The latest reversal is called by geologists the Matuyama-Brunhes boundary (MBB), and occurred approximately 780,000 years ago.
A team of researchers based in Japan and Canada have obtained an improved age for the MBB.
The team studied volcanic ash that was deposited immediately before the MBB.
Once the sediment layer is deposited and buried, or the lava flow has cooled below 500°C, the direction of the earth’s magnetic field as recorded by magnetite grains in these rocks cannot usually be changed by subsequent geological events (except for metamorphism—the process of changes to rock under the influence of elevated pressures and temperatures), even if the direction of the earth’s magnetic field has subsequently changed.
This magnetism in the rocks is thus in essence ‘fossilised’, and so is usually called palaeomagnetism.
Now that you have made some observations about the sedimentary features in the core, it's time to determine the age of the sediments and establish a timeline for the core section.
The relative ages of cores are determined onboard the by examining both the Earth's paleomagnetic record and microfossils preserved within the cores. Maureen Davies, magnetic minerals are like microscopic compasses that become aligned with the Earth's magnetic field at the time the sediments are deposited. Davies with a detailed record of the Earth's paleomagnetic record through time and can be used to help determine ages of sediment cores.This volcanic ash contains small crystals called zircons.Some of these crystals formed at the same time as the ash; thus, radiometric dating of these zircons using the uranium-lead method provided the exact age of the ash.It is not known when the next magnetic reversal will be, or how long the process will take, though it will certainly have a significant impact on the artificial and biological navigational systems of humans and animals.A photograph of the geological section across the Matuyama-Brunhes boundary in Chiba Prefecture, Japan. (b) and (c) Detail of a volcanic ash layer (Byk-E) just below the MBB in the Chiba section.The stripes represented sections in the rock of normal (the same as today) and reversed directions of the earth’s magnetic field, and this has been used as evidence for so-called sea-floor spreading and continental drift.