Volcanoes are "dead" or "active" volcanoes, and there is no strict and scientific standard. Generally speaking, there are 534 volcanoes in the world that have had historical eruptions or recorded historical eruptions as active volcanoes. But history or historical records can be very different for each country or region, some are only three or four hundred years old, others are three or four thousand years old or longer. The definition of active volcanoes based on historical or historical records is impractical. So some volcanologists have come up with an improved definition of active volcanoes with time constraints, that is, volcanoes that have erupted in the past 10,000, 5,000 or 2,000 years are called active volcanoes. Whether to use the year 10000 or 5000 or 2000 will be allowed to be determined according to the specific conditions of different countries and regions.
But the "dead" or "alive" of volcanoes is still relative. Some "dead" volcanoes that have not erupted in the past 10,000 years or more may also erupt again due to deep tectonic or magmatic activity. For example, most of the volcanoes in the Wudalianchi Volcanic Group erupted 10,000 years ago, but the Laoheishan volcano and the Huoshan volcano among them erupted in 1719-1721 AD.
Active magma is the key to judge whether a volcano is "dead" or "alive", and the criterion is as follows: (1) Existing hydrothermal activity or jet phenomena in active volcanic areas; (2) within a small area centered on volcanoes, microseismic activity is significantly higher than that in its periphery; (3) some observable surface deformation occurs in volcanic areas. All the above phenomena are due to the specific activities of the magma system under the volcano, so it is necessary to set up a long-term seismic - topographic observation network in the volcanic region, as well as a variety of geophysical and geochemical methods for detection. This is the basic monitoring and detection research that should be carried out after the volcano has been identified as a dangerous volcano.
Based on the above, we can get the general concept of active volcanoes: Active volcanoes are those that are erupting or have erupted in history and in the past 10,000 years. When an active magma system exists beneath the volcano, the volcano is considered to be risk for eruption and should be placed in a modern volcanic monitoring system.
There is no strict boundary between the three types of volcanoes. Dormant volcanoes can be revived and extinct volcanoes can be "revived". Mount Vesuvius in Italy was thought to be an extinct volcano, at the foot of the volcano, people built towns and opened vineyards on the slopes of the volcano. But in 79 AD, Mount Vesuvius erupted suddenly, and high-temperature volcanic eruptions attacked the unprotected ancient cities of Pompeii and Heragnum, both cities and their inhabitants were completely destroyed and killed.
Pompeii was the most prosperous city in the Roman empire until 79 AD. The city is rich with fertile soil from volcanoes. On August 24,79AD, Mount Vesuvius erupted, within 24 hours, at least 5,000 residents of Pompeii and the city disappeared into lava and ash. Pompeii consequently retained a large number of architectural and artistic relics of the ancient Roman Empire, and became the most famous ancient city site in the world (figure 3-2).
Figure 3-2 Vesuvius Volcano (a) and Ancient site of Pompeii(b)
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