The strength of volcanic eruptions is related to the nature of the lava. The eruption time also vaires, some last for several hours, some can last up to a thousand years. According to their activity, volcanoes can be divided into three types: active volcano, extinct volcano and dormant volcano.
Active volcano refers to volcanoes that are active or erupting periodically. There are about 500 active volcanoes in the world. This kind of volcano is in active period of activity, such as Mount Merapi in Jawa Island. The volcano erupted lasting a period every two years since 20th century. In our country, the most famous is the Mount Qixing in Datun volcanic cluster in Taiwan. On the mainland, there are only records of volcanic eruption of Kadaxier volcanic cluster in the west of Kunlun Mountain in Xinjiang.
Extinct volcano refers to volcanoes that erupted in prehistoric times but has never been active now. Such kind of volcanoes lost their ability to move. Some volcanoes still maintain their complete form, but some have suffered weathering erosion and only left mutilated volcanic relics. In Datong, Shanxi, China, there are 99 isolated volcanic cones within the scope of about 1230 square kilometers, in which the Mount Den is nearly 19000 meters high.
Dormant volcano refers to volcanoes that erupted throughout history but has long been relatively static. The conic form of such kind of volcanoes is well preserved and they are still able to activate or not sure about the capability of activities.
For example, the Tianchi lake in Baitou Mountain in China erupted twice in 1327 and 658 and there were many activities before this. Although there are no eruptions at present, the volcano is in a dormant period judging from the undetectable spewing hot gas from slopes.
It should be noted that there are no strict boundaries between these three tpyes of volcanoes. Dormant volcano can revive and extinct volcano can resuscitate and they are not invariable mutually. People used to think that Mount Vesuvius is a extinct volcano. So people built many towns and cities and opens vineyards. However, in 26 A.D, the volcano erupted suddenly and the hot eruptive mass conquered defenceless two ancient cities, Pompeii and Hera GuNong. The two cities were damaged and residents all died.
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