The origin of the peridotites that form cratonic mantle roots is central in understanding the history and survival of Earth's oldest continents. A major issue over the last decades was the depth of melting that generated the nuclei of cratons. A new evaluation of element partitioning between minerals and melt based on high pressure experimental work irrevocably constrains the pressure of melting to less than 2 GPa that leaves spinel harzburgites as residues. Garnet harzburgites, the dominant rock types of the subcratonic mantle, are generated by subduction via the reaction opx + sp -> grt + ol. Lateral compression and slab stacking leads to lithospheric thickening. The main time of these processes probably occurred before 3.2 Ga ago as indicated by i) the oldest Rhenium depletion ages of peridotites, ii) the existence of crustal components with such ages in the mantle and iii) the age of eclogite xenoliths. The subcratonic mantle was subsequently periodically overprinted by metasomatism. The ages of metasomatism mark the time of the collision of the E- and W-block of the Kaapvaal craton, the time of wide-spread volcanism within the craton (Ventersdoorp magmatism) and of accretion and subduction processes along the rifted margins of the Kaapvaal craton. A stable geothermal gradient was established by the end of the Archean and the subcratonic mantle cooled since then with a rate of 0.07°C/Ma.