Introductory Chapter: Linking Modern and Ancient Volcanic Successions

1. Introduction

Understanding volcanic rocks plays a crucial role in reconstructing the environment in various scales within volcanic rocks formations. Volcanic processes can act in a short time scale and still produce large volumes of eruptive products that commonly can misbalance the sedimentary budget of a sedimentary basin, regardless of their geoenvironmental position (e.g., marine, or terrestrial). Distal ash, on the other hand, can travel hundreds of kilometers away from their source and fall into the background sedimentary environment and can produce a very characteristic and sharp time marker across the entire region. This makes volcanic deposits excellent chronostratigraphy markers [1]. Volcanic processes are also diverse not only by the way coherent and fragmented source materials get generated (e.g., fragmentation style variations, eruption intensity diversity) but also by the way those materials get transported and accumulated. Large volumes of volcanic material can accumulate quickly (hours to days) and alter the entire drainage pattern of large regions. The same accumulated volcaniclastic deposits later can gradually get redeposited and altered by normal surface processes but can provide volcanic detritus over prolonged time along the transportation arteries that eventually lead to marine basins [2, 3]. While this process seems to be a slow and gradual way effectively remove large volume of volcanic deposits from source regions and disperse it over large territories, such processes can also take place in a dramatic and abrupt fashion. Massive breakout lahars can mobilize large volumes of water and damp their sediments suddenly over large areas. On many occasions, like in the Taupo Volcanic Zone in New Zealand, large volumes of caldera-forming silicic eruptions had modified the landscape dramatically and promoted the formation of large lake systems, which, from time to time, initiated breakout lahars moving large volumes of volcaniclasts to other sedimentary basins [4]. Over time, major sedimentary basins from terrestrial to marine produce massive successions of complex multisource volcaniclastic aprons, fans, and basin fills. In ancient settings, such basins and their complex volcaniclastic successions can be the only “messengers” of former high-intensity volcanism, especially if the preserved volcaniclastic rocks are preserved as part of tectonically dissected terrains.

Volcaniclastic sedimentology has evolved in recent years dramatically. Primary eruption-fed processes considered to produce fragmented volcanic materials generate pyroclasts that can start their journey through initial primary volcanic processes that later interact with the normal sedimentary environment, making it increasingly difficult to distinguish the effect of the primary volcanic and the background sedimentary processes. This difficulty manifests in the way how we describe and interpret the preserved volcanic material in the geological record. In the past decades, various terminologies have appeared with an aim to provide a clear method to document objectively volcanic rocks, keeping the descriptive and interpretative aspects of the nomenclature separated [5]. It is apparent that over time, such terminologies have become more and more process related, expressing the strength of the link between primary (eruption-fed) and secondary (background sedimentation-dominated) processes [6, 7, 8]. In the past decades, entire volcaniclastic sedimentology schools have formed with key research groups (Figure 1) with diverse geological backgrounds, demonstrating the vitality of volcaniclastic sedimentology. Moreover, in recent years, the geology-based approach has reemerged, and new research has applied basic geological rules to look at volcanoes through volcano geology perspective [9, 10].

Figure 1.

Bibliometric surveys based on Scopus data for search terms of “volcaniclastic” and “sediment” as well as “pyroclastic” and “sediment” within keyword, title, and abstract documents.

2. Polygenetic vs. monogenetic systems

Volcano types are commonly distinguished along their appearance, volume, and the time required for their formation. Polygenetic volcanic systems are characteristically long-lived volcanoes with a stable melt source and conduit system. Individual eruptions occur in diverse eruption styles, but they activate many times over the total lifespan of the volcano. As a result, a large volcanic edifice will build up that is surrounded by a broad ring plain where mostly valley-filling pyroclastic density currents, hot and cold reworked equivalent of them, accumulate alongside with landscape-draping ash fall beds. Further away we are from the source volcano, the higher the influence of the background sedimentation, resulting in developing extensive volcaniclastic sedimentary basins. On the contrary, small-volume and short-lived, so called “one shot,” eruptions are commonly defined as monogenetic volcanoes [11]. They mostly form complex groups of volcanoes, a volcanic field where the primary volcanic deposits (e.g., directly fed by an eruption) are dispersed over large (100 s km²) areas (Figure 2). While each volcano erupts only once, their eruption record could show great variation of eruption styles as a function of the interaction between the magmatic and external (mostly water) impacts on the individual explosive eruptions. Overall, if the volcanic field is long lived (million years scale), significant volumes of inter-volcano volcaniclastic deposits can accumulate. Over time, volcanic fields can reach mature stages when they can feed from magmas that have evolved over time and produce simple silicic explosive volcanoes such as known from many volcanic fields from the Arabian Peninsula (Figure 2).

Figure 2.

Harat Rahat in Saudi Arabia is a typical mature monogenetic volcanic field with several silicic eruptive centers forming maars, small calderas, and lava domes such as the Holocene Um Rgaibah. The trachytic block-and-ash fan is clearly distinct on a Sentinel Highlight Optimized satellite image.

3. Role of type of volcanism, environment, and volcano instability

Large volumes of silicic magma-dominated volcanism commonly culminate in caldera formation that is associated with large volumes of ignimbrite accumulation. Such processes can be landscape forming as they change not only the hydrology of a large area but also the orography of a large area. Caldera-forming eruptions are common in the geological record, and their volcanic facies architecture can stay relatively intact over millions of years. Their most important characteristic is that an entirely distinct sedimentary system can form within the caldera that is different from the extra-caldera-depositional systems. In a volcanic terrain where a large number of volcanoes can form in a relatively small region, volcanic products can accumulate from different sources. In addition, such closely spaced composite volcanoes can interact with the background sedimentary environment, especially if that is complex and exhibits a multitude of small sedimentary systems across the composite volcanoes (Figure 3). Large stratovolcanoes are commonly associated with convergent plate margins and subduction processes. Along old and long-lived volcanic arcs, volcanoes spaced in a regular fashion result in aligned volcanic fronts that can spread across many climatic zones. Individual volcanoes can provide steady intermediate pyroclast input through medium but occasional high-intensity explosive volcanic eruptions. The sudden input of pyroclasts to the terrestrial environment can behave differently if that occurs in arid or humid climatic conditions. In arid conditions, the preservation potential of primary pyroclastic successions can be good, keeping near-original deposit characteristics intact over longer time; however, occasional high-intensity rainfall events can rapidly modify those features as soil formation is limited, and exposed deposits can be remobilized quickly. On the contrary, in humid climatic conditions, remobilization of pyroclasts due to meteorological and/or volcanic events can trigger massive volcanic mass flows commonly named as lahars (volcanic mud and debris flows).

Figure 3.

Complex terrestrial–marine sedimentary system around the Kronotsky volcano in central eastern Kamchatka, Russia on a Sentinel Short Wave Infra-Red satellite image sensitive for wet zones (green or blue). Note the large lake (Kronotsky Lake), the complex fluvial systems “sampling” volcanic sources of various ages and compositions. Note the deep erosional gullies on the Pleistocene Schmidt Volcano that functions as the main sediment delivery channel. Also, note the complex coastal plain to shallow marine sedimentary system that likely collects volcaniclastic material from a complex, multi-source volcanic terrain (circle).

Major lahars can follow the normal fluvial system, and deposition can interact with fluvio-lacustrine elements. Such systems can form confined long valleys such as observed in the aftermath of the Pinatubo 1991 [12] (Figure 4) eruptions.

Figure 4.

Pinatubo Volcano (Philippines) on a Sentinel False Color satellite image. On the image, fresh sediment-dominated fluvial channels are shown in various gray colors. These fluvial channels functioned as major lahar channels following the Pinatubo 1991 Plinian/ultra-Plinian Volcanic Explosivity Index (VEI) 6 eruption.

Recognition of volcanic instability is dated back to the advent of remote sensing in the late 80s, when peculiar patterns over tens of km² areas recognized along Andean volcanoes were associated with horseshoe-shaped central cone morphology [13]. Volcanic instability is either directly linked to an explosive eruption and/or triggered by the gravitational spreading of the volcano. This means that the type of explosive volcanic activity, the steady degassing of the volcanic system that generate structurally controlled hydrothermal alterations to weaken the growing volcanic edifice, and the environment where the volcano activates together will put the volcanic edifice to a specific course to collapse over time. Large-volume volcanic collapses are known in every geometrical scale across volcanic arcs. Especially in those in arid climate where salt formation is intense such as around the Atacama Basin in Chile, volcanic instability is even accelerated as salt provides good lubrication for the volcanic edifice to slide apart catastrophically [14].

Explosive eruption-triggered volcano collapses are also more frequent than previously thought as the 1980 eruption of Mount St. Helens shed light on the scale of such events (Figure 5). Volcanic collapses, especially those that occur in temperate or tropical climate (e.g., humid conditions) where vegetation cover quickly develops (decadal scale), can initiate new sedimentary regimes as they open large surface areas where unconsolidated volcanoclasts can be remobilized and fed into fluvio-lacustrine sedimentation arteries. Such a process is clearly demonstrated from the Taranaki Volcano in New Zealand and followed dup at Mount St. Helens since the eruption occurred. In some stratovolcanoes such as Taranaki, even the cyclicity of such volcano collapses and their sedimentary responses have been recognized [15]. Individual volcanic particles preserved in ancient rock units carry vital information about the style and type of volcanic eruptions that created those particles, while sedimentary features and the overall facies characteristics can provide information on the transportation and deposition of those individual particles. In arc settings, especially under temperate to tropical, humid climates complex sedimentary environments can form where the volcanism and the background sedimentation together can create complex volcano-influenced sedimentary aprons.

Figure 5.

Mount St. Helens (Washington State) on a Sentinel Short Wave Infra-Red satellite image. Note the horseshoe-shaped scar on the edifice and the connected fluvial system along volcaniclastic material transported away.