Sea floor dating
Then in 1963, Morley, Vine and Matthews showed that marine magnetic anomalies provided evidence for seafloor spreading.
Paleomagnetism is studied on a number of scales: The study of paleomagnetism is possible because iron-bearing minerals such as magnetite may record past directions of the Earth's magnetic field.
These include biomagnetism, magnetic fabrics (used as strain indicators in rocks and soils), and environmental magnetism.
As early as the 18th century, it was noticed that compass needles deviated near strongly magnetized outcrops.
The field is said to be recorded by chemical remanent magnetization (CRM).
A common form of chemical remanent magnetization is held by the mineral hematite, another iron oxide.
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.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.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 constructed apparent polar wander paths for Europe and North America.Magnetic stripes are the result of reversals of the Earth's field and seafloor spreading.New oceanic crust is magnetized as it forms and then it moves away from the ridge in both directions.Magnetic signatures in rocks can be recorded by several different mechanisms.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.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.