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51-670: The connecting three arms form a triple junction . The northernmost branching arm extends north through the Red Sea and into the Dead Sea , while the eastern arm extends through the Gulf of Aden and connects to the Mid-Indian Ocean ridge further to the east. Both of these rifting arms are below sea level and are similar to a mid-ocean ridge . The third rifting arm runs south extending around 4,000 kilometres (2,500 mi) through

102-472: 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 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

153-714: A barrier between the Nile Basin to the north and east and the Congo Basin to the west and south. Lake Rutenzige is fed by several large rivers, the Rutshuru River being one, and drains to the north through the Semliki River into Lake Albert. The Victoria Nile flows from Lake Victoria into the northern end of Lake Albert and exits as the White Nile from a point slightly to the west, flowing north to

204-459: A mid-oceanic ridge will eventually separate the Nubian, Somali and Arabian plates. However, the rifting is studied by a diverse body of research with conflicting hypothetical models, and its future is unknown. Triple junction A triple junction 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 –

255-458: 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

306-667: Is a divergent plate boundary that divides the African (or Somali) and Arabian plate. It extends from the triple junction eastward to the Owen Fracture Zone , where it meets the Aden-Owen-Carlsberg triple junction between the African, Arabian, and Indo-Australian plates. The spreading rate for Aden Ridge is about 17 millimetres per year (0.67 in/year) near the Afar Triple Junction. Before

357-689: Is also the possibility of a subduction zone forming along the eastern side of the Somali plate, caused by the spreading of the EARS and the Mid-Indian Oceanic ridge. To accommodate the compression of the Somali plate due to two extensional edges, the oceanic plate might begin to subduct below the continental plate . Evidence shows the East African Rift System is a typical complex continental-continental rifting event beginning in

408-423: 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 the intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles. In plate tectonics theory during

459-461: 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

510-473: Is considered to be a plume upwelling. Horsts and grabens are very well documented throughout this region. Although they do show and produce crustal extension , for a sufficient ocean basin to form, there needs to be extension that can accommodate for the extensive down fall of the grabens. Listric faults produce the correct model for this sufficient crustal extension. These faults have been documented by Chorowicz (2005) and aid in further verification of

561-476: 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

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612-399: Is found to be approximately 30 million years old. The trap series is dated to a time soon before the major rifting events began. Chorowicz (2005) illustrated the trap series surrounding the newer Neogene volcanics. This helps quantify the amount of crustal extension and gives a model of pre-rifting crustal connection. Seismic tomography compiles P-wave and S-wave data from movements within

663-573: Is now the lake, may have had no outlet other than evaporation. The Lukuga has formed relatively recently, providing a route through which aquatic species of the Congo Basin could colonize Lake Tanganyika, which formerly had distinct fauna. From north to south the mountains include the Lendu Plateau , Rwenzori Mountains , Virunga Mountains and Itombwe Mountains . The Ruwenzori mountains have been identified with Ptolemy 's "Mountains of

714-405: 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

765-621: Is the western branch of the East African Rift , covering parts of Uganda , the Democratic Republic of the Congo (DRC), Rwanda , Burundi and Tanzania . It extends from the northern end of Lake Albert to the southern end of Lake Tanganyika . The geographical term includes the valley and the surrounding mountains. The Albertine Rift and the mountains are the result of tectonic movements that are gradually splitting

816-465: The Albertine Rift ) is characterized by deeper basins , which contain lakes and sediments. The lakes in this area (e.g. Lake Tanganyika and Lake Rukwa) are located in highly rifted basins and have an inter-fingering relationship with faults. Many of the lakes are bounded by normal or strike-slip faults. The extension rate for this rift starts at about 6 millimetres per year (0.24 in/year) in

867-612: 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

918-710: The Philippine and Pacific plates , with the Philippine plate also overriding the Pacific. Here the Japan Trench effectively branches to form the Ryukyu and Bonin arcs . The stability criteria for this type of junction are either ab and ac form a straight line or that the line bc is parallel to CA. Albertine Rift 9°S 34°E  /  9°S 34°E  / -9; 34 The Albertine Rift

969-571: 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

1020-645: The Somali Plate away from the rest of the African continent. The mountains surrounding the rift are composed of uplifted Pre-Cambrian basement rocks , overlaid in parts by recent volcanic rocks . The northern part of the rift is crossed by two large mountain ranges, the Rwenzori Mountains between Lake Albert and Lake Rutanzige (formerly Lake Edward) and the Virunga Mountains between Lake Rutanzige and Lake Kivu . The Virungas form

1071-555: The Afar depression covering more than 200,000 km (77,000 sq mi) and spreading at a rate of 6 to 17 millimetres per year (0.24 to 0.67 in/year). There are many active volcanic areas centralized in the East African Rift System in comparison to the other areas in the Afro-Arabian rift system. Many protruding horsts show successive layers of flood basalts, which can be approximately dated using Ar/Ar-isotope dating . It

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1122-558: The Afar region. The East African Rift is an active rift between the Nubian and Somali protoplates. This rift is caused by elevated heat flow from the mantle under Kenya and the Afar region. Trending NNE to SSW, the East African Rift is composed of a western and an eastern branch. The eastern branch (sometimes called the Gregory Rift ) is characterized by high volcanic activity and the western branch (sometimes called

1173-577: The African plate. At the same time as the subduction in the north, there was mantle upwelling causing the crust to down warp and swell into domes throughout the East African Rift System. The Kenyan dome has been studied extensively. The plume is thought to have begun under Lake Tana in Ethiopia . Based on the environmental correlations and current topographic locations of the Jurassic Upper Limestone and Cretaceous Upper Sandstone,

1224-492: 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

1275-640: 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

1326-870: The Mediterranean. South of the Virungu, Lake Kivu drains to the south into Lake Tanganyika through the Ruzizi River . Lake Tanganyika then drains into the Congo River via the Lukuga River . It seems likely that the present hydrological system was established quite recently when the Virunga volcanoes erupted and blocked the northward flow of water from Lake Kivu into Lake Edward , causing it instead to discharge southward into Lake Tanganyika. Before that Lake Tanganyika, or separate sub-basins in what

1377-562: The Moon". The range covers an area 120 km (75 mi) long by 65 km (40 mi) wide. This range includes Mount Stanley 5,119 m (16,795 ft), Mount Speke 4,890 m (16,040 ft) and Mount Baker 4,843 m (15,889 ft). The Virunga Massif along the border between Rwanda and the DRC consists of eight volcanoes. Two of these, Nyamuragira and Nyiragongo , are still highly active. Isolated mountain blocks further to

1428-575: The Paleogene. It was caused by the far-field stress from the subduction of the Arabian plate under the Eurasian plate, and by mantle upwelling powered by multiple hot spots around the EARS. This crustal uplift has created extension and horst-and-graben structures, and even listric faults suggesting a pre-oceanic basin. If current tectonics continue without change it is thought that an ocean basin with

1479-545: 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

1530-565: The adjacent plates. Following Mackenzie and Morgan's stability model, RRR geometry will continue stably through time until there is a change in the tectonic movement. The Afar Depression is a geological depression that ranges in height from 1,000 to −120 m (3,280 to −390 ft) The area has experienced many domal uplifts, including the Afar dome beginning 40 Mya. This uplift caused massive crustal extension leading to horst and graben structures associated with normal, extensional, faults. The uplift ended in collapse around 25 Mya into

1581-418: 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

Afar Triple Junction - Misplaced Pages Continue

1632-502: 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 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

1683-480: The colder lithosphere to stretch and thin it. The triple rift is thought to have begun in the Late Cretaceous epoch to the Paleogene period. At that time the African plate was experiencing far-field stresses caused by portions of the northern boundary of the African plate subducting under the Eurasian plate. Today, the Arabian plate is experiencing a crustal down pull, or slab pull , that has separated from

1734-532: The countries of Kenya , Uganda , the Democratic Republic of Congo , Rwanda , Burundi , Tanzania , Zambia , Malawi and, finally, Mozambique . This southern rifting arm is better known as the East African Rift or the East African Rift System (EARS), when it includes the Afar Triangle . A rift is the result of pulling apart or extension of lithosphere , including the crust , caused by mantle upwelling where hotter asthenosphere magma rises up into

1785-419: The earth to create a 3D velocity model of the Earth's subsurface. The models distinguish between fast velocity, high anomaly, and slow velocity, slow anomaly, time measurements. Multiple tomography models show a slow anomaly structure beneath southern Africa . Grand et al. (1997) model the large anomaly to extend from the base of the mantle to approximately 1,000 kilometres (620 mi) depth. This slow anomaly

1836-526: The end of the Neogene with a staggering 1,500 metres (4,900 ft) in magnitude. The Ethiopian dome experienced its largest uplift coinciding with the end of the Neogene uplift associated with the Kenyan dome. It has been argued that the current Ethiopian plateau is a result of the most recent uplift of 500 metres (1,600 ft) estimated to be an Oligocene–early Miocene event. But the most accepted argument of

1887-460: The future of this region and the potential for continued extension and subsidence. Some past rifting events have been seen to have an aulacogen (failed arm) together with two successful rifting arms. Some geologists have proposed that the East African Rift System will be the aulacogen in the future, failing to produce an ocean basin, but as of present-day there seems to be no aulacogen and the EARS does not show any evidence of slower rifting. There

1938-410: 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

1989-420: 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 the relative motion at every point along a plate boundary can be calculated from this rotation. But

2040-490: The initial rifting began, Africa was one plate, but as rifting proceeded the plate it began to tear into the Arabian, Somali, and Nubian plates (with the Nubian still sometimes called the African plate). According to McKenzie and Morgan's 1969 classification, the Afar Triple Junction is of ridge-ridge-ridge (RRR) type, describing the movement of the three plates with respect to each other. The Arabian, Somali, and Nubian plates are all divergent margins , or ridges, with respect to

2091-545: The net rock uplift of the Ethiopian Plateau would be 2.2 km (1.4 mi) since c.  150 Mya . The thinned Ethiopian lithosphere could have resulted in ponding from mantle plume and subsequent uplift. Gani et al. (2007) propose that episodic increase of incision of the Ethiopian Plateau suggests episodic growth rates within the plateau, since the incision rates have no correlation to

Afar Triple Junction - Misplaced Pages Continue

2142-542: The north, and declines to the south. The Red Sea Rift is between the African (or Nubian) and Arabian plates. The rift runs along the length of the Red Sea, starting from the Dead Sea to the Afar triple junction. Within the rift, in the Red Sea, there are many volcanoes, including the Jabal al-Tair . The extension rate for this rift varies from about 7 to 17 millimetres per year (0.28 to 0.67 in/year). The Aden Ridge

2193-468: The past climate events. As an effect of Archimedes' principle of isostatic rebound , 2.05 km uplift has occurred within the last 30 million years. Baker et al. (1972) also suggest that the uplift of this area is sporadic and divided by long periods of stability and erosion. Some periods of uplift are recorded at the end of the Cretaceous that resulted in 400 metres (1,300 ft) of uplift and

2244-538: The plateau is the result of the Paleogene flood-basalts . The uplift associated with both domes has resulted in major structural features due to the swelling and warped crustal extension. The two areas of swelling resulted in a large depression between the two domes and subsidence along the coastal regions. The uplift caused by the Ethiopian dome resulted in a massive faulting area of 1,000 metres (3,300 ft) in

2295-446: 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

2346-530: 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

2397-436: 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

2448-701: The south include Mount Bururi in southern Burundi, the Kungwe-Mahale Mountains in western Tanzania, and Mount Kabobo and the Marungu Mountains in the DRC on the shores of Lake Tanganyika . Most of the massifs rise to between 2,000 m (6,600 ft) and 3,500 m (11,500 ft). The Albertine Rift montane forests are important eco-regions. Transitional forests, intermediate between lowland and montane forest, are found at elevations from around 1,000 m (3,300 ft) to 1,750 m (5,740 ft). Montane forest covers

2499-499: 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

2550-433: 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

2601-402: 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 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

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