In his monumental book "The Malay Archipelago" from 1869 Alfred Russel Wallace, who independently of Charles Darwin discoved the role of biologic evolution, wrote:

"It is only when actually gazing on an active volcano that one can fully realize its awfulness and grandeur. Whence comes that inexhaustible fire whose dense and sulphureous smoke for ever issues from this bare and desolate peak? Whence the mighty forces that produced that peak, and still from time to time exhibit themselves in the earthquakes that always occur in the vicinity of volcanic vents? The knowledge from childhood, of the fact that volcanoes and earthquakes exist, has taken away somewhat of the strange and exceptional character that really belongs to them. The inhabitants of most parts of northern Europe sees in the earth the emblem of stability and repose. His whole life-experience, and that of all his age and generation, teaches him that the earth is solid and firm, that its massive rocks may contain water in abundance but never fire; and these essential characeristics of the earth are manifest in every mountain his country contains.

A volcano is a fact opposed to all this mass of experience, a fact so awful a character that, if it were the rule instead of the exception, it would make this earth uninhabitable; a fact so strange and unaccountable that we may be sure it would not be believed on any human testimony, if presented to us now for the first time, as a natural phenomenon happening in a distant country."

14 years later Krakatoa halfway between Java and Sumatra had a terrific eruption, that we classify as a VEI:6 on the Volcano Explosivity Index scale. The major part of Krakatoa island down to 200 m below sea level disappeared. All that remained after the final explosion were 4 sterilized islets in the Sound of Sunda. The series of tsunami waves generated — one 36 m high — killed 34,400 people. In addition a pyroclastic flow from Krakatoa swished 50 km on the surface of the ocean and when making landfall in south Sumatra scorched another 2000 people to death.

When working in Indonesia in the 1980s, I compared the population density of the areas hit by the Krakatoa tsunami 1883 with the population density 100 years later and found that ca 1 million people would get killed had the eruption taken place in the 1980s and not 1883.

Next to the sun, volcanism is the most important environmental factor on planet Earth. Volcanic rocks and their sediments and soils cover more than half of the surface of our planet. Most people don't realize the importance of volcanism because active volcanoes form less than 5% of the surface of the continents.

The 1500 active volcanoes are degassing vast amounts of various elements of great ecologic importance, also when not in eruption. When a volcano has not had an eruption in 10,000 years, we call it inactive. Land volcanoes can be dormant for more than 15 million years, degassing carbon dioxide constantly, and submarine volcanoes for more than 20 million years before they can be classified as dead.

Most of the earth's 1500 active volcanoes are located close to a coast. Many towns, cities and — for cooling purposes — nuclear power plants are also located at a coast. In addition to this >70% of our planet is covered by oceans, and the ocean floors contain probably up to 100,000 volcanoes we do not see. They form mid-oceanic mountain ridges, heaters of the ocean water several 1000s of kms long. New ocean bottom is formed and spread outwards from these submarine volcano chains. Like in Hawaii, individual volcanoes are also formed above geographically restricted magma plumes or hotspots. All paradise islands with atolls and coral reefs in the warm oceans are sitting on the top of a submarine volcano concealed by the cap of light limestone formed by the reef building organisms.

Is it possible to forecast when eruptions will happen? Both No and Yes. It is not now possible to say that a specific volcano will erupt a certain year in the future, but we are now much better at forecasting the length of time between eruptions of volcanoes. How come?

A volcano is fed with hot magma from a magma chamber four or more km below the base of the volcano. A big volcano has a big magma chamber and it takes longer to fill this chamber with molten rock from below. That is why small eruptions from volcanoes with small magma chambers are very common but tens of human generations can pass between the big eruptions. We know that after Krakatoa's VEI:6 eruption in 1883 it will have many small eruptions, but it will take at least 4-5 centuries until Krakatoa has another VEI:6 eruption.

Our latest VEI:6 eruption hit the Philippines with Pinatubo's 'final explosion' 15 June 1991. Its ash hit 12 aircraft in flight between 720-1740 km from the crater and destroyed 10 jet engines. Near the volcano several aircraft on the ground were damaged and 6 airfields in the Philippines were closed down. The volcanic aerosols and ash went through the tropopause and into the stratosphere at 16,000 m. The jet winds brought them in circulation around the globe, a roundtrip took 22 days. The eruption lowered the global temperature 0.6 °C during the next 15 months.

Janus, the ancient Roman god of doorways, duality and transitions of time, has two faces. Volcanoes also have two faces. The friendly face is rejuvenating the soil cover by adding volcanic ash, a natural fertilizer generating richer harvests. When this takes place in a warm and rainy climate the farmers get 3 harvests per year instead of 2. This is part of the explanation why the population has become so dense in east and south Asia. It is also why I started building my database while living on Java in the 1980s.

The hostile face of a volcano spells death and destruction. The sad fact is that people don't realize the two faces because very few volcanoes show their strength all the time like Stromboli in Italy, 'Lighthouse of the Mediterranean' during two millennia does with more than 300 eruptions every week. At Stromboli you don't risk your life unless you stand at the rim of the crater. But Europe's greatest threat is not far away. In the centre of Naples municipality with 3.5 million inhabitants towers Vesuvius — a dwarf compared with Sicily's Etna — but more dangerous.

Vesuvius, seen to the right in the image (photo courtesy NASA ISS004-E-8667) — is Europe's risk volcano No. 1. Pompeii is southeast and Herculaneum at the coast west-southwest of it, both towns were buried by its eruption in 79 CE. Immediately west of Vesuvius is the main part of Naples. This city is threatened also from the west by a swarm of smaller volcanoes, seen as circular areas on the image. Of these the restless Campi Flegrei, the Phlegrean Fields, had its latest eruption in 1538 when Monte Nuovo was formed in Pozzuoli and grew >100 m high in a little more than a week. Vesuvius finished its latest eruption on 4 April, 1944.

We know that Vesuvius as well as Campi Flegrei will have explosive eruptions again and the longer they rest, the more molten rock will be in their magma chambers and the more powerful the coming eruptions will be. Since the lives of millions of people are in danger, both are well monitored. An evacuation plan is now also in place. We know more about Vesuvius than almost all other dangerous volcanoes, since the world's first volcano observatory was established there in 1841.

The Global Volcanism Program at the Smithsonian Institution in Washington D.C. in the United States is the World leader of knowledge about volcanoes and their eruptions. A thick and very useful book 'Volcanoes of the World' has been published by the Smithsonian in three editions. This book should sit in the shelves of every library in every country. After the disastrous Mount St. Helens eruption 18 May 1980 in the State of Washington in the US, funding for volcanology was 10-folded. This led to a great increase of volcano data and an opening of the huge online Global Volcanism Database to everybody. It would help public understanding if journalists consulted this database before talking and writing about volcanic eruptions.

My database has information about almost all the 1500 active volcanoes on Earth and many of their eruptions together with data on the impact of the eruptions in a historical context. I have dated hitherto unknown eruptions, for instance, the first eruption in Syria in our time, but not published my discoveries. The eruption in Syria took place in an area where the lava has been dated as younger than 4000 years. I can date the eruption to year, month and day. Such material is in my database together with new information about known eruptions extracted from old publications and unpublished documents in publicly accessible archives.

Vesuvius map from the 19th century showing the circular Monte Somma caldera in light colour with radiating lava flows in brown. Vesuvius is building its cone in the centre of the caldera. Today the area between the volcano and the Gulf of Naples in the west is densely populated. From Meyers Konversationslexikon.