island

Volcanic island, commonly known as "liudun island", is composed of six connected islands. It is said that a long time ago, a little dragon here fell in love with the fairies in the heavenly palace. He often went to the sky to meet the fairies and broke the heavenly rule. In a rage, the Jade Emperor chased the little dragon here and cut it into six small islands.

island

This entry is compiled and applied by "popular science China" Science Encyclopedia

to examine

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Volcanic island (High Island) is formed by the accumulation of volcanic eruptions (lava, volcanic ash, etc.). It is widely distributed in the circum Pacific region. The area of volcanic islands is generally small. There are single volcanic islands and archipelagic volcanic islands. The famous volcanic island groups include Aleutian Islands and Hawaiian Islands. Volcanic islands can be divided into two types according to their properties: one is oceanic volcanic island, which has no connection with Continental geological structure; the other is volcanic island in continental shelf or continental slope, which has connection with Continental geological structure, but is different from continental island, and belongs to transitional type between continental island and oceanic island.

The essence of phenomenon

formation

According to the theory of plate movement, because of plate movement, the lava flow overflowing from the boundary of plate collision and the growth boundary between plates gradually increased and formed submarine volcanoes. Submarine volcanoes continue to grow upward in the eruption, which will expose the sea surface to form volcanic islands. At first, the submarine volcano is just a lava flow overflowing along the ocean floor rift, and then gradually increases upward. Most of the magma from undersea volcanoes is cooled by the sea before reaching the surface of the sea. Therefore, people have never really seen the scene of submarine volcanic eruption. At most, it's just that the lava springs on the bottom of the sea are constantly emitting new magma to form new igneous rocks. Two members of a diving expedition in the United States risked their lives to explore the Hawaiian Islands volcano. At a depth of 100 feet below the surface of the water, they photographed rivers of lava flowing out of the undersea crater, rushing down the slopes of the volcano to the deeper seabed, while the surrounding water was heated to more than 100 ℃. Submarine volcanoes continue to grow upward in the eruption, which will expose the sea surface to form volcanic islands.

Volcanic islands are not only formed by volcanic eruptions in the ocean, but also by volcanic eruptions on land. In some large caldera lakes on land, some lava domes will be formed. When these lava domes grow higher, they will come out of the lake. A similar case is the Spirit Island in the mezama caldera lake.

form

Volcanic islands are mainly composed of basalt. In geological rock classification, basalt and granite belong to magmatic rock (also called igneous rock). But volcano is a magmatite, which is a dense or foam rock formed by cooling of volcanic magma. The temperature of the magma from the volcanic eruption is as high as 1200 ℃. Due to a certain viscosity, when the terrain is flat, the magma flows very slowly, only a few meters per minute; when it encounters a steep slope, the speed is greatly accelerated. In the process of magma flowing, it carries a lot of water vapor and bubbles. After cooling, it forms basalt with various shapes and a lot of pores.

The granite belongs to intrusive magmatic rock, which is formed by magma intruding into the earth's crust near the surface and cooling and solidification due to temperature drop. Because there is no magma spout, the magma moves slowly and the temperature decreases gradually, so there are obvious crystals in intrusive rocks such as granite, such as quartz, feldspar and mica.

Phenomenon category

Vanishing boundary volcanic island

The subduction under the plate drives the surface material into the upper mantle, and then it is re erupted by volcanoes. For many years, it can form volcanic islands, such as Aleutian Islands.

Growth boundary volcanic island

Because the growth boundary is the place where the plate is torn, the crust is very thin, and the underground magma activity is frequent, many magma will overflow, forming volcanic islands, such as Iceland and Galapagos Islands.

Hot Volcano Island

It is a rare intraplate volcano. The location of this volcanic island is not in the boundary of growth and extinction, but in the intraplate. An upwelling from the core mantle boundary leads directly to the bottom of the lithosphere, creating intraplate volcanic islands in the crust. Such as the Hawaiian Islands, Tahiti, kelgelang island.

Famous volcanic belt

Circum Pacific volcanic belt

The circum Pacific volcanic belt starts from the Cordillera mountains in South America, turns to the Aleutian Islands and Kamchatka Peninsula in the northwest, and extends to the southwest to the Kuril Islands, Japan islands, Ryukyu Islands, Taiwan Islands, Philippines islands and Indonesia islands, with a total length of more than 40000 km, forming a ring structure system opening to the south. The circum Pacific volcanic belt, also known as the circum Pacific Ring of fire, has 512 active volcanoes. Among them, there are more than 30 active volcanoes in the southern segment of the Andes of the lidillera mountains in South America, and 16 active volcanoes in the northern segment. The middle section of the uyeyako volcano is 6723m above sea level, which is the highest active volcano in the world. Further north is the Caribbean region. Along the Pacific coast, there are famous volcanoes, including cheriki volcano, erasu volcano, Santa Ana volcano and tahumurko volcano. There are more than 90 active volcanoes in North America, and the famous ones are Mount Saint Helens, Mount Larsen, Mount Rainier, Mount Shasta, Mount Hood and mount Sanford. The most famous volcanoes in the Aleutian Islands are Katmai volcano and ilyamna volcano. In the Kamchatka Peninsula, there is a frequently active klucchev volcano, striking the Kuril Islands and the island arc of the Japanese archipelago. Famous volcanoes are distributed in the Japanese archipelago, such as Asama mountain, Iwate mountain, shishengyue mountain, ASU mountain and Sanyuan mountain, which are active volcanoes with multiple eruptions. There are many volcanic islands from the Ryukyu Islands to Taiwan Island, such as chiweiyu Island, Diaoyu Island, pengjiayu Island, Penghu Island, Qixingyan Island, Lanyu Island and Huoshao Island, all of which have been formed since the new era. The most active volcanoes are those from the Philippines to the Indonesian islands, such as karakato volcano, Pinatubo volcano, Tayun volcano, Tambora volcano and Pere volcano in the Lesser Antilles Islands, which have erupted many times in recent years.

According to historical records, 80% of the world's modern eruptive volcanoes occur here, mainly in North America, Kamchatka Peninsula, Japan, the Philippines and Indonesia. Indonesia is known as the "country of volcanoes", and the South includes Sumatra. There are nearly 400 volcanoes in the arc trench system composed of Java islands, of which 129 are active volcanoes. During 1966-1970 alone, 22 volcanoes erupted. In addition, submarine volcanic eruptions often occurred, resulting in some new volcanic islands exposed to the sea.

Mid ocean ridge volcanic belt

The mid ocean ridge, also known as the oceanic rift, is distributed in a "W" shape all over the world. From the Arctic basin through Iceland to the South Atlantic Ocean, this section bisects the Atlantic shell and is parallel to the coastlines on both sides. It turns south around the southern end of Africa to the NE and connects with the middle ridge of the Indian Ocean. The mid Indian ridge extends northward to the northern end of the African continent and connects with the East African Rift Valley. The whole mid ocean ridge forms a W-shaped pattern and becomes a global ocean rift with a total length of more than 80000 km. In the middle of the oceanic rift, there are mostly uplifted ridges, 2-3km higher than the sea plains on both sides, so it is called the mid ocean ridge. In the middle of the ridge, there are more grabens 20-30km wide and 1-2km deep, so it is also called the oceanic rift. The volcanoes in the ocean are concentrated in the rift zone, which is called the mid ocean ridge volcanic zone. According to the age determination of the ocean floor rocks, the oceanic rift formed earlier, but the expansion and intense activity of the rift were from Mesozoic to Cenozoic, especially since the Quaternary, which was characterized by volcanic activity.

The distribution of volcanoes in the mid ocean ridge volcanic belt is also uneven. Most of them are concentrated in the Atlantic Rift, starting from Greenland in the north, passing through Iceland, Azores and Cape Verde Islands. The section is more than 10000 kilometers long, and the ridge is composed of basalt, which is the product of volcanic eruption along the ocean rift. Because most of the volcanoes erupt from the bottom of the ocean, they are not easy to be found. According to relevant records, there are only more than 60 active volcanoes on the mid Atlantic ridge. Iceland is located in the mid Atlantic ridge. We can directly observe the volcanoes on Iceland. There are more than 200 volcanoes on the island, including more than 30 active volcanoes. People call it volcanic island. According to the statistics of geologists s and thorarinsson (1960), there have been about 200 volcanic eruptions in the past 1000 years, with an average of 5 years. The famous active volcano is haikela volcano, which has erupted more than 20 times since 1104. The eruption of laki volcano in 1783 was witnessed by people. More than 12 kilometers of lava overflowed from a 25 kilometer long crack. The lava flow covered an area of 565 square kilometers, and the lava flow was more than 70 kilometers long, causing major disasters. In 1963, the volcano erupted in the southern sea area of Iceland. This eruption lasted until 1967, resulting in a new island - sutse volcanic island, which is about 150m above the sea surface and covers an area of 2.8 square kilometers. Six years later, the Haimai volcano in the Westman islands, 32 km northeast of the island, erupted again. The eruption of these volcanoes reflects the characteristics of volcanic eruption in the Atlantic Rift

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