PART II
Eruption


Eruption!Eruptions are exciting. It is almost impossible to think of a volcano without contemplating the awesome power displayed during an eruption. But what exactly is an eruption? How long do eruptions last? How long does a volcano remain dormant between eruptions? Have volcanoes always erupted as they do now and where they do now? These are just a few of the fundamental questions addressed in this section.

We begin with an an overview chapter, Earth's Volcanoes and Eruptions, which serves as an introduction to the various types of eruption that occur at the Earth's surface and how they vary with geographic location. The theory of plate tectonics is used to explain the present day distribution of active volcanoes. In this chapter , active volcanoes are defined by the authors Tom Simkin, Lee Siebert, and Paul Kimberly as those historically active or active within the last 10,000 years.

It is sobering to remember that our own history is pitifully short compared with the lifetime of an average volcano. It is quite possible for a volcano not to have erupted at all since humans have been documenting their surroundings, but for it to be simply dormant right now, between periods of activity. The longer ago that an eruption took place, the harder it is to gather evidence as to what exactly took place as the products of the eruption are subject to erosion processes just like any other rocks. The further back in geological time that we look therefore, the larger the eruptions need to have been before we can identify them. On the other hand, large eruptions are less common than small eruptions now and this has probably always been true. Products from huge caldera forming eruptions are therefore going to be found not in the most recent volcanic deposits, but buried deep in the geological record.

Detailed analysis of all the available evidence from the last 10,000 years has led the authors to deduce the approximate frequency at which we can expect eruptions of various sizes. For example, small eruptions such as occurred at Nevada del Ruiz in Colombia in 1985 killing 35,000 people occur typically several times a year, although fortunately usually without such tragic consequences. The Mount St. Helens event of 1980 occurs on average once each decade. It is unclear however how frequent much larger eruptions are.

Almost as important as predicting the onset of volcanic activity, and certainly more difficult, is predicting when an eruption will cease. With a few notable exceptions, such as Stromboli in the Aeolian Sea offshore Italy, which has been erupting steadily for at least 2,500 years, the average eruption duration is about 7 weeks. Some last for only a day and the longer the dormant period, the larger the final eruption.

Just as earthquakes can be defined using scales of magnitude and intensity, so can volcanic eruptions. Estimates of magnitude and intensity of eruption can be made for very large events millions of years ago using a comparison of the distribution and volume of erupted products (ash and lava) with those of smaller, recent events. In the chapter Sizes of Volcanic Eruptions, David Pyle defines eruption magnitude and intensity and illustrates the use of these properties for understanding the mechanisms driving the colossal eruptions of the past.

Finally in this section, Haraldur Sigurdsson addresses the issue of how the rate of volcanism has varied through geological time. Since there have been no really large caldera forming eruptions since humans have been on Earth, it might be inferred that such things will not occur in the future. However we know that large eruptions occur less frequently than smaller eruptions. In the chapter Volcanic Episodes and Rates of Volcanism, Sigurdsson considers the last 65 million years of Earth history and puts into geological and geographical perspective the fluctuations in the magnitude and intensity of volcanism over this time. There is evidence that there have been several episodes of increased volcanism worldwide in the past and a possible link with climate change. Whether climate change is a cause or effect of variations in the rate of volcanism remains an intriguing question.

Hazel Rymer
The Open University
Milton Keynes, United Kingdom

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