We present new experiments focussing on the transient behaviour of thermal plumes. In a fluid heated from below, plumes develop once the hot thermal boundary layer (TBL) reaches a critical thickness (Howard, 1964). They rise through the fluid owing to their thermal buoyancy and comprise TBL material which empties itself into the plumes. As the TBL becomes exhausted, plumes start disappearing from the bottom up, sometimes even before reaching the upper boundary, depending on the convection intensity. Then, they finally fade away by thermal diffusion. This sequence of events shows that time‐dependence is a key‐factor when interpreting present‐day tomographic images of mantle upwellings. In particular, it could be erroneous to identify the depth of a present‐day slow seismic anomaly with the depth of its origin, or to interpret the absence of a long tail as the absence of a plume.

[1] The Amsterdam-Saint Paul plateau results from a 10 Myr interaction between the South East Indian Ridge and the Amsterdam-Saint Paul hot spot. During this period of time, the structure of the plateau changed as a consequence of changes in both the ridge-hot spot relative distance and in the strength of the hot spot source. The joint analysis of gravity-derived crust thickness and bathymetry reveals that the plateau started to form at ∼10 Ma by an increase of the crustal production at the ridge axis, due to the nearby hot spot. This phase, which lasted 3-4 Myr, corresponds to a period of a strong hot spot source, maybe due to a high temperature or material flux, and decreasing ridge-hot spot distance. A second phase, between ∼6 and ∼3 Ma, corresponds to a decrease in the ridge crustal production. During this period, the hot spot center was close to the ridge axis and this reduced magmatic activity suggests a weak hot spot source. At ∼3 Ma, the ridge was located approximately above the hot spot center. An increase in the hot spot source strength then resulted in the building of the shallower part of the plateau. The variations of the melt production at the ridge axis through time resulted in variations in crustal thickness but also in changes in the ridge morphology. The two periods of increased melt production correspond to smooth ridge morphology, characterized by axial highs, while the intermediate period corresponds to a rougher, rift-valley morphology. These variations reveal changes in axial thermal structure due to higher melting production rates and temperatures.