Figure 1. Histograms of natural magnetization in volcanics of various ages and compositions in North Eurasia (former USSR). Solid columns refer to times of folding and collisions, and open columns refer to times of the rigid crust magmatism associated with extension. S is the overall area of volcanics occurrence, involved in a given histogram (the upper and lower numbers characterize the solid- and open-column histograms respectively). The area-averaged magnetization is estimated from magnetic anomalies (since the sizes of magma bodies substantially exceed the magnetic survey height, the magnetization was calculated from the formula J= T _a/4p for a half-space).