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How are volcanoes born, in which way do they live, and why and when, eventually, do they become extinct? More specifically, why are there any polygenetic central volcanoes at all? Why is the volcanism not simply evenly distributed through the volcanic zone or field? There are certainly volcanic areas where the volcanism is more or less evenly distributed – such as at fast-spreading ridges. But for most other volcanic areas one or more locations within the area dominates the volcanism, resulting in the formation of a specific central volcano. The central volcano erupts much more frequently than its surroundings and, therefore, normally forms an edifice. The edifice is a structure with a certain mechanical strength, most of which derives from the internal structure of the volcano. Later in the evolution of the volcano, it may form a collapse caldera, as well as being subject to lateral collapses, all of which affect its shape. When, eventually, the supply of magma to the volcano is cut off, it becomes extinct, that is, dies.
One of the main aims of the science of volcanology, and that of volcanotectonics in particular, is to understand volcanic unrest periods. By ‘understanding’ I mean that the signals coming from the volcano during the unrest can be interpreted in terms of plausible physical and chemical processes occurring inside the volcano. By volcanic ‘unrest’ we mean an increase in various physical and chemical signals, suggesting that associated processes within the volcano operate at different rates, intensities, or both. By interpreting the unrest period in terms of correct physical processes, there is a chance of assessing the volcanic hazard, namely the probability that the unrest period results in an eruption. Furthermore, when the understanding of the processes giving rise to the signals is accurate, not only the location of the eruption site but also the likely size (volume) of the eruption can be forecasted.
Field studies of volcanotectonic structures offer a way of understanding the processes that take place inside volcanoes before eruptions. Collapse calderas and some other large-scale structures are treated separately (Chapter 5), and here the focus is on sheet intrusions, sills, inclined (cone) sheets, and, in particular, dikes. Since they supply magma to most eruptions, it is important to make detailed and accurate observations and measurements of sheet intrusions in eroded sections of active and inactive (extinct) volcanoes. All the techniques described here apply equally well to inclined sheets, so that the term ‘dike’ in the present context also includes inclined sheets. Most of the techniques also apply to sills; the special aspects of field studies of sills are discussed at the end of the chapter. The observations and measurements provide a better understanding of how dikes propagate, the field conditions that encourage dike arrest, as well as the conditions that encourage their propagation to the surface to feed volcanic eruptions. The field data, when combined with geodetic and seismic monitoring data, can be used to test analytical, analogue, and numerical models on internal processes in volcanoes.
How does magma move or rise from its source chamber to the surface? More specifically, how does magma generate a path to the surface so as to supply magma to an eruption? Or, in general, under what conditions do dike-fed eruptions occur? While these questions have been briefly mentioned in some of the earlier chapters, they and the answers have not been discussed in detail. That I shall do in the present chapter. While magma moves through the crust by different mechanisms (e.g. as diapirs), the main mechanism is magma-driven fractures. The general name for all magma-driven fractures, once solidified, is sheet intrusions or sheets, which include dikes, inclined sheets, and sills. Unless stated otherwise, the theoretical discussion in this chapter applies equally to all these three types of sheets. Here, the focus is on mostly dikes, partly for the simple reason that dikes supply magma to most eruptions. For general theoretical considerations, dike denotes both subvertical dikes, regional and local, and commonly also inclined sheets, although in some instances a distinction will be made between these structures.
Volcanoes are of many types and behave in different ways. Different behaviour is partly because volcanoes are located in different tectonic environments. Many are associated with divergent plate boundaries, others with convergent plate boundaries, and some with transform-fault plate boundaries. In addition, there are volcanoes located within plate interiors, far from plate boundaries. To understand volcano behaviour with a view to being able to forecast volcanic eruptions we must use a variety of scientific techniques and approaches, primarily those of volcanotectonics. The main techniques and approaches for data collection, analysis, and interpretation are discussed in detail in later chapters, but they are briefly summarised here.
Volcanic (or volcano) earthquakes are those that occur inside volcanoes or close to them. The study of these earthquakes in volcanoes is commonly referred to as volcano seismology. Most earthquakes associated with volcanoes occur at comparatively shallow depths, normally of less than 10 km. They differ from other earthquakes at plate boundaries partly in that volcanic earthquakes commonly occur in swarms, that is, clusters in time and space of many comparatively small and similar-sized earthquakes. Volcanic earthquakes provide information about the state of stress in the volcano and of eventual magma-chamber rupture during unrest periods (e.g. Massa et al., 2016). They also indicate the location and, crudely, the size of magma chambers. When a dike (or an inclined sheet) is injected from a magma chamber, the formation of its propagation path (dike-fracture path) produces earthquakes. Thus, accurate monitoring of earthquakes during such events can be used to map out the propagation path of the dike and help assess the likelihood of the dike (or inclined sheet) reaching the surface, causing an eruption.
Polygenetic volcanoes, to a first approximation, behave as is they are elastic. When subject to loading such as magmatic excess pressure in a chamber or overpressure in a dike, the volcano deformation is, so long as the loading is small, roughly linear elastic. When related to pressure changes in the source chamber, the measured deformation is referred to as inflation when the volcano surface rises (during magma-pressure increase) and as deflation when the surface falls or subsides (during magma-pressure decrease). If the loading generates stresses that reach the strength of the rock, then fractures form or reactivate. Slip on shear fractures, that is, faults, commonly triggers earthquakes, which can be used to monitor the state of stress in the volcano as well as magma movement through dike or sheet propagation. Some stresses are sufficiently large to form or reactivate the boundary faults of grabens or the ring-faults of collapse calderas. Similarly, the stresses may result in lateral or sector collapses, that is, landslides. The earthquake activity in volcanoes is treated in Chapter 4, and vertical and lateral collapses in Chapter 5.
A magma chamber is the heart of every polygenetic volcano. Many, presumably most, polygenetic volcanoes have two magma chambers: one shallow crustal chamber and another deep-seated chamber, which we here refer to as a reservoir. Together, the reservoir and the shallow chamber constitute a double magma chamber. The complex interaction between the source reservoir and the chamber determines the frequency of injection of inclined sheets and dikes. Together with the mechanical layering and local stresses in the crustal segment, the double chamber also largely controls the frequency and sizes of eruptions in the volcano to which it supplies magma. We have learned that most shallow chambers evolve from sills and are located in the upper crust. The deep-seated reservoirs, by contrast, are normally located in the lower crust or upper mantle. If located in the crust, they may also evolve from sills; if located in the upper mantle, they may evolve as magma accumulations in regions of low potential energy. The accurate determination of the location of active magma chambers is generally difficult.
When a magma-filled fracture reaches the surface, a volcanic eruption occurs. In Chapters 5 and 7 we have discussed the conditions under which this may happen. Here, the focus is on the likely course of events once an eruption has started. Among the main questions facing scientists and civil authorities during a beginning eruption are: (1) What is the likely size or magnitude of the eruption? (2) What is its likely duration? (3) Is it going to be primarily effusive or explosive or both? All these questions ultimately relate to the hazards and associated risks posed by the particular volcano. For a more reliable assessment of the hazards associated with volcanoes, the frequencies and sizes of their eruptions need to be known and related to a general understanding of the dynamics of eruptions.
One principal aim of volcanotectonic studies is to provide a theoretical framework that makes it possible to make reliable deterministic or probabilistic forecasts of volcanotectonic events. These events, in turn, depend on volcanotectonic processes inside the volcanoes. In the previous chapters we have discussed some of the main observational aspects of volcanotectonics, both geological and geophysical, and defined several of the basic concepts. In order to bring into focus those field observations that are useful for understanding the main processes leading to eruptions, vertical or lateral collapses, and related events, we need to know the basic physics that controls the processes. Here we provide an overview of some principal processes that control volcanotectonic events, emphasising elementary physics, particularly mechanics, and the quantitative aspects of volcanotectonics. Many of these processes are elaborated in later chapters.