Mantle convection

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Simplyfied model of mantle convection: Whole-mantle convection Oceanic spreading.svg
Simplyfied model of mantle convection: Whole-mantle convection

Mantle convection is the very slow creep of Earth's solid silicate mantle as convection currents carry heat from the interior to the planet's surface. [2] [3] Mantle convection causes tectonic plates to move around the Earth's surface. [4]

Contents

The Earth's lithosphere rides atop the asthenosphere, and the two form the components of the upper mantle. The lithosphere is divided into tectonic plates that are continuously being created or consumed at plate boundaries. Accretion occurs as mantle is added to the growing edges of a plate, associated with seafloor spreading. Upwelling beneath the spreading centers is a shallow, rising component of mantle convection and in most cases not directly linked to the global mantle upwelling. The hot material added at spreading centers cools down by conduction and convection of heat as it moves away from the spreading centers. At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of subduction usually at an oceanic trench. Subduction is the descending component of mantle convection. [5]

This subducted material sinks through the Earth's interior. Some subducted material appears to reach the lower mantle, [6] while in other regions this material is impeded from sinking further, possibly due to a phase transition from spinel to silicate perovskite and magnesiowustite, an endothermic reaction. [7]

The subducted oceanic crust triggers volcanism, although the basic mechanisms are varied. Volcanism may occur due to processes that add buoyancy to partially melted mantle, which would cause upward flow of the partial melt as it decreases in density. Secondary convection may cause surface volcanism as a consequence of intraplate extension [8] and mantle plumes. [9] In 1993 it was suggested that inhomogeneities in D" layer have some impact on mantle convection. [10]

Types of convection

Earth cross-section showing location of upper (3) and lower (5) mantle Earth-crust-cutaway-numbered.svg
Earth cross-section showing location of upper (3) and lower (5) mantle
Earth's temperature vs depth. Dashed curve: layered mantle convection. Solid curve: whole-mantle convection. Earth temperature.svg
Earth's temperature vs depth. Dashed curve: layered mantle convection. Solid curve: whole-mantle convection.
A superplume generated by cooling processes in the mantle. Lower Mantle Superplume.PNG
A superplume generated by cooling processes in the mantle.

During the late 20th century, there was significant debate within the geophysics community as to whether convection is likely to be "layered" or "whole". [12] [13] Although elements of this debate still continue, results from seismic tomography, numerical simulations of mantle convection and examination of Earth's gravitational field are all beginning to suggest the existence of whole mantle convection, at least at the present time. In this model, cold subducting oceanic lithosphere descends all the way from the surface to the core–mantle boundary (CMB), and hot plumes rise from the CMB all the way to the surface. [14] This model is strongly based on the results of global seismic tomography models, which typically show slab and plume-like anomalies crossing the mantle transition zone.

Although it is accepted that subducting slabs cross the mantle transition zone and descend into the lower mantle, debate about the existence and continuity of plumes persists, with important implications for the style of mantle convection. This debate is linked to the controversy regarding whether intraplate volcanism is caused by shallow, upper mantle processes or by plumes from the lower mantle. [8]

Many geochemistry studies have argued that the lavas erupted in intraplate areas are different in composition from shallow-derived mid-ocean ridge basalts. Specifically, they typically have elevated helium-3  : helium-4 ratios. Being a primordial nuclide, helium-3 is not naturally produced on Earth. It also quickly escapes from Earth's atmosphere when erupted. The elevated He-3:He-4 ratio of ocean island basalts suggest that they must be sourced from a part of the Earth that has not previously been melted and reprocessed in the same way as mid-ocean ridge basalts have been. This has been interpreted as their originating from a different less well-mixed region, suggested to be the lower mantle. Others, however, have pointed out that geochemical differences could indicate the inclusion of a small component of near-surface material from the lithosphere.

Planform and vigour of convection

On Earth, the Rayleigh number for convection within Earth's mantle is estimated to be of order 107, which indicates vigorous convection. This value corresponds to whole mantle convection (i.e. convection extending from the Earth's surface to the border with the core). On a global scale, surface expression of this convection is the tectonic plate motions and therefore has speeds of a few cm per year. [15] [16] [17] Speeds can be faster for small-scale convection occurring in low viscosity regions beneath the lithosphere, and slower in the lowermost mantle where viscosities are larger. A single shallow convection cycle takes on the order of 50 million years, though deeper convection can be closer to 200 million years. [18]

Currently, whole mantle convection is thought to include broad-scale downwelling beneath the Americas and the western Pacific, both regions with a long history of subduction, and upwelling flow beneath the central Pacific and Africa, both of which exhibit dynamic topography consistent with upwelling. [19] This broad-scale pattern of flow is also consistent with the tectonic plate motions, which are the surface expression of convection in the Earth's mantle and currently indicate convergence toward the western Pacific and the Americas, and divergence away from the central Pacific and Africa. [20] The persistence of net tectonic divergence away from Africa and the Pacific for the past 250 myr indicates the long-term stability of this general mantle flow pattern [20] and is consistent with other studies [21] [22] [23] that suggest long-term stability of the large low-shear-velocity provinces of the lowermost mantle that form the base of these upwellings.

Creep in the mantle

Due to the varying temperatures and pressures between the lower and upper mantle, a variety of creep processes can occur, with dislocation creep dominating in the lower mantle and diffusional creep occasionally dominating in the upper mantle. However, there is a large transition region in creep processes between the upper and lower mantle, and even within each section creep properties can change strongly with location and thus temperature and pressure. [24]

Since the upper mantle is primarily composed of olivine ((Mg,Fe)2SiO4), the rheological characteristics of the upper mantle are largely those of olivine. The strength of olivine is proportional to its melting temperature, and is also is very sensitive to water and silica content. The solidus depression by impurities, primarily Ca, Al, and Na, and pressure affects creep behavior and thus contributes to the change in creep mechanisms with location. While creep behavior is generally plotted as homologous temperature versus stress, in the case of the mantle it is often more useful to look at the pressure dependence of stress. Though stress is simply force over area, defining the area is difficult in geology. Equation 1 demonstrates the pressure dependence of stress. Since it is very difficult to simulate the high pressures in the mantle (1MPa at 300–400 km), the low pressure laboratory data is usually extrapolated to high pressures by applying creep concepts from metallurgy. [25]

Most of the mantle has homologous temperatures of 0.65–0.75 and experiences strain rates of per second. Stresses in the mantle are dependent on density, gravity, thermal expansion coefficients, temperature differences driving convection, and the distance over which convection occurs—all of which give stresses around a fraction of 3-30MPa.

Due to the large grain sizes (at low stresses as high as several mm), it is unlikely that Nabarro-Herring (NH) creep dominates; dislocation creep tends to dominate instead. 14 MPa is the stress below which diffusional creep dominates and above which power law creep dominates at 0.5Tm of olivine. Thus, even for relatively low temperatures, the stress diffusional creep would operate at is too low for realistic conditions. Though the power law creep rate increases with increasing water content due to weakening (reducing activation energy of diffusion and thus increasing the NH creep rate) NH is generally still not large enough to dominate. Nevertheless, diffusional creep can dominate in very cold or deep parts of the upper mantle.

Additional deformation in the mantle can be attributed to transformation enhanced ductility. Below 400 km, the olivine undergoes a pressure-induced phase transformation, which can cause more deformation due to the increased ductility. [25] Further evidence for the dominance of power law creep comes from preferred lattice orientations as a result of deformation. Under dislocation creep, crystal structures reorient into lower stress orientations. This does not happen under diffusional creep, thus observation of preferred orientations in samples lends credence to the dominance of dislocation creep. [26]

Mantle convection in other celestial bodies

A similar process of slow convection probably occurs (or occurred) in the interiors of other planets (e.g., Venus, Mars) and some satellites (e.g., Io, Europa, Enceladus).

See also

Related Research Articles

<span class="mw-page-title-main">Plate tectonics</span> Movement of Earths lithosphere

Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s.

<span class="mw-page-title-main">Lithosphere</span> Outermost shell of a terrestrial-type planet or natural satellite

A lithosphere is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time scales of up to thousands of years or more. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy.

<span class="mw-page-title-main">Asthenosphere</span> Highly viscous, mechanically weak, and ductile region of Earths mantle

The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~80 and 200 km below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

<span class="mw-page-title-main">Mantle plume</span> Upwelling of abnormally hot rock within Earths mantle

A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.

<span class="mw-page-title-main">Earth's mantle</span> A layer of silicate rock between Earths crust and its outer core

Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×1024 kg (8.84×1024 lb) and thus makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometers (1,800 mi) making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.

<span class="mw-page-title-main">Large igneous province</span> Huge regional accumulation of igneous rocks

A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive and extrusive, arising when magma travels through the crust towards the surface. The formation of LIPs is variously attributed to mantle plumes or to processes associated with divergent plate tectonics. The formation of some of the LIPs in the past 500 million years coincide in time with mass extinctions and rapid climatic changes, which has led to numerous hypotheses about causal relationships. LIPs are fundamentally different from any other currently active volcanoes or volcanic systems.

<span class="mw-page-title-main">Iceland hotspot</span> Hotspot partly responsible for volcanic activity forming the Iceland Plateau and island

The Iceland hotspot is a hotspot which is partly responsible for the high volcanic activity which has formed the Iceland Plateau and the island of Iceland.

<span class="mw-page-title-main">Geothermal gradient</span> Rate of temperature increase with depth in Earths interior

Geothermal gradient is the rate of change in temperature with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate boundaries, temperature rises in about 25–30 °C/km (72–87 °F/mi) of depth near the surface in most of the world. However, in some cases the temperature may drop with increasing depth, especially near the surface, a phenomenon known as inverse or negative geothermal gradient. The effects of weather, the Sun, and season only reach a depth of roughly 10–20 m (33–66 ft).

<span class="mw-page-title-main">Geodynamics</span> Study of dynamics of the Earth

Geodynamics is a subfield of geophysics dealing with dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as seafloor spreading, mountain building, volcanoes, earthquakes, faulting. It also attempts to probe the internal activity by measuring magnetic fields, gravity, and seismic waves, as well as the mineralogy of rocks and their isotopic composition. Methods of geodynamics are also applied to exploration of other planets.

<span class="mw-page-title-main">Crustal recycling</span> Tectonic recycling process

Crustal recycling is a tectonic process by which surface material from the lithosphere is recycled into the mantle by subduction erosion or delamination. The subducting slabs carry volatile compounds and water into the mantle, as well as crustal material with an isotopic signature different from that of primitive mantle. Identification of this crustal signature in mantle-derived rocks is proof of crustal recycling.

<span class="mw-page-title-main">Large low-shear-velocity provinces</span> Structures of the Earths mantle

Large low-shear-velocity provinces, LLSVPs, also called LLVPs or superplumes, are characteristic structures of parts of the lowermost mantle of Earth. These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP. Both extend laterally for thousands of kilometers and possibly up to 1,000 kilometres vertically from the core–mantle boundary. The Pacific LLSVP is 3,000 kilometers across and underlies four hotspots that suggest multiple mantle plumes underneath. These zones represent around 8% of the volume of the mantle.

<span class="mw-page-title-main">Geodynamics of terrestrial exoplanets</span>

The discovery of extrasolar Earth-sized planets has encouraged research into their potential for habitability. One of the generally agreed requirements for a life-sustaining planet is a mobile, fractured lithosphere cyclically recycled into a vigorously convecting mantle, in a process commonly known as plate tectonics. Plate tectonics provide a means of geochemical regulation of atmospheric particulates, as well as removal of carbon from the atmosphere. This prevents a “runaway greenhouse” effect that can result in inhospitable surface temperatures and vaporization of liquid surface water. Planetary scientists have not reached a consensus on whether Earth-like exoplanets have plate tectonics, but it is widely thought that the likelihood of plate tectonics on an Earth-like exoplanet is a function of planetary radius, initial temperature upon coalescence, insolation, and presence or absence of liquid-phase surface water.

<span class="mw-page-title-main">Lithosphere–asthenosphere boundary</span> Level representing a mechanical difference between layers in Earths inner structure

The lithosphere–asthenosphere boundary represents a mechanical difference between layers in Earth's inner structure. Earth's inner structure can be described both chemically and mechanically. The lithosphere–asthenosphere boundary lies between Earth's cooler, rigid lithosphere and the warmer, ductile asthenosphere. The actual depth of the boundary is still a topic of debate and study, although it is known to vary according to the environment.

Lid tectonics, commonly thought of as stagnant lid tectonics or single lid tectonics, is the type of tectonics that is believed to exist on several silicate planets and moons in the Solar System, and possibly existed on Earth during the very early part of its history. The lid is the equivalent of the lithosphere, formed of solid silicate minerals. The relative stability and immobility of the strong cooler lids leads to stagnant lid tectonics, which has greatly reduced amounts of horizontal tectonics compared with plate tectonics. The presence of a stagnant lid above a convecting mantle was recognised as a possible stable regime for convection on Earth, in contrast to the well-attested mobile plate tectonics of the current eon.

<span class="mw-page-title-main">Earth's crustal evolution</span>

Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface.

The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust and ends at the top of the lower mantle at 670 km (420 mi). Temperatures range from approximately 500 K at the upper boundary with the crust to approximately 1,200 K at the boundary with the lower mantle. Upper mantle material that has come up onto the surface comprises about 55% olivine, 35% pyroxene, and 5 to 10% of calcium oxide and aluminum oxide minerals such as plagioclase, spinel, or garnet, depending upon depth.

Ridge push is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.

<span class="mw-page-title-main">Plate theory (volcanism)</span>

The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:

  1. Changes in the configuration of plate boundaries.
  2. Vertical motions.
  3. Thermal contraction.

Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.

References

  1. ↑ Carlo Doglioni, Giuliano Panza: Polarized Plate Tectonics]. Advances in Geophysics, Volume 56, 2015.
  2. ↑ Kobes, Randy. "Mantle Convection". Archived from the original on 9 June 2011. Retrieved 26 February 2020. Physics Department, University of Winnipeg
  3. ↑ Ricard, Y. (2009). "2. Physics of Mantle Convection". In David Bercovici and Gerald Schubert (ed.). Treatise on Geophysics: Mantle Dynamics. Vol. 7. Elsevier Science. ISBN   9780444535801.
  4. ↑ Moresi, Louis; Solomatov, Viatcheslav (1998). "Mantle convection with a brittle lithosphere: thoughts on the global tectonic styles of the Earth and Venus". Geophysical Journal International. 133 (3): 669–82. Bibcode:1998GeoJI.133..669M. CiteSeerX   10.1.1.30.5989 . doi:10.1046/j.1365-246X.1998.00521.x.
  5. ↑ Gerald Schubert; Donald Lawson Turcotte; Peter Olson (2001). "Chapter 2: Plate tectonics". Mantle convection in the earth and planets. Cambridge University Press. pp. 16 ff. ISBN   978-0-521-79836-5.
  6. ↑ Fukao, Yoshio; Obayashi, Masayuki; Nakakuki, Tomoeki; Group, the Deep Slab Project (2009-01-01). "Stagnant Slab: A Review" (PDF). Annual Review of Earth and Planetary Sciences. 37 (1): 19–46. Bibcode:2009AREPS..37...19F. doi:10.1146/annurev.earth.36.031207.124224.
  7. ↑ Gerald Schubert; Donald Lawson Turcotte; Peter Olson (2001). "§2.5.3: Fate of descending slabs". Cited work. pp. 35 ff. ISBN   978-0-521-79836-5.
  8. 1 2 Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy. Wiley-Blackwell. ISBN   978-1-4051-6148-0.
  9. 1 2 Kent C. Condie (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann. p. 5. ISBN   978-0-7506-3386-4.
  10. ↑ Czechowski L. (1993) Geodesy and Physics of the Earth pp 392-395, The Origin of Hotspots and The D” Layer
  11. ↑ Ctirad Matyska & David A Yuen (2007). "Figure 17 in Lower-mantle material properties and convection models of multiscale plumes". Plates, plumes, and planetary processes. Geological Society of America. p. 159. ISBN   978-0-8137-2430-0.
  12. ↑ Donald Lawson Turcotte; Gerald Schubert (2002). Geodynamics (2nd ed.). Cambridge University Press. ISBN   978-0-521-66624-4.
  13. ↑ Gerald Schubert; Donald Lawson Turcotte; Peter Olson (2001). Cited work. p. 616. ISBN   978-0-521-79836-5.
  14. ↑ Montelli, R; Nolet, G; Dahlen, FA; Masters, G; Engdahl ER; Hung SH (2004). "Finite-frequency tomography reveals a variety of plumes in the mantle" (PDF). Science. 303 (5656): 338–43. Bibcode:2004Sci...303..338M. doi:10.1126/science.1092485. PMID   14657505. S2CID   35802740.
  15. ↑ Small-scale convection in the upper mantle beneath the Chinese Tian Shan Mountains, http://www.vlab.msi.umn.edu/reports/allpublications/files/2007-pap79.pdf Archived 2013-05-30 at the Wayback Machine
  16. ↑ Polar Wandering and Mantle Convection, http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1972IAUS...48..212T&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES
  17. ↑ Picture showing convection with velocities indicated. "IRIS Image Gallery". Archived from the original on 2011-09-28. Retrieved 2011-08-29.
  18. ↑ Thermal Convection with a Freely Moving Top Boundary, See section IV Discussion and Conclusions http://physics.nyu.edu/jz11/publications/ConvecA.pdf
  19. ↑ Lithgow-Bertelloni, Carolina; Silver, Paul G. (1998). "Dynamic topography, plate driving forces and the African superswell". Nature. 395 (6699): 269–272. Bibcode:1998Natur.395..269L. doi:10.1038/26212. ISSN   0028-0836. S2CID   4414115.
  20. 1 2 Conrad, Clinton P.; Steinberger, Bernhard; Torsvik, Trond H. (2013). "Stability of active mantle upwelling revealed by net characteristics of plate tectonics". Nature. 498 (7455): 479–482. Bibcode:2013Natur.498..479C. doi:10.1038/nature12203. hdl: 10852/61522 . ISSN   0028-0836. PMID   23803848. S2CID   205234113.
  21. ↑ Torsvik, Trond H.; Smethurst, Mark A.; Burke, Kevin; Steinberger, Bernhard (2006). "Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle". Geophysical Journal International. 167 (3): 1447–1460. Bibcode:2006GeoJI.167.1447T. doi: 10.1111/j.1365-246x.2006.03158.x . ISSN   0956-540X.
  22. ↑ Torsvik, Trond H.; Steinberger, Bernhard; Ashwal, Lewis D.; Doubrovine, Pavel V.; Trønnes, Reidar G. (2016). "Earth evolution and dynamics—a tribute to Kevin Burke". Canadian Journal of Earth Sciences. 53 (11): 1073–1087. Bibcode:2016CaJES..53.1073T. doi:10.1139/cjes-2015-0228. hdl: 10852/61998 . ISSN   0008-4077.
  23. ↑ Dziewonski, Adam M.; Lekic, Vedran; Romanowicz, Barbara A. (2010). "Mantle Anchor Structure: An argument for bottom up tectonics". Earth and Planetary Science Letters. 299 (1–2): 69–79. Bibcode:2010E&PSL.299...69D. doi:10.1016/j.epsl.2010.08.013. ISSN   0012-821X.
  24. ↑ Weertman, J.; White, S.; Cook, Alan H. (1978-02-14). "Creep Laws for the Mantle of the Earth [and Discussion]". Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. 288 (1350): 9–26. Bibcode:1978RSPTA.288....9W. doi:10.1098/rsta.1978.0003. ISSN   1364-503X. S2CID   91874725.
  25. 1 2 Borch, Robert S.; Green, Harry W. (1987-11-26). "Dependence of creep in olivine on homologous temperature and its implications for flow in the mantle". Nature. 330 (6146): 345–48. Bibcode:1987Natur.330..345B. doi:10.1038/330345a0. S2CID   4319163.
  26. ↑ Karato, Shun-ichiro; Wu, Patrick (1993-05-07). "Rheology of the Upper Mantle: A Synthesis". Science. 260 (5109): 771–78. Bibcode:1993Sci...260..771K. doi:10.1126/science.260.5109.771. ISSN   0036-8075. PMID   17746109. S2CID   8626640.
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