Under the crust, there is a "liquid zone" between 100 and 150 kilometers, where contains molten silicate material that is volatile under high temperature and pressure, known as magma. Once it breaks out of the crust, if forms a volcano. There are about 2000 known volcanoes on earth; 50000 active volcanoes have been discovered, most of which are on land and a few of them are underwater. Volcanoes are not evenly distributed on earth and they all appear in the fault zone of the crust. In terms of the world, volcanoes are mainly located in the Pacific Rim and the rigion from Indonesia to Myanmar, Himalayas and Central Asia to Mediterranean area. Now 99% active volcanoes concentrate in the two main regions.
Volcanoes have a long history. Some volcanoes erupted before human history but they are no longer active. Such volcanoes are calld extinct volcanoes. But there are some extinct volcanoes that suddenly erupt as the crust changes and people call them as dormant volcanoes. In the history of human, there are some volcanoes that erupt sometimes and people call them active volcanoes. Volcanic activities can produce a variety of substances. In the ejected solid material, there are generally broken pieces of rock, debris and ash, etc; in the ejected liquid material, there is nerally lava flow, water and all kinds of aqueous sloutions and mud flow of water, debris and ash, etc; in the ejected gas, there is generally water vapor, carbon, hydrogen, nitrogen, sulfur and other oxides. Besides, in the volcanic aticities, there are often ejecting out invisible light, electricity, magnetic, acoustic and radioactive substances, etc. These substances can sometimes kill people or cause failure of electricity, meters, etc and make wrecked plane, ship, etc. volcanic eruptions, which is a catastrophic natural phenomenon, can cause huge loss of existence and property of human beings in the short run. However, after the volcanic eruption, it can provide fertile soil, thermal energy and various mineral resources as well as tourism resources.
Many books described volcanic eruption in detail. For instance, in the "Heilongjiang anecdote", there are descriptions of eruption of volcanic cluster in Five Connected lakes in Heilongjiang: In the southeast of Moergen (Nenjiang), fire appeared on land and stones flied in the air. The noise was very aloud and the fire continued several days. After that, the land became pool. This happened in 58 years of Kangxi dynasty.
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