234

Bioelectromagnetism

FIGURE 6.1 Reversals of the GMF during the past 5 million years (5 Ma) (Mankinen and Wentworth, 2003).

(Reproduced with permission from Mankinen and Wentworth (2003), Copyright 2003, USGS; Mankinen and

Dalrymple (1979), Copyright 1979, John Wiley & Sons.)

Polarity chrons or epochs are long periods during which the GMF remained oriented in one direction

for most of the duration of the chron (Mankinen and Wentworth, 2003). As shown in Figure 6.1, the

reversals of the GMF during the past 5 million years (5 Ma) are illustrated by Mankinen and Wentworth

(2003), modifed from Mankinen and Dalrymple (1979).

Te periods during which the geomagnetic polarity was “normal” (that is the same as at present)

are indicated in black, whereas periods of reversed polarity are indicated in white (Mankinen and

Wentworth, 2003) (Figure 6.1). Most chrons are interrupted by shorter periods, called “subchrons,” dur­

ing which the feld fips to the opposite of the dominant orientation during the longer parent chron

(Mankinen and Wentworth, 2003).