Where in Earth's Interior Can Convection Cells Be Found?

February 16, 2022 Post a Comment

Convection Cells

Mantle Dynamics

Y. Ricard , in Treatise on Geophysics, 2007

7.02.vi.vii A Complex Lithosphere: Plates and Continents

The lithosphere is part of the convection cell, and plate tectonics and mantle convection cannot exist separated. The fact that the cold lithosphere is much more sticky and concentrates well-nigh of the mass heterogeneities of the mantle, makes it behaving to some extent like a membrane on top of a less viscous fluid. This suggests some illustration between pall convection and what is chosen Marangoni convection (Marangoni, 1840–1925). Marangoni convection (Nield, 1964) is controlled by temperature-dependent surface tension on top of sparse layers of fluids.

The Earth's mantle is certainly not controlled past surface tension, and Marangoni convection, strictly speaking, has nothing to practise with mantle convection. Still, the equations of thermal convection with cooling from the top and with a highly gluey lithosphere can be shown to be mathematically related (through a change of variables) to those of Marangoni convection (Lemery et al., 2000). There are big differences between drapery convection and surface driven convection simply this analogy has sometimes been advocated as a 'top-down' view of the pall dynamics (Anderson, 2001). More classically, the interpretation of plate cooling in terms of ridge-push forcefulness (Turcotte and Schubert, 1982), or the analysis of tectonic stresses using thin sheet approximations (England and Mckenzie, 1982) belong to the same approach that emphasizes the importance of the lithosphere as a stress guide and as a major source of density anomalies.

Due to the complexities of the lithosphere properties, the purlieus condition at the surface of the Earth is far from being a uniform free-slip status. Both continents and tectonic plates impose their own wavelengths and specific boundary weather on the underlying convecting asthenosphere. Of course, the position of the continents and the number and shape of the plates are themselves consequences of mantle convection. The plates manifestly organize the large-scale flow in the mantle (Hager and O'connell, 1979; Ricard and Vigny, 1989). They impose a complex boundary condition where the angular velocity is piecewise abiding. The continents with their reduced heat flow (Jaupart and Mareschal, 1999) also impose a laterally variable oestrus flux purlieus status.

Convection models with continents have been studied numerically (Gurnis and Hager, 1988; Grigné and Labrosse, 2001, Coltice et al., 2007) and experimentally (Guillou and Jaupart, 1995). Continents with their thick lithosphere tend to increase the thickness of the top boundary layer and the temperature beneath them (see Figure 9 ). Hot rising currents are predicted under continents and downwellings are localized along continental edges. The existence of a thick and stable continental root must be due to a chemically lighter and more viscous subcontinental lithosphere (Doin et al., 1997). The ratio of the heat flux extracted beyond continents compared to that extracted across oceans increases with the Rayleigh number. This suggests that the continental geotherms were not much different in the past when the radiogenic sources were larger; information technology is mostly the oceanic heat flux that was larger (Lenardic, 1998). Simulating organized plates self-consistently coupled with a convective drape has been a very difficult quest. The attempts to generate plates using T-dependent or simple nonlinear rheologies have failed. Although in 2-D some successes can be obtained in localizing deformation in plate-like domains, (Schmeling and Jacoby, 1981; Weinstein and Olson, 1992; Weinstein, 1996), they are obtained with stress exponents (e.g., north ≥ vii) that are larger than what can be expected from laboratory experiments (n ∼ 2). The issues are however worst in iii-D. Generally, these early models do not predict the important shear motions between plates that is observed (Christensen and Harder, 1991; Ogawa et al., 1991).

Some authors accept tried to mimic the presence of plates by imposing plate-like surface boundary weather condition. These studies have been performed in 2-D and 3-D (Ricard and Vigny, 1989; Gable et al., 1991; King et al., 1992; Monnereau and Quéré, 2001). Although they have confirmed the profound issue of plates on the wavelengths of convection, on its fourth dimension dependence and on the surface oestrus flux, these approaches cannot predict the evolution of surface plate geometry. Effigy 10 illustrates the organizing effect of plates in spherical, internally heated compressible convection with depth-dependent viscosity (Bunge and Richards, 1996). To obtain a self-consistent generation of surface plates, more than complex rheologies that include brittle failure, strain softening, and damage mechanisms must be introduced (e.g., Bercovici, 1993, 1995; Moresi and Solomatov, 1998; Auth et al., 2003). The existence of plates seems also to crave the existence of a weak sublithospheric asthenosphere (Richards et al., 2001). In the terminal years, the starting time successes in computing 3-D models that spontaneously organize their height purlieus layer into plates have been reached (Tackley, 1998, 2000c, 2000d, 2000e; Trompert and Hansen, 1998a; Stein et al., 2004). Although the topological characteristics of the predicted plates and their time evolution may be still far from the observed characteristics of plate tectonics, and often too episodic (stagnant-hat convection punctuated by plate-like events), a very important quantum has been made by modelers (come across Figure 11 ).

The Globe'due south plate boundaries keep the memory of their weakness over geological times (Gurnis et al., 2000). This implies that the rheological backdrop cannot be a elementary fourth dimension-contained function of stress or temperature but has a long-term memory. The rheologies that have been used to predict plates in convective models remain empirical and their estimation in terms of microscopic behavior and damage theory remains largely to be washed (Bercovici and Ricard, 2005). Reviews on the rapid progress and the limitations of self-coherent convection models tin can be plant in Bercovici et al. (2000), Tackley (2000a), and Bercovici (2003).

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Mantle Dynamics

P.J. Tackley , in Treatise on Geophysics, 2007

vii.10.ii.3.two Steady-country 2-D flows

Steady-land two-D Bénard convection cells follow the laminar stretching law, whether kinematic ( Olson et al., 1984b) or convective (McKenzie, 1979). The fit to theory is reasonable for both nondiffusive and diffusive heterogeneity (Olson et al., 1984b). Information technology is instructive to sympathise the resulting stretching regime equally a baseline for more than interpreting more than complex flows.

Heterogeneities are stretched into spirals of lamellae that are locally oriented subparallel to the streamlines, because different streamlines are rotating at dissimilar rates. For instance, near the meridian of the domain the lamellae are locally subhorizontal. In a completely heated-from below steady-state Bénard convection prison cell the interior of the cell (far from the boundaries) undergoes almost rigid-trunk rotation with an associated low stretching rate, simply about the boundaries, where heterogeneities pass close to stagnation points in the corners, the cumulative strain can increase essentially (due east.g., the attribute ratio of the strain ellipse (a/b) by a factor of 1000) over one rotation (McKenzie, 1979). It is interesting to note that strain does non increase monotonically, but rather equally large oscillations with a superimposed linear trend (McKenzie, 1979). The parts of these oscillations where the strain decreases take been termed 'unmixing'. The add-on of internal heating makes the situation more than circuitous equally there is no uniformly-rotating expanse in the middle, but again the cumulative strain (a/b) undergoes considerable oscillation and near the edge of the 'jail cell' can attain a cistron of 100 over one rotation (McKenzie, 1979). The highest strain rates (hence velocity gradients) occur near the downwelling. For finite-sized bodies (initially cylinders), Hoffman and McKenzie (1985) found that for pure basal heating the corporeality of stretching was rapid for a blob initially situated near the edge of the flow, but with internal heating the hulk became deformed only non greatly stretched in one revolution.

Steady corner flows relevant to mid-body of water ridges and subduction zones take too been studied (McKenzie, 1979). The virtually astringent strain (a/b ∼ 30) was experienced in the corner above a subducting slab, with the principle centrality of the strain ellipse aligned subparallel to the streamline. Again, strain does not necessarily increase monotonically with time.

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Auroral structures: Revealing the importance of meso-calibration M-I coupling

Larry R. Lyons , ... Yukitoshi Nishimura , in Cross-Scale Coupling and Energy Transfer in the Magnetosphere-Ionosphere-Thermosphere System, 2022

2.iv.iii Omega bands and DAPS

The menstruation channels associated with the Harang aurora become part of the dusk convection cell. Menstruation channels further to the due east often end up within the morning-side convection prison cell, which then class strong eastward flows recently identified as dawnside auroral polarization streams (or DAPS) (Liu et al., 2020). In that location they flow east while staying poleward of the most intense morning-side aurora. More intense aurora occurs within the region of upwards Region ii currents, whereas DAPS lie within the region of downward Region i currents and oft display a steep slope at the purlieus between Regions 1 and 2 currents.

DAPS are associated with a major disturbance that only occurs in the midnight-to-morning sector known as omega bands. Omega bands are large-calibration, auroral folds that movement eastward in the morning time sector with velocities of a few to several hundred chiliad/southward (Akasofu, 1974; Andre and Baumjohann, 1982; Opgenoorth et al., 1983). They have a tendency to occur during periods of enhanced convection (e.1000., Solovyev et al., 1999), and they typically occur forth the poleward edge of the diffuse auroral region that lies inside the upwardly Region 2 currents, and have wavelengths of ~ 500–1000 km (Henderson, 2012; Lyons and Walterscheid, 1985). They protrude poleward from the equatorward portion of the auroral oval toward the higher breadth portion of the oval and are separated past dark regions (Henderson, 2012). Auroral torches protrude more strongly poleward (Akasofu and Kimball, 1964), though they tin can be seen to evolve into omega bands every bit they move eastward. Omega bands are associated with substantial magnetic field perturbations on the footing that tin exist every bit large as several hundred nT with periods of several to 10s of minutes (Kawasaki and Rostoker, 1979; Rostoker and Barichello, 1980), consistent with them beingness a major disturbance.

Henderson (2012) took advantage of global imaging from the NASA Polar spacecraft and showed examples where auroral streamers evolved into torch-like structures when they reached the diffuse auroral region, and presently evolve into articulate, east moving, omega bands. These observations demonstrate that flow channels on the morning side, appearing to be within the morning convection prison cell, tin evolve into omega bands when they extend sufficiently earthward in the plasma sheet. We accept been able to see this connection in the THEMIS ASI images, since they embrace a large-enough region of the postmidnight auroral region when sky atmospheric condition are articulate. While a detailed study of such events has non yet been performed using the THEMIS ASI observations, an example was shown past Nishimura et al. (2010b) and is given in Fig. 2.9. In Fig. 2.nine, the first console shows a PBI near the auroral poleward boundary that extended equatorward to near the equatorward auroral region every bit a streamer every bit seen in the second console. Connexion to the newly formed torch structure is seen in the tertiary, and its east propagation equally tin can be seen in the fourth panel. The same process is seen in Fig. ii.10, where high-altitude measurements of the aurora from the Polar spacecraft VIS low resolution camera bear witness the sequence of an omega band formation from a PBI evolving into an auroral streamer that afterwards gives ascent to the torch formation (figure adapted from Henderson, 2012).

Liu et al. (2018) showed clearly that the poleward edge of omega bands corresponds to the equatorward boundary of DAPS, and then that it is reasonable to advise that the omega bands are associated with the strong shear flow at this purlieus and with the reduced entropy of the plasma inside flow channels.

The physical process by which flow channels pb to strong SAPS and DAPS flows is discussed afterwards.

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Drape Dynamics

E.Grand. Parmentier , in Treatise on Geophysics, 2007

7.07.2.i A Historical Perspective

Turcotte and Oxburgh (1967) identified the moving lithospheric plates (Chapter half dozen.02 ) as the conductive thermal boundary layer at the superlative of pall convection cells, thus establishing a physical relationship between upper-mantle thermal structure and drapery dynamics. During the emergence of plate tectonics, measurements of seafloor depth and oestrus menses in the oceans provided the first direct evidence on the thermal structure of the upper mantle. Heat menstruum generally decreased with crustal age but showed great variability, particularly at young ages. This variability is now understood to be due primarily to hydrothermal circulation of seawater through permeable crustal rocks. In dissimilarity, seafloor depths showed a much more than systemic variation with seafloor historic period. The age dependence of isostatic seafloor depth, which depends on the depth-averaged temperature, was recognized as a stronger observational constraint than estrus flow on upper-mantle thermal structure ( Langseth et al., 1966; McKenzie, 1967; Vogt and Ostenso, 1967; McKenzie and Sclater, 1969; Sleep, 1969).

The thermal construction that develops due to conductive cooling should be, at least to get-go social club, only a function of crustal historic period, rather than an contained function of both altitude from spreading center and plate velocity. Observations of seafloor depth and age generally confirmed this age dependence. The Turcotte and Oxburgh boundary layer hypothesis, in the elementary form suggested originally, indicated that the thermal purlieus layer should keep to thicken every bit the square root of age so that old seafloor would continue to subside. This did not appear to explicate observations showing a relatively uniform depth of sometime seafloor which required that the thermal purlieus layer evolve to a nearly constant thickness. Models for upper-mantle thermal structure that were consistent with average depth of seafloor as a office of age from the growing collection of observations treated the conductive cooling of horizontally moving upper mantle, equally in the simple boundary layer theory, but with the added assumption of a uniform temperature at a prescribed depth (Langseth et al., 1966; McKenzie, 1967). A relatively uniform seafloor depth at old ages requires a machinery to transport heat to bottom of the thermal boundary layer, thus reducing the charge per unit at which it thickens. This set the phase for enquiry in the post-obit several decades and is still a source of continuing report and argue.

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Auroral geospace

Evgeny 5. Mishin , Anatoly Five. Streltsov , in Nonlinear Moving ridge and Plasma Structures in the Auroral and Subauroral Geospace, 2022

3.one.1.three Plasma convection

The magnetosphere connects with the ionosphere by virtually equipotential magnetic field lines. Thus, the electric potential maps downward and drives a similar two-cell convection in the ionosphere, equally illustrates Fig. 3.1.2A with the MLT-MLAT distribution of plasma drifts forth the equipotential lines averaged over many orbits of the Orbiting Geophysical Observatory (OGO) 6 satellite (Heppner, 1972). Here, MLT (MLAT) stands for magnetic local time (geomagnetic latitude) defined by the axis of the geomagnetic dipole. Likewise depicted hither are Region 1 (R1) Birkeland currents get-go detailed by Iijima and Potemra (1978). These currents originate from the jump of the normal component of the convection electric field (polarization charge) at the polar cap boundary. Inertialess electrons menses along B toward (from) the positive (negative) charge region, hereby creating down (upward) field-aligned current on the morning (evening) side.

Following the OGO fleet, numerous spacecraft have detailed the variation of the convection/FACs patterns with the Imf strength and clock angle by averaging over many orbits (e.g., Carter et al., 2016; Weimer et al., 2017 and references therein). Shown in Fig. 3.one.2B are MLT-MLAT maps of FACs obtained from the Agile Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) data set with the location of the auroral oval, obtained from the Imager for Magnetopause-to-Aurora Global Exploration (Epitome) far ultraviolet (FUV) cameras. The maps are obtained by binning the data spanning two solar cycles under the same solar current of air and geophysical atmospheric condition.

Ground-based networks, such as the Super Dual Auroral Radar Network (SuperDARN) consisting of high frequency (HF) radars located in the high- and midlatitude regions of both hemispheres, can provide a global boilerplate picture and "instant" snapshots of the convection pattern. Equally an example, Fig. three.1.2C (Nishitani et al., 2019) shows a pattern of Northern Hemisphere ionospheric convection from the SuperDARN data averaged over a few years. In addition, shown in Fig. 3.1.2D and E are MLT-MLAT "instant" distributions of the electric potential and FACs in the ionosphere only before an individual substorm (V. Thousand. Mishin et al., 2017). These maps are calculated from footing-based magnetometer data using the magnetogram inversion technique or MIT (e.thousand., V. M. Mishin, 1991 and references therein). MIT reconstructs ii-D distributions of large-calibration equivalent ionospheric currents, FACs, and plasma convection with a 1-min time resolution.

In general, the convection design in Fig. 3.one.2 comprises two opposite-rotating big-scale vortices adjacent to the polar cap antisunward flow. As well conspicuously seen are convection and current structures at lower latitudes. These are the Iijima and Potemra Region two (R2) currents connecting the inner edge of the plasmasheet with the auroral boundary and the band current with the subauroral ionosphere merely equatorward of the auroral oval. Region 2 currents originate from polarization charges (positive/negative at sunset/dawn) created past hot plasma particles convecting sunward due to the dawn–dusk electrical field and gradient-curvature globe-trotting azimuthally charged particles (electrons—eastward, ions—westward). The resulting polarization field is opposite to the driving dawn–dusk electric field and hence changes the convection pattern in the inner magnetosphere. The corresponding R2 FACs flow into/from the ionosphere at the evening/morning side.

Closure of FACs in the ionosphere creates ionospheric currents or electrojets (recall Fig. one.fourB and C), specially in the auroral oval with the greater Hall conductivity due to energetic electron atmospheric precipitation (e.thousand., Robinson et al., 1987). The Hall electric current is carried mainly by magnetized electrons. Therefore, its direction for equipotential magnetic field lines is opposite to the convection flow. It is worth introducing here the notion of the DP-2 magnetic disturbance (Disturbance Polar of the 2nd type) created by the two-cell ionospheric current organization.

Attributable to current continuity, FACs are determined by the divergence of the perpendicular current. The latter in the ideal MHD with isotropic force per unit area follows from the momentum equation (Eq. 2.1.122):

(3.1.2) ${\frac{j_{∥}}{B} |}_{M} = - {\int_{0}^{s} \frac{d {due south}^{'}}{B} \nabla_{⊥} \cdot j_{⊥} |}_{K} = {\int_{0}^{s} \nabla_{⊥} \cdot [(\underset{inertial}{\underset{︸}{\ ρ_{grand} d U_{⊥} / d t}} + \underset{diamagnetic}{\underset{︸}{\nabla_{⊥} P}}) \times b] \frac{d s^{'}}{B^{2}} |}_{M} \to \overset{\nabla_{⊥} P = j \times B; P (s) = const}{\overset{︷}{\to}} \overset{Vasyliunas'south (1970) formula}{\overset{︷}{j_{∥}^{(Yard)} = b_{Thousand} \cdot {[\nabla_{⊥} \tilde{V} \times \nabla_{⊥} P]}_{M}}}$

Here $\tilde{V} = \int_{0}^{due south} B^{- 1} d {south}^{'}$ is the book of a flux tube of unit magnetic flux, "Thousand" stands for "equatorial Magnetosphere," and the integral is taken forth a field line from the equatorial plane point where $j_{∥} = 0$ to the ionospheric foot point (Vasyliunas, 1970). In other words, FACs announced when the isocontours of $\tilde{V}$ and force per unit area misalign. For case, the flux tube volume in the band current region depends primarily on the radial distance and the azimuthal current flows in the equatorial plane with B poleward (||z). Therefore, the main contribution to duskside R2 FACs comes from the azimuthal slope of the band current (RC) pressure level.

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Energetic particle dynamics, precipitation, and conductivity

Christine Gabrielse , ... Yiqun Yu , in Cantankerous-Scale Coupling and Energy Transfer in the Magnetosphere-Ionosphere-Thermosphere System, 2022

iv.2.1.1 Big-scale convection

As described in Section 1.one, large-scale convection occurs in the magnetosphere and high-latitude ionosphere. The shape and intensity of the two-cell convection pattern differ depending on the solar current of air and the IMF. Mapping of the large-scale convection pattern in the loftier-latitude ionosphere equally a function of the IMF clock angle—a measure of the degree to which the solar wind magnetic field points in the ± B _Z vs ± B _Y direction—has been progressively detailed over the years with increased observations both in space and from the ground [e.thou., Heppner (1977) using Ogo 6; Heppner and Maynard (1987) using Dynamics Explorer 2; Rich and Hairston (1994) using DMSP; Weimer (1995) using DE satellite data; Ruohoniemi and Greenwald (1996) using ground-based HF radar data; Papitashvili and Rich (2002) using DMSP; Ruohoniemi and Greenwald (2005) and Cousins and Shepherd (2010) using SuperDARN HF radar information; Haaland et al. (2007) using Cluster data]. Fig. four.six, which is replicated from Haaland et al. (2007) , illustrates the convection cells. During − IMF B _Y, the convection pattern is symmetric about midnight. During + IMF B _Y, the convection design tilts so that the primal, faster part of the antisunward convection about the poles lies postmidnight, but the nightside, equatorward portion (in the oval) lies premidnight. When Imf B _Z is negative—resulting in dayside reconnection—large-scale convection is enhanced as compared to + IMF B _Z. During + IMF B _Z, 3 or even four convection cells may form from reconnection between the IMF and preexisting open flux in the tail lobe (Dungey, 1963; Reiff and Burch, 1985; Cowley and Lockwood, 1992).

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HISTORY OF GEOLOGY SINCE 1962

U.B. Marvin , in Encyclopedia of Geology, 2005

Sea-floor Spreading: 1960 and 1961

In 1960, Hess circulated the preprint of a book chapter in which he described the ocean floors as the exposed surface of the mantle. He proposed that large-scale convection cells in the drape create new ocean floor at the ridges, where the ascension limbs diverge and movement to either side until they cool and plunge downwardly into the drapery at the trenches ( meet TECTONICS | Sea Trenches) or below continental margins (see TECTONICS | Convergent Plate Boundaries and Accretionary Wedges). He now explained guyots equally volcanic peaks eroded on oceanic ridge crests and carried to the depths by the moving seafloor. Where limbs rise below continents they split up them apart and raft the granitic fragments in opposite directions until grounding them over zones of downward-welling. Hess suggested that inasmuch equally granite is too buoyant to sink into the mantle, some fragments of continents have survived since the beginning of geologic time, while the ocean floors have been swept make clean and replaced past new mantle material every 300–400 my.

Hess further theorized that the mantle consists of peridotite, an olivine-rich stone that becomes hydrated to serpentinite at the ridges by reaction with heated waters released from depth. He favoured serpentinite partly because of its ease of recycling and partly because he believed information technology would be impossible to achieve the uniform 4.5 km thickness of the bounding main floors with dykes and lava flows. Hess acknowledged that drape convection was regarded as too radical an idea to be widely accepted by geologists and geophysicists, but he pointed out that his model would account for many phenomena in a coherent fashion: the formation of the ridge-rift arrangement and the trenches, the youth and uniform thickness of the ocean floors, the thinness of pelagic sediments, and moving continents. Hess called his chapter an essay in 'geopoetry'.

In 1961, Dietz published a iii-page article in Nature in which he, besides, argued that convection in the mantle, moving at a few centimetres per year, could produce the overall structure of the bounding main basins: the ridges form over sites of rising and diverging limbs; the trenches form at sites of converging and down-welling limbs, and the fracture zones are shears betwixt regions of slow and fast pitter-patter. Dietz suggested that the mantle consists of eclogite, a dumbo pyroxene-garnet rock, and that the body of water floors are congenital of basaltic dykes and pillow lavas, formed by the partial melting of the eclogite at the ridges. But he added that the actual stone compositions were of less importance than the fact that the ocean floors must exist recycled mantle material. Dietz pointed out, equally had Hess, that rising convection currents rift continents autonomously and acquit the sialic fragments en bloc to sites of down-welling and compression, where folded mountain ranges grade on their margins. He added that the pelagic sediments ride downward into the depths on the surfaces of the plunging oceanic slabs where they are granitized and welded onto the undersides of the continents – thus contributing to the persistent continental complimentary-board, despite steady erosion of the continental surfaces toward base level.

Dietz chosen this procedure 'sea-floor spreading'. This concept, he argued, requires geologists to think of World's outer layers in terms of their relative strengths, then we should begin referring to the 'lithosphere', a celebrated proper name for Globe's outermost layer, which is relatively strong and rigid to a compatible depth of well-nigh 70 km (now mostly taken as 100 km) nether both continents and oceans. Beneath the lithosphere lies the weaker, more yielding, 'asthenosphere' on which the lithosphere moves in response to convection currents. The asthenosphere had previously been hypothesized on the basis of seismic evidence.

Dietz made the first proffer (subsequently confirmed) that the oceanic deep-sea hills are a chaos topography, adult when strips of juvenile sea floor have ruptured under stress as the floors move outward. Finally, he referred to two papers in press, one by Victor Vacquier et al., and 1 by Ronald Bricklayer and Arthur Raff, reporting the discovery of linear magnetic anomalies on the Pacific flooring. Some of the magnetic lineations appeared to exist kickoff by up to 1185 km along the Mendocino fracture zone. Dietz suggested that the lineations are developed normal to the direction of convective pitter-patter of the ocean floor. Noting that neither the fracture zones nor the magnetic lineations impinge upon the continental margins, he suggested that both lost their identity equally the Pacific flooring slipped beneath the American continents.

Rarely are ideas subsequently seen as basic to a g new system of thought in science, appreciated at full value when they first appear. That was the instance with body of water-floor spreading, which failed to grab the attention of more than a few readers at a fourth dimension when most geologists and geophysicists were unprepared to have seriously the idea of mantle convection, much less that of the new notion of spreading seafloors. However, Dietz's paper elicited a favourable letter to Nature from J Tuzo Wilson at the University of Toronto, and information technology inspired Ewing to redirect a big portion of Lamont's research into testing the ocean-floor spreading hypothesis. Ewing outfitted two ships with upgraded seismic reflection profilers to measure out the thickness of pelagic sediments across the oceans. He was to discover well-nigh no sediments on the ridges and a small thickening of the layers towards the edges of the continental shelves. Ewing took the earliest photographs of deep sea sediments and observed ripple marks, which, until then, had been used as diagnostic of shallow waters.

In the next few years, as data favourable to seafloor spreading accumulated, a regrettable negative reaction developed toward Dietz. Even though his model differed from that of Hess, the belief spread that he had 'stolen' Hess's basic idea and rushed information technology into print nether his own name. This impression nonetheless persists to some degree, even though Dietz's choice of basaltic rather than serpentinized ocean floors is the ane universally accepted today.

In 1986, Menard reviewed this controversy in his The Sea of Truth, a personal business relationship of the sequence of events that led to plate tectonics. Menard, who had refereed the pre-publication manuscripts of both Hess and Dietz, wrote that he felt certain – and said and then at the time – that Dietz was unaware of Hess'southward preprint when he wrote his own newspaper. All the same, Menard believed that merely 1 person can accept priority for an idea and it should be Hess, then he urged Dietz, if only for appearances sake, to add a footnote to his adjacent publication on seafloor spreading conceding credit for the thought to Hess. Dietz did and then, and the two men made their peace in print. Subsequently, Menard came to realise that, in fact, more than than one person can be struck with the aforementioned idea, particularly at a time when new information are coming in and are being freely discussed. He cited several boosted examples that took identify during the race towards plate tectonics.

Finally, Menard remarked that the priority for this thought probably should go to Arthur Holmes in Great britain, who had favoured convection in the curtain as a ruling cistron in global tectonics from the late 1920s until he died in 1965. Holmes described a basaltic layer which becomes a kind of endless travelling belt equally it moves from ridges to trenches carrying continental fragments along with it. Over the years, Holmes inverse his diagrams somewhat, but they all prove the limbs of a convection prison cell rising under a continental slab, which is stretched and pulled apart into 2 fragments that are rafted to either side leaving behind new ocean floors. Holmes did not depict the creation of new ocean floor at a spreading oceanic ridge; he showed the ridges equally existence wholly or partially sialic. In that important respect his idea differed from those of Hess and Dietz. Even so, Holmes'south basic model was, without question, a predecessor of sea-flooring spreading.

History books do not necessarily aid u.s. in resolving disputes. In 1973, Allan Cox of the US Geological Survey omitted Dietz'due south article from his collection of the landmark papers that led to plate tectonics. And in the entry on Hess in Volume 17 of The Lexicon of Scientific Biography, published in 1990, we read that his hypothesis of sea-flooring spreading was the most important innovation leading to plate tectonics, simply that it was given its name by Dietz who, "with Hess's preprint in hand", published the first commodity on information technology in 1961. Fortunately, others knew better. In 1966, the Geological Society of America would present its highest laurels, the Penrose Medal, to Hess for his research on the petrology of ultramafic rocks and for his provocative tectonic hypotheses including that of the spreading ocean floor. In 1988, the Penrose Medal went to Dietz for his "world-class, innovative contributions in three divisions of the geosciences: ocean-floor spreading, recognition of terrestrial bear on structures, and the meteorite touch of the Moon'south surface".

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Seismology and the Structure of the World

R.A. Dunn , D.W. Forsyth , in Treatise on Geophysics, 2007

one.12.i Depth Extent of Anomalous Construction

I of the early geodynamic questions nearly mid-bounding main ridges was whether the upwelling is passive (driven simply by the viscous drag of separating plates) or active (a buoyantly rising limb of a convection jail cell). If convection is driven largely by cooling from above, then the downwelling limb, in the form of the sinking lithospheric slabs, is expected to exist cooler than its surroundings and to extend deep into the curtain. If driven past heating from below, every bit from heat released from the core, then there should likewise be hot, upwelling limbs rising from deep in the drapery. In that location are a multifariousness of geological observations that favor dominantly passive mid-ocean ridges, such as the relatively compatible thickness and limerick of the oceanic crust, and seismological studies accept clearly shown that there is no deep, low-velocity root beneath typical spreading centers corresponding to a hot, upwelling limb.

The anomalous seismic velocity structure beneath typical mid-sea ridges appears to extend no deeper than most 200 km, except perhaps in the vicinity of a hot spot like Iceland. There are several lines of evidence. The thickness of the transition zone between the 410 and 660 km-depth discontinuities is one indicator of temperature of the upper mantle, because the 2 discontinuities are governed by phase changes with Clapeyron slopes of opposite sign. Global maps of the topography of these discontinuities based on precursors to SS resulting from underside reflections (SdS phases) show a correlation of greater transition-zone thickness with the location of subduction zones, indicating colder temperatures in the vicinity of descending slabs, but in that location is no correlation of transition-zone thickness with mid-ocean ridges (Flanagan and Shearer, 1998; Shearer, 2000). A local study of P-to-SV conversions at these discontinuities beneath the Due east Pacific Ascent (EPR) using receiver role analysis of data recorded on an array of ocean-lesser seismometers (OBSs) found that the time deviation betwixt the P410s and P660s conversions is most exactly equal to the global average (Shen et al., 1998), indicating that any thermal anomaly is bars to the upper drapery above the 410-km discontinuity. A model study of multiple Due south phases in the vicinity of the EPR also found no vertical displacement of either the 410- or 660-km discontinuities (Melbourne and Helmberger, 2002).

Global or regional tomographic studies based on variations in body-wave traveltimes sometimes show mid-ocean ridge structure extending downwards to depths of 300–400 km (Masters et al., 2000), merely, because ray paths in the upper mantle are relatively steep, there typically is a trade-off between amplitude of the anomaly and the depth extent. If velocity variations in the inversion of traveltimes to structure are damped to prevent unrealistic oscillations, the tendency is to force the model to smear big-amplitude, shallow anomalies beyond their actual depth extent. For the Mantle Electromagnetic and Tomography (MELT) Experiment crossing the EPR, a 'squeezing' test was performed in which model anomalies were progressively limited to shallower depths. The variation in quality of fit showed that anomalous structure extends to at least 200 km depth (Hammond and Toomey, 2003), but probably no deeper than 300 km ( Figure 1 ). Global or regional tomographic models based on Rayleigh surface waves or surface and body waves combined (Nishimura and Forsyth, 1989; Masters et al., 2000; Ekstrom, 2000; Ritzwoller et al., 2004; Ritsema, 2005; Priestley and McKenzie, 2006; Maggi et al., 2006a; Zhou et al., 2006) observe that anomalous structure in the vicinity of nearly ridges is confined to the uppermost 150–200 km of the mantle ( Figure 2 ).

Global studies employing Beloved waves, which have relatively poor depth resolution in the mantle, sometimes study anomalies extending several hundred kilometers beneath spreading centers (Gung et al., 2003; Gu et al., 2005a; Zhou et al., 2006), in dissimilarity to tomographic images with Rayleigh waves, which have much better vertical resolution ( Figure iii ). Vertical smearing due to the poor Love-moving ridge resolution could be responsible for the discrepancy, simply synthetic tests advise that the departure is real, indicating that polarization anisotropy reverses from the usual SH > SV in shallow oceanic mantle to SH < SV at depths greater than 200 km beneath mid-ocean ridges. (Love waves are sensitive to horizontally propagating, horizontally polarized Southward waves, or SH, while Rayleigh waves are sensitive to horizontally propagating, vertically polarized S waves, or SV.) Anisotropy in the oceanic mantle is thought to be caused primarily by the lattice-preferred orientation of olivine crystals; SH < SV may bespeak vertical upwelling to depths on the social club of 300 km (Gu et al., 2005a; Zhou et al., 2006), merely the absence of an accompanying Rayleigh-wave anomaly or a deflection of the 410-km aperture indicates that the upwelling is probably passively induced with no accompanying thermal anomaly.

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The Crust

P.A. Candela , in Treatise on Geochemistry, 2003

iii.12.1.4 The Terrestrial Heat Engine

The cardinal engine of the Earth's endogenous fluid advection system is the irreversible loss of rut from the interior of the planet to the surface, where it is ultimately radiated into infinite. The idealized Rayleigh–Bernard convection prison cell that we can use to model convection of Earth systems tin can be crudely represented as a Carnot Cycle. At the bottom of the convection jail cell, matter is heated and consequently expands; the fluid buoyantly rises and cools adiabatically upon expansion, because information technology has been removed physically from the heat source. In this step, the fluid does "P–V" (expansion) work, as well equally work against gravity. To complete the cycle, the fluid loses heat at a cold (normally upper) boundary, contracts, and consequently sinks. During its downward menses, the fluid is compressed, and also loses gravitational potential energy. The working fluids in "solid–globe" convection systems may be solid drapery, salt (and other sediments), magma, or geothermal fluids; farther, upwardly motion of i fluid (e.chiliad., salt) may exist coupled with the downward move of other "fluids" (e.thousand., sediment). We tin can consider the Earth to comprise nested Carnot-convection engines, including the grand convection arrangement of the chaff and pall. The upwelling of magmas and related fluids into the crust can result in the generation of crustally derived magmas. The mantle- and lower crustal-derived magmas are themselves buoyant in the chaff, and transport matter to the mid- and upper crust. While in the crust, these magmas can drive the advection and upward transport of magmatic water and locally derived metamorphic water. Further, deep-crustal fluids, also as more shallowly circulating meteoric fluids are commonly driven to convect, creating large-calibration element redistribution systems (come across Chapter three.06 ).

Convection may ship crustal fluids and their components reversibly, i.e., with no internet transfer of fluid or fluid components from 1 region to another. Alternatively, net transfer of matter may be effected by changes in phase (e.g., solidification of a magma) or by changes in solubility of some fluid-solute components due to changes in temperature, or pressure level, or by chemical reaction along the flow path. For minerals that increase in aqueous solubility with increasing temperature (i.e., those with prograde solubility), there is a net transfer of solute materials from high temperature to low temperature. This is the case for quartz (except for some restricted regions of $P – T$ space) and results in the web of quartz veins that are and then common in the upper continental crust. Yet, some minerals, such as anhydrite, decease in solubility with increasing temperature. This phenomenon, referred to commonly as retrograde solubility, results in the precipitation of minerals in the hotter portions of aqueous convective systems (run into too Affiliate three.eighteen). For this reason, oxidized sulfur is precipitated at depth in the oceanic crust. Chemic reaction tin fix materials at higher or lower levels. For instance, significant concentrations of HCl can occur in magmatic volatiles, and can go reactive upon cooling. Further, magmatic volatiles can as well comprise SO₂, which upon disproportionation yields H₂Southward past the reaction

(two) $four H_{2} O + iv {And so}_{2} \to 3 H_{2} {SO}_{iv} + H_{two} Southward$

promoting the fixation of metallic sulfides in magmatic–hydrothermal systems.

The potential for focusing fluid flow around a betoken estrus source such every bit a magma, with or without significant outward advection of magmatic volatiles, provides for strong lateral (i.e., longitudinal and latitudinal) concentration of ore metals, alteration, and associated chemical and isotopic anomalies. Focusing of geothermal fluids appears to be a critical link in the creation of hydrothermal ores. Unfocused hydrothermal fluid menstruation produces just geothermal systems, non ore deposits. This focusing is of import and is shown by the mutual creation of large positive geochemical anomalies relative to the rather subdued magnitude of the associated negative anomalies (although in the report of ores as geochemical entities, we must always be cognizant of the inherent economic bias toward the report of not only higher grade and tonnage deposits, but toward positive as opposed to negative anomalies). The presence of cupolas and other apophyses in the roof zones of magma chambers creates thermal point sources that can focus fluid catamenia. Structures such as releasing bends (Henley and Berger, 2000) human action to focus upward surges of magma, forming vertically elongated, high aspect ratio cupolas (Sutherland-Dark-brown, 1976). These weather condition obtain most efficiently in local environments of dilation in otherwise compressional regimes. In the New Guinea Fold Chugalug, at least four major magmatic–hydrothermal Cu–Au (Ag) deposits (Grasberg, Porgera, Ok Tedi, and Frieda River) are located forth zones of local dilation. Loma et al. (2002) proposed that, during orogenesis and crustal thickening, strike-skid motion occurred forth fracture zones roughly parallel to the convergence vector. Localized zones of dilation opened where these fracture zones intersected major orthogonal faults and other structural discontinuities, facilitating igneous intrusion and subsequent mineralization. The up advection and subsequent emplacement of magma in a favorable structural surround led to aqueous fluid advection and focusing, and ultimately chemical element redistribution. Finally, nigh hydrothermal ore formation tin be thought of as a type of metamorphism that results in element redistribution. Whereas the hydrothermal alteration commonly associated with hydrothermal anomalies is metasomatic in nature, metamorphism sensu stricto tin also be important in ore formation. In the example of greenstone and turbidite hosted mesothermal aureate deposits, there is a suggested link betwixt regional metamorphism and the generation of vein-hosted gold mineralization. Metamorphism, together with deformation, can provide a source of fluid, a source of metals, a source of focusing, or all three.

The confluence of thermally induced density variations brought about past melting in the lower crust and upper mantle, together with the structural/tectonic filters operating in the Earth'south brittle upper chaff, generally provide the virtually-point sources of heat and affair required for focusing hydrothermal fluid flow. However, whether fed from underlying magma, metamorphic dewatering, or circulating meteoric water, fluid focusing is not sufficient for the germination of a mineralized system. Hobbs and Ord (1997) point out that fluid focusing is ineffective if the fluid-menses vectors are at low angles relative to isotherms and isobars. However, magmas and other fluids may exist advected almost adiabatically into the upper crust. Only when high potential gradients are present, as when fluids are brought into close proximity with the surface, volition they menstruum at high angles to isotherms. Farther, magmas at lithostatic pressure may exist emplaced into rocks wherein the ambient fluids in fractures are most hydrostatic. Under conditions of strong thermal and mechanical gradients, pregnant changes in solubility of ore and gangue minerals can occur. Of grade, strong gradients in chemical potential can as well bring about mineral precipitation. Chemical mixing of different fluids or interaction of fluids with rocks with which they are strongly out of equilibrium effect in strong gradients of chemical potential (due east.yard., the formation of skarns at intrusion/limestone contacts). Generally, the combination of thermally induced density changes in a gravity field, together with strong gradients in thermal, mechanical, and chemical potentials, bulldoze hydrothermal chemical element redistribution in the chaff. In the next section of this chapter, I will hash out the general bug related to changes in temperature, pressure, and chemic composition affecting the ship and precipitation of ore metals in hydrothermal systems.

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PLATE TECTONICS

R.C. Searle , in Encyclopedia of Geology, 2005

Plates as Parts of the Mantle Convection Bicycle

Plate motions are ultimately driven past the Earth's heat energy, and they are intimately related to the mantle convection that is driven by this estrus. Ane view of plates is that they simply correspond the surficial parts of mantle convection cells: as hot, ductile drapery rises to the surface, it cools and becomes brittle – a plate – and then moves every bit a rigid cake over the surface before beingness subducted, gaining temperature and becoming ductile again. Recent results from seismic tomography advise that, around the rim of the Pacific, sheets of cold material descend below subduction zones deep into the lower mantle, implying a stiff coupling of mantle movement and subducted plates.

However, the coupling is not perfect. There are some parts of the mid-bounding main ridge (divergent plate boundaries) where it seems that the deeper mantle (beneath the asthenosphere) may exist descending rather than rising. I such place is the so-called Australo-Antarctic Discordance s of Australia. Moreover, some plates, such equally Africa, are well-nigh entirely surrounded by ridges and take very few subduction zones on their boundaries. In such cases, a rigid coupling of plates to convection cells would imply the unusual scenario of upwelling along an expanding ring, with a downwelling column inside it. In fact, 1 of the advantages of plate tectonics is that information technology allows partial decoupling of plate motions from deeper mantle flow via the ductile asthenosphere.

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Bacon Wisithir1961

Where in Earth's Interior Can Convection Cells Be Found?

Convection Cells

Mantle Dynamics

7.02.vi.vii A Complex Lithosphere: Plates and Continents

Mantle Dynamics

vii.10.ii.3.two Steady-country 2-D flows

Auroral structures: Revealing the importance of meso-calibration M-I coupling

2.iv.iii Omega bands and DAPS

Drape Dynamics

7.07.2.i A Historical Perspective

Auroral geospace

3.one.1.three Plasma convection

Energetic particle dynamics, precipitation, and conductivity

iv.2.1.1 Big-scale convection

HISTORY OF GEOLOGY SINCE 1962

Sea-floor Spreading: 1960 and 1961

Seismology and the Structure of the World

one.12.i Depth Extent of Anomalous Construction

The Crust

iii.12.1.4 The Terrestrial Heat Engine

PLATE TECTONICS

Plates as Parts of the Mantle Convection Bicycle

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