Introduction

Volcanoes are one of the most exciting landforms on Earth today, and frequent volcanic eruptions around the world every year are a constant reminder of the dynamic nature of the Earth as a planet. With many well-documented major volcanic eruptions in the last two decades (e.g., the eruptions of Mount St. Helens in 1980 in the northwestern United States, El Chichón in 1982 in Mexico, Galungung in 1982 in Indonesia, Hawaii from 1992 to present, Mt. Pinatubo in 1991 in the Philippines, Mt. Unzen from 1991 to 1994 in Japan, and Soufriere Hills, Montserrat from 1996 to present), it may appear that the frequency of eruptions is increasing and that volcanic eruptions are a relatively recent or increasingly significant phenomenon in the Earth's history. However, the remains of ancient volcanic rock successions around the world indicate that volcanic activity has been a fundamental aspect of the evolution of our planet throughout its history. The apparent increase in the frequency of eruptive activity in recent times is probably largely an artefact of improved surveillance opportunities, technology, and media coverage. In this chapter the significance of volcanic activity in the history and evolution of the Earth will be examined. In particular, we will explore how volcanism plays a fundamental role in the geological cycle and the dynamic mechanism that drives the geological cycle, plate tectonics. We will also explore how far back in time modern plate tectonics can be recognized as the driving force for the geological cycle and associated volcanism.