Pangaea, which existed from about 350 to 200 Ma, was not the first supercontinent. In 1966, Tuzo Wilson proposed that there has been a continuous series of cycles of continental rifting and collision; that is, break-up of supercontinents, drifting, collision, and formation of other supercontinents. Pangaea was preceded by Pannotia (600 to 540 Ma), by Rodinia (1,100 to 750 Ma), and by other supercontinents before that.

4. Magnetic reversals: In section 4.2 we saw that rocks could retain magnetic information that they acquired when they were formed. However, Earth’s magnetic field is not stable over geological time. For reasons that are not completely understood, the magnetic field decays periodically and then becomes re-established. When it does re-establish, it may be oriented the way it was before the decay, or it may be oriented with the reversed polarity. During periods of reversed polarity, a compass would point south instead of north. Over the past 250 Ma, there have a few hundred magnetic field reversals, and their timing has been anything but regular. The shortest ones that geologists have been able to define lasted only a few thousand years, and the longest one was more than 30 million years, during the Cretaceous (Figure 4.5.64.5.6\PageIndex{6}). The present “normal” event has persisted for about 780,000 years.

The rate at which the line unspooled was measured by counting the number of knots that passed through the sailor’s hands in a certain amount of time. Since it was originally measured as the number of knots per unit time, speed over water traditionally uses the unit of “knots” to indicate speed. One knot (kt) = 1 nautical mile per hour = 1.15 mph = 1.85 kph.

Other map types include bathymetric maps (Figure 2.4.42.4.4\PageIndex{4}). These are similar to topographic maps for terrestrial locations, with lines connecting areas of equal depth.