TOKYO -- The recent eruption of Mount Ontake, Japan's worst postwar volcanic disaster, provided graphic evidence of the threat posed by active volcanoes.
Japan has 110 active volcanoes, some 7% of the world total. Active volcanoes are those that can erupt at any moment.
Why are people allowed to climb such dangerous mountains? And what exactly is an active volcano?
The Japan Meteorological Agency and some other expert organizations define volcanoes as mountains created by eruptions during the past 2.6 million years.
There are about 450 volcanoes across the nation. The Coordinaing Committee for the Prediction of Volcanic Eruption, the agency's panel of experts, considers a volcano to be active if it has erupted in the last 10,000 years.
There are some 1,500 active volcanoes around the world, including the famous Mount Pinatubo in the Philippines and Mount Tambora in Indonesia.
The 110 active volcanoes that dot the Japanese Archipelago are located throughout the nation, with the northernmost being Mount Moyoro, one of Hokkaido's Etorofu Islands, which is controlled by Russia, and the southernmost the Nikko Seamount, near the island of Iwo Jima.
Of these active volcanoes, 89 are located in eastern Japan, including Mount Fuji, and 21 in western Japan, including Mount Sakurajima.
Dangerous or not
Volcanoes used to be classified into three categories: active, which erupt regularly; dormant, which erupted long ago but are currently at rest; and extinct, which have not erupted in historical times.
From the viewpoint of Earth's history of 4.6 billion years, however, intervals of several hundreds of years are very short periods of time and meaningless for determining whether a volcano can erupt. As this view became popular among volcanologists, the Meteorological Agency in the 1960's started considering all volcanoes with a history of eruption to be active.
In the 1970's, the eruption prediction committee redefined active volcanoes as those that have a history of eruption and those that are showing signs of fumarolic activity, which means a vent on the mountain is spewing steam and gases. The committee designated 77 active volcanoes in Japan according to this definition.
Progress in volcanology in ensuing years has led to many cases in which evidence of past eruptions of a volcano was found in materials like volcanic ash despite the lack of records of such events.
In 1991, the committee again changed its definition of an active volcano to those that have erupted in the past 2,000 years or so and that are showing signs of fumarolic activity. Accordingly, the number of active volcanoes in Japan was raised to 83.
In 2003, the panel expanded the time frame for past eruptions to the last 10,000 years and designed 110 active volcanoes in 2011.
Toshitsugu Fujii, who heads the prediction committee, noted that the definition of an active volcano is based on its record of eruptions. This is basically the probability approach based on the assumption that a volcano that has erupted in the last 10,000 years is more likely to erupt again, he said. "Given that we don't know the route magma travels, we cannot completely rule out the possibility that a volcano with no history of eruption may still erupt."
In 1979, Mount Ontake, until then regarded as an extinct volcano, erupted. That event prompted the Meteorological Agency to stop using the classifications of "dormant" and "extinct" volcanoes.
An official at the agency's Seismology and Volcanology Department said it is now known that some volcanoes erupt just once in tens of thousands of years. "It is difficult to determine whether a volcano may erupt in the future by using only its data for historical times," the official explained. "The classifications of dormant and extinct volcanoes are not appropriate."
The Meteorological Agency monitors around the clock 47 volcanoes that are showing signs of strong volcanic activity.
In 2007, the agency introduced a warning system to indicate the risk of volcanic eruptions in five grades for 30 of the 47 volcanoes it is monitoring.
The Volcanic Alert Levels are divided into five categories depending on the scope of areas that must be warned and actions that should be taken by local residents.
The warning level for Mount Ontake at the time of the eruption was the lowest Level 1, which signifies that no particular response or action is required. There is no restriction on climbing a Level 1 mountain.
In contrast, Mount Sakurajima is currently designated at Level 3, which means climbing the mountain is restricted.
Level 1 should not be interpreted as a guarantee that there is no risk. Mount Ontake erupted suddenly without showing any telltale signs of crustal movements.
At the current level of knowledge and technology used to predict volcanic eruptions, it is difficult to reliably detect all the subtle signs of eruption.
A Level 1 designation does not mean the volcano is entirely safe to climb. Mountain climbers should know that an active volcano can erupt at any time.
Mount Fuji, which is climbed by hundres of thousands of people every year, is no exception.
The 2011 giant earthquake that struck wide areas in eastern Japan caused major crustal movements all over the country. Experts warn that these movements could stimulate volcanic activity in various parts of the nation.
During the 19th century, there were more than four massive volcanic eruptions in Japan that emitted 300 million cubic meters or more of volcanic ash and other materials, according to Fujii. "The eruption of Mount Ontake should be a cautionary tale about the threat of volcanoes for all Japanese," he said.
The volcanic disaster, which claimed more than 50 lives, has underscored the limitations of the science of predicting volcanic eruptions.
Eruptions occur much less frequently than other natural disasters such as earthquakes and floods. But once it happens, a major volcanic eruption would cause devastating damage over wide areas. The government should learn lessons from what happened at Mount Ontake and take steps to better prepare the nation for such events.