There is a "liquid zone" between 100 and 150 km under the crust. In this liquid zone, there is a molten silicate substance containing gas volatiles at high temperature and high pressure, that is, magma. Once it rushes out from the weak part of the earth's crust, a volcano is formed.

There are about 2000 known "extinct volcanoes" on the earth; There are 523 "active volcanoes" discovered, including 455 on land and 68 underwater. Volcanoes are unevenly distributed on the earth, and all appear on the fault zone in the crust. Worldwide, volcanoes are mainly concentrated around the Pacific Ocean and Indonesia, passing through Myanmar, Himalayas, Central Asia and the Mediterranean Sea to the north. Today, 80% of the active volcanoes on the earth are distributed in these two belts.

Volcano has a long history. Some volcanoes have erupted before human history, but they are not active now. Such a volcano is called an "extinct volcano"; However, some "extinct volcanoes" will suddenly erupt with the change of the earth's crust, which is called "dormant volcano"; Volcanoes that erupt from time to time in human history are called "active volcanoes".

Volcanic activity can eject a variety of substances, and among the ejected solid substances, there are generally rocks, debris and volcanic ash crushed by blasting. Among the ejected liquid substances, there are generally lava flows, water, various aqueous solutions and mud flows mixed with water, debris and volcanic ash. Among the ejected gas substances, there are usually water vapor and oxides of carbon, hydrogen, nitrogen, fluorine and sulfur. In addition, in volcanic activities, visible or invisible light, electricity, magnetism, sound and radioactive substances are often ejected, which can sometimes kill people, or make power and instruments fail, resulting in plane and ship crashes.

The intensity of volcanic eruption is related to the nature of lava. The eruption time is long or short, ranging from a few hours to thousands of years. According to volcanic activity, volcanoes can be divided into three categories: active volcanoes, extinct volcanoes and dormant volcanoes. The dormant volcano refers to the eruption recorded in human history, but its activity has not been seen since then. There are about 500 active volcanoes in the world.

Volcanic eruption will cause great losses to human beings, lives and property in a short time. This is a catastrophic natural phenomenon. However, after volcanic eruption, it can provide abundant land, heat energy, various mineral resources and tourism resources.

Many books describe the volcanic eruption in detail. For example, Biography of Heilongjiang Province describes the eruption of two volcanoes in Wudalianchi Volcano Group in Heilongjiang Province. "In the southeast of Mulgen (now Nenjiang), one day there was an underground fire, stones soared, and the sound shook the fields. The more days the fire goes out, the land becomes a pond. This is the fifty-eighth year of Kangxi. "

Volcanic types

(a) According to the classification of volcanic activity

1, active volcano

Refers to a volcano that is still active or erupts periodically in modern times. This volcano is at the peak of its activity. For example, Mount Merapi in Zhaotu Island has been erupting every two or three years since this century. The recent volcanic activity in China is most famous in Qixing Mountain, the main peak of Datun Volcano Group on the island of Taiwan Province Province. On the mainland, only in 1995, there was a record of the volcanic eruption of the Yutian Caldarcy Volcano Group in the western section of Kunlun Mountain in Xinjiang. The volcanic eruption formed a flat-topped volcanic cone. The top of the cone is 4900 meters above sea level, the cone height is 145 meters, the cone bottom diameter is 642 meters, the cone top diameter is 175 meters, and the crater depth is 56 meters.

2. extinct volcano

Refers to a volcano that erupted in prehistoric times but has never been active in history. This volcano has lost its ability to move. Some volcanoes remain intact, while others have weathered and eroded, leaving only incomplete volcanic remains. Within about 50 square kilometers of Fiona Fang, there are two isolated volcanic cones, of which the height of the Langwoshan volcanic cone is nearly120m.

3. dormant volcano

Refers to a volcano that has erupted in history but has been relatively static for a long time. This kind of volcano has preserved a complete volcanic male body and still has volcanic activity ability, or it cannot be concluded that it has lost its volcanic activity ability. For example, China Baitoushan Tianchi erupted twice in 1597 and 1792, and there were many activities before that. Although there is no eruption activity at present, some unfathomable blowholes on the hillside keep spewing high-temperature gas, indicating that the volcano is currently in a dormant state.

It should be noted that there are no strict boundaries between these three types of volcanoes. Dormant volcanoes can be resurrected, and dead volcanoes can also be "resurrected", and they are not static. In the past, Mount Vesuvius was always considered as an extinct volcano. At the foot of the volcano, many towns and vineyards were built on the hillside of the volcano. However, in 79 AD, Vesuvius erupted suddenly, and the high-temperature volcanic eruption in Attack and Capture created two unsuspecting ancient cities, Pompeii and Heragonom. Cities and residents were destroyed and killed.

(two) according to the type of volcanic eruption.

Volcanism is influenced by factors such as magma properties, underground magma chamber pressure, volcanic channel shape and volcanic eruption environment (land or underwater), which makes volcanic eruption have the following types.

1, fissure eruption

Magma overflows the surface along a huge crack in the earth's crust, which is called fissure eruption. There is no strong explosion phenomenon in this eruption, and the ejecta are mostly basic molten slurry, which often forms a lava platform with a wide coverage after condensation. For example, the Permian Emeishan basalt distributed in the border area of Sichuan, Yunnan and Guizhou in southwest China and the Tertiary Hannuoba basalt north of Zhangjiakou in Hebei belong to fissure eruption. Modern fissure eruption is mainly distributed in the mid-ocean ridge at the bottom of the ocean. Only Iceland can see this kind of volcanic eruption on the mainland, so it is also called Icelandic volcano.

2. Central eruption

Underground magma is ejected from the surface through a tubular volcanic passage, which is called central eruption. This is the main form of modern volcanic activity, which can be subdivided into three types:

Quiet type: when the volcano erupts, only a large amount of hot magma flows out of the crater quietly and flows slowly along the hillside, just like cooking rice soup from a rice cooker. Overflow is mainly alkaline molten slurry with high temperature, low viscosity and easy flow. Less gas, no explosion, Hawaii volcano is its representative, also known as Hawaiian type.

Explosive; When the volcano erupts, it explodes violently, and at the same time, it ejects a lot of gas and pyroclastic materials, and the molten slurry ejected is mainly medium-acid molten slurry. 1902, 12, 16 In February, Mount Bailey in the West Indies broke out, also known as Bailey type.

Intermediate type: it belongs to the transitional type between calm eruption and explosive eruption. This type is dominated by intermediate-basic lava eruption. If there is an explosion, the explosive force is not great. It can erupt smoothly for months or even years and is characterized by intermittent eruption. Take stron Boulder Volcano on the Lipari Islands near the Italian west coast as an example. The volcano erupts about every 2-3 minutes, and its flame can still be seen 50 kilometers away at night. So it is also called Stromboli style.

3. Infiltration eruption

Magma penetrates the earth's crust and overflows the surface in a large area, which is called penetrating eruption. This is an ancient way of volcanic activity, which no longer exists in modern times. Some scholars believe that the Archean crust is thin and the underground magma temperature is high, which often causes penetrating magma ejection.

earthquake

Earthquake is the rapid vibration of the earth's surface, which was also called earthquake in ancient times. Just like wind, rain, lightning, landslides and volcanic eruptions, they are natural phenomena that often occur on the earth.

It originates from a point underground, which is called the focus. Vibration comes from the source and propagates in the earth. The closest point on the ground to the earthquake source is called the epicenter, which is the place where the vibration was first received. Ground vibration is the most intuitive and common manifestation of earthquakes. Strong earthquakes at the bottom of the sea or in coastal areas will cause huge waves, which is called tsunami. Earthquakes are extremely frequent, and there are about 5 million earthquakes every year in the world.

The structure of the ball is like an egg, which can be divided into three layers. The middle layer is the "yolk"-the core; In the middle is "egg white"-mantle; The outer layer is the "eggshell"-the shell. Earthquakes usually occur in the earth's crust. The earth is constantly rotating, and the interior of the crust is constantly changing. The resulting force led to the deformation, fracture and dislocation of the crust and strata, so an earthquake occurred. The place where an earthquake occurs underground is called the source. The place from the source vertically upward to the surface is called the epicenter. The distance from the epicenter to the source is called the focal depth. Earthquakes with a focal concentration of less than 70km are shallow earthquakes, earthquakes with a focal concentration of 70km to 300km are moderate earthquakes, and earthquakes with a focal concentration of more than 300km are deep earthquakes. The earthquake with the deepest focal depth was 1963, which occurred in the northern part of Irian Jaya province, Indonesia, with a focal depth of 786 kilometers. Earthquakes of the same size have different damage to the ground because of different focal depths. The shallower the source, the greater the damage, but the smaller the spread, and vice versa.

The distance from a place to the epicenter is called epicentral distance. Earthquakes with epicentral distance less than 1000 km are called near earthquakes, earthquakes with epicentral distance between 100- 1000 km are called near earthquakes, and earthquakes with epicentral distance greater than1000 km are called teleseisms. Among them, the farther the epicentral distance, the smaller the impact and damage.

The ground vibration caused by earthquake is a complex movement, which is the result of the joint action of longitudinal wave and shear wave. In the epicenter, longitudinal waves made the ground jump up and down. Shear waves make the ground shake horizontally. Because longitudinal waves travel faster and decay faster, while shear waves travel slower and decay slower, they are far from the epicenter, so you often can't feel the jump up and down, but you can feel the horizontal shaking.

The magnitude of the earthquake itself is expressed by magnitude, and the magnitude is determined according to the energy of elastic waves released during the earthquake. China generally uses the Richter scale. Earthquakes with magnitude less than 2.5 are usually called small earthquakes, earthquakes with magnitude 2.5-4.7 are called inductive earthquakes, and earthquakes with magnitude greater than 4.7 are called destructive earthquakes. For every magnitude difference of 1, the energy released by the earthquake is about 30 times different. For example, an earthquake of magnitude 7 is equivalent to 30 earthquakes of magnitude 6 or 900 earthquakes with magnitude difference of 0. 1, and the average difference of energy released is 1.4 times.

When a large earthquake occurs in a certain place, a series of earthquakes often occur within a period of time, the largest of which is called the main earthquake, the earthquake before the main earthquake is called the foreshock, and the earthquake after the main earthquake is called the aftershock.

Earthquakes have a certain temporal and spatial distribution law. In terms of time, earthquakes have periodic phenomena of alternating active periods and quiet periods. From a spatial point of view, the distribution of earthquakes has certain zones, called seismic zones, which are mainly concentrated in the Pacific Rim and Mediterranean-Himalayan seismic zones. The Pacific seismic belt almost concentrates more than 80% of the shallow earthquakes (0 km ~ 70 km) in the world, and all moderate earthquakes (70 km ~ 300 km) and deep earthquakes release about 80% of the total energy.

The degree of ground vibration at a certain point during an earthquake is called seismic intensity. China divides the earthquake intensity into 12 degrees.

Although both magnitude and intensity can reflect the strength of an earthquake, their significance is the same. The same earthquake has only one magnitude, but the intensity varies from place to place, and the intensity value varies from place to place. For example, on1February 1990 10, an earthquake of magnitude 5. 1 occurred in Changshu-Taicang. Some people say that Suzhou is level 4 and Wuxi is level 3, which is wrong. No matter where it is, it can only be said that an earthquake of magnitude 5. 1 occurred in Changshu-Taicang, but this time, the earthquake intensities in shaxi town, Taicang, Suzhou and Wuxi were 6 degrees, 4 degrees and 3 degrees respectively.

Seismic intensity is a frequently used term. There are qualitative and quantitative standards for dividing strength. On the China Earthquake Intensity Table (see the table below), people's feelings and the degree of earthquake damage of ordinary houses are described, which can be used as the basic basis for determining the intensity.

Cause of earthquake

There are many reasons for the vibration of the earth's surface. According to the causes of earthquakes, earthquakes can be divided into the following categories:

1. Crustal earthquake

Earthquakes caused by dislocation and fracture of rock strata in deep underground are called tectonic earthquakes (Figure1-1). This kind of earthquake has the highest frequency and the greatest destructive power, accounting for more than 90% of the global earthquakes.

2. Volcanic earthquake

Earthquakes caused by volcanism, such as magmatism and gas explosion, are called volcanic earthquakes. Volcanic earthquakes can only occur in volcanic active areas, and earthquakes in volcanic active areas only account for about 7% of global earthquakes.

3. Collapse earthquake

The earthquake caused by the collapse of underground caves or the top of mines is called collapse earthquake. Such earthquakes are relatively small in scale and few in frequency. Even if it exists, it often occurs in limestone areas with dense caves or large underground mining areas.

4. Induced earthquake

Earthquakes caused by reservoir impoundment and oil field water injection are called induced earthquakes. This kind of earthquake only occurs in some specific reservoir areas or oil fields.

5. Artificial earthquake

The ground vibration caused by underground nuclear explosion and explosive blasting is called artificial earthquake. Artificial earthquakes are earthquakes caused by human activities. Such as vibration caused by industrial blasting and underground nuclear explosion; High-pressure water injection in deep wells and water storage in large reservoirs increase the pressure on the earth's crust and sometimes induce earthquakes.

The place where seismic waves are generated is called the source. The vertical projection of the source on the ground is called the epicenter. The depth from the epicenter to the source is called the focal depth. Generally, the focal depth less than 70km is called shallow earthquakes, the depth of 70-300km is called Zhongyuan earthquake, and the depth greater than 300km is called deep earthquake. Destructive earthquakes usually occur in shallow earthquakes. For example, the focal depth of the Tangshan earthquake in 1976 was 12km.