Global events that periodically resurface nearly the entire planet will leave a crater-free surface: craters then occur and aren't subsequently modified until the next global event. Resurfacing events occurring frequently everywhere will produce a surface with many craters in the process of being resurfaced. Thus, the end-members can be distinguished by observing the extent to which the craters have experienced some degree of tectonic deformation or volcanic flooding. Initial surveys of the crater population suggested that only a few percent of the craters were heavily deformed or embayed by subsequent volcanism, thus favoring the "catastrophic resurfacing" end member. A number of geophysical models were proposed to generate a global catastrophe, including The portion of the planet with large rift zones and superposed volcanoes was found to correlate with a low crater density and an unusual number of heavily deformed and obviously embayed craters. The tessera regions of the planet seem to have a slightly higher than normal percentage of craters, but a few of these craters appear to be heavily deformed. These observations, combined with global geologic mapping activities, lead to scenarios of geologic surface evolution that paralleled the catastrophic geophysical models. The general vision is that the tessera regions are old and date to a past time of more intense surface deformation; in rapid succession the tessera ceased deforming and volcanism flooded the low-lying areas; currently geologic activity is concentrated along the planet's rift zones. Turcotte (1993) suggested that Venus has episodic tectonics, whereby short periods of rapid tectonics are separated by periods of surface inactivity lasting on the order of 500 Ma. During periods of inactivity, the lithosphere cools conductively and thickens to over 300 km. The active mode of plate tectonics occurs when the thick lithosphere detaches and founders into the interior of the planet. Large scale lithosphere recycling is thus invoked to explain resurfacing events. Episodic large scale overturns can occur due to a compositionally stratified mantle where there is competition between the compositional and thermal buoyancy of the upper mantle. This sort of mantle layering is further supported by the 'basalt barrier' mechanism, which states that subducted basaltic crust is positively buoyant between the mantle depths of 660–750 km, and negatively buoyant at other depths, and can accumulate at the bottom of the transition zone and cause mantle layering. The breakdown of mantle layering and consequent mantle overturns would lead to dramatic episodes of volcanism, formation of large amounts of crust, and tectonic activity on the planet's surface, as has been inferred to have happened on Venus around 500 Ma from the surface morphology and cratering.