccia lead to the interpretation of an emplacement mechanism different from that of rhyolite lava flows.

#### **4.3 Facies architecture**

Within a single flow unit, the vertical and lateral lithofacies distribution and association allows to recognize a characteristic structural partitioning of lava flow interior [11, 97]: (i) basal zone; (ii) core or central zone; and (iii) upper zone.

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


#### **Figure 9.**

*(a) Overview of the outcrop showing the contact between the Quaranta (QRT) and Pozzaroni (PZZ) SLLFs. The upper portion of the Quaranta lava flow (BAS-MSS) has been* in situ *paleo-weathered into a whitish sandy saprolite with groups of corestones (CS). The basal zone of the Pozzaroni lava flow is composed of a discontinuous bed of monogenetic matrix-supported breccia (br) overlaid by flow-bedded trachydacite. Both breccia and stratified basal trachydacite are deformed in convoluted folds. dt =debris. Boxes refer to b and c details. (b) Closeup of the breccia at the base of PZZ eruptive unit. The flow lamination in the clasts is randomly oriented. (c) Detail of the recumbent fold and convoluted structures at the base of PZZ formed during the flow for the high viscosity of the lava.*

layering. Massive lava is generally uniform and dense. Lava shows poorly developed, vertical, or locally inclined columnar joints a few to several meters across (i.e., Sorgente del Fiora and Piancastagnaio eruptive units). Subhorizontal sheeting joints are also present and are more conspicuous near the top and base of the central zone. Flow layering is parallel to aligned crystals and vesicle-rich layers and is interpreted as planes of weakness imparted by the flowage. In some units (i.e., Sorgente del Fiora and Vivo d'Orcia eruptive units), lenses and irregular zones of welded breccia (lithofacies f; **Table 2**) and scoriaceous agglomerate (lithofacies g; **Table 2**), with an individual thickness of 0.5–1.5 m, form interlayers between otherwise massive lava banks. We interpret that these interlayers resulted from intraflow fragmentation processes within a single eruptive unit, probably related to stresses caused by movement in the overlying flow.

iii. The upper zone of Monte Amiata SLLFs trachydacite is characterized by a moderately more vesicular groundmass and a planar morphology forming subhorizontal plateaus and poorly inclined sides. It does not show the typical structure of lithophysae, strong vesiculation, and scoriaceous or blocky autobreccia classically described for the top of large silicic lava flows [12, 92]. The absence of these surface structures is confirmed by the stratigraphic log of the Sorgente del Fiora unit in the DL core (**Figure 4**), even if it cannot be excluded that in other units, they were present but currently poorly exposed or subsequently eroded.

## **4.4 Geochemical and petrographic characteristics**

The whole chemical composition of the lava flows (LF) and domes (LD) at Monte Amiata ranges from latite to trachydacite (SiO2 = 57–68 wt%; Na2O+K2O=7–9 wt%; **Figure 10**) [40, 42, 44, 57, 58, 100]. In variable proportion through the entire sequence, millimetric to pluri-decimetric in size, meta-sedimentary xenoliths and microgranular magmatic enclaves (ME) are present. The ME compositions range from trachybasalt to latite (SiO2 = 47–59 wt%; Na2O+K2O=5–8 wt%; **Figure 10**; [58]). A careful selection (**Table 3**) of data from literature shows that all the SLLF samples are among the most evolved rocks of the suite and classify them as trachydacite (SiO2 = 64–68 wt%; Na2O+K2O=8–9 wt%; **Figure 10**) having a normative q > 20% (q = Q/(Q + or + ab + an)\*100) [101].

The SLLFs are highly porphyritic (about 40% according to [41]), medium- to coarse-grained, with maximum dimensions of the phenocrysts rarely exceeding 1 cm (**Table 4**). The most abundant phenocrysts are plagioclase, K-feldspar, orthopyroxene, and biotite, and less abundant are apatite, ilmenite, and quartz, rarely present clinopyroxene (**Table 4**). The presence of fragmented crystals is peculiar, especially of K-feldspar (**Figure 11a** and **b**). Content in mafic magmatic enclaves is scarce, while tabular meta-sedimentary xenoliths are more frequent.

Instead, the lava domes, coulées, and short lava flows are medium to high porphyritic ranging from 26 to 34% [41, 56], with similar mineral paragenesis, but are characterized by the distinctive presence of K-feldspar megacrysts (from 1 to 5–6 cm long) coupled with abundant microgranular magmatic enclaves [40, 42, 44, 57].

The SLLFs are generally porphyritic to glomeroporphyritic with a glassy groundmass commonly showing perlitic fractures. Different groundmass microtextures are observed, also in the same unit: (i) glassy groundmass microlite-free (**Figure 11c**) or *Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*

**Figure 10.** *T.A.S. (Total Alkali-Silica) diagram for products of Monte Amiata volcano: Silicic lava flows and lava domes (LF + LD; light gray dots), magmatic enclaves (ME; dark gray diamonds), and SLLF units studied in this work (red dots; Table 2). Data from [39–42, 44, 57].*

(ii) with ultra-microlites aligned to the flow direction (**Figure 11d**); (iii) glassy groundmass locally devitrified with scattered spherulites (**Figure 11e**); and (iv) heterogeneous groundmass with flow banding in elongated bands or lenses generally of a darker color (**Figure 11f**). Flow bands are normally enriched in crystal fragments (**Figure 11g**); (v) highly vesicular with fibrous glass and large flattened vesicles (**Figure 11h**).
