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).