Introductory Concepts

In this chapter, the importance of volcanism throughout Earth history, the diversity of volcano types and the complex process spectrum that operates in volcanic settings are discussed. Volcanoes occur in a variety of forms and sizes, their life spans vary enormously, and the processes, deposits, and stratigraphic successions produced by different types of volcanoes can be extremely variable. Since there is such a diversity of rock and deposit types in the complex environments of volcanic terrains, the interpretation of their possible origins (lavas, intrusions, associated autoclastic, pyroclastic, epiclastic, and chemical deposits) can be difficult. This is especially so for ancient rock sequences, which may have been lithified, hydrothermally altered, metamorphosed, deformed, and tectonically dismembered. As well as some essential genetic terms, we also introduce the facies approach to documenting deposit characteristics. Understanding the spatial and age relationships between facies and associations of facies is crucial in deciphering the stratigraphic architecture, and chronology of events. By doing this, models of the deposits produced by particular events can be developed, and eruptive histories can be reconstructed in both modern and ancient successions. We also urge caution in assessing the tectonic setting of Earth’s early volcanism during the Hadean and Archean (45% of the Earth history) because plate tectonics did not operate through most of this huge period of time (see Chap. 17 for discussion of this intriguing topic).