Geomagnetic polarity reversals
Recognize reversals in places where we have extensive history of volcanism - many rocks that have cooled through their Curie temperatures
Roughly half of rocks have normal polarity; half have reversed polarity
Reversals are worldwide & restricted to specific times in the past
Pattern of normal & reversed polarity is sufficiently distinctive that we can assign ages to rocks in which we can recognize their TRM
- Pattern is a song with two notes of different duration; distinctive rhythms in a well-known song