After Early Jurassic time the Central Atlantic Ocean began opening between West Gondwana and Laurussia, whereas between Europe and North America the spreading began in the Cretaceous. The oldest seafloor in the Central Atlantic is estimated to be about 175 Ma (Klitgord & Schouten, 1986). However, it is not clear to what extent the two continental landmasses already had been moving apart before that time (through thinning of the continental crust without ocean floor formation). The Late Triassic to Early Jurassic dike swarms and sediment-filled grabens of eastern North America (e.g., Newark Basin) and in West Africa suggest that rifting began well before 175 Ma, but the amount of rift-related extension between the two continents is poorly known. From a paleomagnetic viewpoint, however, there is no serious discordance between the Early Jurassic Laurussia and West Gondwana paleopoles (Chapter 5, Tables 5.10 and 5.11), so only after 175 Ma do the paleopoles need to be compared with rotation parameters that take the ocean spreading into account.

The Atlantic Ocean is, of course, still widening today. Technological developments in the last decade have allowed precise measurements of this increase in width. Using geodetic techniques (e.g., Very Long Baseline Interferometry or VLBI) the distance between Onsala, Sweden and Westford, Massachusetts, USA is seen to be increasing by 13.4 ± 0.7 mm per year, that is, about 10 cm during the eight-year duration of the measurements (Figure 6.1, from B. O. Ronnang, pers. comm., 1990). Similar measurements have documented higher plate velocities in the Pacific Ocean.