Chapter 11. Volcanism

Introduction

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Explain the relationships between plate tectonics, the formation of magma, and volcanism
  • Describe the range of magma compositions formed in differing tectonic environments, and discuss the relationship between magma composition, including gas content, and eruption style
  • Explain the geological and eruption-style differences between different types of volcanoes, especially shield volcanoes, composite volcanoes, and cinder cones
  • Understand the types of hazards posed to people and infrastructure by the different types of volcanic eruptions
  • Describe the behaviours that indicate a volcano is ready to erupt, and how we monitor those behaviours and predict eruptions
  • Summarize the types of volcanoes that have erupted in British Columbia over the past 2.6 Ma, and the characteristics of some of those eruptions

What Is a Volcano?

A volcano is any location where magma comes to the surface, or has done so within the past several million years. This may or may not be a mountain. Some volcanic eruptions happen on land, in which case they are called subaerial eruptions. However, volcanic eruptions can also happen underwater, such as on the ocean floor or under a lake.  These kinds of eruptions are called subaqueous eruptions.

Canada has a lot of volcanic rock, but most of it is old- some of it billions of years old. Only in B.C. and the Yukon are there volcanoes that have been active within the past 2.6 Ma (Pleistocene or younger), and the vast majority of these are in B.C. We’ll look at those in some detail toward the end of this chapter, but a few of them are shown on Figures 11.1 and 11.2.

Mt. Garibaldi, near Squamish B.C., is one of Canada’s tallest (2,678 m) and most recently active volcanoes. It last erupted approximately 10,000 years ago. [Steven Earle CC-BY 4.0]

Figure 11.1 Mt. Garibaldi, near Squamish B.C., is one of Canada’s tallest (2,678 m) and most recently active volcanoes. It last erupted approximately 10,000 years ago. [Steven Earle CC-BY 4.0]

Mt. Garibaldi (background left, view from the north) with Garibaldi Lake in the foreground. Mt. Price is at centre. The dark flat–topped peak is The Table. All three volcanoes were active during the last glaciation. [Steven Earle CC-BY 4.0]

Figure 11.2 Mt. Garibaldi (background left, view from the north) with Garibaldi Lake in the foreground. Mt. Price is at centre. The dark flat–topped peak is The Table. All three volcanoes were active during the last glaciation. [Steven Earle CC-BY 4.0]

Studying volcanoes helps us understand the geological evolution of Earth, and significant changes in climate. A key reason, however, is that understanding volcanoes helps us to save lives and property. Over the past few decades, volcanologists have made great strides in their ability to forecast volcanic eruptions and predict the consequences. This has already saved thousands of lives, but the knowledge was not cheaply bought. Some of this understanding cost the lives of the very people trying to learn about volcanoes in order to save others.