Volcano and earthquake.

Craters are cracks in the earth that release heat and gas. Volcano is a cone-shaped mountain formed by magma and other high-temperature substances deep in the earth passing through cracks in the earth's crust and spewing out. Earth science is also called accumulation mountain. There are active volcanoes and extinct volcanoes. Active volcano is a volcano that erupts frequently or periodically in human history. Dead volcanoes are volcanoes that have never erupted in human history, but no one knows that they will suddenly wake up in their sleep.

There are two largest volcanic activity zones on the earth: the Pacific Rim volcanic zone and the Mediterranean volcanic zone. There are more than 850 active volcanoes in the world (more than 700 on land and more than 0/00 on the seabed), of which 3/4 are distributed in the volcanic belt around the Pacific Ocean, becoming the "ring of fire" worn by the earth. The eruption of active volcano is irregular. It seems that he is a mighty man with infinite divine power, and the power generated during eruption is unimaginable. 1980 On May 18, Mount St. hellens erupted in Washington, USA, and precious photos were taken by satellite. Analysis shows that the shock wave of volcanic eruption passes through the 200-kilometer-thick atmosphere, releasing energy equivalent to more than 500 atomic bombs dropped by the United States in Hiroshima that year. The fiery magma destroyed houses, bridges, roads, forests, people and animals.

At least 20 cities around the world were completely destroyed by erupting volcanoes in an instant. The earliest record is ancient Greece in 1470 BC. At that time, the bustling ancient city of Knossos was razed to the ground by the sudden eruption of santorini volcano, and the 50-meter-high waves swept across the islands and coasts of the Eastern Mediterranean, and the Minoan Civilization Center and Crete, 30 kilometers away, were instantly destroyed.

In 79 AD, the Italian volcano Vesuvius, under the volcanic eruption, buried four prosperous ancient castles nearby, namely Pompeii, Stebia, Gerkulum and Auronqi. All these have left eternal regrets for human civilization. It also makes people realize that our so-called brilliant civilization is so pale and powerless in the face of the forces of nature, and the disaster caused by volcanic eruption is so irresistible. Volcanic eruption not only directly destroys everything, but also causes a series of disasters-fire, tsunami, debris flow and flood, forming a volcanic lake that may burst at any time. Volcanic ash is very small, and it floats far away or rises to the sky with the wind, and spreads for a long time, leading to reduced visibility, air crashes, traffic accidents, and even "Leng Xia" in climate variation. 1783, Japan's Asayama volcano erupted, leading to "Leng Xia" in Japan, and even freezing damage in the northeast. Volcanic ejecta: Carbon dioxide, sulfur dioxide, hydrogen, chlorine, hydrogen sulfide, etc. It will also pollute the air, form acid rain and produce the greenhouse effect.

Volcanic fire:1977 65438+1On 7 October, Nyiragongo volcano in Africa erupted, burning 430 square kilometers of tropical rain forests in Zaire and Rwanda.

Tsunami caused by volcano:1On August 27th, 883, Krakato volcano in the Eta Strait of Indonesia erupted, which triggered the biggest tsunami in human history, and set off wild waves as high as 30-40m, engulfing all ships in this sea area. All houses, vehicles, people and animals along the coast of Java Island and Sumatra Island were involved in the rough sea. In Indonesia alone, 36,000 people were killed in the tsunami, and the economic losses were incalculable.

Debris flow caused by volcano: Palicot volcano erupted in February 1943, and the nearby hillside was covered with volcanic ash with a thickness of 60-70cm. When hurricanes and rainstorms swept through Mexico, mudslides were formed, which instantly buried three villages and dozens of villagers at the foot of the mountain and destroyed more than 600 square kilometers of farmland.

Flood caused by volcano:1985165438+1October 13 The eruption of the Lewis volcano in Tolima province, Colombia, instantly melted the snow for thousands of years and caused flash floods. The flood spread over 30,000 square kilometers, causing 25,000 deaths, 130 people were homeless, and 65,433 people were homeless.

Volcanic mountain type

There are many kinds of volcanoes, which can be divided into three types according to their activity: extinct volcanoes, dormant volcanoes and active volcanoes. An 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. A dormant volcano refers to a volcano that has erupted in history but has been relatively static for a long time. These volcanoes all have well-preserved volcanic shapes, and they still have volcanic activity, or it cannot be concluded that they have lost volcanic activity. 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. An 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.

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.

When a volcano erupts, the outlet from which gas, magma, solids and other substances are discharged is called a crater. Generally speaking, the central eruption has only one crater at a time, but most volcanoes erupt many times. Some of the subsequent eruptions came from the original crater, but most of them erupted on its side, thus forming a new crater, called a parasitic crater. In addition to the central eruption, there is also a fissure eruption, that is, the eruption material is ejected from the cracks in the earth's crust. It formed more craters. For example, the eruption of Laki volcano in Iceland, in a 32-kilometer-long crack, lava continuously flows out from 105 crater. Most craters are round in shape, which are generally divided into five categories: symmetrical craters, asymmetrical craters, cracked craters, craters without obvious craters and sunken craters. There is a special kind of crater called crater. It is because the volcano erupted so violently that a huge amount of magma washed away volcanic neck and the surrounding rocks at once. The crater diameter caused by this eruption is often more than 5 kilometers. The violent eruption not only formed the crater, but also greatly reduced the height of the volcano. For example, the eruption of Mount Tambora produced a crater with a diameter of 12km, and the altitude dropped by 1400 meters.

earthquake

The vibration of the earth's surface caused by the sudden release of slowly accumulated energy inside the earth is called an earthquake. Due to the constant movement and change of the earth, different parts of the earth's crust are subjected to such forces as squeezing, stretching and twisting. Energy is also gradually accumulated. In some fragile places, rock strata are prone to sudden rupture, resulting in fracture and dislocation, which will lead to earthquakes.

Several basic concepts about earthquakes;

Earthquake source: the place where the earth directly breaks;

Earthquake: the place on the ground facing the source;

Focal depth: the distance from the focal point to the epicenter;

Epicenter distance: the distance from the epicenter to any observation point on the ground;

Extreme earthquake zone: the most severely damaged area after the earthquake.

Magnitude is a measure of earthquake magnitude, which is divided according to the energy released by the earthquake and expressed by "magnitude".

The magnitude is calculated by the records of seismic instruments. The stronger the earthquake, the greater the magnitude. The magnitude difference is one level and the energy difference is about 30 times.

Magnitude classification of earthquakes:

Weak earthquake: earthquake with magnitude less than 3;

Sensible earthquake: an earthquake with magnitude equal to or greater than 3 and less than or equal to 5;

Moderate earthquake: an earthquake with a magnitude greater than 5 and less than 6;

Strong earthquake: an earthquake with magnitude equal to or greater than 6. Among them, magnitude 8 or above is also called a giant earthquake.

Intensity is the degree of damage to buildings such as ground and houses caused by earthquakes, which is expressed by "degree". China divides the earthquake intensity into 12 degrees.

Magnitude and seismic intensity are two completely different concepts. Magnitude is only related to the energy released by the earthquake, and it is a measure of earthquake magnitude, so an earthquake has only one magnitude. Intensity indicates the degree of damage to the ground caused by the earthquake, which varies from place to place, but there is only one epicenter intensity.

Generally speaking, the greater the magnitude, the higher the intensity. The intensity of the same earthquake is different in different places with different epicentral distances (generally speaking, the epicenter area is the most damaged, and its intensity value is called epicenter intensity. With the increase of epicentral distance, the losses caused by the earthquake gradually decrease. In addition to magnitude and epicentral distance, intensity is also related to focal depth, geological structure and rock properties.

There are many kinds of volcanoes, which can be divided into three types according to their activity: extinct volcanoes, dormant volcanoes and active volcanoes. An 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. A dormant volcano refers to a volcano that has erupted in history but has been relatively static for a long time. These volcanoes all have well-preserved volcanic shapes, and they still have volcanic activity, or it cannot be concluded that they have lost volcanic activity. 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. An 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.

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.

When a volcano erupts, the outlet from which gas, magma, solids and other substances are discharged is called a crater. Generally speaking, the central eruption has only one crater at a time, but most volcanoes erupt many times. Some of the subsequent eruptions came from the original crater, but most of them erupted on its side, thus forming a new crater, called a parasitic crater. In addition to the central eruption, there is also a fissure eruption, that is, the eruption material is ejected from the cracks in the earth's crust. It formed more craters. For example, the eruption of Laki volcano in Iceland, in a 32-kilometer-long crack, lava continuously flows out from 105 crater. Most craters are round in shape, which are generally divided into five categories: symmetrical craters, asymmetrical craters, cracked craters, craters without obvious craters and sunken craters. There is a special kind of crater called crater. It is because the volcano erupted so violently that a huge amount of magma washed away volcanic neck and the surrounding rocks at once. The crater diameter caused by this eruption is often more than 5 kilometers. The violent eruption not only formed the crater, but also greatly reduced the height of the volcano. For example, the eruption of Mount Tambora produced a crater with a diameter of 12km, and the altitude dropped by 1400 meters.

On our earth, volcanic activities usually occur in some unusual geological backgrounds, most of which are on the boundaries of huge plates that make up the lithosphere. About 80% of active volcanoes on the earth and their related volcanic activities occur when two plates meet, and one plate swoops under the other. The subduction plate, on the one hand, increases the local pressure due to compression, on the other hand, it itself melts into magma; At this time, if there are cracks or weak points in the upper compression plate, the magma with great pressure will be ejected from these places to form a volcano. There is also a different situation, that is, on the mid-ocean ridge axis, magma gushes from the mantle and separates to both sides of the ridge, forming a new ocean floor. This volcanic activity actually takes place underwater.

Volcanic eruption

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 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. People can enjoy this volcano to their heart's content.

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. 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.

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. 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.

The form of volcanic eruption is mainly determined by two aspects: one is the nature of magma, and the other is the ratio of solid, gas and liquid during eruption. Magma can be divided into basic magma and acidic magma according to its silica content. Basic magma has low viscosity and high fluidity, and is also called basalt magma. The volcanic activity caused by this magma is less, and the magma can flow long distances on the ground, forming a strange landscape. Acidic magma has high viscosity and poor fluidity, which leads to intense volcanic activity. In addition, the eruption dominated by gas is the strongest, which is called "explosion"; The eruption dominated by solids takes the second place, which is called "ejection"; The weakest, mainly liquid, is called "flow"; Some of the three are stronger, called "jet".

There are many volcanoes in the world. In the past 400 years, there have been more than 500 active volcanoes. Although there are many volcanoes, they are regularly distributed. People divide volcanoes into four zones: the Pacific Rim volcanic zone, the Atlantic volcanic zone, the Mediterranean volcanic zone and the East African volcanic zone. The Pacific volcanic belt includes the east and west banks, and Taiwan Province Province of China is on this volcanic belt. El Salvador, known as the "volcano country", is also in this volcanic belt. Llullaillaco, Chile, the highest active volcano in the world, is 6723 meters above sea level; The highest extinct volcano-Aconcagua volcano in Chile, with an altitude of 6964 meters, is also in this zone. The most striking submarine volcanic eruption in the Atlantic volcanic belt and the subsequent volcanic island. There, sometimes you can see the strange sight of submarine volcanic eruption, and an island will emerge from the original Wang Yang in a few days. Because the newly formed volcanic island material is still relatively loose and can't resist the impact of the waves, it often disappears suddenly like magic. The Mediterranean volcanic belt is famous for its frequent volcanic activities, and the world-famous Mount Vesuvius is on this volcanic belt. Many volcanoes in this area are still active today, such as Mount Etna in Sicily, which has never stopped erupting since 6000 BC. The African continent is called the East African Rift Valley, which is formed by the tension on both sides and is also a place where volcanic activity is very active. On both sides of the rift valley, there are volcanoes. The extinct volcano still spews out steam, hot springs flow in, and the active volcano is steaming and smoky. There are two world-famous volcanic wonders here: one is the mouth of Ngorongoro volcano, the second largest crater, and the other is the lava lake of Nyiragongo volcano. Kilimanjaro, the first peak in Africa, and Mount Kenya, the second peak, are covered with snow all year round, which is very beautiful.

There are about 660 volcanoes in China, most of which are extinct. Active and dormant volcanoes are mainly distributed in Taiwan Province Province, Kunlun Mountain in Heilongjiang Province, Changbai Mountain and Tengchong in Yunnan Province. There are more than 230 buildings in Heilongjiang and Jilin, among which Wudalianchi and Baitou Mountain are famous. Inner Mongolia has the largest number of volcanoes in China, with more than 270 volcanoes. There is the largest volcanic group in China, including more than 100 volcanoes, of which there are as many as 56 in Dali volcanic group. Taiwan Province Province, located in the volcanic belt around the Pacific Ocean, is the most violent place of volcanic activity in China. Penghu Islands, Huoshao Island and Diaoyu Island are all volcanic islands erupted by undersea volcanoes. The most famous Datun Volcano Group in Taiwan Province Province is located in the northern part of the island, and consists of more than 10 volcanoes with an altitude of about 1000 meters.

Earthquake:

1, philosophers and thinkers of various nationalities have made various explanations on the causes of earthquakes, and China's contribution in this field is unparalleled. As far back as 2700 years ago in the Western Zhou Dynasty, Bai Yang's father thought that "the sun can't come out and the shade can't be steamed, so there is an earthquake." This is the philosophical theory of Yin and Yang.

2. Since ancient times, due to the backwardness of science, human beings have always attributed earthquake disasters to God's natural forces. Many ethnic groups believe that the earth is supported by some animals, and when these animals move, earthquakes will occur. For example, some tribes in India believe that there are several elephants standing on the tortoise carrying the earth, and an earthquake will occur when the elephants move. It is generally believed in Japan that the earthquake was caused by the underground catfish turning over. If the catfish is stopped, the world will be peaceful. In ancient Greece, it was said that the sea god hit the seabed with a trident when he was angry, causing earthquakes and tsunamis.

3. Since 1960s, scientists have gradually put forward the theory of plate tectonics. The lithosphere on the earth's surface consists of several huge plates. These plates either separate from each other or collide and subduct. Plate boundaries are often earthquakes. An area where volcanic activity is particularly active. However, the principle of intraplate earthquakes is still unclear.

4. 1996, a strong earthquake of magnitude 8.3 occurred in San Francisco, USA, causing serious losses. After the earthquake, it was found that there were dislocations on both sides of the 430-kilometer-long San Andreas fault on the west coast of North America. American seismologist Fred put forward the theory of elastic resilience through research. According to this theory, rocks are deformed by earthquake movement. When the deformation exceeds a certain degree, the rock breaks and dislocates violently, and then the deformed rock bounces back to its original state, which is the process of earthquake occurrence. The picture shows the St. Frederick fault, the nest of the earthquake.

Modern science explains the causes of earthquakes as follows:

Because the earth is constantly moving and changing, it gradually accumulates huge energy, causing sudden rupture of rock strata or dislocation of original faults in some fragile areas of the crust. This is an earthquake.

Most earthquakes occur in the earth's crust.

Earthquakes can be divided into four types: tectonic earthquakes, volcanic earthquakes, subsidence earthquakes and induced earthquakes.

Tectonic earthquake means that under the action of tectonic movement, when the local stress reaches and exceeds the strength limit of rock stratum, the rock stratum will suddenly deform or even rupture, and the energy will be released at once, causing ground motion. This kind of earthquake is called tectonic earthquake, accounting for more than 90% of the total number of earthquakes.

Volcanic earthquake refers to an earthquake that occurs in the deep underground due to the fracture or collapse of the overlying strata due to the loss of a large amount of magma, the reduction of underground pressure or the lack of time to replenish magma. The number of such earthquakes is small, accounting for only about 7% of the total number of earthquakes.

Collapse earthquake is a local earthquake caused by underground cave or goaf collapse. Collapse earthquakes are all the results of gravity, with small scale and low frequency, accounting for only about 3% of the total number of earthquakes.

Artificial earthquakes and induced earthquakes are earthquakes caused by artificial blasting, mining, military construction and underground nuclear tests. Earthquakes caused by human production activities Some fault activities are called induced earthquakes, mainly including reservoir earthquakes, deep well pumping and water injection induced earthquakes, nuclear tests induced earthquakes, and mining activities and irrigation can also induce earthquakes. Since the storage of Xinfengjiang Reservoir in Guangdong Province in June 1959+00, 337 earthquakes have been recorded by the end of June 1987, among which an earthquake with magnitude of 6. 1 occurred in June/962, resulting in cracks 82 meters long in the concrete dam.

There are many kinds of volcanoes, which can be divided into three types according to their activity: extinct volcano, dormant volcano and active volcano. An 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. A dormant volcano refers to a volcano that has erupted in history but has been relatively static for a long time. These volcanoes all have well-preserved volcanic shapes, and they still have volcanic activity, or it cannot be concluded that they have lost volcanic activity. 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. An 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.

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.

When a volcano erupts, the outlet from which gas, magma, solids and other substances are discharged is called a crater. Generally speaking, the central eruption has only one crater at a time, but most volcanoes erupt many times. Some of the subsequent eruptions came from the original crater, but most of them erupted on its side, thus forming a new crater, called a parasitic crater. In addition to the central eruption, there is also a fissure eruption, that is, the eruption material is ejected from the cracks in the earth's crust. It formed more craters. For example, the eruption of Laki volcano in Iceland, in a 32-kilometer-long crack, lava continuously flows out from 105 crater. Most craters are round in shape, which are generally divided into five categories: symmetrical craters, asymmetrical craters, cracked craters, craters without obvious craters and sunken craters. There is a special kind of crater called crater. It is because the volcano erupted so violently that a huge amount of magma washed away volcanic neck and the surrounding rocks at once. The crater diameter caused by this eruption is often more than 5 kilometers. The violent eruption not only formed the crater, but also greatly reduced the height of the volcano. For example, the eruption of Mount Tambora produced a crater with a diameter of 12km, and the altitude dropped by 1400 meters.

On our earth, volcanic activities usually occur in some unusual geological backgrounds, most of which are on the boundaries of huge plates that make up the lithosphere. About 80% of active volcanoes on the earth and their related volcanic activities occur when two plates meet, and one plate swoops under the other. The subduction plate, on the one hand, increases the local pressure due to compression, on the other hand, it itself melts into magma; At this time, if there are cracks or weak points in the upper compression plate, the magma with great pressure will be ejected from these places to form a volcano. There is also a different situation, that is, on the mid-ocean ridge axis, magma gushes from the mantle and separates to both sides of the ridge, forming a new ocean floor. This volcanic activity actually takes place underwater.

Volcanic eruption

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 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. People can enjoy this volcano to their heart's content.

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. 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.

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. 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.

The form of volcanic eruption is mainly determined by two aspects: one is the nature of magma, and the other is the ratio of solid, gas and liquid during eruption. Magma can be divided into basic magma and acidic magma according to its silica content. Basic magma has low viscosity and high fluidity, and is also called basalt magma. The volcanic activity caused by this magma is less, and the magma can flow long distances on the ground, forming a strange landscape. Acidic magma has high viscosity and poor fluidity, which leads to intense volcanic activity. In addition, the eruption dominated by gas is the strongest, which is called "explosion"; The eruption dominated by solids takes the second place, which is called "ejection"; The weakest, mainly liquid, is called "flow"; Some of the three are stronger, called "jet".

There are many volcanoes in the world. In the past 400 years, there have been more than 500 active volcanoes. Although there are many volcanoes, they are regularly distributed. People divide volcanoes into four zones: the Pacific Rim volcanic zone, the Atlantic volcanic zone, the Mediterranean volcanic zone and the East African volcanic zone. The Pacific volcanic belt includes the east and west banks, and Taiwan Province Province of China is on this volcanic belt. El Salvador, known as the "volcano country", is also on this volcanic belt, the highest active volcano in the world-Ueyaco in Chile.