The theory of plate tectonics states that the earth's surface is seriously deformed only at plate boundaries. When a continent breaks up, divergent plate boundaries form, and new oceans and more but smaller continents take shape. At a convergent boundary, oceanic crust is lost, island arcs form and mountain ranges rise. Collisions between continents combine them into supercontinents. New oceans are evidence for divergence, while the scars of ancient collisions, called sutures, testify to the existence of former oceans closed by collisions.

TRACKING THE DRIFT OF CONTINENTS

Plate tectonics has presented us with a world where the shapes, sizes and positions of continents and ocean basins forever change. Luckily, it also offers the means to reconstruct their past arrangements. To this end we use magnetic anomalies and transform faults to rotate the plates by simple geometric operations into their proper positions at any moment in the past (Section 6.6). It is essential that we have enough old ocean crust at hand to provide us with the data. Because of subduction, forever swallowing the ocean rock record like an anaconda ingesting a pig, this procedure limits us to the last 100 my.

For most of the Mesozoic and the entire Paleozoic, not to forget several billion Precambrian years, we must use other, less satisfactory means. A good set of paleomagnetic measurements will suffice to give a continent its proper orientation and latitude, provided we do not mix data from normal and reversed polarity intervals, but it will not provide its longitude (Figure 6.1).