Earthquakes are divided into natural earthquakes and artificial earthquakes. Natural earthquakes are mainly tectonic earthquakes, which are caused by the fracture and dislocation of rocks deep underground, so that the long-term accumulated energy is suddenly released and spread in all directions in the form of seismic waves, making houses shake and move to the ground. Tectonic earthquakes account for more than 90% of the total number of earthquakes. Followed by earthquakes caused by volcanic eruptions, known as volcanic earthquakes, accounting for about 7% of the total number of earthquakes. In addition, earthquakes will also occur in some special circumstances, such as cave collapse (collapse earthquake) and large meteorites hitting the ground (meteorite impact 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.
Thermal convection of mantle materials. It is driven by the energy generated by the decay of radioactive elements in the earth. It is the external expression of energy release inside the earth. The internal energy release mainly takes the following forms: earthquake, volcano, plate movement and geological structure. The earthquake is one of them.
There is a seismic source in the earth, which releases energy (seismic wave) and causes vibration in a certain range.
Other geological disasters or natural disasters can also indirectly induce earthquakes.
Thermal convection of mantle materials. It is driven by the energy generated by the decay of radioactive elements in the earth. It is the external expression of energy release inside the earth. The internal energy release mainly takes the following forms: earthquake, volcano, plate movement and geological structure. The earthquake is one of them.
Surface processes such as precipitation, wind, ocean currents and rivers are all driven by the external energy of the earth, namely the sun. Volcanic eruption
Volcanic eruption is the release of magma and other ejecta from the crater to the surface in a short time. Because the magma contains a large number of volatiles and the confining pressure of the overlying strata, these volatiles cannot overflow when dissolved in the magma. When the magma rises close to the surface, the pressure decreases and the volatiles are released sharply, thus forming a volcanic eruption. Volcanic eruption is a peculiar geological phenomenon, a manifestation of crustal movement and the strongest display of heat energy inside the earth on the surface.
Due to the influence of magmatic properties, underground magma chamber pressure, volcanic tunnel shape, volcanic eruption environment (land or underwater) and other factors, the forms of volcanic eruption are very different, and generally there are the following categories:
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 Emei Mountain 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:
(1) Quiet type: When a volcano erupts, only a large amount of hot lava quietly overflows from the crater and flows slowly along the hillside, just like rice soup cooked in an electric 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. People can enjoy this volcano to their heart's content.
(2) Explosive type: when the volcano erupts, it produces a violent explosion, and at the same time, it ejects a lot of gas and pyroclastic materials, and the molten slurry ejected is mainly medium-acidic molten slurry. 19021216 In February, the eruption of Mount Pere in the West Indies shocked the whole world. It spewed out thick magma, and at the same time spewed out a lot of pumice and fiery volcanic ash. This volcanic eruption, which killed 26,000 people, falls into this category, also known as Pei Lei type.
(3) Intermediate type: it belongs to the transitional type between calm eruption and explosive eruption, and 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. Its representative is the stron Boulder volcano on the Lipari Islands near the Italian west coast. The volcano erupts about every 2-3 minutes, and the flame of volcanic eruption can still be seen 50 kilometers away at night, so it is known as the "Mediterranean lighthouse". Also known as Stromboli style. Some people think that Wudalianchi volcano in Heilongjiang belongs to this type.
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.
Three stages of volcanic eruption
1, gas explosion
During the gestation period of volcanic eruption, due to gas dissolution and earthquake swarm, the degree of cracks in overlying rocks increased and the pressure decreased, while the amount of gas dissolved in magma increased, the volume of magma gradually expanded, the density decreased and the internal pressure increased. When the internal pressure greatly exceeds the external pressure, the gas explodes violently in the fracture density area of overlying rock, which breaks the rock and opens the channel of volcanic eruption. First, the debris is ejected, and then the magma erupts.
2. Formation of eruption column
After the gas explosion, the gas sprayed the cuttings and deep magma in the channel into the sky with great jet force, forming a high eruption column. The eruption column can be divided into three areas:
(1) air impact zone: located at the lower part of the eruption column, equivalent to one tenth of the height of the whole eruption column. Because the gas rushes out of the crater with great speed and strength, although the density of ejected rocks and other substances far exceeds the density of the atmosphere, it will also be thrown into the sky. When rising in the volcanic tunnel, the speed of air explosion gradually accelerates. When it ejects the surface and shoots into the sky, its speed gradually decreases due to the pressure of the atmosphere and the consumption of jet energy. Materials washed into the air by the air begin to fall at different heights according to their gravity.
(2) Convection zone: located in the upper part of the gas impact zone, it is called convection zone because the gas impact speed of the eruption column slows down, the gas in the gas column diffuses outward, and the gas in the atmosphere is constantly replenished, forming convection of gas inside and outside the eruption column. Dense matter in this area began to decline. Substances whose density is less than that of the atmosphere continue to rise under the action of atmospheric buoyancy. The height of the gas column in the convection area is relatively large, accounting for about seven tenths of the total height of the eruption column.
(3) Diffusion zone: located at the top of the eruption column, the pressure between the eruption column and the upper atmosphere is basically balanced. The eruption column keeps rising, and the gas and low-density substances in the column diffuse horizontally, so it is called diffusion zone. The volcanic ash brought into the sky will form a volcanic ash cloud, which can float in the air for a long time, which will have a great impact on the regional climate and even cause disasters. The column height in this area accounts for about two-tenths of the total column height.
3. The collapse of the eruption column
In the process of rising, the eruption column carries debris with different particle sizes and densities, and collapses at different heights and stages according to the gravity. There are four main factors that determine the collapse speed of the eruption column:
(1) The crater radius is large, the gas impulse is small, and the column collapses quickly;
(2) If the content of cuttings in the eruption column is high, the particle size is large and the density is high, the column will collapse quickly;
(3) If many solid rocks in the eruption column return to the air repeatedly, the column will collapse quickly;
(4) If surface water is added to the eruption column, the density of the column can be increased and the column will collapse quickly. On the contrary, if the eruption column stays in the air for a long time, it will slowly collapse.
Volcanic eruptions are inconsistent. Just like Kilauea volcano in Hawaii, lava has already quietly flowed out in advance. Because lava flows slowly, it only destroys property and does not endanger life. However, pyroclastic eruption or steam explosion (or violent steam explosion) like 1883 Krakato volcano in Indonesia caused heavy casualties.
Volatile substances play an important role in the process of volcanic eruption, which is both the product and the driving force of volcanic eruption. During the whole process from magma generation to volcanic eruption, the activities of volatile substances have played a role.
British scientists believe that the eruption of super volcano may destroy mankind.
British scientists believe that human beings may be destroyed in a super volcanic eruption. Stephen cerf of the University of Great Britain told an e-magazine reporter that there was no way to stop the disaster. At present, scientists are busy making various strategies to resist "external threats", such as how to prevent asteroids from colliding with the earth, but rarely consider that the main danger may come from the inside of the earth.
Geophysicists assert that the eruption intensity of some volcanoes is hundreds of times that of the past, and the earth experienced such a large-scale disaster not long before the emergence of civilization.
American geologists discovered the shallow volcanic ash dead layer in Yellowstone National Park earlier, believing that its formation was caused by the super volcanic eruption 620,000 years ago. Therefore, some funnel-shaped craters can still be seen here, which are all craters formed after destructive volcanic eruptions.
In the report to the British government's working group on natural disasters, the consequences of this super volcano eruption have been described in detail-a large area will be covered by lava, and dust and ash scattered into the atmosphere will prevent a lot of sunlight from reaching the earth's surface, which will undoubtedly change the global climate.
According to the research of Michael Ram Pi Nuo of new york University, 74,000 years ago, the super eruption of Sumatra volcano caused global cooling and destroyed 3/4 plants in the northern hemisphere.
Hazards of volcanoes
The most powerful and spectacular volcanic eruptions often occur in subduction zones. The volcano here may erupt again after hundreds of years of silence, and once it erupts, its power is particularly fierce. This kind of volcanic eruption often brings disasters to mankind.
Affect the global climate
When a volcano erupts, a large amount of volcanic ash and gas will have a great impact on the climate. Because in this case, dark days, storms and even mud rain will bother local residents for months. Volcanic ash and gas are sprayed high into the air, and they will spread far away with the wind. These volcanic materials will block the sunlight and cause the temperature to drop. In addition, they will filter out some wavelengths of light, making the sun and the moon look like they are covered with a halo or have strange colors, especially at sunrise and sunset, which can form a strange natural landscape.
destroy the environment
A large amount of volcanic ash ejected by volcanic eruption combined with heavy rain to form debris flow, which can destroy roads and bridges, flood nearby villages and cities and make countless people homeless. Mud formed by soil and rock fragments can flood the whole city.
Although the rocks are covered by volcanic ash clouds, when the volcano just erupted, you can still see huge rocks spewing into the air.
Resurrect life
Volcanic eruption has a far-reaching impact on the natural landscape. Land is the most precious resource in the world, because it can breed all kinds of plants to support everything. If volcanic eruption can cover farmland with volcanic ash less than 20 cm thick, it is really a blessing for farmers, because these volcanic ash are rich in nutrition and can make the land more fertile.
1. After the lava disintegrated, weeds and liverworts began to appear.
2. Pteridophytes grow on the hillside where rope-like lava flows.
Volcanic ash enriches the surrounding land, and the local grapes are harvested every year.