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85-843: A triple junction , the Chile Triple Junction , occurs on the seafloor of the Pacific Ocean off Taitao and Tres Montes Peninsula at the southern coast of Chile . Here, three tectonic plates meet: the Nazca plate, the South American plate , and the Antarctic plate . The eastern margin is a convergent boundary subduction zone under the South American plate and the Andes Mountains , forming

170-456: A ring current . This current reduces the magnetic field at the Earth's surface. Particles that penetrate the ionosphere and collide with the atoms there give rise to the lights of the aurorae while also emitting X-rays . The varying conditions in the magnetosphere, known as space weather , are largely driven by solar activity. If the solar wind is weak, the magnetosphere expands; while if it

255-647: A date arrived at by interpreting magnetic anomalies . Subduction under the South American continent began about 140 Mya, although the formation of the high parts of the Central Andes and the Bolivian orocline did not occur until 45 Mya. It has been suggested that the mountains were forced up by the subduction of the older and heavier parts of the plate, which sank more quickly into the mantle . 15°S 85°W  /  15°S 85°W  / -15; -85 Triple junction A triple junction

340-448: 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

425-425: A history of producing massive earthquakes , including the largest ever recorded on earth, the moment magnitude 9.5 1960 Valdivia earthquake . A second triple junction occurs at the northwest corner of the plate where the Nazca, Cocos, and Pacific plates all join off the coast of Colombia . Yet another triple junction occurs at the southwest corner at the intersection of the Nazca, Pacific, and Antarctic plates off

510-413: A permanent magnetic moment. This remanent magnetization , or remanence , can be acquired in more than one way. In lava flows , the direction of the field is "frozen" in small minerals as they cool, giving rise to a thermoremanent magnetization . In sediments, the orientation of magnetic particles acquires a slight bias towards the magnetic field as they are deposited on an ocean floor or lake bottom. This

595-400: A presently accelerating rate—10 kilometres (6.2 mi) per year at the beginning of the 1900s, up to 40 kilometres (25 mi) per year in 2003, and since then has only accelerated. The Earth's magnetic field is believed to be generated by electric currents in the conductive iron alloys of its core, created by convection currents due to heat escaping from the core. The Earth and most of

680-499: A region can be represented by a chart with isogonic lines (contour lines with each line representing a fixed declination). Components of the Earth's magnetic field at the surface from the World Magnetic Model for 2020. Near the surface of the Earth, its magnetic field can be closely approximated by the field of a magnetic dipole positioned at the center of the Earth and tilted at an angle of about 11° with respect to

765-456: A simple compass can remain useful for navigation. Using magnetoreception , various other organisms, ranging from some types of bacteria to pigeons, use the Earth's magnetic field for orientation and navigation. At any location, the Earth's magnetic field can be represented by a three-dimensional vector. A typical procedure for measuring its direction is to use a compass to determine the direction of magnetic North. Its angle relative to true North

850-406: 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

935-596: A third of NASA's satellites. The largest documented storm, the Carrington Event , occurred in 1859. It induced currents strong enough to disrupt telegraph lines, and aurorae were reported as far south as Hawaii. The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere ( ionospheric dynamo region ) and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of

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1020-515: A year or more mostly reflect changes in the Earth's interior , particularly the iron-rich core . Frequently, the Earth's magnetosphere is hit by solar flares causing geomagnetic storms, provoking displays of aurorae. The short-term instability of the magnetic field is measured with the K-index . Data from THEMIS show that the magnetic field, which interacts with the solar wind, is reduced when

1105-463: Is 1–2 Earth radii out while the outer belt is at 4–7 Earth radii. The plasmasphere and Van Allen belts have partial overlap, with the extent of overlap varying greatly with solar activity. As well as deflecting the solar wind, the Earth's magnetic field deflects cosmic rays , high-energy charged particles that are mostly from outside the Solar System . Many cosmic rays are kept out of

1190-548: Is approximately dipolar, with an axis that is nearly aligned with the rotational axis, occasionally the North and South geomagnetic poles trade places. Evidence for these geomagnetic reversals can be found in basalts , sediment cores taken from the ocean floors, and seafloor magnetic anomalies. Reversals occur nearly randomly in time, with intervals between reversals ranging from less than 0.1 million years to as much as 50 million years. The most recent geomagnetic reversal, called

1275-461: 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

1360-453: Is called compositional convection . A Coriolis effect , caused by the overall planetary rotation, tends to organize the flow into rolls aligned along the north–south polar axis. A dynamo can amplify a magnetic field, but it needs a "seed" field to get it started. For the Earth, this could have been an external magnetic field. Early in its history the Sun went through a T-Tauri phase in which

1445-431: Is called detrital remanent magnetization . Thermoremanent magnetization is the main source of the magnetic anomalies around mid-ocean ridges. As the seafloor spreads, magma wells up from the mantle , cools to form new basaltic crust on both sides of the ridge, and is carried away from it by seafloor spreading. As it cools, it records the direction of the Earth's field. When the Earth's field reverses, new basalt records

1530-526: 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

1615-450: Is distorted further out by the solar wind. This is a stream of charged particles leaving the Sun's corona and accelerating to a speed of 200 to 1000 kilometres per second. They carry with them a magnetic field, the interplanetary magnetic field (IMF). The solar wind exerts a pressure, and if it could reach Earth's atmosphere it would erode it. However, it is kept away by the pressure of

1700-408: Is generally reported in microteslas (μT), with 1 G = 100 μT. A nanotesla is also referred to as a gamma (γ). The Earth's field ranges between approximately 22 and 67 μT (0.22 and 0.67 G). By comparison, a strong refrigerator magnet has a field of about 10,000 μT (100 G). A map of intensity contours is called an isodynamic chart . As the World Magnetic Model shows,

1785-518: Is represented by a field of a magnetic dipole currently tilted at an angle of about 11° with respect to Earth's rotational axis, as if there were an enormous bar magnet placed at that angle through the center of Earth. The North geomagnetic pole ( Ellesmere Island , Nunavut , Canada) actually represents the South pole of Earth's magnetic field, and conversely the South geomagnetic pole corresponds to

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1870-405: 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

1955-407: Is shown below . Declination is positive for an eastward deviation of the field relative to true north. It can be estimated by comparing the magnetic north–south heading on a compass with the direction of a celestial pole . Maps typically include information on the declination as an angle or a small diagram showing the relationship between magnetic north and true north. Information on declination for

2040-600: Is shown in the image. This forms the basis of magnetostratigraphy , a geophysical correlation technique that can be used to date both sedimentary and volcanic sequences as well as the seafloor magnetic anomalies. Paleomagnetic studies of Paleoarchean lava in Australia and conglomerate in South Africa have concluded that the magnetic field has been present since at least about 3,450  million years ago . In 2024 researchers published evidence from Greenland for

2125-464: Is strong, it compresses the magnetosphere and more of it gets in. Periods of particularly intense activity, called geomagnetic storms , can occur when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar System. Such a wave can take just two days to reach the Earth. Geomagnetic storms can cause a lot of disruption; the "Halloween" storm of 2003 damaged more than

2210-489: Is tearing as well as deforming as it is subducted (Barzangi and Isacks). The subduction has formed and continues to form the volcanic Andes Mountain Range. Deformation of the Nazca plate even affects the geography of Bolivia , far to the east (Tinker et al.). The 1994 Bolivia earthquake occurred on the Nazca plate; this had a magnitude of 8.2 M w {\displaystyle M_{w}} , which at that time

2295-420: Is the declination ( D ) or variation . Facing magnetic North, the angle the field makes with the horizontal is the inclination ( I ) or magnetic dip . The intensity ( F ) of the field is proportional to the force it exerts on a magnet. Another common representation is in X (North), Y (East) and Z (Down) coordinates. The intensity of the field is often measured in gauss (G) , but

2380-423: 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

2465-490: The Boothia Peninsula in 1831 to 600 kilometres (370 mi) from Resolute Bay in 2001. The magnetic equator is the line where the inclination is zero (the magnetic field is horizontal). The global definition of the Earth's field is based on a mathematical model. If a line is drawn through the center of the Earth, parallel to the moment of the best-fitting magnetic dipole, the two positions where it intersects

2550-505: The Brunhes–Matuyama reversal , occurred about 780,000 years ago. A related phenomenon, a geomagnetic excursion , takes the dipole axis across the equator and then back to the original polarity. The Laschamp event is an example of an excursion, occurring during the last ice age (41,000 years ago). The past magnetic field is recorded mostly by strongly magnetic minerals , particularly iron oxides such as magnetite , that can carry

2635-612: 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

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2720-470: The North and South Magnetic Poles abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors. The magnetosphere is defined by the extent of Earth's magnetic field in space or geospace . It extends above

2805-701: The Peru–Chile Trench . The southern side is a divergent boundary with the Antarctic plate, the Chile Rise , where seafloor spreading permits magma to rise. The western side is a divergent boundary with the Pacific plate , forming the East Pacific Rise . The northern side is a divergent boundary with the Cocos plate , the Galapagos Rise . The subduction of the Nazca plate under southern Chile has

2890-780: 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. Geomagnetism Earth's magnetic field , also known as the geomagnetic field , is the magnetic field that extends from Earth's interior out into space, where it interacts with

2975-571: 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

3060-483: The electrical conductivity σ and the permeability μ . The term ∂ B /∂ t is the partial derivative of the field with respect to time; ∇ is the Laplace operator , ∇× is the curl operator , and × is the vector product . The first term on the right hand side of the induction equation is a diffusion term. In a stationary fluid, the magnetic field declines and any concentrations of field spread out. If

3145-402: The ionosphere , several tens of thousands of kilometres into space , protecting Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects Earth from harmful ultraviolet radiation . Earth's magnetic field deflects most of the solar wind, whose charged particles would otherwise strip away

3230-491: The solar wind , a stream of charged particles emanating from the Sun . The magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in Earth's outer core : these convection currents are caused by heat escaping from the core, a natural process called a geodynamo . The magnitude of Earth's magnetic field at its surface ranges from 25 to 65 μT (0.25 to 0.65 G). As an approximation, it

3315-402: The Earth's dynamo shut off, the dipole part would disappear in a few tens of thousands of years. In a perfect conductor ( σ = ∞ {\displaystyle \sigma =\infty \;} ), there would be no diffusion. By Lenz's law , any change in the magnetic field would be immediately opposed by currents, so the flux through a given volume of fluid could not change. As

3400-420: The Earth's magnetic field cycles with intensity every 200 million years. The lead author stated that "Our findings, when considered alongside the existing datasets, support the existence of an approximately 200-million-year-long cycle in the strength of the Earth's magnetic field related to deep Earth processes." The inclination is given by an angle that can assume values between −90° (up) to 90° (down). In

3485-434: The Earth's magnetic field. The magnetopause , the area where the pressures balance, is the boundary of the magnetosphere. Despite its name, the magnetosphere is asymmetric, with the sunward side being about 10  Earth radii out but the other side stretching out in a magnetotail that extends beyond 200 Earth radii. Sunward of the magnetopause is the bow shock , the area where the solar wind slows abruptly. Inside

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3570-437: The Earth's surface are called the North and South geomagnetic poles. If the Earth's magnetic field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses would point towards them. However, the Earth's field has a significant non-dipolar contribution, so the poles do not coincide and compasses do not generally point at either. Earth's magnetic field, predominantly dipolar at its surface,

3655-492: 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

3740-640: 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

3825-545: 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

3910-571: The Solar System by the Sun's magnetosphere, or heliosphere . By contrast, astronauts on the Moon risk exposure to radiation. Anyone who had been on the Moon's surface during a particularly violent solar eruption in 2005 would have received a lethal dose. Some of the charged particles do get into the magnetosphere. These spiral around field lines, bouncing back and forth between the poles several times per second. In addition, positive ions slowly drift westward and negative ions drift eastward, giving rise to

3995-407: The basis for magnetostratigraphy , a way of dating rocks and sediments. The field also magnetizes the crust, and magnetic anomalies can be used to search for deposits of metal ores . Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century. Although the magnetic declination does shift with time, this wandering is slow enough that

4080-681: The coast of southern Chile . At each of these triple junctions an anomalous microplate exists, the Galapagos microplate at the northern junction and the Juan Fernandez microplate at the southern junction. The Easter Island microplate is a third microplate that is located just north of the Juan Fernandez Microplate and lies just west of Easter Island . The Carnegie Ridge is a 1,350-kilometre-long (840 mi) and up to 300-kilometre-wide (190 mi) feature on

4165-435: The current strength are within the normal range of variation, as shown by the record of past magnetic fields recorded in rocks. The nature of Earth's magnetic field is one of heteroscedastic (seemingly random) fluctuation. An instantaneous measurement of it, or several measurements of it across the span of decades or centuries, are not sufficient to extrapolate an overall trend in the field strength. It has gone up and down in

4250-465: The electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz force ). These effects can be combined in a partial differential equation for the magnetic field called the magnetic induction equation , where u is the velocity of the fluid; B is the magnetic B-field; and η = 1/σμ is the magnetic diffusivity , which is the reciprocal of the product of

4335-429: The existence of the magnetic field as early as 3,700 million years ago. Starting in the late 1800s and throughout the 1900s and later, the overall geomagnetic field has become weaker; the present strong deterioration corresponds to a 10–15% decline and has accelerated since 2000; geomagnetic intensity has declined almost continuously from a maximum 35% above the modern value, from circa year 1 AD. The rate of decrease and

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4420-420: The fluid is sustained by convection , motion driven by buoyancy . The temperature increases towards the center of the Earth, and the higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This

4505-429: The fluid moved, the magnetic field would go with it. The theorem describing this effect is called the frozen-in-field theorem . Even in a fluid with a finite conductivity, new field is generated by stretching field lines as the fluid moves in ways that deform it. This process could go on generating new field indefinitely, were it not that as the magnetic field increases in strength, it resists fluid motion. The motion of

4590-410: 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

4675-601: The inclination. The inclination of the Earth's field is 90° (downwards) at the North Magnetic Pole and –90° (upwards) at the South Magnetic Pole. The two poles wander independently of each other and are not directly opposite each other on the globe. Movements of up to 40 kilometres (25 mi) per year have been observed for the North Magnetic Pole. Over the last 180 years, the North Magnetic Pole has been migrating northwestward, from Cape Adelaide in

4760-554: The intensity tends to decrease from the poles to the equator. A minimum intensity occurs in the South Atlantic Anomaly over South America while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia. The intensity of the magnetic field is subject to change over time. A 2021 paleomagnetic study from the University of Liverpool contributed to a growing body of evidence that

4845-405: The interior. The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core. The mechanism by which the Earth generates a magnetic field is known as a geodynamo . The magnetic field is generated by a feedback loop: current loops generate magnetic fields ( Ampère's circuital law ); a changing magnetic field generates an electric field ( Faraday's law ); and

4930-426: 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

5015-414: The last few centuries. The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century. At this rate of decrease, the field would be negligible in about 1600 years. However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual. A prominent feature in

5100-422: The liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary , which is about 3,800 K (3,530 °C; 6,380 °F). The heat is generated by potential energy released by heavier materials sinking toward the core ( planetary differentiation , the iron catastrophe ) as well as decay of radioactive elements in

5185-487: The magnet is suspended so it can turn freely. Since opposite poles attract, the North Magnetic Pole of the Earth is really the south pole of its magnetic field (the place where the field is directed downward into the Earth). The positions of the magnetic poles can be defined in at least two ways: locally or globally. The local definition is the point where the magnetic field is vertical. This can be determined by measuring

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5270-643: The magnetic field once shifted at a rate of up to 6° per day at some time in Earth's history, a surprising result. However, in 2014 one of the original authors published a new study which found the results were actually due to the continuous thermal demagnitization of the lava, not to a shift in the magnetic field. In July 2020 scientists report that analysis of simulations and a recent observational field model show that maximum rates of directional change of Earth's magnetic field reached ~10° per year – almost 100 times faster than current changes and 10 times faster than previously thought. Although generally Earth's field

5355-467: The magnetic orientation is aligned between Sun and Earth – opposite to the previous hypothesis. During forthcoming solar storms, this could result in blackouts and disruptions in artificial satellites . Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation . Over hundreds of years, magnetic declination is observed to vary over tens of degrees. The animation shows how global declinations have changed over

5440-484: The magnetosphere is the plasmasphere , a donut-shaped region containing low-energy charged particles, or plasma . This region begins at a height of 60 km, extends up to 3 or 4 Earth radii, and includes the ionosphere. This region rotates with the Earth. There are also two concentric tire-shaped regions, called the Van Allen radiation belts , with high-energy ions (energies from 0.1 to 10  MeV ). The inner belt

5525-704: The non-dipolar part of the secular variation is a westward drift at a rate of about 0.2° per year. This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD. Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as paleomagnetic secular variation or paleosecular variation (PSV) . The records typically include long periods of small change with occasional large changes reflecting geomagnetic excursions and reversals. A 1995 study of lava flows on Steens Mountain , Oregon appeared to suggest

5610-519: The north pole of Earth's magnetic field (because opposite magnetic poles attract and the north end of a magnet, like a compass needle, points toward Earth's South magnetic field. While the North and South magnetic poles are usually located near the geographic poles, they slowly and continuously move over geological time scales, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred thousand years, Earth's field reverses and

5695-435: The north poles, it must be attracted to the south pole of Earth's magnet. The dipolar field accounts for 80–90% of the field in most locations. Historically, the north and south poles of a magnet were first defined by the Earth's magnetic field, not vice versa, since one of the first uses for a magnet was as a compass needle. A magnet's North pole is defined as the pole that is attracted by the Earth's North Magnetic Pole when

5780-486: The northern hemisphere, the field points downwards. It is straight down at the North Magnetic Pole and rotates upwards as the latitude decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwards until it is straight up at the South Magnetic Pole. Inclination can be measured with a dip circle . An isoclinic chart (map of inclination contours) for the Earth's magnetic field

5865-490: The ocean floor of the northern Nazca plate that includes the Galápagos archipelago at its western end. It is being subducted under South America with the rest of the Nazca plate. The absolute motion of the Nazca plate has been calibrated at 3.7 cm/year (1.5 in/year) east motion (88°), one of the fastest absolute motions of any tectonic plate. The subducting Nazca plate, which exhibits unusual flat slab subduction ,

5950-446: The ozone layer that protects the Earth from harmful ultraviolet radiation. One stripping mechanism is for gas to be caught in bubbles of the magnetic field, which are ripped off by solar winds. Calculations of the loss of carbon dioxide from the atmosphere of Mars , resulting from scavenging of ions by the solar wind, indicate that the dissipation of the magnetic field of Mars caused a near total loss of its atmosphere . The study of

6035-418: The past for unknown reasons. Also, noting the local intensity of the dipole field (or its fluctuation) is insufficient to characterize Earth's magnetic field as a whole, as it is not strictly a dipole field. The dipole component of Earth's field can diminish even while the total magnetic field remains the same or increases. The Earth's magnetic north pole is drifting from northern Canada towards Siberia with

6120-422: The past magnetic field of the Earth is known as paleomagnetism. The polarity of the Earth's magnetic field is recorded in igneous rocks , and reversals of the field are thus detectable as "stripes" centered on mid-ocean ridges where the sea floor is spreading, while the stability of the geomagnetic poles between reversals has allowed paleomagnetism to track the past motion of continents. Reversals also provide

6205-417: The planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of electrically conducting fluids. The Earth's field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core , with a radius of 1220 km, and a liquid outer core . The motion of

6290-464: 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

6375-446: 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

6460-518: 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

6545-415: The reversed direction. The result is a series of stripes that are symmetric about the ridge. A ship towing a magnetometer on the surface of the ocean can detect these stripes and infer the age of the ocean floor below. This provides information on the rate at which seafloor has spread in the past. Radiometric dating of lava flows has been used to establish a geomagnetic polarity time scale , part of which

6630-530: 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

6715-411: The rotational axis of the Earth. The dipole is roughly equivalent to a powerful bar magnet , with its south pole pointing towards the geomagnetic North Pole. This may seem surprising, but the north pole of a magnet is so defined because, if allowed to rotate freely, it points roughly northward (in the geographic sense). Since the north pole of a magnet attracts the south poles of other magnets and repels

6800-436: 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

6885-411: The solar wind would have had a magnetic field orders of magnitude larger than the present solar wind. However, much of the field may have been screened out by the Earth's mantle. An alternative source is currents in the core-mantle boundary driven by chemical reactions or variations in thermal or electric conductivity. Such effects may still provide a small bias that are part of the boundary conditions for

6970-443: 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

7055-499: 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

7140-433: 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

7225-565: Was the strongest instrumentally recorded earthquake occurring deeper than 300 km (190 mi). Aside from the Juan Fernández Islands , this area has very few other islands that are affected by the earthquakes resulting from complicated movements at these junctions. The precursor of the Nazca plate, Juan de Fuca plate , and the Cocos plate was the Farallon plate , which split in the late Oligocene , about 22.8 Mya ,

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