240

Bioelectromagnetism

FIGURE 6.4 Records of geomagnetic polarity reversal frequency and dipole moment since the Cambrian (Biggin

et al., 2012). (a) Te marine magnetic anomaly record (MMA), and plots of inverse chron length (black bars), and

reversal frequency. Te black and white stripes on the top represent the year of geomagnetic polarity. Black is the

same normal polarity as at present, and white is the reverse polarity. Cretaceous Normal Superchron (~83–125 Ma).

(b) Reversal frequency from the MMA and magnetostratigraphic studies (chequered area indicates insufcient

data) alongside virtual (axial) dipole moment (spatially normalized feld intensity) measurements. ORS, Ordovician

(Moyero) Reversed Superchron (~460–490 Ma); PCRS, Permian–Carboniferous (Kiaman) Reversed Superchron

(~267–313 Ma). (Reproduced with permission from Biggin et al. (2012), Copyright 2012, Springer Nature.)

FIGURE 6.5 Te average reversal frequency and eruption ages of large igneous provinces (LIPs) (ofset by +50

Myr) that have not yet been subducted (Biggin et al., 2012). ORS, Ordovician (Moyero) Reversed Superchron (~460–

490 Ma); PCRS, Permian–Carboniferous (Kiaman) Reversed Superchron (~267–313 Ma); CNS, Cretaceous Normal

Superchron (~83–125 Ma). (Reproduced with permission from Biggin et al. (2012), Copyright 2012, Springer Nature.)