Past plate motions

Abstract

The role of the Earth's magnetic field Introduction It should be clear from the preceding chapter that it is possible, without too much difficulty, to calculate the relative motions of pairs of plates at any location along their common boundary and to see what may occur in the future. This chapter deals with the past motions of the plates and shows how to reconstruct their previous interactions from evidence they have left. Two important facts together make it possible to determine past plate motions. The first is that the Earth's magnetic field has not always had its present (normal) polarity with the ‘north’ magnetic pole close to the north geographic pole and the ‘south’ magnetic pole close to the south geographic pole. Over geological history the magnetic field has intermittently reversed. Thus, there have been times in the past when the north magnetic pole has been located close to the present-day south geographic pole and the south magnetic pole has been located close to the present-day north geographic pole; then the field is said to be reversed. The second fact is that, under certain circumstances (discussed in Section 3.2) rocks can record the Earth's past ( palaeo ) magnetic field. Together, these facts enable us to estimate dates and past positions of the plates from magnetic measurements.