Changes in the Earth’s Crust

Abstract

In the previous chapter we looked at how the isotopic compositions of various elements, particularly radiogenic elements, are effective in throwing light on the history of the earth, acting as “fossils” that record the evolutionary process of the earth. Isotopic ratios were used to trace the movement of material and elements in the earth’s interior. The paleomagnetic methods to be discussed in this chapter are effective in tracing past tectonic movements of the earth’s crust, such as continental drift. Here the remanent magnetization in rocks provides their location in the paleomagnetic coordinate system, and with this as a guide it is possible to track the trajectory of the movement. The remanent magnetization in rocks is used as a tectonic tracer. Paleomagnetic methods have been applied to a great number of problems in earth science since the 1950’s, and have recorded striking achievements. These methods have made a particularly major contribution to establishing the foundation of plate tectonics theory, which has revolutionized earth science in the latter half of the 20th century by providing conclusive proof of the ocean floor spreading theory and the continental drift theory. In this chapter we will first explain briefly the mechanism behind rock magnetism, which is the basis of paleomagnetism, and then discuss general paleomagnetic methods. We will begin in the following section by taking a look at the history of crustal movements, which is gradually being brought to light through such research.