![Fig. 2. Time variations of four signi¢cant kinematic, geographic or dynamic indicators of hotspot motion (all on the same time scale in million years BP). All display a step-like (Heavyside) change in velocity at V40^50 Ma. Velocity patterns are shown as full red or black dashed lines, depending on whether the change happened at 40 or 50 Ma. (a) Distance between observed and predicted positions for the Hawaiian hotspot [17]. Predicted positions are based on the hypothesis that the Reunion and Hawaiian hotspots have remained ¢xed with respect to each other; the dated track of the Reunion hotspot is transferred to the Paci¢c plate following kinematic parameters discussed by [17], notably those in the Adare trough between E and W Antarctica. (b) and (c) Latitudinal evolution of the Hawaiian and Reunion hotspots based on data from [34^38]. (d) Along track true polar wander velocity at 10 Ma intervals [39].](/figures/fig-2-time-variations-of-four-signic-cant-kinematic-1bl5l1ra.webp)
![Fig. 4. A schematic cross-section of the dynamic Earth going through its rotation axis, outlining the sources of the three types of plumes/hotspots identi¢ed in this paper: the ‘primary’ or main, deeper plumes possibly coming from the lowermost mantle boundary layer (DQ in the broad sense) are the main topic of the paper; the ‘secondary’ plumes possibly coming from the top of domes near the depth of the transition zone at the locations of the superswells are indicated [46,47]; the ‘tertiary’ hotspots may have a super¢cial origin, linked to tensile stresses in the lithosphere and decompression melting [9,10]. There are on the order of 10 primary (deeper) plumes forming a girdle around the two antipodal domes upwelling below the central Paci¢c and Africa. At present only plume tails and no plume heads are active and close to the surface, and the number of plumes in a single cross-section is less. The £uid mechanics aspects are based on the experimental study of thermochemical plumes by Davaille et al. [57,58], and the lower mantle domes are based on seismic tomography [25,26]. The location of possible avalanches [63] at the downwellings of the lower mantle quadrupolar convection cells are indicated by sagging in the transition zone, though no such event is thought to be presently active.](/figures/fig-4-a-schematic-cross-section-of-the-dynamic-earth-going-1jdfco5e.webp)
![Fig. 3. Primary plumes and superswells shown on a tomographic map of shear wave velocity at 2850 km depth [25]. Only the positive (fast, cold) anomalies are shown in blue shades. The negative (slow, hot) anomalies are in white (the complete tomographic picture is seen in Fig. 1b). The locations of the Paci¢c and African superswells are indicated as large pink dots. The seven primary hotspots identi¢ed in the paper are shown as smaller red dots. Three hotspots that could be part of the primary group (see text) are shown as green dots with red edges. The primary hotspots tend to form above hot regions but away from both superswells and the cold (subduction-related?) belts.](/figures/fig-3-primary-plumes-and-superswells-shown-on-a-tomographic-190awqcg.webp)
Q2. What is the common reason for the shallowness of the reservoir?
Since a shallow reservoir would be likely to be sampled by mid-ocean ridges, it is often considered that the primitive reservoir lies deep in the mantle, con¢ned to the transition zone at the bottom of the upper mantle, or even deeper in the lower mantle (but see [23]).
Q3. What is the likely origin of primary plumes?
Primary hotspots can be traced in the upper mantle down to the transition zone; they can only be produced by plumes which originate from instabilities out of a thermal boundary layer.
Q4. How much rms velocity is the kinematic analysis of Mu«ller?
The kinematic analysis of Mu«ller et al. [15] shows that inter-hotspot motions between the four Indo-Atlantic hotspots are also less than about 5 mm/a.
Q5. What is the likely explanation for the avalanche?
On the other hand, cold subducted material, accumulated at the base of subduction zones, in the transition zone, along the great circle of quadrupolar convection in the lower mantle, may trigger a major avalanche in the lower mantle [61^63].
Q6. What is the reason for the debates that have opposed apparently conjectured, endmember?
Mixing the three distinct types of hotspots, with the hope of establishing a single origin, could be the reason for most of the debates that have opposed apparently con£icting, endmember models for the last decades.
Q7. What is the main source of information on the TPW curves?
And primary hotspots would be their main source of information on their time history, being the passive markers of readjustments in the two-cell geometry of the lower mantle reservoirs.
Q8. What is the definition of a primary plume?
In this framework, a primary plume could be a thermochemical plume issuing from an instability involving higher chemical density anomalies.
Q9. What is the origin of the primary plumes?
Note that surface motions of these secondary plumes could also re£ect lower mantle convection, hence be consistent with those derived for primary plumes.
Q10. How long can a large head and a small tail last?
Fluid mechanics arguments show that the joint presence of a very large head and a small but long enduring tail can only be produced at depthsmuch in excess of the transition zone [53^56].
Q11. What are the three types of hotspots?
The three types of hotspots may simply correspond to the three boundary layers between the core^mantle boundary and the surface of the Earth.