Chapter 7 Volcanism

Learning Objectives

After having carefully read this chapter and completed the exercises within it and the questions at the end, you should be able to:
  • Explain the relationships between plate tectonics and the formation of magma and volcanism,
  • Describe the range of magma compositions formed in differing tectonic environments, and discuss the relationship between magma composition (and 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, to people and to infrastructure, posed by the different types of volcanic eruptions,
  • Describe the symptoms that we can expect to observe when a volcano is ready to erupt, and the techniques that we can use to monitor those symptoms and predict eruptions,
  • Understand why so many humans have chosen to live near to volcanoes and some of the other attractions of volcanoes, and
  • Describe some of the ways that volcanic eruptions contribute to Earth systems.
A volcanic eruption is what happens when magma comes to surface. A volcano forms where there have been repeated eruptions at the same location over at least months—in the case of some small cinder cones—or for tens of thousands to a few million years for larger volcanoes. Eruptions can take place on the ocean floor (or even under the water of lake), in which case they are called sub-aqueous eruptions, or they can take place on land, where they are called sub-aerial eruptions.  Not all volcanic eruptions produce the volcanic mountains with which we are familiar; in fact most of the Earth’s volcanism takes place along the sea-floor spreading ridges and does not produce volcanic mountains at all—not even sea floor mountains.

Indonesia has one of the greatest concentrations of volcanoes on Earth, and Indonesia’s most active volcano is Mt. Merapi, which started erupting again in December 2020 and continued into early 2021 (Figure 7.0.1).  Mt. Merapi has had a significant eruption 74 times over the past 473 years, or once every 6 years on average. That is quite exceptional for a volcano of this type, most of which erupt only a few times a century, many much less than that.

Figure 7.0.1 Mount Merapi Eruption in April 2020

Mount Merapi is situated in the central part of the Island of Java, Indonesia (Figure 7.0.2), and it is one of over 20 volcanoes on the island. All of Java’s volcanoes are related to the subduction of the Australia Plate under the Sunda Plate and under Java.  A described in Section 7.1, water from the subducting plate rises into the hot mantle rock beneath the overriding plate (in this case the Sunda Plate) and that leads to flux melting that produces the magma that feeds Java’s volcanoes.

Figure 7.0.2 The Topography of Java Showing a Chain of Active Volcanoes Along the Central Part of the Island.
The study of volcanoes is critical to our understanding of the geological evolution of the Earth.  Volcanism has contributed significantly to our oceans, atmosphere, and biosphere, and has also had (and still has) significant implications for climate and climate change. Perhaps most important of all, understanding volcanic eruptions allows us to save lives and property. Over the past few decades, volcanologists have made great strides in their ability to forecast volcanic eruptions and to predict their consequences, and this has already saved thousands of lives.
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