How man learnt to live with lava

The 1990s were designated the International Decade of Natural Disaster Reduction (IDNDR) by the United Nations, and considerable research effort went into achieving a better understanding of volcanic processes and a better public understanding of volcanic phenomena. Some eruptions were predicted successfully and thousands of lives were saved through evacuation and well-coordinated aid programmes.

The IDNDR followed the 1985 disaster at Nevado del Ruiz in Colombia, where more than 22,000 people died in one night following a small eruption and a large mudslide. It seemed obvious that a better understanding of the workings of active volcanoes and increased public awareness could dramatically reduce the human cost of natural disasters.

To some extent this view seems to have been correct, as a much larger eruption, at Mount Pinatubo in the Philippines in 1991, was successfully predicted. Although several hundred people died during and immediately after the eruption - mainly due to mudflows following a typhoon and disease in the refugee camps - about 250,000 people were evacuated. This eruption is often used as an example of the success of modern volcanology. It is certainly true that the eruption forecasting was good and, crucially, communication between scientists, officials and the population was excellent. There is much evidence, though, that the education of the public and the local political and economic situation play an important role in determining whether a disaster is averted or not.

Famous historical eruptions have captured the public imagination. Hollywood films use dramatic special effects to glamorise volcanology and, to some extent, educate audiences. The historic eruptions of Vesuvius (destroying Pompeii and Herculaneum in AD 79) and Krakatoa (killing more than 36,000 in 1883) are well known to us through books and films, but many ancient civilisations also had stories and myths relating to volcanoes and volcanic events. There are lessons that can be learnt from past major eruptions and their effects on the local population or on civilisation in general. The interdependence of nations in the global economy now means that a major eruption can have significant global economic consequences.

The risk that a community is exposed to from a volcanic hazard depends on its vulnerability, and this may be expressed by the relationship: Risk = Hazard x Vulnerability. The "hazard" may be any volcanic phenomenon, such as ash fall, lava or mud or pyroclastic (volcanic debris) flow, gas release or tsunami, or a combination of these. The hazard depends on the type of volcano, the time since its last eruption, its geographical location, the local climate and the time of year. The longer a volcano is dormant between eruptions, the larger the eruption tends to be, so that while Kilauea (Hawaii) and Etna (Sicily) are well-known active volcanoes, since they erupt almost continuously, the direct risk to the human population is limited.

The risk to property and the environment is not diminished, however. The location and timing of a volcanic event affects the risk, since wind directions and strengths in the upper atmosphere vary with latitude and season. This means that an ash eruption might be transported around the globe at one time, but remain confined to a small region at another.

The local area is always affected by volcanic activity, whatever the type of eruption, for anything up to several tens of square kilometres. For example, lava flows consist of rivers of molten rock travelling at between 1m and 30km an hour (depending on the type of lava) for distances up to several tens of kilometres at temperatures up to 1,000°C. The more explosive eruptions tend to eject large quantities (between 0.1 km³ and several tens of cubic kilometres) of ash and rock fragments into the atmosphere and much of these fall out locally. Ground surges of these materials may follow collapse of an eruption column forming a cloud of volcanic debris. Most eruptions are accompanied by some gas release, and this may escape into the upper atmosphere and have global consequences or may form aerosols with water vapour at lower elevations and cause acid rain that falls locally. Eruptions may be triggered by earthquakes and vice versa, and those occurring on islands or close to the sea may produce tsunamis with effects reaching tens or even hundreds of kilometres.

Vulnerability is a measure of the number of people in the affected area and the local infrastructure. Obviously the risk increases with the number of people living near a volcano, but it also depends on the type of eruption and therefore the hazard. As the global population increases, the risk of volcanic disaster increases because more and more people live near active volcanoes. However, improved global communications reduce the vulnerability of communities by raising both awareness of hazards and levels of preparedness.

These two books address the issues of the human response to volcanism in contrasting ways. Both authors have previously written books on volcanoes; and there seem to be more and more books on the subject every month. So is there really room for more? There is overlap between the books - after all, there is only a finite number of well-documented historical eruptions to write about - but both of them fall into the "must have" category for the professional volcanologist, keen amateur and interested lay person alike.

Both are meant for a wide audience, but neither could fairly be described as a coffee-table book. Both contain a massive amount of detail and fact and are similar in size and price. But they are refreshingly different.

Alwyn Scarth is a Yorkshireman who trained as a geographer and lectured in geography at the University of Dundee. He has had a lifelong fascination with volcanoes, and his previous publications range from the varied landforms that volcanoes create to the volcanic features in Homer's Odyssey . Haraldur Sigurdsson is an Icelander by birth and so has a natural affinity for volcanoes, although much of his professional life has involved lecturing in the geosciences at universities in the United States. His research is wide ranging and his writing prolific. He was editor-in-chief of the Encyclopedia of Volcanoes , which has been heralded as "a veritable Who's Who of modern volcanology" and "the most important volcano book of the century".

Scarth has chosen 15 volcanoes and famous eruptions and documented them in detail. However, this is anything but a dry academic read. Using eyewitness source material (that in Latin, Italian, Spanish and French translated by himself), he brings the eruptions to life in a gripping, heart-rending manner.

Touching "human interest" stories precisely detailed make this a compelling read, beautifully illustrated with colour photographs and line drawings. The book seeks to show how, over the millennia, people have lived with volcanoes, and how their reactions of fear, fascination, religious fervour, awe, enterprise and courage have remained essentially unchanged despite recent technological advances. Although our ability to recognise and act on the warning signs of an eruption has improved, only a few attempts at diverting slow-moving lava flows have been successful, and there is still no possibility of preventing an eruption.

Sigurdsson considers the question of how our ideas and understanding of volcanic processes have varied historically. Volcanology as a science is surprisingly young. Well into the 19th century, the Neptunists were arguing strongly for precipitation of all rock formations from a universal ocean. Plutonists argued that internal heat was the driving force for mountain uplift, folding and deformation, using volcanoes, basalts and granites as evidence for melting of rocks within the earth at high temperatures. And both camps tried to reconcile their scientific findings with biblical passages on the creation.

Only with the acceptance of the theory of plate tectonics in the second half of the 20th century (1960s onwards) could volcanoes and their products be understood in their global context. The problem is that there are many different types of volcanoes, and the characteristic eruptions also vary widely, from devastating explosions to lava flows and quiet gas discharges. Although it seems obvious to us now, it was not always clear that these were manifestations of similar subsurface processes. Geophysical techniques for monitoring volcanic processes have been available only for the past few decades.

Sigurdsson's book is generously illustrated with black-and-white photographs and line drawings, many from his extensive volcanic art collection. He aims to chart the way in which ideas have evolved in considerable detail. Ideas move in and out of favour, and rival groups support opposing theories at different times. It was, therefore, not possible to structure the book in either a strongly chronological manner or by following a single concept from initial conception to universal acceptance or rejection. This makes Sigurdsson's a slightly less easy read than Scarth's book, but it is no less of an academic achievement and source of insight and comment on the history of our science.

We tend to take the present-day understanding of volcanoes for granted. These two books provide insight into the thought processes and actions of past generations. They are informative, fascinating and sobering for the professional volcanologist, anyone attracted by volcanoes and, indeed, anyone interested in human resourcefulness.

Hazel Rymer is Royal Society research fellow in earth sciences, Open University.