together and had since moved apart. Although he produced an abundance of circumstantial evidence, his theory met with little acceptance for two reasons: (1) no mechanism for continental drift was known, and (2) there was no way to reconstruct the movements of the continents over time. Keith Runcorn and Edward A. Irving constructed apparent polar wander paths for Europe and North America. These curves diverged, but could be reconciled if it was assumed that the continents had been in contact up to 200 million years ago. This provided the first clear geophysical evidence for continental drift. Then in 1963, Morley, Vine and Matthews showed that marine magnetic anomalies provided evidence for seafloor spreading. Fields Paleomagnetism is studied on a number of scales: Geomagnetic secular variation is the small-scale changes in the direction and intensity of the Earth's magnetic field. The magnetic north pole is constantly shifting relative to the axis of rotation of the Earth. Magnetism is a vector and so magnetic field variation is studied by palaeodirectional measurements of magnetic declination and magnetic inclination and palaeointensity measurements. Magnetostratigraphy uses the polarity reversal history of the Earth's magnetic field recorded in rocks to determine the age of those rocks. Reversals have occurred at irregular intervals throughout Earth's history. The age and pattern of these reversals is known from the study of sea floor spreading zones and the dating of volcanic rocks. Principles of remanent magnetization The study of paleomagnetism is possible because iron-bearing minerals such as magnetite may record past directions of the Earth's magnetic field. Magnetic signatures in rocks can be recorded by several different mechanisms. Thermoremanent magnetization Iron-titanium oxide minerals in basalt and other igneous rocks may preserve the direction of the Earth's magnetic field when the rocks cool through the Curie temperatures of those minerals. The Curie temperature of magnetite, a spinel-group iron oxide, is about 580 °C, whereas most basalt and gabbro are completely crystallized at temperatures below 900 °C. Hence, the mineral grains are not rotated physically to align with the Earth's field, but rather they may record the orientation of that field. The record so preserved is called a thermoremanent magnetization (TRM). Because complex oxidation reactions may occur as igneous rocks cool after crystallization, the orientations of the Earth's magnetic field are not always accurately recorded, nor is the record necessarily maintained. Nonetheless, the record has been preserved well enough in basalts of the