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36-599: Although usually called the " Seven Islands of Izu " ( 伊豆七島 in Japanese), there are in fact more than a dozen islands and islets. Nine among them are currently inhabited. The Izu islands stretch south-east from the Izu Peninsula on Honshu and cover an area of approximately 301.56 km (116.43 sq mi). There are nine populated islands with a total population of 24,645 people (as of 2009) spread over 296.56 km (114.50 sq mi). The largest of them

72-448: A few are stable through time ( stable in this context means that the geometrical configuration of the triple junction will not change through geologic time). The meeting of four or more plates is also theoretically possible, but junctions will only exist instantaneously. The first scientific paper detailing the triple-junction concept was published in 1969 by Dan McKenzie and W. Jason Morgan . The term had traditionally been used for

108-458: A single point, for the triple junction to exist stably. These lines necessarily are parallel to the plate boundaries as to remain on the plate boundaries the observer must either move along the plate boundary or remain stationary on it. The point at which these lines meet, J, gives the overall motion of the triple junction with respect to the Earth. Using these criteria it can easily be shown why

144-429: Is Izu Oshima (8,346 inhabitants, 91.06 km (35.16 sq mi)), the smallest Toshima (292 inhabitants, 4.12 km (1.59 sq mi).) Of the inhabited islands, seven are traditionally referred to as the "Izu Seven": Oshima, Toshima, Niijima, Kozujima, Miyakejima, Hachijojima, and Mikurajima, though Shikinejima and Aogashima are sometimes included as well. Each of the islands has its unique character: Oshima

180-461: Is believed to have caused the formation of the Pacific plate about 190 million years ago. By assuming that plates are rigid and that the Earth is spherical, Leonhard Euler 's theorem of motion on a sphere can be used to reduce the stability assessment to determining boundaries and relative motions of the interacting plates. The rigid assumption holds very well in the case of oceanic crust , and

216-526: Is demonstrated below – as the perpendicular bisectors of the sides of a triangle always meet at a single point, the lines ab, bc and ca can always be made to meet regardless of relative velocities. RTF junctions are less common, an unstable junction of this type (an RTF(a)) is thought to have existed at roughly 12 Ma at the mouth of the Gulf of California where the East Pacific Rise currently meets

252-565: Is noted for its active volcano Mt Mihara and camellias , Hachijo-kojima for its former penal colony, Mikurajima for dolphin watching, Niijima for its numerous beaches, Kozujima for its white sandy shores, Hachijojima for its well-preserved unique culture, and Miyakejima for the 2001 volcanic eruption. During the Edo period , Nii-jima, Miyake-jima, and Hachijō-jima served as places of exile for criminals. The subtropical Ogasawara Islands , which are also administratively part of Tokyo, lie further to

288-405: Is retained with time as the plates involved move. This places restrictions on relative velocities and plate boundary orientation. An unstable triple junction will change with time, either to become another form of triple junction (RRF junctions easily evolve to FFR junctions), will change geometry or are simply not feasible (as in the case of FFF junctions). The inherent instability of an FFF junction

324-423: Is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can be described according to the types of plate margin that meet at them (e.g. fault–fault–trench, ridge–ridge–ridge, or abbreviated F-F-T, R-R-R). Of the ten possible types of triple junctions only

360-750: The Fuji-Hakone-Izu National Park . The four southernmost islands are not administered under any town or village in Hachijō Subprefecture, but are unincorporated areas . Torishima is now uninhabited but is an important bird refuge. Deserted islands between Aogashima and Ogasawara Islands, namely Bayonaise Rocks ( Beyonēzu Retsugan ), Smith Island ( Sumisu-tō ), Torishima , and Lot's Wife ( Sōfu-iwa ) do not belong to any municipality , because both Hachijō Town and Aogashima Village claim administrative rights. They are directly controlled by Hachijō Subprefecture instead. Though

396-527: The Meiji era . In addition, Hachijokojima, which became temporarily uninhabited during the 1965s, and Udoneshima and Torishima , where there are temporary residents, were once inhabited islands. And there are also other smaller uninhabited islands. As this name of the Seven Islands of Izu continued to be used even today, the islanders of the islands other than the Seven Islands feel discriminated., and

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432-612: The Mid-Atlantic Ridge , and an associated aulacogen , the Benue Trough , in the Niger Delta region of Africa. RRR junctions are also common as rifting along three fractures at 120° is the best way to relieve stresses from uplift at the surface of a sphere; on Earth, stresses similar to these are believed to be caused by the mantle hotspots thought to initiate rifting in continents. The stability of RRR junctions

468-571: The San Andreas Fault zone. The Guadeloupe and Farallon microplates were previously being subducted under the North American plate and the northern end of this boundary met the San Andreas Fault . Material for this subduction was provided by a ridge equivalent to the modern East Pacific Rise slightly displaced to the west of the trench. As the ridge itself was subducted an RTF triple junction momentarily existed but subduction of

504-610: The Aogashima villagers in particular have petitioned not to use it at every opportunity. In connection with this, the affiliated organization of Tokyo, the "Izu Seven Islands Tourism Federation," was renamed to "Tokyo Islands Tourism Federation". However, this name is still used in sightseeing pamphlets at the travel agencies, such as those of Tokai Kisen that operate ferries to the various islands. 33°49′N 139°37′E  /  33.817°N 139.617°E  / 33.817; 139.617 Triple junction A triple junction

540-492: The Euler poles are distant from the triple junction concerned. The definitions they used for R, T and F are as follows: For a triple junction between the plates A, B and C to exist, the following condition must be satisfied: where A v B is the relative motion of B with respect to A. This condition can be represented in velocity space by constructing a velocity triangle ABC where the lengths AB, BC and CA are proportional to

576-640: The FFF triple junction is not stable: the only case in which three lines lying along the sides of a triangle can meet at a point is the trivial case in which the triangle has sides lengths zero, corresponding to zero relative motion between the plates. As faults are required to be active for the purpose of this assessment, an FFF junction can never be stable. McKenzie and Morgan determined that there were 16 types of triple junction theoretically possible, though several of these are speculative and have not necessarily been seen on Earth. These junctions were classified firstly by

612-449: The Izu volcanic arc . The Izu arc ends there at a tectonic triple junction . Volcanic activity is frequent in the area. 31 people were killed when the research vessel Kaiyō Maru no 5 was destroyed during the 1953 eruption of Myōjin-shō . Volcanic activity, including the release of harmful gases, forced the evacuation of Miyake-jima in 2000. Residents were allowed to return permanently to

648-617: The Izu Archipelago are oceanic islands that formed relatively recently (within a few million years) without any previous connection to mainland Japan. In contrast to isolated Pacific islands, such as Hawaii and the Galápagos , the Izu Islands are located near the mainland and have thus been frequently colonized by various species by overseas dispersal from the mainland or from adjacent islands. This make them interesting for

684-545: The RRF configuration could be stable under certain conditions. An RRR junction is always stable using these definitions and therefore very common on Earth, though in a geological sense ridge spreading is usually discontinued in one direction leaving a failed rift zone . There are many examples of these present both now and in the geological past such as the South Atlantic opening with ridges spreading North and South to form

720-410: The geological details but simply by defining the properties of the ridges , trenches and transform faults involved, making some simplifying assumptions and applying simple velocity calculations. This assessment can generalise to most actual triple junction settings provided the assumptions and definitions broadly apply to the real Earth. A stable junction is one at which the geometry of the junction

756-426: The intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles. In plate tectonics theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction). One of these divergent plate boundaries fails (see aulacogen ) and the other two continue spreading to form an ocean. The opening of

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792-501: The island in February 2005 but were required to carry gas masks in case of future volcanic emissions. To handle the various types of natural disasters threatening the region, including tsunamis, storm, floods, and volcanism, Tokyo metropolitan government has developed prevention and safety measures, including hazard maps and evacuation guidance, radios, signs, and a transport system for emergency supplies. A chain of volcanic islands,

828-412: The islands, by cargo-passengers boats , jetfoils , and aircraft , is supported by harbours on all inhabited islands and five airports (small islands can be reached by helicopter). There are 5 airports, 15 harbors, and 19 fishing ports. Flights from Tokyo take 30 minutes, while boats take 7–10 hours and jetfoils make the route in about two hours. Transportation on the islands is considered important to

864-456: The population on the Izu Islands has been dropping, the pace is less dramatic than on other isolated Japanese islands. The divergent Hachijō language is spoken on the islands. The primary industries are fisheries, agriculture, and tourism. The most scenic spots on the islands are crowded with tourists during summers. Popular tourist activities include swimming, scuba diving, surfing, fishing, bird watching and trekking. Transportation between

900-464: The purely kinematic point of view where the plates are rigid and moving over the surface of the Earth. No knowledge of the Earth's interior or the geological details of the crust are then needed. Another useful simplification is that the kinematics of triple junctions on a flat Earth are essentially the same as those on the surface of a sphere. On a sphere, plate motions are described as relative rotations about Euler poles (see Plate reconstruction ), and

936-552: The quality of life, which is why about 215 km (134 mi) of paved main roads have been constructed to serve various kinds of vehicles. There was no electricity on the islands before 1953, but by 1962 98% of the area was receiving electricity. The islands occupy the northern portion of the Izu–Bonin–Mariana Arc which extends to the Izu Peninsula and Mount Fuji on the Honshū mainland which are northern extensions of

972-446: The radius of the Earth at the equator and poles only varies by a factor of roughly one part in 300 so the Earth approximates very well to a sphere. McKenzie and Morgan first analysed the stability of triple junctions using these assumptions with the additional assumption that the Euler poles describing the motions of the plates were such that they approximated to straight line motion on a flat surface. This simplification applies when

1008-518: The relative motion at every point along a plate boundary can be calculated from this rotation. But the area around a triple junction is small enough (relative to the size of the sphere) and (usually) far enough from the pole of rotation, that the relative motion across a boundary can be assumed to be constant along that boundary. Thus, analysis of triple junctions can usually be done on a flat surface with motions defined by vectors. Triple junctions may be described and their stability assessed without use of

1044-530: The ridge caused the subducted lithosphere to weaken and 'tear' from the point of the triple junction. The loss of slab pull caused by the detachment of this lithosphere ended the RTF junction giving the present day ridge – fault system. An RTF(a) is stable if ab goes through the point in velocity space C, or if ac and bc are colinear. A TTT(a) junction can be found in central Japan where the Eurasian plate overrides

1080-436: The same velocity space diagrams in the following way. The lines ab, bc and ca join points in velocity space which will leave the geometry of AB, BC and CA unchanged. These lines are the same as those that join points in velocity space at which an observer could move at the given velocity and still remain on the plate boundary. When these are drawn onto the diagram containing the velocity triangle these lines must be able to meet at

1116-619: The seven inhabited islands of the Izu Islands that belong to the Tokyo Islands of Tokyo Prefecture , Japan . The name comes from the fact that there were the seven main inhabited islands in the Izu Islands during the Edo period : Izu Oshima , To-shima , Niijima , Kozushima , Miyakejima , Mikurajima , and Hachijojima . In fact, in addition to these seven islands, the Izu Islands have two other inhabited islands, Aogashima and Shikinejima , which have been inhabited since

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1152-506: The south Atlantic Ocean started at the south of the South American and African continents, reaching a triple junction in the present Gulf of Guinea , from where it continued to the west. The NE-trending Benue Trough is the failed arm of this junction. In the years since, the term triple-junction has come to refer to any point where three tectonic plates meet. The properties of triple junctions are most easily understood from

1188-417: The south. They form a far-flung archipelago of over thirty (30) islands some 1,000 km (621 mi) due south of Tokyo. The Izu Islands are divided into two towns (Oshima and Hachijojima) and six villages (the remaining inhabited islands.) Three subprefectures are formed above the municipalities as branch offices of the metropolitan government . All the islands (more than a dozen in total) lie within

1224-674: The studies of ecological and evolutionary processes. Campanula (Bellflower) colonized the entire archipelago in a single event. Similarly, the Euhadra snails, endemic to Japan, populated the islands in a single event and all individuals on inhabited islands possess an identical haplotype . The Apodemus mice, on the other hand, colonized the islands from the mainland in two independent events. 34°44′N 139°24′E  /  34.733°N 139.400°E  / 34.733; 139.400 Seven Islands of Izu The Seven Islands of Izu ( Japanese : 伊豆七島 = Izu Shichitō) are

1260-499: The types of plate boundaries meeting – for example RRR, TTR, RRT, FFT etc. – and secondly by the relative motion directions of the plates involved. Some configurations such as RRR can only have one set of relative motions whereas TTT junctions may be classified into TTT(a) and TTT(b). These differences in motion direction affect the stability criteria. McKenzie and Morgan claimed that of these 16 types, 14 were stable with FFF and RRF configurations unstable, however, York later showed that

1296-433: The velocities A v B , B v C and C v A respectively. Further conditions must also be met for the triple junction to exist stably – the plates must move in a way that leaves their individual geometries unchanged. Alternatively the triple junction must move in such a way that it remains on all three of the plate boundaries involved. McKenzie and Morgan demonstrated that these criteria can be represented on

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