**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).

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 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).*

*Introductory Chapter: Linking Modern and Ancient Volcanic Successions DOI: http://dx.doi.org/10.5772/intechopen.110313*

#### **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
