Hot Spot Effect Research Articles

Ballast gas for heat transfer control in fixed-bed reactors

Controlled oxidation reactions of hydrocarbons are practiced today in fixed-bed, fluid bed or transport bed reactors with oxygen or air as the oxidant. In fixed-bed reactors, due to the exothermicity of the oxidation reactions, heat removal and temperature control are critical in achieving safe and optimum reaction conditions that maximize conversion and selectivity. Industrial reactors often control hot spot formation by using an appropriate diluent (ballast) gas. Steam, nitrogen, methane and carbon dioxide are the most commonly used ballast gases. The scope of this paper is to elucidate the benefits of ballast gas use as a tool to optimize heat transfer and reduce the hot spot effect in controlled oxidations with fixed-bed reactors. Ballast gas does not participate in the reaction, but it is used to control heat removal and flammability. A homogeneous one-dimensional reactor model was used to study and compare two oxidation processes: (1) ethylene oxidation to ethylene oxide and (2) o-xylene oxidation to phthalic anhydride. For both processes hot spot temperature and conversion decreased as methane replaced nitrogen in the process ballast gas. Selectivity increased in the ethylene oxide case but decreased in the phthalic anhydride case, indicating that ballast gas effect on selectivity depends on the reaction mechanism. Thus, there is an optimum ballast gas composition that will optimize hot spot temperature and reactor yield.

METHOD OF SPHERICAL HARMONICS IN THE RADIATIVE TRANSFER PROBLEM WITH NON-LAMBERTIAN SURFACE

The paper reports on the spherical harmonics method (MSH) for solving the radiative transfer problem in the vertically inhomogeneous atmosphere bounded by a non-Lambertian surface. The surface bi-directional reflectance function is assumed to be symmetrical with respect to the principal plane, otherwise it can be of an arbitrary shape and can include directional signatures as a hot spot effect or a specular reflection. The lower boundary conditions, taken in the form of Marshak, are represented analytically on the basis of recurrent formulas obtained here. For calculating the angular structures of radiation fields, the new computationally efficient smoothing procedure is developed based on the correction function of MSH. It enables one to essentially reduce the order of MSH preserving the high accuracy of solution. In conjunction with the predominantly analytical way of solving the differential system of equations of MSH, this combination allows one to achieve exceptionally high speed of computations.

Dynamic scheme for reducing hot-spot effects in multipath networks

The presence of hot spots in a multistage interconnection network (MIN) can impair message transmission, increasing average delay time and jeopardising system performance. To reduce the hot-spot impact in the cyclic gamma interconnection network, a MIN design with multiple disjoint paths between every communication pair, a static routing scheme is presented to trim down average system delay time when the majority of message patterns is asynchronous. To further reduce average system delay time regardless of transmission patterns, a dynamic routing and rerouting scheme is proposed. Extensive simulation has been conducted to evaluate the performance of the proposed schemes and some other schemes. Results show that the proposed dynamic scheme can reduce the hot-spot impact more effectively at no extra cost.

Coexistence of the hot-spot effect and flux-flow instability in high-Tcsuperconducting films

Voltage jumps were observed in current-voltage $(I\ensuremath{-}V)$ characteristics of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ superconducting film. The measurements demonstrate that the observed voltage instability originates from two different mechanisms, depending on the temperature and the magnetic field. This is inferred from the shape of the $I\ensuremath{-}V$ characteristics, the temperature and magnetic-field dependencies of the critical current, and the critical voltage associated to the voltage jumps. Relevant models indicate that the voltage jumps at low temperatures are due to the hot-spot effect, whereas those at temperatures near ${T}_{c}$ are due to flux-flow instabilities.

Seismic anisotropy of oceanic upper mantle: Shear wave splitting methodologies and observations

We develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy. To obtain high‐quality estimates of receiver splitting at island stations, we devise a stacking method that finds the optimum splitting parameters and 95% error region for teleseismic shear‐wave phases from a suite of earthquake events. To obtain additional measurements in oceanic regions, we develop techniques to measure splitting parameters and errors for SS phases that sample upper mantle anisotropy at their bounce points. However, we find the data are often of low resolution, and anomalous characteristics are sometimes found that make splitting difficult to interpret. Ten second splitting of SS is observed across the BANJO seismic array, but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce. The receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the Pacific region, with fast polarization azimuth (ϕ) in the lithosphere oriented in the fossil spreading direction and ϕ in the asthenosphere oriented in the absolute plate motion direction. This model has been proposed to explain surface wave data in the Pacific Ocean, but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted. While the island splitting measurements could reflect the influence of individual hotspot upwellings, hotspot effects do not appear to be universally dominant. We propose that splitting observations alternatively indicate broad‐scale differences in the underlying character of oceanic upper mantle anisotropy, associated with coherant patters of lithospheric structure and asthenospheric flow. In particular, splitting, surface wave models, and regional studies all support a model where lithospheric anisotropy throughout the South Pacific has been erased or reoriented toward the absolute plate motion direction, whereas more limited observations in the North Pacific indicate that the fossil lithospheric signature appears to be preserved.

The Sensitivity of the OSAVI Vegetation Index to Observational Parameters

OSAVI is a soil-adjusted vegetation index optimized for agricultural monitoring. The SAIL–SOILSPECT–PROSPECT–KUUSK model was applied to test the sensitivity of OSAVI to a number of observational and environmental parameters, with the constraint that the parameter ranges were chosen to be representative of conditions encountered during operational monitoring by SPOT and Landsat of the U.K. sugar beet crop in the summer months. Within these defined limits, variations in solar and viewing angles introduced no more than 4% systematic (and rectifiable) error in estimation of crop canopy cover while errors associated with atmospheric correction were of a similar magnitude. Although sugar beet has large leaves close to the ground and was expected to generate a large hot spot effect, the effect of uncertainty in leaf size appears to be negligible. Leaf angle distribution is effectively accounted for by the use of crop cover as an index of productivity, except when the crop is wilting, but this is unlikely to occur in sugar beet at the time of satellite overpass. Subject to the uncertainties of modeling, the estimation of crop canopy cover within an absolute error of 5% appears to be achievable.

Distributions of Chemical Composition for Ancient and Current MORB: Consequences and Perspectives

We compare the chemical composition, back-tracted depth and crustal thickness of crust formed more than 80 m.y. ago with the zero age crust that is being produced today. Through this comparison, we can evaluate whether the steady state assumption of geophysical depth-age modelling is justified, and whether there have been large scale changes in the distribution of ridge depths with time. It is important to note that the appropriate comparison is between the isochronous distributions of depth and chemistry. There is not a single depth of ocean ridges today, nor is there a single chemical composition. Therefore the critical question is not whether there is some analogue for some ocean ridge today vs. some ocean ridge in the past, but instead whether there is a statistically significant difference in the distributions of depth and chemistry between the global system of ridges today and that in the Mesozoic. The twenty years of drilling by the ocean drilling project have provided a significant number of basement samples from older ocean crust. However, these samples are mostly not directly comparable to the zero-age data set of dredged glasses from active ridges, and hence interpretation of their chemical composition requires some care: (1) many sites are located on bathymetric highs that were created off-axis, or were created by nearridge hot spots. While these sites may be important for the overall distribution of depths, their chemistry is often anomalous. The data we have considered have been selected in order to avoid a hot spot effect, first by eliminating all holes on shallow crust with La/Sm N greater than 1, and then by eliminating those sites which appear to have come from hot spot margins, (2) despite the pervasive alteration, some old holes have small amounts of glass for which major element compositions have been determined that are directly comparable to glass compositions for zero age crust. However, when glasses analyses are not available, it is necessary to find proxies for major elements that are relatively unaffected by alteration. Many studies over the years have shown, that for modest degrees of alteration, the high field strengh elements and the R E E are generally minimally affected by alteration. This is particularly true for ratios among these elements, since any dilution effects by addition of water and alteration minerals will have even less effect on the ratios of unaffected elements than on the absolute concentrations; (3) there is also the issue of quality of measurement and the fact that most drill core analyses are carried out on whole rocks. The phenocryst content of the whole rock has a major effect on parameters such as Na8.0 and Fe8.0. These various criteria lead to our selection of sites and data for investigation of this problem. Despite that the elimination of data is far greater than the data we are able to include, the final data set is well distributed around the globe, and include multiple sites on old crust in the Atlantic, Pacific and Indian ocean basins. Comparison of the chemical and crustal distributions between ancient and current MORB show that the oceanic crust older than 80 m.y. exhibit statistically lower Na8.0, Zr/Y, Sm/Yb N and higher Fe8.0 and crustal thickness (Fig. 1). These systematics suggest that the mantle was hotter in this time period by about 50~ that the zero age crust was several hundred meters shallower and the crust 1-2 km thicker (Humler et al; 1998). Data from ocean ridges today suggest that there are temperature variations in the upper mantle of as much as 240~ from the hottest to the coldest ridges (Klein and Langmuir, 1987). Within this large amplitude of thermal variation within the earth, there is clearly room for significant temporal changes in the ambient temperature of the upper mantle. The petrological results from the old drillsites suggest that

Interconnection network front-end controller combining to reduce hot spots effects

Multistage interconnection networks are very promising for shared-memory multiprocessor systems. These networks offer flexibility, scalability, and good performance-cost ratio. However, under a non-uniform traffic pattern, the performance of multistage interconnection networks suffers greatly because of hot spot traffic contention. The potential performance degradation due to even moderate hot spot traffic was found to be very significant, severely reducing all memory access, not just to shared locations [1]. Several techniques have been proposed to reduce the effects of hot spot and tree saturation contentions in multistage interconnection networks. These include multibuffered switching nodes, multipath networks, and request combining [2]. Request combining strategy was found to be an effective method of reducing the tree saturation problem [1]. Some request combining approaches have been proposed [1,3–5]. This paper proposes a new request combining based architecture to reduce the hot spot performance degradation. This approach is referred to as interconnection network front-end controller combining (IN-FEC). In IN-FEC, the interconnection network combines requests to the shared memory location, and the memory front-end controllers are used to decombine the combined request and distribute the results. Simulation results show considerable throughput enhancement under IN-FEC for an Omega network.

Ergodization of the magnetic field at the plasma edge by the dynamic ergodic divertor of TEXTOR 94

The structure of the resonant helical magnetic field of the dynamic ergodic divertor (DED) which is planned for TEXTOR is presented. The ergodic features are investigated by field line tracing calculations by the Gourdon code. The island widths and the related Chirikov parameters are determined by Poincaré plots. The deduced values are smaller than the ones predicted by the standard theory and a correction of the standard theory is presented. The onset of stochastization by the nonlinear interaction of neighboring island chains as well as remnant islands and barriers for field line diffusion at the maximum perturbation level, for which the Chirikov parameter is slightly above 1, are shown. The effect of hot spots sweeping over the limiter surface due to the rotation of the perturbation field is demonstrated. Local effects of the helical field and the divergence of a bundle of field lines are shortly worked out. Poincaré plots for lower mode operation are shown, which can penetrate deeper into the plasma to act at the q=2 surface. The accuracy of the numerical tool is demonstrated to be sufficient for these problems.

Calculations of the soil hot-spot effect using the coherent backscattering theory

The shadowing theory has been widely used to account for the hot-spot effect for the canopy and soil. However, it is not valid for soils that are composed of fine particles and do not have well-defined shadows. The coherent backscattering theory is used in this study to calculate both the magnitude and the angular width of the hot-spot peak for clay and silt soils. The soil hot-spot magnitude is calculated by solving the vector radiative transfer equation. The dense nature of the soil medium is taken into account in the calculations of the hot-spot angular width. The results are also linked with the Hapke hot-spot empirical function. The effects of the wavelength dependence and of particle size and shape also are exainined.

The effects of grain boundary diffusion anisotropy on via electromigration failure

Electromigration performance of multilevel interconnect vias is investigated using a three-dimensional computer model. The model uses the finite-element method to obtain self-consistently the temperature and current density distributions in order to calculate electromigration fluxes. The model includes the polycrystalline grain structure of the tracks as well as stress-migration and concentration gradient backfluxes. While in single level systems, failure can be analyzed with two-dimensional models because the fluxes are homogeneous, the inclusion of the third dimension along the track thickness is necessary for multilevel systems. In addition to the effects of hot spots, current crowding, and microstructure, it is found that the anisotropy of the grain boundary diffusion plays an important role in determining the locations of void formation. The microstructural details of the track at the interface with the via, including grain boundary geometry, are very important for electromigration studies.

A semidiscrete model for the scattering of light by vegetation

An advanced bidirectional reflectance factor model is developed to account for the architectural effects exhibited by homogeneous vegetation canopies for the first orders of light scattering. The characterization of the canopy allows the simulation of the relevant scattering processes as a function of the number, size, and orientation of the leaves, as well as the total height of the canopy. A turbid medium approach is used to represent the contribution to the total reflectance due to the light scattering at orders higher than 1. This model therefore incorporates two previously separate approaches to the problem of describing light scattering in plant canopies and enhances existing models relying on parameterized formulae to account for the hot spot effect in the extinction coefficient. Simulation results using this model compare quite favorably with those produced with a Monte Carlo ray‐tracing model for a variety of vegetation cases. The semidiscrete model is also inverted against a well‐documented data set of bidirectional reflectance factors taken over a soybean canopy. It is shown that the inversion of the model against a small subset of these measurements leads to reasonable values for the retrieved canopy parameters. These values are used in a direct mode to simulate the bidirectional reflectance factors for solar and viewing conditions significantly different from those available in the subset of soybean data and compared with the full set of actual measurements.

FFT Thermoelastic Solutions for Moving Heat Sources

An analytical frequency domain solution is obtained using the spatial Fourier transform for thermal and thermoelastic fields due to an arbitrary heat source or thermal distribution moving at constant speed over the surface of an insulated, traction free elastic half space. Conversions between the space and frequency domains for the input and output are performed efficiently and robustly using FFT techniques. The method is validated by comparison to the analytical result for the moving line heat source in which it is shown that numerical evaluation of the analytical solution is problematic for large speeds or distances from the heat source. The utility of the method is illustrated on the constant patch moving heat source and discretely distributed multiple heat sources known as the “hot spot” problem. It is shown, through several examples, that the effect of hot spots on surface displacement and tangential stress is small. Finally, this conclusion is generalized by quantifying the frequency domain solution for the moving heat source problem as a low pass filter.

The hotspot effect in heterogeneous vegetation canopies and performances of various hotspot models

In an earlier paper (Qin and Goel, 1995), we reviewed various analytical hotspot models and evaluated them for homogeneous vegetation canopies by comparing them against Diana—a computer graphics, radiosity based, model. In this paper, we complete their evaluation by studying the hotspot effect of heterogeneous canopies (trees and crops). For this purpose, we first describe a computer graphics based method with which the cross‐correlation (or overlap) function that drives the hotspot effect can be accurately evaluated. We find that all the analytical hotspot models, more or less, overestimate the cross‐correlation function for tree canopies. Except the hybrid model, they perform worse in the principal cone (PC) than in the principal plane (PP); for big leaves or at low LAI or small spacing than for small leaves or at high LAI or large spacing. We investigate in detail various elements of heterogeneity (spacing distance between trees/plants, mutual shadowing between leaves and between tree crowns, crown porosity (within‐crown gaps), row structure (for crops), fraction of nonleaf organs, and nonrandom dispersion of vegetation elements etc.), and identify their influences on the canopy hotspot effect. The result shows that mutual shadowing, which is quite significant at high LAI and/or small spacing or under high solar/view zenith in PP, reduces the canopy hotspot and causes cross‐correlation function to decrease with LAI (for a fixed leaf size) rather than increase with LAI when mutual shadowing is neglected. The effect of spacing on the cross‐correlation function is significant if crown is used as the unit and insignificant (except for low LAI at low solar zenith angle) if leaf is used as the unit and LAI is kept fixed when spacing varies. At low LAI and/or small spacing (for a fixed LAI), crown porosity strongly affects the overlap function that uses crown as the unit, and causes geometric‐optics based hotspot models to considerably overestimate the overlap function. The large proportion of nonleaf elements in a canopy with small spacing and low or medium LAI often results in an asymmetric distribution of the canopy hotspot in PP. The row structure in row‐planted crops causes the zenithal width of hotspot to depend on the angle between solar azimuth and row direction and some unusual fluctuations of the cross‐correlation function (even a secondary peak) in PC. We present one modified expression for cross‐correlation function to account for mutual shadowing which is neglected in existing analytical models using leaf as a unit, and another for overlap function to allow for within‐crown porosity which is neglected in hotspot models using crown as the unit. Both expressions improve the accuracy in representing hotspot phenomenon

Structural analysis of the segmentation of the Central Indian Ridge between 20�30?S and 25�30?S (Rodriguez Triple Junction)

The morphological characteristics of the segmentation of the Central Indian Ridge (CIR) from the Indian Ocean Triple Junction (25°30′S) to the Egeria Transform Fault system (20°30′S) are analyzed. The compilation of Sea Beam data from R/VSonne cruises SO43 and SO52, and R/VCharcot cruises Rodriguez 1 and 2 provides an almost continuous bathymetric coverage of a 450-km-long section of the ridge axis. The bathymetric data are combined with a GLORIA side-scan sonar swath to visualize the fabric of the ridge and complement the coverage in some areas. This section of the CIR has a full spreading rate of about 50 mm yr−1, increasing slightly from north to south. The morphology of the CIR is generally similar to that of a slow-spreading center, despite an intermediate spreading rate at these latitudes. The axis is marked by an axial valley 5–35 km wide and 500–1800 m deep, sometimes exhibiting a 100–600 m-high neovolcanic ridge. It is offset by only one 40km offset transform fault (at 22°40′S), and by nine second-order discontinuities, with offsets varying from 4 to 21 km, separating segments 28 to 85 km long. The bathymetry analysis and an empirical orthogonal function analysis performed on across-axis profiles reveal morphologic variations in the axis and the second-order discontinuities. The ridge axis deepens and the relief across the axial valley increases from north to south. The discontinuities observed south of 22°S all have morphologies similar to those of the slow-spreading Mid-Atlantic Ridge. North of 22°S, two discontinuities have map geometries that have not been observed previously on slow-spreading ridges. The axial valleys overlap, and their tips curve toward the adjacent segment. The overlap distance is 2 to 4 times greater than the offset. Based on these characteristics, these discontinuities resemble overlapping spreading centers (OSCs) described on the fast-spreading EPR. The evolution of one such discontinuity appears to decapitate a nearby segment, as observed for the evolution of some OSCs on the EPR. These morphological variations of the CIR axis may be explained by an increase in the crustal thickness in the north of the study area relative to the Triple Junction area. Variations in crustal thickness could be related to a broad bathymetric anomaly centered at 19°S, 65°E, which probably reflects the effect of the nearby Reunion hotspot, or an anomaly in the composition of the mantle beneath the ridge near 19°S. Other explanations for the morphological variations include the termination of the CIR at the Rodriguez Triple Junction or the kinematic evolution of the triple junction and its resultant lengthening of the CIR. These latter effects are more likely to account for the axial morphology near the Triple Junction than for the long-wavelength morphological variation.

An evaluation of hotspot models for vegetation canopies

Abstract Hotspot effect is a typical angular reflectance signature of vegetation canopies. In this paper, the basic principles for various physical models for the hotspot effect of vegetation canopies are reviewed. The performance of several analytical models for homogeneous canopies are evaluated by comparing with a computer‐graphics based model—Diana. Almost all of these models perform better for a canopy with smaller leaves, higher LAI or when the solar zenith angle is high. Among these models, the mixed model, which quantitatively takes into account of the leaf size, shape and orientation by adding a geometrical‐optical kernel to modeling of the hotspot for the continuous canopies, performs best in most cases. The influences of both canopy structure and sun geometry on hotspot effect, especially hotspot width, are examined and the topics for further study are suggested.

Ion-induced crystallization and amorphization at crystal/amorphous interfaces of silicon

New empirical equations describing the rate of ion-induced crystallization at a Si crystal/amorphous interface have been developed. In our model, crystallization/amorphization at the interface arises from formation of hot spots and of knock-ons in the collision cascades. It is presumed that the hot spots induce amorphous-to-crystal transformation which lowers the free energy, similarly to heating to high temperatures, and that the bond rearrangement by a series of displacements by knock-ons and recombination to the original lattice point in collision cascades can lead to both crystal-to-amorphous and amorphous-to-crystal transformations. In both hot-spot and knock-on effects, the presence of di-vacancies under irradiation with ion beams is assumed to prohibit crystallization. The model can explain the experimental observation that the crystallization/amorphization rate is scaled by X = ø 1 2 exp(C v /2kT) , the product of the root of the flux and the inverse of root of the Boltzmann factor for the motion of the di-vacancies. Crystallization rate in the hot-spots derived assuming that an incident ion induces spontaneous crystallization within a characteristic volume along the track reveals that the radius is 10 atomic distances and the thickness of is about 0.3 monolayer for 1.5 MeV Xe ions. The calculated crystallization/amorphization rate fits to experimental results over a wide temperature range.

Self-heating hotspot effects in HTS thin films

The current-voltage (I-V) characteristics associated with self-heating hotspots in HTS thin films patterned into several different geometries have been investigated under both current-biased and voltage-biased conditions. The hysteretic behavior of the I-V characteristics as a function of sample temperature are in agreement with behavior predicted by resistive hotspot models for superconducting bridges. Additionally, the effect of magnetic fields on the current required to generate, as well as maintain, a normal hotspot has been investigated. Finally, the characteristics of voltage pulses resulting from a step increase in current have been examined to study the propagation of hot spots in HTS films.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Gravity anomalies and crustal thickness variations along the Mid‐Atlantic Ridge between 33°N and 40°N

Multibeam bathymetry and gravity data have been obtained along an ∼800‐km‐long section of the Mid‐Atlantic Ridge from just south of the Hayes fracture zone at 33°N to the northern edge of the Azores Platform near 40°N. A three‐dimensional analysis of these gravity and topography data, combined with results from earlier seismic refraction studies in this area, reveal two different scales of crustal heterogeneity. A systematic, along‐axis, segment‐scale (λ&lt;20–100 km) variation in crustal thickness is present with the thickest crust (&gt;8–9 km) near the middle of spreading segments and the thinnest crust (&lt;3–4 km) near segment offsets. The magnitude of this along‐axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset. There also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs, inferred to be thinner crust, beneath the “inside corner” highs adjacent to these offsets. This segment‐scale crustal heterogeneity is similar to that reported from the Kane‐to‐Atlantis section of the Mid‐Atlantic Ridge and from parts of the intermediate‐spreading southern Mid‐Atlantic Ridge. It is superimposed on a second, longer wavelength variation in gravity and crustal thickness associated with the Azores hot spot. The most pronounced effect of the Azores hot spot on the Mid‐Atlantic Ridge occurs between 38°N and 40°N where the ridge axis rapidly shoals by more than 1000 m, the crust thickens by over 2 km, and the rift valley largely disappears. The absence of a deep axial rift valley on the Azores Platform, and near the midpoints of some ridge segments along the Mid‐Atlantic Ridge to the south, agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate. The transition from a rift valley to an axial high morphology at these spreading rates (∼11 mm/yr half rate) occurs at a crustal thickness of about 9±1km.

Mantle temperature anomalies along the past and paleoaxes of the Galápagos spreading center as inferred from gravity analyses

To better understand the effects of hot spots on mid‐ocean ridge thermal structure, we investigate the subsurface density structure of the Galápagos spreading center and nearby lithosphere. Using shipboard gravity and bathymetry data, we obtain maps of mantle Bouguer anomalies (MBA) by removing from the free‐air gravity the attractions of seafloor topography and a 6‐km‐thick model crust. Comparison of observed and theoretical MBA profiles along isochrons for ages 0.0–7.7 Ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle. This result is consistent with the notion that seafloor topography along the Galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. Along the ridge axis, the MBA decreases from the east and west toward the Galapagos hot spot by ∼90 mGal, reaching a minimum nearest the hot spot at 91°W. Seafloor topography mirrors the MBA along axis, increasing by ∼1.1 km toward the hot spot. These variations in MBA and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50±25°C toward the hot spot. The predicted crustal thickening of 2–4 km nearest the hot spot accounts for 70–75% of the along‐axis MBA and bathymetry anomalies; mantle density variations account for the rest of the anomalies. From the crustal isochron of age 7.7 Ma to the present‐day axis, the along‐isochron amplitudes of MBA decrease from ∼150 to ∼90 mGal. The corresponding along‐isochron bathymetry anomalies decrease from ∼1.7 to ∼1.1 km. These observations along the paleoaxes of the Galapagos spreading center indicate that the axial temperature anomaly was 70% hotter in the past (86±25°C) and has steadily decreased to 50±25°C as the ridge axis migrated away from the Galapagos hot spot. These along‐isochron temperature anomalies, however, have remained well below that estimated for the hot spot itself (200°C), indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10–20 times greater than that along the ridge axis over the past 7.7 m.y.