**3. Volcanic deposits**

In general terms, a volcanic deposit can be defined as a stratigraphic unit that is generated directly or indirectly by a volcanic process. This includes lavas, any type of primary volcaniclastic deposits, in addition to any epiclastic deposits that are directly derived from the erosion, reworking, and redeposition of primary volcanic deposits. Unlike with non-volcanic sedimentary deposits, which tend to preserve their characteristics and appearance over long distances and wide extensions, volcanic deposits may change drastically over very short distances, which makes it difficult to determine their lateral and spatial correlations when the outcrops are non-continuous. For this reason, facies—defined as a body or fraction of rock or sediment that has a unique defining character that allows it to be differentiated from other facies or fractions of rock or sediment [10, 11]—are widely used in the reconstruction of volcanic environments [1, 8, 9, 12]. When describing volcanic deposits or volcanic facies, there will be always a set of factors (e.g., physical, chemical, and biological) that will help to define their origin (e.g., eruption, transport, and depositional mechanisms), source area, and depositional environment [13]. An important aspect to be considered here is that eruption processes are very rapid, much more so than the geological timescale. Thus, volcanic deposits, in particular those that present a very wide spatial extent, will represent very valuable isochrones in the geological record, which will be crucial to stratigraphic corrections. Therefore, the correct identification and interpretation of volcanic deposits are so important. In each case, the degree of details used in the stratigraphic division will depend on the type of study we want to do, the degree of exposure of the selected materials, and the level of knowledge we have.

The genesis or mode of formation of volcanic deposits is not always evident, so initially, as Cas and Wright [1] propose, it is better to use descriptive terms (e.g., lava flow, intraformational breccias, deposit of supported matrix, rhyolitic) rather than terms that imply a certain genesis (e.g., ignimbrite, co-ignimbritic breccia). For this reason, both during field work and while writing it once the data have been prepared, it is always advisable to first adequately describe the materials and, at the end of the process, to propose an interpretation of them, since a well-done description, in addition to being more impartial, will always remain and can be useful for later work, while interpretations and genetic models always depend on each particular author and on the trends (writing style) of the moment.

### **3.1 Lithological characteristics of ancient volcanic deposits**

## *3.1.1 Lava flows*

Lava flows are the products of effusive volcanism and may form stratigraphic units of variable extents and thicknesses depending on the composition of the erupting magma and the size of the eruption. The internal characteristics of lava flows are mostly preserved in ancient volcanic terrains (**Figure 4**), but their superficial aspects may have been partially or totally obliterated by further erosion or burial by younger sediments. In fact, on many occasions when we will only observe a cross section of the lava flow, and like so, we will be able to distinguish aspects such as macroscopic texture, autobrecciation, columnar jointing, the presence of scoriae at the base and/ or top of the deposit, but rarely we will see the original surface or base of the lava flow. On some occasions, it will be difficult to distinguish between true lava flows and younger sills that have intruded into previous sediments along a stratigraphic plane. This may be relevant when trying to interpret the cores extracted from boreholes drilled in volcanic successions, as they may have a very similar aspect. Another important aspect of paleo lava flows is the different degree of alteration that they may have experienced, as this may severely affect their original texture and mineralogy.

*Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770*

#### **Figure 4.**

*Field photographs of a) a vesicular andesitic lava flow (upper Ordovician, Catalan Pyrenees, NE Spain, 450 ma); vesicles are now filled with secondary minerals, b) classical porphyritic texture in dacitic lavas (Camprodon, Catalan Pyrenees, 300 ma) and (c) of a dacitic dome with well-developed columnar jointing (El Querforadat, Catalan Pyrenees, 300 ma) (credits: Joan Martí).*

#### *3.1.2 Volcaniclastic deposits*

The main lithological criteria to consider in the study and characterization of volcaniclastic deposits are the nature of the clastic components, namely the morphology of the grains and the resulting texture of the deposit, in addition to the petrological and geochemical characteristics of the volcanic components and the identification of the alteration products [1, 9, 14]. First, we must analyze the nature of the grains. It is necessary to identify whether it is a primary volcaniclastic deposit or, on the contrary, it has been formed by weathering and erosion of preexisting materials. Although the lithological characterization of the deposit may not be sufficient to make this discrimination, such that other criteria may be also necessary, it helps to obtain essential information when trying to characterize the deposit and identify its origin [12].

The aspects that must be analyzed to define the texture of a deposit are grain size distribution, the degree of sorting of the different fragment populations, their shape, their degree of rounding, and the fabric (**Figure 5**) [1, 8, 15, 16]. The characteristics of paleovolcanic deposits, which are generally compacted, altered, and sometimes deformed and metamorphosed, complicate their study compared with recent deposits. Although there are few difficulties in identifying the nature of the different components—for example, it is almost impossible to obtain an absolute grain-size distribution of these deposits—nonetheless, the relative size comparison of the fragments at a macroscopic level, in addition to the point count, can give us a first-order approximation on the size distribution.

Also, due to the consolidation of most ancient deposits, it is not possible to carry out a three-dimensional grain morphology study. Furthermore, alteration processes tend to modify pyroclastic morphologies (e.g., [17]). However, a detailed petrographic study can reveal some primary morphological characteristics of the volcanic grains and, consequently, reveal their fragmentation mechanism. Likewise, the degree of rounding can be examined without difficulty at macroscopic and microscopic levels, which also gives us information about the transport mechanisms of the deposits.

Other textural aspects, such as the orientation of crystals and fragments, the presence of lineations, geometry of the spaces between the clasts, etc., can provide information on the nature of the deposit and the transport and deposition mechanisms. For example, the products derived from the explosive activity of siliceous magmas almost always contain pumice fragments. Due to its vitreous nature and its high content of vesicles, this component is easily altered by post-depositional processes. However, the texture of pumice fragments can remain relatively preserved (**Figure 5**); this occurs in those cases where the vesicle content of the original fragments was relatively low due to the stretching they underwent when emplaced at high temperature (ignimbrites and welded pumice deposits), now appearing as clayey aggregates with a frayed appearance, while being preferentially oriented. However, this texture is not exclusive to welded rocks, but can also appear in deformed rocks that contained stretched or unstretched pumice fragments (**Figure 6**), or simply by compaction of devitrified pumice fragments at edaphic levels [17]. The interpretation in each case should be based not only on the textural aspect, which will be similar in all of them, but also on the relationships with the other deposits of the same sequence.

Another lithological criterion that must be taken into account is the petrological and geochemical composition of the rocks, although this is only feasible in the case of lava or other massive volcanic rocks, or in some particular cases of primary pyroclastic deposits formed almost entirely of juvenile material. In most cases, volcaniclastic rocks, whether primary or derived from the reworking of preexisting rocks, contain a variable number of lithic fragments of diverse compositions and origins. In consolidated rocks, such as most ancient volcaniclastic rocks, the impossibility of separating the different components of the deposit makes chemical analysis of the rock unfeasible, since the presence of lithic fragments contaminates the true composition of the eruptive magma. Likewise, the existence of alteration (hydrothermal, diagenetic, and/or meteoric), a common characteristic of paleovolcanic rocks, also makes it difficult to identify the chemical composition of juvenile volcanic components. However, the mineralogical composition of volcanic fragments can be established in most cases by petrographic analysis, although when alteration processes have been important, the original mineralogical composition can also be obliterated.

#### **Figure 5.**

*Field photographs of two ignimbrites with eutaxitic texture, a) from the upper Miocene (6 ma) (Central Andes, Argentina), and b) from the upper Ordovician (450 ma) (eastern Pyrenees, Spain). Despite the difference in age between the two rocks they show a very similar appearance (credits: Joan Martí).*

*Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770*

#### **Figure 6.**

*Microphotographs of a) a vesicular pumice texture in a fallout deposit, with vesicles now filled with secondary minerals (Gréixer rhyolitic succession, Catalan Pyrenees, NE of Spain, 300 ma), and b) elongated pumice fragments (fiamme) in a) strongly welded ignimbrite (upper carboniferous, Campelles, Catalan Pyrenees, 450 ma). In both cases, pumice fragments are now devitrified and transformed into clay aggregates. (credits: Joan Martí).*

### **3.2 Geometry of paleovolcanic deposits**

Geometry defines the three-dimensional shape of the deposit and will be controlled by the topography of the terrain, the volume of deposit, the transport and deposition mechanisms of volcanic materials, the existence of non- and post-depositional erosive processes, and the existence of subsequent deformations [1, 8, 12].

Three-dimensional deposit geometries are difficult to observe in ancient volcanic terrains. The paleovolcanic successions correspond to parts of complex volcanic edifices that are very rarely well preserved, such that the resulting successions and their geometry will depend on the relationship between deposition, erosion, and deformation. In general, volcanic materials are easily altered and eroded, so that a large part of them will disappear, partially forming the epiclastic volcaniclastic deposits. Moreover, paleovolcanic terrains may have been affected by further tectonic movements, so they may have been incorporated into different tectonics units that may have hidden them or have altered their original lateral continuity. This implies that the lateral extension of paleovolcanic deposits and, consequently, their relative age cannot be always established. Only in cases of rapid accumulation of large volumes of pyroclastic materials—as is the case of intra-caldera deposits in collapse calderas (e.g., [18])—can most of the original materials be preserved (**Figure 7**).

On the other hand, the presence of discontinuities in volcanic terrains is frequent and may have a relatively local significance, with the corresponding erosive episodes generally being the result of an eruptive event rather than a tectonic uplift (**Figures 7** and **8**) [2]. Likewise, the presence of strong dips in this type of terrain does not necessarily imply the existence of tectonic pulses. On the contrary, the volcanic edifices may initially have steep slopes that will determine the geometry of subsequent deposits. Thus, the interpretation of the geometry of paleovolcanic deposits must be done with great care, since otherwise the existence of phenomena may be assumed that had never really occurred [2]. A good recommendation when interpreting the geometry of ancient volcanic deposits is to compare it with current analogs in which its three-dimensional representation at the regional level can be deduced.

### **3.3 Sedimentological characteristics**

Sedimentary structures occur before deposition (i.e., erosional features), during deposition (stream-generated structures), and after deposition (bioturbations,

#### **Figure 7.**

*Panorama of the post-Variscan Permo-carboniferous volcano-sedimentary formations at Erillcastell (Catalan Pyrenees, NE Spain, 300–270 ma), where an entire intra-caldera succession (Erillcastell Fm.) is preserved [16]. Also observe the discontinuities between some of the formations, which indicate the existence of inter-formational tectonic movements. (credit: Joan Martí).*

deformations in soft sediments) of sedimentary aggregates. Together with the textural aspects, they inform us about the characteristics of the emplacement and deposition processes. Sediments can basically be transported in two ways, particle by particle or en masse, resulting in different structures in both cases, although not always exclusive, such that each sediment must be analyzed in detail and evaluated on its own merits [1].

These types of transport mechanisms can occur both in primary pyroclastic materials and in other types of sediments, although the transport medium is usually gas in the former, while in the latter it is frequently water. This implies differences in the morphological and textural characteristics of the sediment components, which will allow us to identify the genetic character of the deposit.

Volcaniclastic deposits tend to present elements (either textural or sedimentary structures) that allow us to reconstruct the directions of the paleocurrents. In epiclastic materials, the existence of ripples, dunes, cross-stratification, imbrications, angle of repose, etc., constitute the basic elements for this task. Pyroclastic materials can also exhibit unidirectional sedimentary structures, especially in the case of dilute PDC (i.e., pyroclastic surge) deposits (**Figure 8**). Massive deposits, such as dense PDC deposits or lahars, may present other types of structures such as imbrications, lineations of elongated elements (crystals, stretched pumice fragments, plant remains, etc.) that also allow the direction of flow to be identified.

## **3.4 Fossils**

The use of fossils as paleoenvironmental indicators is essential not only for nonvolcanic successions, but also for volcanic ones. In paleovolcanic successions, the

*Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770*

#### **Figure 8.**

*Succession composed of primary phreatomagmatic pyroclastic deposits mostly emplaced by dilute PDCs, showing a wide diversity of sedimentary structures (middle Miocene, México) (credit: Joan Martí).*

presence of fossils in the interbedded epiclastic, but also in the pyroclastic deposits, can provide information about the age of the rocks and their depositional environment, although it is not always easy to know whether the fossils were deposited *in situ* or were transported and redeposited. The presence of fossils (vertebrate, invertebrate and plants) within pyroclastic deposits (e.g., [19]) and lavas (e.g., [20]) is common. In addition to the stratigraphic, paleoenvironmental, and paleoclimatological information that this represents (e.g., [21, 22]), we can also obtain information on the emplacement temperature of the deposit and the direction and sense in which it was emplaced (e.g., [23, 24]). Also significant is the presence of fossils in the post-eruptive successions of maars (e.g., [25]), or the presence of vertebrate tracks on volcanic and associated deposits [26], which helps to decipher their paleoenvironmental evolution.

## **3.5 Factors that alter the original characteristics of volcanic deposits**

Volcanic materials may undergo, including from the stages immediately after their emplacement, a series of transformations that can imply significant changes in their texture, mineralogy, and chemistry. The metastable nature of volcanic glass, the main component in this type of rocks, favors these changes. In paleovolcanic terrains, there is also the superposition of large-scale erosive and re-sedimentation processes, such as sediment gravity flows or debris avalanches, diagenesis, tectonic deformations, and even metamorphism. For all these reasons, the lithological characteristics that we can identify in modern deposits are not always comparable or identifiable with the same clarity in ancient materials.

One of the conflicting points in the interpretation of the alteration processes experienced by volcanic materials is the distinction between the results produced by processes such as weathering, hydrothermal alteration, or diagenesis. In all cases, the result of the transformations of the affected rocks is the devitrification of the glass and the formation of secondary minerals replacing the original glass, filling the pores of the rock, and/or partially or totally replacing the primary minerals. Consequently, there will be a change in the chemical composition of the original volcanic components, although the intensity of this change will depend on the intensity of the alteration processes and the original composition and texture of the rock (**Figure 9**).

#### **Figure 9.**

*Examples of microphotographs of volcanic and subvolcanic rocks from the Permo-carboniferous terrains (305–285 ma) of the Catalan Pyrenees (NE Spain) showing different degrees of alteration. a) Dacitic lava flow, showing a porphyritic texture with phenocrysts of quartz and plagioclase. b) Granodioritic dyke, showing a porphyritic texture with phenocrysts of quartz and partially altered plagioclase. c) Crystal-rich (phenocrysts of quartz, plagioclase, and biotite), totally devitrified (to microcrystalline quartz aggregates) pumices, in a partially welded ignimbrite. d) Crystal-rich (phenocrysts of quartz, plagioclase, and biotite), totally devitrified (to microcrystalline clay aggregates) fiamme pumice fragments, in a very crystal-rich ignimbrite matrix. e) Eutaxitic texture in an ignimbrite matrix with devitrified fiamme transformed into clay aggregates. f) Crystal-rich dilute PDC deposits. (credits: Joan Martí).*

## *Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770*

Weathering includes a set of processes, such as the chemical action of the atmospheric air and rainwater and of plants and bacteria, as well as the mechanical action associated with temperature changes, by means of which the rocks exposed on the Earth's surface alter until they become soils [27, 28]. Weathering depends on the climatic conditions prevailing in the deposition area and on the composition of the rocks. The changes produced by weathering tend to cause a compositional zonation in the rock, in most times grading vertically in intensity but also from an unaltered zone in the interior to zones with variable alteration in the external part. In the altered parts, weathering can imply a loss of original porosity in the rock, although sometimes this can also increase due to the dissolution of the original glass components (e.g., [29, 30]).

Hydrothermal alteration includes the chemical, mineralogical, and textural changes that occur in rocks due to thermal and chemical changes in the environment in the presence of hot water, steam, or gas [31, 32]. Hydrothermal alteration involves ion exchange reactions, mineral phase transformations, mineral dissolution, and the precipitation of new mineral phases [1]. In most volcanic zones, hydrothermal alteration is associated with the proximal zones, which are characterized by the presence of fluids secreted directly from the residual magma chamber, or *via* the percolation of meteoric water that is heated at depth by this magmatic heat source. These hydrothermal fluids experience a convective-type permanent circulation that allows an almost continuous transformation of the host rocks. However, in distal areas of pyroclastic and lava deposits, a particular type of hydrothermal (or autohydrothermal) alteration can also occur, caused by the magmatic gases themselves that have been trapped in the deposit ("vapor phase alteration"), producing rapid devitrification of its vitreous components and precipitation of secondary mineral phases in the vesicles and pores of the rock [33–37].

Hydrothermal alteration can produce quartz aggregates, amorphous silica, potassium feldspar, albite, calcite, montmorillonite, illite, kaolinite, alunite, chlorite, zeolites, and low-grade metamorphic minerals, depending largely on the composition of the rock and the origin and composition of the hydrothermal fluids. Likewise, hydrothermal alteration is responsible for the presence of important epithermal mineralizations of precious metals and metallic sulfides that are associated with many volcanic zones, especially in paleovolcanic terrains (e.g., [38]). Sometimes hydrothermal alteration is also responsible for the existence of pseudo-eutaxitic or clastic textures, since it tends to give patching-type structures (e.g., [1, 39, 40]), which can confuse massive lava flows for volcaniclastic deposits. This implies that extreme care must be taken when examining the textures of paleovolcanic rocks and using other criteria such as their relationship with other deposits, their geometries, lateral variations, before making a final diagnosis of their nature. Special attention should be given to rocks that have undergone subsequent deformation and therefore present penetrative schistosity, since the recrystallization of clay minerals, micas, and chlorites is more important and can enhance the presence of pseudo-clastic textures.

Diagenetic changes group all those that can take place within sediment following its deposition and burial, except those that are due to metamorphism or weathering on the Earth's surface. There has been an ongoing discussion among researchers about the exact demarcation limits under which diagenesis occurs. In our case, we consider as such the chemical, mineralogical, and textural changes that occur slowly and at low temperatures (between 20 and 300°C), which occur in the sediment (volcanic rocks) after its burial, although being relatively close to the surface to be able to withstand pressures less than 1 kbar (~ 3 km deep).

Diagenesis is a process associated with the lithification of sediments, which includes compaction, cementation, recrystallization, authigenic mineralization, and the growth of concretions or nodules [41]. Diagenetic processes occur in the early stages of burial of deposits and are associated with the circulation of interstitial fluids, these being mostly meteoric water. Unlike what happens with weathering or hydrothermal alteration, diagenesis affects the entire rock more homogeneously, especially in terms of changes in texture, although there may be zoning in the appearance of sequences of secondary minerals. The diagenesis of volcaniclastic rocks is of great importance in the exploration of hydrocarbons, since it produces a modification of the original porosity of the rock, generating a secondary porosity and favoring the maturation of hydrocarbons (see [42]).

In rocks that contain abundant primary volcanic components (pyroclastic rocks), diagenesis can be favored by the existence of previous compositional and textural changes in the glass produced during their transport and initial weathering. However, in the case of intra-caldera succession (e.g., [43, 44]), the rapid emplacement of thick ignimbritic successions prevents weathering and favors the vapor phase and a very early diagenesis (or hydrothermal alteration) but at a much higher temperature than it would be in a normal burial process.

Metamorphic transformations, whether due to regional or contact metamorphism, constitute a higher degree extension of diagenesis, although they should not be confused with hydrothermal alteration, which generally has a much more localized effect [1]. Metamorphism produces significant mineralogical and textural changes and, if accompanied by deformation, can completely obliterate the initial texture of the rock, especially when there are intermediate to high-grade transformations. In low-grade metamorphic transformations (green schist facies), it is possible, however, to still recognize some primary textural aspects such as welding textures, vitroclasts, or perlite fractures (**Figure 10**).

In all these alteration processes, the fundamental factor is the metastable character of the volcanic glass, the resulting products reflecting its original composition. The devitrification of basaltic or silicic glasses can give rise to totally different products even when they have been generated under very similar conditions. Palagonitization is a typical alteration of basaltic glass in both subaqueous and subaerial conditions, characterized by its transformation first into an apparently amorphous substance (palagonite), due to the initial hydration process, and later into smectites and zeolites. Palagonitization is explained as the result of hydrothermal or diagenetic alteration [8, 45–50], especially in submarine environments, although it can also be explained because of hydrovolcanic processes (e.g., [51]). Unlike basaltic composition glasses, the devitrification of silica glasses gives rise to the formation of perlite textures in the initial stages of hydration (without emplacement) and the majority formation of zeolites, clay minerals, and potassium feldspar in the more advanced stages. A singular case is the formation of tonsteins (e.g., [52]) and bentonites (e.g., [53]) that correspond, respectively, to layers rich in kaolinite (illite and smectites) interbedded between deposits of marl, slates, and especially coal layers and to layers that are dominated by smectites. In both cases, it is the product of the alteration of layers of silicic ashfall deposits.

In addition to the alterations described above, it should be noted that volcanic terrains are places where contemporaneous crustal movements take place and because many of them are associated with orogenic belts, where subsequent penetrative deformation may take place. Deformation can significantly change the original stratigraphic relationships and deposit geometry. Likewise, deformation can also

*Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770*

**Figure 10.**

*a) Microphotographs of vitroclasts in an ignimbrite matrix b), perlite fractures in a highly welded, rheomorphic pyroclastic deposit (Gréixer-Coll de pi rhyolitic succession, Catalan Pyrenees, NE Spain, 300 ma) (credits: Joan Martí).*

produce important changes at macroscopic and microscopic scales, causing important problems for the identification of primary textures. For this reason, in paleovolcanic terrains that have undergone tectonic transformations, it is important to carefully study the textures of the volcaniclastic rocks, in order to not confuse aspects superimposed by the deformation of the original elements of the rocks. In the same way, at a regional and outcrop level, it is necessary to know how tectonic deformation has affected the original structure of the area and, if possible, to conduct a palinspastic restoration of the terrain to its original position (e.g., [54]).
