Island arccontinental marginA long string of islands in a continuous arc.And strong volcanic activity, seismic activity andOrogenyThe long curvilinear ocean associated with the processIsland chain。Such geological features asaleutian -AlaskaIsland arc and Kuril Kamchatka island arc.[1]
The island arc refers to a long string of islands on the continental margin.The islands are mainly mountainous, and the west Pacific island arc adjacent to the abyssal trench is the most typical island arc. It is located in the chimeric zone of the Pacific plate, the Eurasian plate and the Indian Ocean plate, with unstable crust and many volcanic earthquakes.According to statistics, there are more than 500 active volcanoes in the world, more than half of which are concentrated in the island arc belt;95% of the global earthquake energy is also released here.Magma eruption caused by frequent volcanic activities has made the island arc belt the most mineral rich area in the world.[1]
characteristic
Announce
edit
Geological characteristics
Volcanic activity and volcanic rocks: island arc is an area of intense volcanic activityNeogenetothe quaternary periodDuring this period, thousands of volcanoes had once been active.Two thirds of them are associated with the island arc.Layered lavaFused tuffandVolcaniclastic rockIs a feature.Volcanoes are generally distributed parallel to the axis of the island arc.It usually erupts suddenly at the place 200~300 kilometers away from the land side of the trench. This boundary is called "volcanic front", which may reflectSubduction zoneAt the beginning of intermediate (or above) melting, the front is the area with the most frequent volcanic activity.On the side close to the trench, there is no volcanic activity, but towards the mainland, the volcanic activity weakens rapidly.Volcanic zoneThere is homology belowIntrusive rock。
Island arc volcanic rocksAndesiteandbasaltMainly, andesite belongs toContinental crustIs different fromOceanic basinBasic basalt on the inner island.Because of this difference, as early as 1912, P. Marshall proposed“Andesite line”The line is considered as the boundary line of the outer edge of the island arc.
The volcanic rocks of the island arc are zoned, which is closely related to the seismic activity of the island arc.Adjacent to the trenchTholeiiteThe occurrence of a series of volcanic rocks, whose material source is relatively shallow, is located inShallow focus earthquakeSourceBelt region;Further away from the trench are high potassium calc alkaline andAlkaline volcanic rockSeries, inDeep focus earthquakeSource area.Potassium content in volcanic rocks and Rb, Sr, etcLarge ion lithophile elementThe content of K, Rb, Sr, etc. is closely related to the depth of the underlying seismic source zone. The content of K, Rb, Sr, etc. increases towards the tilt direction of the seismic source zone, indicating that the plateSubduction zoneProbably island arc areaMagmaAnd the formation of various magma sourcesSource depthof
gravity anomaly: The island arc itself is a positive gravity anomaly, and the adjacent trench is a negative gravity anomaly. The difference between the two is more than 400 milligals, reflecting that the crust is in an unbalanced state.The negative gravity anomaly reflects that the high-density rocks move down to deeper parts, while the water layer or sedimentary layer thickens in situ;The positive gravity anomaly reflects the distribution ofmantleAnd volcanic rocks.The gravity anomaly measured in the trench arc basin system can be used to explain the changes of density and structure in the crust.
Heat flow: trench arc basin system has significant heat flow anomaly.At one side of the trench, the heat flow is lower than the normal value;stayVolcanic island arcOne side andBack arc basin, heat flow is higher than normal.The low heat flow is caused by the cold and compact crustal materialupper mantleThe high heat flow is caused by the upper mantle material（Magma）Caused by upward flow.[2]
Topographic characteristics
An island arc is an archipelago arranged in an arc, also known as“Island chain”、“Festoon islands”。The convex surface of the arc often points to an ocean floorSea basinThe center of;The concave faces the mainland, becoming the natural barrier of the mainland.
Island arc is important to separate ocean and marginal seaTectonic geomorphologyunit。It's a long stretchArc archipelago。Japanese scholarSeiya UedaAccording to others, the island arc has three main characteristics:
1. Modern volcanic activity;
2. Deep sea trenches with a depth of more than 6000 meters;
3. There is an earthquake source up to 70km deep.Island arcs are mainly distributed in the western Pacific Ocean.The total length of the Golden Globe Island Arc is 40000 kilometers.[3]
The island arc is an archipelago that is located near the mainland and protrudes into a circular arc towards the ocean.They are often arranged in the form of flowers, so they can be collectively calledFestoon islandsAmong them, the most spectacular is the Huacai Islands on the eastern edge of Asia, which are composed ofaleutian , Qiandao, Japan, RyukyuLuzonEqual island arc;The Greater Antilles and Lesser Antilles in Central America are also typical colorful islands formed by the combination of island arcs.The island arc sometimes appears alone, mostlyVolcanic arcDominant;However, there are often two parallel island arcs inside and outside, some of which may be submerged below the sea surface.The outer arc is out of some strongly changed sedimentary rocks orMetamorphic rockComposition;The inner arc is caused by a volcano with a new formation ageAndesiteIt is dominated by qualitative rocks.[4]
On the edge of the ocean parallel to the island arc, there are often deep and narrow arc trenches, like those outside the Luzon arcMariana TrenchMariana Trench is the deepest trench in the world, and the deepest abyss can reach a depth of 11034 meters.Island arcs and trenches are often places with frequent and intense earthquake and volcanic activities,SourceIt is very deep, causing deep earthquakes more than 70 kilometers deep, so it has special tectonic significance.Trenches, volcanoes and deep earthquakes are the three characteristics of island arcs.[4]
According to the newly establishedPlate tectonic theory, the island arc is actually the continental andOcean plateThe position of the results produced during the polymerization is appropriatesubduction zoneAbove.[4]
topographic features
Most island arcs are composed of two parallel, bow shaped islands.such a sort ofDouble island arcThe inner column of is composed of a series of erupting volcanoes, while the outer column is composed of non volcanic islands.In the case of only a single arc, many of the islands that make up it are volcanic.
An island arc, in typical cases, has a land block on the concave side, or a partially closed, particularly shallow sea.Along its convex side, there are long and narrowDeep trench。The maximum depth of the ocean is found in such a submarine depression, as inMariana Trenchandtonga trench That's the case.
Destructive earthquakeIt often occurs at the location of island arc.These earthquakes are similar to those recorded in other parts of the worldShallow focus earthquakeDifferent, they originate from the place 600 km deep below the base of the island arcDeep focus earthquakeevent.These earthquakesSourceIt tends to be deeper and deeper towards the concave side of the island arc.
Volcanic island arcContinuously rising, suffering intenseDenudation, debris material is transported to the surroundingSea basinAnd deep-sea trench, formingTurbiditeAnd other coarseclastic rock。Island arc areaMetamorphic rockThe belt appears, and the outside of the island arc is a high-pressure and low-temperature areaMetamorphic zoneThe inner side is a high-temperature and low-pressure regional metamorphic belt, the former is blueAmphibole、Jadeite, and andalusite, the latter is represented byAndalusiteThe two metamorphic belts are parallel to each otherBimetamorphic zone。[2]
Topographic classification
Announce
edit
According to the geomorphic characteristics of island arcs, island arcs can be divided into:
2. Double arc type, consisting of an outer arc parallel to the trench (island arc without volcanic activity or sedimentation at present) and an inner arc (volcanic activity still exists at present), such as the Indonesian archipelago arc;
3. Multi arc type, there is a residual arc on the continental side of the double arc type (no modern volcanic activity).according toBack arc basin(For example, the island arcs in the western Pacific have back arc basins, while those in South AmericaAndesNo back arc basin behind the chain), back arcTectonic activity(compression, tension or shear) and other characteristics can also be used as indicators of island arc classification.[5]
Topographic distribution
Announce
edit
Island arcs are also distributed in various oceans, with the Pacific Ocean being the most developed, especially the Western Pacific Ocean.The Kamchatka Kuril Island Arc, Japan Island Arc, Ryukyu Island Arc and Taiwan Philippines Island Arc, with a total length of more than 10000 kilometers, are calledEast Asian festoon islandsIs the longest island arc in the world's oceans.
The island arc is a volcanic and seismic zone.As the island arc one trench system is located on the outer side of the mainland, it is the springboard for the mainland countries on its inner side to move towards the sea, and also the coastal defense outpost of these countries;Also due to the island arc 1 -oceanic trenchDepartment inOceanic marginThe island arc is also an important place for hegemonic powers to control the sea, with many important naval bases, such asMariana IslandsInGuam，Japan IslandsInYokosuka、Sasebo, Ryukyu IslandsZhongcheng Bay，Greater AntillesInGuantanamo BayEtc.
The island is mainly mountainous and faces deep trenches.The western Pacific island arc is the most typical, which is divided into two sections: the northern section consists ofKuril Islands, Japan Islands, Ryukyu Islands, Taiwan Island andthe philippine islandsComposition, facing the Pacific Ocean, is the East Asia Pacific Island Arc!The southern section is composed ofAndaman Islands、Nicobar Islands、sumatra、JavaIt is composed of Nusa Tenggara Islands and protrudes towards the Indian Ocean, known as the Sunda Island Arc in the Indian Ocean.Two sections of island arcSulawesijoin.The western Pacific island arc is located atPacific plate 、Eurasian plate Chimeric zone with Indian Ocean plate, with unstable crustVolcanic earthquake。According to statistics, there areActive volcanoMore than 500, more than half of which are concentrated in the island arc zone.[5]
Most island arcs occur along the western edge of the Pacific Basin.The few exceptions are the East Indian Island Arc, the West Indian Island Arc and the South Atlantic OceanScotia Arc 。According to popular theory, the island arc is formed in twoLithospheric plate(the huge rigid plates that make up the earth's surface).When colliding, one of the plates has heavyOceanic crust, plunging downward and being squeezed into the locally melted lower mantle beneath the second plate with lighter oceanic crust.The island arc is due tobasaltandAndesiteErupt fromOverthrust plateOn the surface.Basalt is believed to be derived from semi molten mantle, while andesite may be generated from the partial melting of sediments on the falling plate and its surface.[5]
be locatedcontinental marginParallel to the trenchArc archipelago。Its distribution is consistent with that of the trench, to the west of the Pacific OceanActive continental marginMainly, separateOceanic basinandMarginal basinThe importance ofTectonic geomorphologyUnit is also the most active tectonic activity zone in the world.The convex side of the island arc towards the ocean is parallel to the trench, and the concave side isBack arc basin；It is composed of trench and back arc basinTrench arc basin system。An island arc extends hundreds to thousands of kilometers in length and about 200 to 300 kilometers in width.The global island arc (including mountain arc) has a total length of 40000 km.Seiya UedaThe areas that are considered to meet the following three conditions can be called island arcs: ① there are modernVolcanism；②With a trench parallel to it with a depth of more than 6000 meters; ③There are earthquake sources with a depth greater than 70km.Therefore, island arcs are not only limited to arc islands in the ocean, but also include arc mountains (mountain arcs) on the west coast of Central America and South America.[5]
Topographical origin
Announce
edit
Trench arc basin system cross section structure model
Plate tectonic theoryThe origin of island arc is explained comprehensively: oceanlithospherealong withMantle convectionfromMid ocean ridgeGrowth and expansion, while in the correspondingActive continental marginsubductionSubmergenceThe island arc is the product of the submergence of the oceanic plate.When the oceanic plate submerges under the continental plate, the friction between the two platesmantleMaterial warming, partial melting,MagmaUpwelling erupts from the surface to form a volcano, forming an inner arc.On the side close to the ocean, due to the oceanic plateSubductionAnd drag the seabed down to form a deep trench.In the process of subduction, oceanic sediments proliferate on the side of the island arc near the trench, which widens the island arc and forms an outer arc.The back arc area is formed due to secondary expansionBack arc basin。
According to the crustal structure of the island arc, there are two different genetic types of the island arc.Typical type of type I island arcContinental crust, with granitic basement.They are usually composed of older large islands and distributed adjacent to the continent. It is generally believed that they are fragments separated from the continent due to back arc spreading, such asJapanese archipelago；Another type of transitional crust between ocean and continent under island arc is generally composed of young islands, which are far away from the continent and spread in the ocean, such asMariana Islands。When one oceanic plate subducts under another oceanic plate, island arcs develop and grow on the basis of oceanic crustMagmaAnd the accumulation of sediments makes the crust of this kind of intra oceanic island arc gradually thicken and transform into continental crust.[2]
