Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism

Abstract

This chapter discusses how to apply the most significant aspects and concepts of modern volcanology to the study the ancient volcanic terrains, where volcanic successions appear exposed in discontinuous outcrops, with various degrees of deformation, which are often manifested in the presence of metamorphosed and hydrothermally altered volcanic rock assemblages. The way to understand paleovolcanism is through the identification and interpretation of the products of past volcanic activity in terms that is equivalent to what is done in modern terrains, despite the difficulty of having to characterize and recompose all those subsequent geological processes that have been superimposed upon them. This chapter summarizes the most fundamental aspects of the study of ancient volcanic terrains, paying special attention to the definition of facies associations, the characterization of their spatial and genetic relationships, and their paleoenvironmental and paleogeographic significance, as well as to the possible causes of the original facies modification. The implications for the presence of volcanism in the dynamics of sedimentary basins and its relationship with different geodynamic environments are also analyzed.

1. Introduction

Volcanic deposits are present since the very beginning of the geological record, thus confirming that volcanism has been a main component of Earth’s evolution. Active volcanism has important implications for our society. There is of course the constant threat that volcanic activity represents for the immediate areas around active volcanoes, but there are also risks at greater distances, even globally, depending on the intensity of this activity. On the other hand, volcanic systems represent an important source of natural resources (e.g., geothermal energy, mineral deposits) (Figure 1a). In addition, volcanism provides relevant information to understand the dynamics of the Earth’s mantle and crust, which is essential to understand the evolution our planet, in addition to other planetary bodies with similar characteristics. For these reasons, characterizing past volcanic events in the same way that we characterize active volcanism is essential to decipher the meaning of these past volcanic episodes in a local, regional, or even global context. Unfortunately, the degree of preservation of ancient volcanic deposits, which may have suffered important transformations due to erosion, diagenesis, hydrothermal alteration, tectonic deformation, etc., may not always be conclusive to recognizing their nature and significance, hence complicating their comparison with modern deposits (Figure 1b). However, the study of ancient volcanic successions, even if this represents an additional handicap, should employ the same concepts and methods (e.g., stratigraphic correlations, facies analysis, textural characterization, componentry analysis) that are used in the characterization and interpretation of historical eruption products (e.g., [1]).

Figure 1.

Sketch illustrating the main concepts of modern and paleovolcanism and their main observational differences. a) Current state of an active central volcano and sedimentation in and adjacent basin. Primary volcanic deposits (lava flows, pyroclastic (air fall, PDC, lahars, etc.)) deposits contribute to the infill of the basin together with deposits resulting from their erosion and redeposition (reworking) by external (epiclastic) processes. Black arrows indicate the sense of movement of magma inside the volcanic edifice, lava flows, eruption column, and PDC. b) State of the same depositional environment after volcanic activity has ceased and the volcanic edifice has been partially dismantled, producing volcanic epiclastic deposits that will mostly contain fragments from the primary volcanic deposits. c) Same scenario after hypothetical compressional tectonics and further erosion (white line).

The same transformation that volcanology has undergone regarding the study of active or recent volcanic areas—from a basically descriptive to a more interpretative and quantitative science—should also affect the study of ancient volcanic terrains. The presence of volcanic episodes in the geological record should not be regarded as an isolated or sporadic event that does not necessarily need to be relevant to interpret the remainder of rocks found at a particular site [2]. On the contrary, we should try to deduce the tectonic controls that conditioned the rise and accumulation of magmas and their geodynamic significance, the mechanisms responsible for their eruptions, or to determine the influence that volcanic activity had had on sedimentation in the basin. In this sense, the invariance over time of the physical parameters that control the ascent and eruption of magmas allows ancient and modern volcanic products to be compared and interpreted in the same terms.

In this chapter, I will revise the main concepts that should be considered in the study of ancient volcanic succession to achieve the same degree of accuracy and information as in the study of recent volcanism.

2. Definitions

Before starting with the identification criteria for paleovolcanic rocks, it is necessary to review some definitions referring to the processes and products of paleovolcanism. The first aspect that requires our attention is the proper definition of paleovolcanism.

There is no specific age from which the limit between ancient volcanism (or paleovolcanism) and modern volcanism can be distinguished, since we can find relatively recent terrains that have been strongly altered and eroded (e.g., some volcanic islands or active calderas hosting geothermal fields), and ancient terrains that preserve a large part of the original characteristics of their volcanic materials (e.g., Permo-Carboniferous volcanism in some areas). For this reason, and although there is no specific definition for the term paleovolcanism, we will consider it as the volcanism recorded in the stratigraphic record of regions that have undergone erosion, diagenesis, tectonics, hydrothermal, and/or metamorphic processes, thus causing significant changes in the original volcanic facies.

The second aspect that needs our attention is the difference between processes and products (Figure 2). Processes refer to those aspects concerning the origin as well as the transport and emplacement mechanisms of volcanic materials, while the term products should be understood as the result of these processes. While this is not particularly problematic when referring to a lava flow—and in this case, both the process and the product will receive the same name, lava flow—the situation is much more complex when we refer to any type of clastic volcanic deposits (e.g., covering the full spectrum of primary, eruption-fed products to secondary, epiclastic successions). For this reason, it is necessary to distinguish here between process and product and to use an appropriate nomenclature that allows them to be differentiated. As an example, we can use the term pyroclastic density current (PDC), which refers to the flow processes of transport and deposition of primary pyroclastic material (e.g., [3, 4]). It is therefore incorrect to use this term as a particular type of fragmentary volcanic deposit. The term PDC deposit, as a general term to refer to an indeterminate type of deposit produced by such a process, or the terms ignimbrite, block-and-ash flow, dilute PDC deposit, dense PDC, deposit, etc., applied to specific deposits derived from PDCs with different characteristics, are appropriate to refer to the products of this process.

Figure 2.

Difference between processes and products in volcano-sedimentary environments: a) formation of an eruption column that will rise into the atmosphere and will disperse horizontally controlled by the predominant winds and initiation of a PDC that will run away from the vent on the volcano slopes controlled by gravity (Mount Saint Helens 1980 eruption, (photo by H. Glicken-U.S. Forrest Service. Credit: USGS). b) Fallout deposit formed by the deposition of pumice fragments from the eruption column and of an ignimbrite deposited from a PDC (Tenerife, Canary Islands) (credit: Joan Martí).

This complexity increases when referring to paleovolcanic materials where the discrimination between primary and secondary products (derived from the weathering and erosion of the former) is not always straightforward (e.g., [1, 5]). Likewise, terms such as ignimbrite, block-and-ash, Plinian fall deposit, although they refer to a deposit, imply a specific type of process in each case. In paleovolcanism, it is not always straightforward to identify the genetic characteristics of a deposit based on its lithological features, hence I recommend using purely descriptive terms (see below), even though this description may fit for the products of different processes, and there will be time to apply more precise terms if the information obtained eventually allows it.

To clarify the nomenclature of clastic volcanic materials, Fisher [6, 7] established two groups of definitions, the first group is non-genetic, based on the lithological characteristics of volcanic materials in order to differentiate the different products, and the second group comprises definitions focused on differentiating between their genetic mechanisms. Some of these definitions were reviewed by Fisher and Schmincke [8] and Fisher and Smith [9]. According to Fisher’s definitions, the term volcaniclastic includes the entire spectrum of clastic materials composed in part or entirely of volcanic fragments originating from any particle formation mechanism (i.e., pyroclastic, hydroclastic, epiclastic, autoclastic), transported by any mechanism, deposited in any physiographic environment, or mixed with any other volcaniclastic type, or with any type of non-volcanic fragments in any proportion whatsoever. This non-genetic term allows products to be identified without the need to attribute origins or processes to them.

The main fragmentation processes that generate volcaniclastic deposits are pyroclastic, hydroclastic, autoclastic, and epiclastic (Figure 3) [7, 9]. Pyroclasts are formed by direct fragmentation of magma due to the rapid exsolution and explosive expansion of the gases it contains. Hydroclasts are formed by explosive or non-explosive water-magma interactions that result in frozen glass particles. Autoclastic fragmentation is caused by the mechanical friction of lava flows that are being emplaced or by the gravitational collapse of domes or spines. Finally, epiclastic fragments are lithic fragments and crystals derived from any type of preexisting rock by weathering and erosion—in this case, volcanic or volcaniclastic. Fisher and Smith [9] suggested that to understand volcanic facies and sedimentation differences between volcanic and non-volcanic areas, fragmentation processes must be clearly separated from particle transport processes (e.g., wind, pyroclastic flows, flowing water, ice, avalanches), since these terms refer to the processes that create the particles and that cannot change from one type of particle to another simply by changing the transport agent.

Figure 3.

Examples of products originated by different fragmentation processes. a) Pumice-rich ignimbrite resulting from pyroclastic fragmentation of the erupting magma at the conduit (Cerro Galán ignimbrite, Central Andes, Argentina, 2 Ma). b) Hyaloclastites originated by hydrofragmentation of a subglacial basaltic lava flow (precaldera deposits, Deception Island, Antarctica, unknown age). c) Autobrecciated sub-aerial andesitic lava flow (Coll de Vanses andesites, Catalan Pyrenees, NE Spain, 300 Ma). d) Epiclastic deposit (ep) formed by water erosion and redeposition of a previous ignimbrite (ig). (Castellar de N’Hug Permian red beds, Catalan Pyrenees, NE Spain, 285 Ma) (credits: Joan Martí).

In this chapter, I follow Fisher’s conceptual recommendations, and therefore, an attempt is made to clearly differentiate between processes (fragmentation, transport, and deposition mechanisms) and their products (rocks and deposits), bearing in mind that before deducing the process, we must describe and correctly interpret the products, which is not always possible.

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.

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.

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).

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.

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).

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.

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).

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).

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.

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).

3.3 Sedimentological characteristics

Sedimentary structures occur before deposition (i.e., erosional features), during deposition (stream-generated structures), and after deposition (bioturbations, 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 non-volcanic successions, but also for volcanic ones. In paleovolcanic successions, the 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).

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, 34, 35, 36, 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).

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).

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, 46, 47, 48, 49, 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 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]).

4. Stratigraphy of paleovolcanic successions

An important aspect in the characterization of paleovolcanic terrains is the correct description and interpretation of their stratigraphy. We have already highlighted some of the most relevant difficulties in merely identifying ancient volcanic deposits, not to mention the challenges to establish their lateral relationships and the relative ages. The development of a detailed stratigraphy can help alleviate these difficulties and correctly interpret the succession of events that make up a given succession of deposits. Likewise, the completion of such a stratigraphy is essential to be able to interpret such successions in terms of eruptive sequences.

The stratigraphic divisions to be established, as well as their geological mapping, will depend on the type of study to be carried out. However, there must be some general criteria that allow us to establish a stratigraphy that can be compared with other similar examples and, therefore, can always be interpreted in the same way. When investigating a given area, the first step must always be the objective description of the local stratigraphic succession in terms of lithostratigraphic units able to be mapped and correlated. Lithostratigraphy consists of the description, identification, and interpretation of rock units (see [55]). Individual units must be described and defined based on their general lithological characteristics and their interrelationships with adjacent units. Stratigraphy of volcanic terrains, both modern and ancient, should try to identify the stratigraphic and chronological order in which the products of an eruption, a series of eruptions, or of an interruptive period (epiclastic deposits or reworked volcanics) appear in the geological record (Figure 11) [2]. Therefore, the most logical way to describe the stratigraphy of volcanic terrains is by using the same principles as classical stratigraphy (e.g., [56]), that is, to identify and group the different existing units based on a hierarchy that allows for the identification of a temporal succession of events or units of eruptive activity [2, 8]. In recent volcanism, the identification of the different lithostratigraphic units can be done without too much difficulty since the products of the different eruptions can nearly always be easily distinguished. Likewise, the variations in the compositional trend of the magmas, eruptive styles, or other characteristics that allow the deposits from different eruptions to be grouped in cycles of volcanic activity are equally identifiable. However, in older terranes, due to the complications discussed above, establishing the correct lithostratigraphy is not always possible. Despite this, attempts should be made to use the same lithostratigraphic subdivisions, since an accurate interpretation of a paleovolcanic zone must include (or at least attempt) the identification and interpretation of the different volcanic episodes recorded in the succession of deposits. Martí et al. [2] have provided a detailed review of the principles of volcanic stratigraphy and how they should be applied in field studies of volcanic terrains. I direct the reader to this contribution to be informed about the methods used in volcanic stratigraphy.

Figure 11.

Example of a volcano-sedimentary succession in which different volcanic units (lava flows (L), primary pyroclastic deposits (P), and epiclastic deposits (E)) all having originated by reworking of volcanic material (Tenerife, Canary Islands). The presence of paleosoils (Pa) separating some of the deposits is also visible. (credit: Joan Martí).

As we have seen previously, the different deposits can be identified based on their lithology (mineralogy, petrology, alteration, color, degree of welding, grain-size distribution), geometry, and relative stratigraphic position. Within a volcano-sedimentary succession, the existence of different deposits corresponding to the same eruption (i.e., Member) and of different members constituting a formation can be established based on the presence of first-order discontinuities such as paleosoils, erosional surfaces, or interbedded epiclastic deposits. However, the presence of erosional surfaces or interbedded epiclastic deposits does not always indicate a significant interruption in eruptive activity. This can be especially important when trying to reconstruct an eruptive sequence from a poorly exposed succession of deposits. Recall that some pyroclastic deposits are emplaced in a highly turbulent regime (e.g., [57]), so they can erode previously formed deposits without indicating a change of eruption. In the same way, the existence of rainfall, sometimes torrential, associated with volcanic eruptions is a common fact, and this may cause some primary pyroclastic deposits to be partially reworked during the eruption itself (e.g., [58]). In this case, the maturity of the epiclastic deposit (degree of reworking) will be a criterion to consider in its identification.

Establishing the age of the deposits forming a particular stratigraphic succession is crucial to determine the eruptive history of a particular volcanic system. This will permit distinguishing between several eruptions and also establishing the existence of possible cycles of activity. However, knowing the “absolute” (radiogenic) age is not always possible, since it will depend on the quality (degree of alteration) of the samples, their mineralogy, and the limitations of the method itself. The same restrictions or uncertainties apply with dating based on flora found in ancient pyroclastic or associated deposits. Therefore, what is essential is to establish at least the relative chronology of the set of deposits studied.

The importance of establishing a correct stratigraphy for correlation purposes relies on the fact that pyroclastic materials can be deposited over wide extensions, sometimes exceeding the limits of the basin itself, which means that these volcaniclastic horizons occasionally constitute excellent correlation levels. As previously mentioned, we should also consider that a volcanic eruption represents a very short period of time (generally hours or a few days), which, when translated to the geological scale, means an instant; in this way they can be considered as a physical representation of an isochron.

In recent years, magnetostratigraphy, which uses variations within the stratigraphic sequence of the magnetic properties of rocks (magnetic susceptibility and direction of remanent magnetism), has emerged as an excellent method for geological correlations (see [59]) and particularly in old volcanic terranes (see e.g., [60]). In this sense, we must consider that magnetostratigraphy, together with radiometric or fossil dating, allows us to obtain not only relative ages but also an absolute timescale of the volcanic succession.

In any case, it must be kept in mind that when going backward (toward older terrains) in the examination of paleovolcanic terrains, the geological timescale is progressively less well defined, so we may find that simple stratigraphic unit levels are representing very important periods of time, even several million years long, as the degree of preservation of volcanic materials becomes worse proportional to the age of the terrain. However, in fact, a volcanic level—and especially those of pyroclastic origins—represents an instant not only on the geological timescale, but also on the human timescale. For this reason, we must be very careful in interpreting the chronostratigraphic value of volcanic units in ancient terrains since each deposit by itself represents a single event in geological time but corresponds to the culmination of long geodynamic and magmatic processes that may extend significantly longer than the observed stratigraphic succession.

Volcanic deposits may show significant lateral variations from the vicinity to the vent to the areas away from it (e.g., [61]). In this sense, it is worth mentioning that proximal to distal definition is far not as fixed as in normal sedimentary environment. In volcanic systems, these can be in a very broad range, even with similar eruption styles but different eruption intensity, eruption rate, etc. This is particularly important in paleovolcanic systems where we have limited spatial knowledge about them system. So what we see in the cross sections offered by most outcrops needs to be scale up to 3D to be able to provide an “intelligent guess” for the location’s position relevant to the source.

Depending on the distance to the vent, volcanic deposits can be proximal, intermediate, or distal. For example, fall deposits will progressively decrease in thickness and grain size with the distance from the vent. For ballistically emplaced deposits, a more or less radial distribution can be observed around the vent, but the deposits associated with the horizontal dispersion of the eruptive column will present a distribution that will depend on the orientation of the prevailing winds, although the proximal to distal distribution will be as mentioned before. The deposits generated by PDCs may also present significant lateral variation with distance from the vent. In the case of deposits emplaced from dense PDCs, they may correspond to thick units (intra-formational breccias) in the proximal zones (Figure 12a), massive ignimbrites in the intermediate zones (Figure 12b), and co-ignimbritic ash layers in the most distal areas (Figure 12c). Deposits emplaced from dilute PDCs, especially those associated with tuff ring or cone-type edifices, present a very characteristic distribution from the vent to the distal zones, with significant changes in their internal sedimentary structures. However, in ancient volcanic terrains, especially for those in which later tectonic processes have been important, it is possible that only parts of the geological record corresponding to volcanic activity have been preserved, so that these variations from proximal to distal will probably only be assumed on the basis of variations in the deposit’s thickness, grain size, or rock type [61, 62, 63].

Figure 12.

Example of a) proximal (co-ignimbrite lag breccias), b) intermediate (units of ignimbrites showing a characteristic planar basal contact), and c) distal (distal, > 100 km away from the vent, strongly indurated (silicified) ash fallout deposits), deposits from the Permo-carboniferous volcanism of the Catalan Pyrenees (NE Spain) (credits: Joan Martí).

Within this ideal model of proximal-distal (referred to the vent area) variations in volcanic terrains (Figure 1a and 12), proximal areas are mainly represented by lava flows, domes, and coarse-grained primary pyroclastic deposits or epiclastic volcaniclastic materials generated by erosion and gravitational processes, which act on the steep slopes of the volcanic edifice. In deeply eroded terrains, proximal areas may also include different groups of subvolcanic intrusive rocks (stocks, sills, and dykes). The presence of a significant fumarolic alteration is also a good guide to identify proximal zones in paleovolcanic terrains. The intermediate areas are mostly represented by the terminal parts of lava flows and thick successions of PDC, as well as some fallout deposits and their reworked products. Increasing the distance from the vent also increases the amount of re-sedimented pyroclastic material and epiclastic deposits. Finally, the distal areas will be formed by fine-grained fallout deposits interbedded with abundant non-volcanic sedimentary material.

5. Depositional environment

An important aspect in the study of ancient volcanic terrains is to identify the corresponding depositional environments. These include all the physical, chemical, biological, and geological aspects that affect sedimentation within a specific area, being possible to distinguish between local environments (e.g., marine and non-marine, fluvial, lacustrine, wind, deep, shallow), defined in geomorphological terms, and tectonic environments with a much broader regional implication [13].

The explosive character of some volcanic activity is widely recognized in subaerial environments, and the distinctive characteristics of the products of this type of volcanic activity are well established. However, in relatively shallow subaquatic environments, explosive volcanic activity with characteristics similar to subaerial ones and with nearly identical products can also occur [64, 65, 66, 67, 68, 69, 70]. Likewise, this may be the case for PDC deposits originating in a subaerial environment, but that were ultimately emplaced in a subaqueous environment. In this case, there are no particular characteristics that allow them to be distinguished from purely subaerial or subaquatic deposits of the same type [71, 72, 73, 74, 75]. Furthermore, it will be necessary to identify the existing lateral variations within the volcanic deposits and the characteristics of the interbedded epiclastic deposits in order to accurately determine the depositional environment. In deep submarine environments, the hydrostatic pressure of the water column inhibits the vesiculation of magmas (e.g., [76, 77]), such that the volcanic activity will be predominantly effusive, regardless of the type of magma, until the volcanic edifice grows enough for the magma column to reach sufficiently superficial levels where it can experience explosive vesiculation, thus beginning to generate the first pyroclastic products [70, 77].

Depositional environments will also depend strongly on the tectonic setting where they develop, as this will control the geometry of sedimentary basins, rate of subsidence, location of volcanic vents, types of volcanism, local tectonics, and the general sedimentation rate.

Finally, it is worth mentioning that in paleovolcanic terrains in which the mixture of pyroclastic material with sedimentary material (with different proportions of each) is frequent, subsequent transformations experienced by volcanic deposits can cause significant changes with respect to the primary composition and texture of these rocks. However, both the composition and the secondary texture resulting from these transformations can serve to establish groups of deposits based on compositional and textural characteristics that may help their spatial and temporal correlation (Figure 13).

Figure 13.

Comparison between two microphotographs of crystal-rich, pumice rich ignimbrites from a) Cerro Galan, Argentine (2 ma) and b) Prats d’Aguiló, Catalan Pyrenees, NE Spain (300 ma). Both show a very similar texture, with a similar content of phenocrysts of the same composition (quartz, Q; plagioclase, Pl; biotite, Bi), pumice fragments (P), vitric in the Gerro Galán ignimbrite and devitrified (clay aggregates) in the Prats d’Aguiló ignimbrite, and lithic fragments (L). (credits: Joan Martí).

6. Volcanism, basin dynamics, and sedimentation

The presence of volcanic deposits is frequent in many sedimentary basins. The existence of volcanic episodes in the sedimentary record represents an important source of information to understanding the geological evolution of that particular time frame. Specifically, the presence of volcanism responds to geodynamic conditions that favor the genesis and rise of magmas and that, on the other hand, can translate into suitable tectonic conditions for the development of a subsidence structure [78]. In paleovolcanic terrains, where erosion and tectonics may have obliterated their original characteristics, the location of vent zones or proximal areas will help to infer the position of the main fault zones that controlled volcanism and that occasionally may also be associated with basin subsidence—even when these faults may have been reactivated in subsequent tectonic movements (e.g., [54, 79]). Likewise, volcanic deposits, and especially those derived from explosive eruptions, constitute a valuable tool for establishing stratigraphic correlations within and outside the limits of the basin, as well as a precise geochronology of the host sedimentary successions (e.g., [80]). On the other hand, the interaction between volcanic processes (which are generally catastrophic) and sedimentary processes sometimes leads to the appearance of “anomalous” deposits within the sedimentary successions, which can be predisposed to erroneous interpretations if the nature of volcanic processes is not well known [81, 82]. Thanks to the current approach of volcanology that pursues the study and understanding of volcanic processes, bringing together many other aspects besides the pure identification and classification of volcanic rocks, it is possible to obtain a much broader vision of these phenomena, which helps in the interpretation of other geological problems, such as basin analysis.

The formation of a sedimentary basin is a geodynamic process that frequently implies the existence of a fracture network that allows the progressive subsidence of the blocks it delimits, thus accommodating sedimentation [83]. Similarly, volcanic episodes respond to geodynamic processes that lead to the formation of magmas at depth and hence facilitate their rise to the surface. However, the formation and ascent of magmas do not always imply the existence of eruptive processes. Only when tectonic conditions are adequate (especially in the upper crust) can magma reach the surface. These conditions imply a locally distended stress field that favors the opening of fractures and consequently the rise of magma through them (e.g., [84]). In many cases, these fractures are the ones that delimit the basin and control its subsidence, so the location of the volcanic centers is directly related to the structure of the basin. Therefore, in paleovolcanic terrains, the reconstruction of volcanic stratigraphy that indicates the position of vents may help to infer the structure of the basin where volcanic deposits have been emplaced.

Due to the fact that the physical conditions that control the release of magma to the surface do not vary with time, the study of volcanic processes is useful, above all, in the reconstruction of those basins that have undergone subsequent tectonic transformations. In current basins, the application of geophysical methods allows obtaining an adequate understanding of their structure and dynamics—although these methods may be of little value in the interpretation of ancient basins. However, the reconstruction of the volcanic episodes helps to know the initial structural conditions that controlled them and, therefore, allows one to deduce which were the tectonic features that directed the dynamics of the basin. Moreover, in deeply eroded paleovolcanic terrains, it is sometimes possible to observe the roots of large volcanic complexes (stratovolcanoes, collapse calderas, etc.) represented by different sets of subvolcanic intrusions and faults [85, 86, 87], whose orientation may depart from the regional ones; hence, the structural reconstruction of such paleovolcanic settings needs to be conducted and understood at different spatial and temporal scales when these complexities appear.

The influence of volcanic activity on sedimentation can be significant in various aspects. The sedimentation rate in a basin with volcanism can be much higher than in a non-volcanic basin with similar characteristics. This may represent a significant increase in the rate of subsidence of the basin, while it can significantly reduce the time required to become clogged. This fact can be accentuated in the case of volcano-tectonic basins or especially in the case of large collapse calderas, where the deposition of successions of volcano-sedimentary materials, several hundreds of meters thick, is carried out over very short periods of time (e.g., [43]). This can be misleading if the observer does not properly separate both processes on the timeline.

In a sedimentary basin where there is a direct influence of volcanic activity, sedimentation will be significantly affected by the simultaneous presence of eruptions that generate large volumes of pyroclastic materials and by the growth and subsequent dismantling of volcanic edifices. When studying the response of the sedimentary system to the presence of volcanism, we must make a distinction between syn-eruptive periods and inter-eruptive periods [82]. The syn-eruptive periods are characterized by the instantaneous production, geologically speaking, of large volumes of volcaniclastic sediments and other volcanic products that may be remobilized and deposited through different sedimentation processes (e.g., [88]). These periods are short and are separated by relatively longer inter-eruptive periods during which volcanism has little or no influence on the sedimentary system and which will consequently be characterized by a significant decrease in sediment production. In the stratigraphic record, the existence of these syn- and inter-eruptive periods can be identified based on the lithological and sedimentological characteristics of the deposits. In this sense, we must take into account that the resulting deposits will depend on the relative importance of these two types of periods.

The presence of volcaniclastic sedimentation implies some notable differences with respect to typical siliciclastic sedimentation [9, 82, 89, 90]. First, the resulting deposits will mostly be made up of fragments derived directly from eruptive activity rather than by weathering of preexisting rocks (Figure 14). In contrast, pyroclastic deposits are sediments generated over very short intervals of time and can be emplaced in the form of thick layers that may cover the topography more or less homogeneously or fill valleys and topographically depressed areas, resulting in efficient erosion. In a volcanic terrain with a predominance of explosive activity, erosion rates are high not only due to the existence of a high volume of unconsolidated material, but also to the destruction of the vegetation, which acts as a regulating agent for sedimentation from volcanic processes [82, 88, 91]. The loss of vegetation and the relative impermeability of the pyroclastic material, due to its fine grain size or poor sorting compared with soils, causes an increase in the amount of material that can be remobilized, which significantly increases the volume and periodicity of the total discharge into the basin [82].

Figure 14.

Field example of an ignimbrite deposit (Ig) eroded by an epiclastic crystal-rich sandstone (E) which incorporates fragments of the ignimbrite (Igf). The lack of a paleosoil separating both deposits is indicative of a short time lapse between the two (credit: Joan Martí).

The style of volcanism is of great importance to the development of the basin infilling successions. The volume of volcaniclastic material determines the extent of the influence of eruptions on sedimentation. In paleovolcanic terrains where a large part of the vent areas may have been eroded, it is necessary to identify the primary or secondary character of all the deposits that form the stratigraphic record if we want to know the evolution of the non-volcanic sedimentation in the basin and the influence of eruptive activity on it. The identification of the syn- and inter-eruptive periods serves to interpret the stratigraphic record in terms of cycles of eruptive activity, which, when combined with the identification of compositional criteria, allows establishing the relationship between sedimentation and magma compositions. In this way, we can observe how variations in the volume and extension of volcanic material supplied by the eruptions to the basin may depend on variations in the degree of explosiveness of magmas, this in turn being related to variations in their chemical composition (and volatile content).

Finally, we should note that the epiclastic processes that act in volcanic terrains do not differ from those that can be found in non-volcanic terrains. However, differences may exist in the resulting deposits due variations in the density of fragments, as a consequence of their variable degrees of vesiculation; this may affect their hydraulic classification and, consequently, the texture and sedimentary structures of the resulting deposits [1, 92]. A common feature of most paleovolcanic sequences is the presence of crystal-rich deposits of a different nature (e.g., [1, 58, 66, 93, 94, 95]). The main features that these rocks present can be relatively similar despite their potential diversity of origins (Figure 9c,d,f), making it possible that they could be materials of pyroclastic or epiclastic origin or a combination of both. The correct characterization of these crystal-rich deposits, particularly those of epiclastic origins, will permit us to know the influence of volcanism on sedimentation in the basin, its nature, and quite possibly the location of the source areas of the volcaniclastic materials. For example, in the description of many ancient terrains we can find rocks of an ambiguous nature, rich in crystals and with a clay matrix, which are generically called graywackes (e.g., [96, 97]), and which in many cases, among other origins, correspond to volcaniclastic (pyroclastic or epiclastic) deposits. The presence of these deposits in the stratigraphic record remark the importance of volcanism as a source of sediments and offer good samples for radiometric dating because of the minerals (e.g., zircon) they usually contain.

7. Final remarks and future perspectives

In the previous sections, we have briefly outlined some of the main aspects that need to be considered when working in paleovolcanic terrains. Probably, the most important is the fact that volcanic activity has not fundamentally changed over time (uniformitarianism), so that we can confidently assume that the volcanic processes and products that we observe at present are the same as those represented in the geological past. Therefore, their characterization and interpretation may be carried out in the same way. This observation, however, also allows us to consider another important aspect of volcanic activity, which is that, despite the existence of certain broad similarities, each volcanic area may have particular characteristics, which suggest that it should be studied independently. The criteria that must be taken into account when studying an ancient volcanic terrain should be as similar as possible to the ones used in recent volcanic areas. At present, it is not enough to give a good description of the different volcanic or volcano-sedimentary deposits, an essential step in any volcanological study, but they must be interpreted in terms of their volcanological significance and their influence on associated depositional environments. Therefore, the same should be applied to the study of paleovolcanic terrains.

Many paleovolcanic terrains are associated with important mineral deposits (e.g., Kurokos, in Japan [98, 99]; the Pyrite Belt of the Iberian Peninsula [40, 100]). Volcanic areas represent regions with a high thermal flux and the existence of hydrothermal fluids, many of them directly associated with the magmatic source, which means a high possibility for the accumulation of mineralizing elements. Likewise, volcaniclastic materials can constitute important hydrocarbon reservoirs (e.g., [42, 101]). Diagenetic changes of fresh volcanic deposits can reduce their permeability; however, the dissolution of volcaniclastic materials can contrastingly increase permeability and increase reservoir quality. The study of paleovolcanic terrains in terms of modern volcanology may also provide results that are quite helpful in the study of recent terrains with similar characteristics, where such processes may be inferred, yet not observed directly due to the lack of deep erosion and tectonics. Similarly, the exploration of high-enthalpy geothermal reservoirs in active volcanic areas can greatly benefit from the study of ancient analogs coming from deeply eroded volcanic terrains where the roots of the volcanic edifices are well exposed and the geometry and distribution of fossil geothermal reservoirs can be observed (e.g., [102, 103, 104]).

Another aspect to highlight in the study of ancient volcanic terrains is that they offer a good source of information to analyze plate tectonic evolution and the formation of sedimentary basins (e.g., [105]). In this sense, we can deduce the geodynamic framework in which the basins develop by studying the nature of the associated volcanism. The reconstruction of the position of eruptive vents will give information on the tectonic structure of the basin, since it will allow us to infer the distribution of the fractures through which the magma ascended to the surface; this will thereby provide clues for the reconstruction of the corresponding stress field. Therefore, the presence of volcanic products in the sedimentary record of a basin should not be necessarily considered as an isolated event. Volcanic activity must be interpreted as an effect with the same causes that condition the existence of some sedimentary basins, although on some occasions it can even become the direct cause that conditions their formation, as is the case of some collapse calderas and volcano-tectonic depressions (e.g., [106, 107]).

Based on what has been exposed in this contribution, I hope that a clear idea can be drawn for the importance of a correct interpretation of the volcanic episodes that we find in the geological record. A good description of the products generated by these volcanic processes is the first step to be able to understand their true meaning. However, the reconstruction of the volcanic episodes, in terms of their geodynamic framework, volcano-tectonic environment, basin dynamics, and eruptive mechanisms, based on a correct identification and interpretation of deposits, should be the main objective that we must consider when beginning a study of paleovolcanic terrains.

Acknowledgments

This research has been partially funded by the Spanish grants MINECO CGL2017-84901-C2 and MICINN PID 2020-114273GB-C21. I warmly thank the scientific editor Karoly Nemeth and the IntexOpen reviewers for their comments, which have contributed to improve the manuscript. English text has been reviewed and revised by Grant George Buffett (www.terranova.barcelona).