THE late Archaean eon (3 to 2.5 billion years ago) seems to have been a time of profound geological changes(1-5) : rapid growth of the continents, of the volume of cratonic sediments and of the area of the continental shelves was accompanied by widespread emplacement of potassium-rich granitic rocks, leading to the ’cratonization’ of the continental crust(3,6). There is also evidence for climate change(7,8) at this time, possibly reflecting changes in the composition of the atmosphere and the volume of the oceans. The period ends with the Huronian glaciation, the first web documented glacial episode(9), and this transition appears to be marked by a strong increase in the intensity of the geomagnetic held(10). Here we present simulations of mantle convection at the end of the late Archaean, and show that a breakdown of two-layer convection (a ’flush’ instability(11)) occurring at this time could have led to a period of volcanic and tectonic activity that might account for the reported geological and palaeoenvironmental changes.