Hot Thermal Anomaly Research Articles

Deflected Mantle Flow and Shearing‐Aligned Lithospheric Melt Under the Strike‐Slip Dead Sea Rift

AbstractContinental rifting is one of the fundamental tectonics of the Earth evolution while our current understandings on the dynamic mechanism of the strike‐slip ones are relatively limited. Here, we have utilized three kinds of core‐refracted shear waves (including PKS, SKKS, and SKS) and employed the shear‐wave splitting technique to systematically investigate the azimuthal anisotropy of the upper mantle under the typical strike‐slip Dead Sea rift. There are a total of 1,855 well‐determined anisotropic measurements from 187 stations with dominantly N‐S fast orientations. We have proposed a new model from a joint analysis of multiple newly available geophysical observations to interpret the resulting anisotropy as mainly due to the absolute‐plate‐motion‐driven mantle flow deflected by the thick lithosphere of the eastern Arabian plate. Relatively larger splitting times are locally revealed at the rift zone and attributed to additional lithospheric anisotropy from the shearing‐oriented melt pockets whose existence further induces complex anisotropy with slight difference of fast orientations between the upper and lower layer anisotropy. The overall rift‐parallel fast orientations, when combined with the absence of low‐velocity and hot thermal anomalies in the mantle transition zone, rule out the role of mantle plume or edge‐driven convection in the rift development and further infer the Dead Sea rift to evolve in a passive mode.

Receiver function image of the mantle transition zone beneath western China: Fragmented subduction and counterflow upwelling

A uniform image of the mantle transition zone (MTZ) beneath western China and neighboring regions is produced through Variable Bin Radius Stacking of receiver functions. We utilized a large data set of 218,050 receiver functions from 1,991 broadband seismic stations. Our results, after 3-D velocity corrections, show significant lateral variations in topography of the 410- and 660-km discontinuity and thickness of the MTZ. The observed lateral variations of the MTZ correlate with seismic-velocity anomalies identified in independent tomographic studies, which are interpreted as cold and hot thermal anomalies from lithospheric downwellings and mantle upwellings, respectively. In the southern Tibetan Plateau, the MTZ topography reveals four segmented zones of up to ∼20 km thicker-than-average MTZ from west to east interfingered with regions of thin-to-normal MTZ. These segmented MTZ thickenings likely originate from a series of cold lithospheric fingers associated with the fragmented subduction of the Indian lithosphere, while the thin-to-normal MTZ may result from an absence of the subducting Indian slab. These observations provide novel evidence for the proposed fragmentation of the Indian subduction. Moreover, we observe regions of thickened MTZ under the Tien Shan orogen and the Qaidam block, which likely result from the foundering of cold delaminated/broken-off lithospheric blocks triggered by the underthrusting of the Junggar and Tarim blocks. Regions of thinned MTZ beneath the Tien Shan region are additionally observed, which could be attributed to counterflow upwellings.

The Dynamic Crust of Northern Afar and Adjacent Rift Margins: New Evidence From Receiver Function Analysis in Eritrea and Ethiopia

AbstractAfar is undergoing the final stages of continental rifting and hosts the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rifts. To better understand the nature of the crust and continental breakup in the region, we calculate teleseismic receiver functions across northeastern Afar and the Danakil microplate, using new data from a regional deployment in Eritrea. We estimate the Moho depth and bulk crustal VP/VS ratio using the H‐κ stacking method. The heterogeneity of our crustal thickness estimates (∼19–35 km) indicates that the Danakil microplate has undergone stretching and crustal thinning. By investigating the relationship between crustal thickness and topographic elevation, we estimate the regional crustal bulk density as ρc ≈ 2,850 ± 20 kg m−3, which is higher than expected, given the crustal thickness of the region. We show that topography is 1.5 ± 0.4 km higher than would be expected due to crustal isostasy alone. We propose that this topography is supported by the same hot mantle upwelling suggested to be responsible for the onset of rifting in East Africa. Uplift is generated due to the presence of a hot thermal anomaly beneath the plate and by thinning of the lithospheric mantle. Our results are consistent with a number of independent constraints on the thermal structure of the asthenospheric and lithospheric mantle. Evidence of melt within the crust is provided by anomalously high VP/VS ratios of >1.9, demonstrating that magma‐assisted extension continues to be important in the final stages of continental breakup.

Two million years of evolution of the southern central andes retroarc, Payenia Volcanic Province, Argentina, from the study of the Cerro Nevado Volcanic Complex

New K–Ar ages, major and trace element analyses, and Sr and Nd isotopic data for the Cerro Nevado Volcanic Complex (CNVC) coupled with those previously published allow us to better document the hot thermal anomaly which affected the upper mantle beneath the Andean back-arc Payenia Volcanic Province and generated alkaline magmatism. These data, along with previously published ones from this and neighbor volcanic centers, suggest that this anomaly stagnated there, during at least the last 2 million years. Geochemical features from the CNVC basaltic magmas indicate that they were sourced within an OIB-type mantle with an EMI signature, similar to that underlying the neighboring Auca Mahuida Shield Volcano to the south, and attributed to the rise of a mantle thermal anomaly. We identify at least five events of partial melting and seven eruptive phases from 1.88 to 0.75 Ma. Continental crust contamination in the San Rafael Block crustal basement occurred coupled with fractional crystallization processes, probably in magma chambers and across several cycles, producing the trachybasaltic, trachyandesitic and trachytic magmas of the Cerro Nevado volcanic cone (1.34–1.22 Ma).

Mantle Transition Zone Receiver Functions for Bermuda: Automation, Quality Control, and Interpretation

AbstractThe origin of the Bermuda rise remains ambiguous, despite, or perhaps because of, the existence of sometimes incongruous seismic wave‐speed and discontinuity models in the sub‐Bermudian mantle. Hence, whether Bermuda is the surface manifestation of a mantle plume remains in question. Using the largest data set of seismic records from Bermuda to date, we estimate radial receiver functions at the Global Seismographic Network station BBSR in multiple frequency bands, using iterative time‐domain deconvolution. Motivated by synthetic experiments using axisymmetric spectral‐element forward waveform modeling, we devise a quality metric for our receiver functions to aid in the automation and reproduction of mantle transition zone discontinuity studies. We interpret the complex signals we observe by considering the mineralogical controls on mantle transition zone discontinuity structure, and conclude that our results are likely to be indicative of a thicker than average mantle transition zone. Our result is incompatible with the canonical model of a whole mantle plume in an olivine dominated mantle; however, considerations of phase transitions in the garnet system would allow us to reconcile our observations with the possible presence of a through‐going hot thermal anomaly beneath Bermuda.

The Formation of Hot Thermal Anomalies in Cold Subduction‐Influenced Regions of Earth's Lowermost Mantle

AbstractThe Earth's lowermost mantle is characterized by two large low shear velocity provinces (LLSVPs). The regions outside the LLSVPs have been suggested to be strongly influenced by subducted slabs and, therefore, much colder than the LLSVPs. However, localized low‐velocity seismic anomalies have been detected in the subduction‐influenced regions, whose origin remains unclear. Here, three‐dimensional geodynamic calculations are performed, and they show that linear, ridge‐like hot thermal anomalies, or thermal ridges, form in the relatively cold, downwelling regions of the lowermost mantle. Like the formation of Richter rolls due to sublithosphere small‐scale convection (SSC), the thermal ridges form as a result of SSC from the basal thermal boundary layer and they extend in directions parallel to the surrounding mantle flow. The formation of thermal ridges in subduction regions of the lowermost mantle is very sensitive to the thermal structures of the subducted materials, and thermal heterogeneities brought to the bottom of the mantle by subducting slabs greatly promote the formation of thermal ridges. The formation of thermal ridges is also facilitated by the increase of core‐mantle boundary heat flux and vigor of lowermost mantle convection. The thermal ridges may explain the low‐velocity seismic anomalies outside of the LLSVPs in the lowermost mantle. The results suggest that the relatively cold, subduction‐influenced regions of the Earth's lowermost mantle may contain localized hot anomalies.

Mantle Transition Zone Thickness Beneath the Middle East: Evidence for Segmented Tethyan Slabs, Delaminated Lithosphere, and Lower Mantle Upwelling

AbstractWe use a very large seismic data set to provide a comprehensive image of the mantle transition zone (MTZ) beneath the Middle East. We utilized the technique of Common Conversion Point stacking of P wave receiver functions to investigate the topography on the 410‐ and 660‐km discontinuities defining the upper and lower boundaries of the MTZ. Our results show significant topography on the 410‐ and 660‐km discontinuities and corresponding variations in the MTZ thickness. The MTZ topography is broadly consistent with the results of seismic tomography studies, implying the presence of both cold thermal anomalies imparted by detached Tethyan slabs and lithospheric segments and hot thermal anomalies induced by upwelling of lower mantle material. The MTZ topography in the northern Middle East is dominated by the presence of patches of cold material that are intermittently separated by regions of hot to normal MTZ. Our results suggest that instead of a continuous slab, the Tethyan slab in the Middle East is strongly segmented along the strike of the subduction boundary. Furthermore, we find evidence for a significant gap in subduction extending from the eastern edge of the Cyprean arc to NW Iran. The southern Middle East is dominated by the processes related to the mantle upwelling beneath the Afar depression. Our results imply that buoyant lower mantle material enters the MTZ beneath the Afar depression and then spreads laterally to the northeast beneath the western Arabia, flowing within the MTZ and in the upper mantle.

Thermal infrared imaging of geothermal environments by UAV (unmanned aerial vehicle)

Unmanned aerial vehicles (UAVs) are increasingly being used for aerial surveys and monitoring of geothermal environments. Advances in UAV technology have made it a valuable option for project and environmental managers to consider. The many advantages of UAVs are encouraging users to employ these platforms in many different and new applications. In this study, we use a small (<2 kg), affordable quadcopter to safely and accurately map heat patterns of cryptic geothermal fumaroles. An aerial thermal infrared camera is used to map heat signatures of a geothermal feature in the Wairakei–Tauhara geothermal field near Taupo, New Zealand. The platform consists of a battery-powered quadcopter mounted with a FLIR Tau 320 camera and sensor. The 700 m2 area was flown and images mosaicked and processed using Pix4D and ArcGIS 10.2. The orthorectified image with pixel size of 0.5 cm showed a warm to hot thermal anomaly. A high level of resolution was achieved by flying at altitudes unsafe for conventional manned aircraft. Despite the battery life, navigation capabilities, and flight regulations limiting UAV application, remote sensing using UAV can revolutionize the exploration of geothermal thermal energy sources and the unique ecosystems they create.

The effect of thermal weakening and buoyancy forces on rift localization: Field evidences from the Gulf of Aden oblique rifting

Abstract On the basis of field and geophysical data, analog and numerical models, we here discuss the role of buoyancy forces arising from thickness variations in the lithosphere during rifting. In the Gulf of Aden, an oceanized Tertiary oblique rift, several successive directions of extension and associated normal faults suggest that transient stress rotations occurred during rifting. Especially, rift-parallel faults (070°E) overprinted the early divergence-perpendicular normal faults (110°E). Moreover, some first-order differences are noticeable between the western part of the Gulf, which deformed under the Afar hot spot influence, and the eastern part. In the western Gulf of Aden, the ocean–continent transition (OCT) and the oceanic ridge have cut obliquely through the inherited and reactivated Mesozoic basins (100°E to 140°E). The OCT trend is parallel to the overall Gulf trend (070°E). In the eastern part, the oceanization occurred within few syn-rift 110°E-trending basins and the OCT trends mostly perpendicular to the divergence direction. Here, we propose that this contrast is strongly controlled by the Afar hot spot: during rifting times, the hot spot likely induced a hot thermal anomaly in the western asthenosphere. This may have triggered both thermal buoyancy forces and thermal weakening of the lithosphere that helped localizing the rift obliquely. In such localized rift, rift-perpendicular trending crustal buoyancy forces (i.e. around 160°E) have enhanced rift-parallel normal faults (070°E) during final rift localization into a narrow zone strongly oblique to the early syn-rift basins. As a consequence of the Afar hot spot, in the west, the ridge is long and straight; in the east, the ridge segments are rather long too (although less than in the west) as the ridge initiated parallel to the OCT; in between, the ridge is more segmented as both the hot spot influence gradually decreases eastward and the ridge initiated obliquely to the OCT.

Bent-shaped plumes and horizontal channel flow beneath the 660 km discontinuity

Recent high-resolution seismic imaging of the transition zone topography beneath the Hawaiian archipelago shows strong evidence for a 1000 to 2000 km wide hot thermal anomaly ponding beneath the 660 km boundary west of Hawaii islands [Q. Cao et al. Seismic imaging of transition zone discontinuities suggests hot mantle west of Hawaii. Science (2011), 332, 1068–1071]. This scenario suggests that Hawaiian volcanism may not be caused by a stationary narrow plume rising from the core–mantle boundary but by hot plume material first held back beneath the 660 km discontinuity and then entrained under the transition zone before coming up to the surface. Using a cylindrical model of iso-chemical mantle convection with multiple phase transitions, we investigate the dynamical conditions for obtaining this peculiar plume morphology. Focusing on the role exerted by pressure-dependent thermodynamic and transport parameters, we show that a strong reduction of the coefficient of thermal expansion in the lower mantle and a viscosity hill at a depth of around 1800 km allow plumes to have enough focused buoyancy to reach and pass the 660 km depth interface. The lateral spreading of plumes near the top of the lower mantle manifests itself as a channel flow whose length is controlled by the viscosity contrast due to temperature variations ∆η T. For small values of ∆η T, broad and highly viscous plumes are generated that tend to pass through the transition zone relatively unperturbed. For higher values (10 2 ≤ ∆η T ≤ 10 3), we obtain horizontal channel flows beneath the 660 km boundary as long as 1500 km within a timescale that resembles that of Hawaiian hotspot activity. This finding could help to explain the origin of the broad hot anomaly observed west of Hawaii. For a normal thermal anomaly of 450 K associated with a lower mantle plume, we obtain activation energies of about 400 kJ/mol and 670 kJ/mol for ∆ ηT = 10 2 and 10 3, respectively, in good agreement with values based on lower mantle mineral physics. If an increase of the thermal conductivity with depth is also included, our model can predict both long channel flows beneath the 660 km discontinuity and also values of the radial velocity of local blobs sinking in the lower mantle of around 1 cm/yr, in good agreement with those inferred for the sinking rate of cold remnants of the subducted lithosphere.

The thermo-chemical and physical structure beneath the North American continent from Bayesian inversion of surface-wave phase velocities

[1] We jointly invert local fundamental-mode and higher-order surface-wave phase-velocities for radial models of the thermo-chemical and anisotropic physical structure of the Earth's mantle to ∼1000 km depth beneath the North American continent. Inversion for thermo-chemical state relies on a self-consistent thermodynamic method whereby phase equilibria and physical properties (P-, S-wave velocity and density) are computed as functions of composition (in the Na2O-CaO-FeO-MgO-Al2O3-SiO2 model system), pressure and temperature. We employ a sampling-based strategy to solve the non-linear inverse problem relying on a Markov Chain Monte Carlo method to sample the posterior distribution in the model space. A range of models fitting the observations within uncertainties are obtained from which any statistics can be estimated. To further refine sampled models we compute geoid anomalies for a collection of these and compare with observations, exemplifying a posteriori filtering through the use of additional data. Our thermo-chemical maps reveal the tectonically stable older eastern parts of North America to be chemically depleted (high Mg#) and colder (>200°C) relative to the active younger regions (western margin and oceans). In the transition zone the thermo-chemical structure decouples from that of the upper mantle, with a relatively hot thermal anomaly appearing beneath the cratonic area that likely extends into the lower mantle. In the lower mantle no consistent large-scale thermo-chemical heterogeneities are observed, although our results do suggest distinct upper and lower mantle compositions. Concerning anisotropy structure, we find evidence for a number of distinct anisotropic layers pervading the mantle, including transition zone and upper-most lower mantle.

Mantle reflectivity structure beneath oceanic hotspots

This study applies high-resolution Radon transform to a large set of SS precursors and explores the mantle reflectivity structure beneath 17 potentially ‘deep-rooted’ hotspots. The combined reduced time (τ) and ray parameter (p) information effectively constrains the depth, spatial distribution and sharpness of upper-/mid-mantle reflectors. The olivine to wadsleyite phase boundary is deeper than the ocean and global averages and produces a dominant τ–p domain signal. Laterally coherent observations of the deep 410-km seismic discontinuity, thin upper mantle transition zone and weak/absent 520-km reflector beneath hotspots make compelling arguments for large-scale, hot thermal anomalies in the top 400–600 km of the mantle. On the other hand, a relatively ‘flat’ and weak reflector at ∼653 km is inconsistent with ringwoodite to silicate perovskite + magnesiowüstite transformation at temperatures greater than 2000 K. The lack of a negative correlation between topography and temperature implies (1) average or below-average temperatures at 600–700 km depths or (2) high temperatures and a dominating majorite garnet to Ca perovskite phase transformation. The proper choice between these two scenarios will directly impact the origin and depth of mantle plumes beneath hotspots. We further identify lower-mantle reflectors at 800–950 and 1100–1350 km depths beneath a number of the hotspots. Their presence implies that the chemistry and thermodynamics of the mid-mantle may be more complex than suggested by seismic tomography.

Seismological imaging of the Antarctic continental lithosphere: a review

Tomographic analysis of seismic surface waves can map upper mantle structure under the Antarctic plate with reliability and fairly good detail, making the best use of the relatively limited dataset currently available. The large-scale features of Antarctic upper mantle agree with global views of Earth structure under oceans and continents. Low seismic velocities map the hot thermal anomaly under mid-ocean ridges down to approximately 150 km, stronger and wider under faster-spreading ridges. Cold continental roots show as seismically fast material under the older part of the continent (East), while the West Antarctic Rift System has a clearly slow wave signature. The seismically imaged lithosphere has variable thickness under the craton. It appears rather regular and about 220 km thick under Dronning Maud Land, but it deepens in the region stretching below Enderby Land, Gamburtsev Mountains, to Wilkes Land, where it reaches its maximum thickness, in excess of 250 km. This variability in lithospheric thickness is analogous to what has been found under other continents. The high velocity anomaly imaging cratonic roots appears to spread out, as a cool halo, off the passive continental margins, but terminates sharply towards the West Antarctic Rift System. The fast/slow contact runs under the Transantarctic Mountains and it is particularly sharp in the Ross Embayment, where seismically slow material, imaged down to 250 km, can be interpreted as the deep-seated hot anomaly related to a mantle plume.

Why is the number of superplumes so limited?

Numerical simulation shows that the hot plumes can be formed by the self-accelerated process if local thermal perturbation is present. Since the local thermal perturbation may exist extensively near the CMB and the transition zone around 670 km, the number of the hot plumes should be great. The cold plume seems to be originated by the subduction slabs. Because the subduction belts distribute extensively on the Earth’s surface, the number of the cold plume should be also great. However, the observed number of the superplumes is much less than the supposed ones. A simple 2D model of whole mantle convection is designed to investigate the interaction between plumes and thermal anomalies. The hot thermal anomalies (HTA) are imposed at CMB and a depth of about 670 km discontinuity, corresponding to two plausible locations of the hot plumes origination. And, the cold thermal anomalies (CTA) are imposed near the surface and 670 km discontinuity, corresponding to the subducted slabs and possible accumulation of slabs at 670 km discontinuity. The numerical results show that the fixed HTA does not originate a new plume. It draws the hot plume to move towards itself and stabilizes the plume at its own position. The imposed free HTA moves towards the hot plume, and finally merge with the plume. The situation for the CTA is similar to that for the hot one. From the numerical results, we can reach the following conclusions. It is very difficult for the HTA/CTA to originate a new hot/cold plume in a convective flow field. The boundary instability is suppressed by the developed plumes. Instead of originating new hot/cold plumes, the HTA/CTA interact with the plumes to form huge superplumes. This might be one reason for the question, why the number of the superplumes is so limited.

The potential influence of radiative heat transfer on the formation of megaplumes in the lower mantle

Recent seismic tomographic models have revealed broad, low-velocity anomalies in the lower mantle beneath Africa and the central Pacific which suggest a break in the symmetry between hot and cold regions in lower mantle dynamics. We have considered the possible impact from radiative heat transfer, which can be described by a nonlinear temperature-dependent coefficient in the thermal conductivity, in 2D numerical simulations. Results for Rayleigh numbers up to 10 6 show a strong stabilizing influence from radiative heat transfer on mantle upwellings and the production of extremely hot thermal anomalies in the interior. This nonlinearity is responsible for producing a strong attractor in the mantle convective system, which greatly simplifies its time-dependent dynamics. The possible link of the simplified lower mantle time-dependent dynamics with polar wander is discussed. The main point here is that slow time dependence of the huge anomalies in the lower mantle can be the main mechanism controlling long-term rotational dynamics.

Lower mantle thermal structure deduced from seismic tomography, mineral physics and numerical modelling

The long-wavelength thermal anomalies in the lower mantle have been mapped out using several seismic tomographic models in conjunction with thermodynamic parameters derived from high-pressure mineral physics experiments. These parameters are the depth variations of thermal expansivity and of the proportionality factor between changes in density and seismic velocity. The giant plume-like structures in the lower mantle under the Pacific Ocean and Africa have outer fringes with thermal anomalies around 300–400 K, but very high temperatures are found in the center of the plumes near the base of the core-mantle boundary. These extreme values can exceed +1500 K and may reflect large hot thermal anomalies in the lower mantle, which are supported by recent measurements of high melting temperatures of perovskite and iron. Extremely cold anomalies, around −1500 K, are found for anomalies in the deep mantle around the Pacific rim and under South America. Numerical simulations show that large negative thermal anomalies of this magnitude can be produced in the lower mantle, following a catastrophic flushing event. Cold anomalies in the mid-lower mantle have modest magnitudes of around −500 K. A correlation pattern exists between the present-day locations of cold masses in the lower mantle and the sites of past subduction since the Cretaceous. Results from correlation analysis show that the slab mass-flux in the lower mantle did not conform to a steady-state nature but exhibited time-dependent behavior.

Temperature‐dependent Newtonian and non‐Newtonian convection: Implications for lithospheric processes

Convection studies of temperature‐dependent Newtonian and non‐Newtonian rheology in aspect‐ratio seven boxes have been carried out for volume averaged Rayleigh numbers between O(105) and O(107). Large lateral viscosity variations, O(104) are found for non‐Newtonian cases with total temperature‐dependent viscosity variations up to 250. Flow structures for both rheologies are distinguished by large‐scale circulations with relatively stable descending limbs. Much larger hot thermal anomalies are found near the surface for non‐Newtonian rheology. Lithospheric thinning is facilitated by non‐Newtonian rheology because of stress‐softening and the lubrication of the descending limbs by hot diapirs.

Penetrative convective flows induced by internal heating and mantle compressibility

The effects of internal heating on anelastic, compressible convection have been investigated in a spherical shell, with the ratio of inner to outer radii appropriate to lower mantle convection. The internal heating and the compressibility are characterized by two dimensionless numbers: R, the ratio of two Rayleigh numbers, and D, the dissipation number. Isothermal, stress‐free boundary conditions are applied at the top and at the bottom of the shell. The equations have been solved, with a time‐dependent finite difference code, in a temperature, vorticity, stream function formulation (axisymmetrical configuration) for Rayleigh numbers Rab ranging from the critical Rc up to 2000 Rc. The neutral surface computed over the (D,R) space leads to an unexpected crest shape which is correlated with a shift toward higher‐degree harmonics of the most unstable modes. The phenomenon of penetrative convection is generated, for the weakly nonlinear cases, by the combination of large internal heating rates R and large dissipation numbers D. Small cells appear at the top of the shell, counterrotating with respect to the surrounding global circulation. The onset of time dependence (at Rab = Rtd) and the evolution of bottom topography have been monitored over the (D,R) space by increasing the Rayleigh numbers gradually from the critical. With no internal heating the stabilizing effect of compressibility delays the transition to time dependence. However, the combined effects of internal heating and compressibility drastically reduce the threshold of time dependence to Rtd = 3 Rc, for R = 16, D = 0.6. The study of the topographic deflection of the bottom interface conducted with the same set of (D, R, Rab) values shows that this deflection always decreases with the increase of the Rayleigh number. To reach situations as close as possible to mantle conditions we have focused on D = 0.3 and R = 16 for Rayleigh numbers up to 2000 Rc. In these highly nonlinear regimes, penetrative convection cells are still generated at the top or at the bottom of the shell and evolve by breaking the surrounding chaotic circulations or by pairing with the closest corotating cells. The destruction of these transient cells by viscous dissipation leaves a persisting hot thermal anomaly. The internal heating combined with compressibility reinforces the chaotic character of convection. As a consequence, the bottom topographic deflections display highly time‐dependent behavior, and the thermal anomaly spectra contain energy up to degree 100. Serious assumptions are imposed on the modeling of the Earth's mantle by the mathematical and numerical difficulties of the problem, in particular by the introduction of an isothermal stress‐free upper boundary. In spite of these limitations we propose that penetrative convection, due to the combination of internal heating and compressibility, could be a mechanism able to generate spontaneously layered convection and local melting in the mantle.