I am fascinated by the idea of the earth as a giant magnet for a couple of reasons. One is that what we call the North Pole is in fact the South Pole of the earth as a magnet (as you should know from school opposite poles attract and the north poles of magnets point north). And thus, Australia should be at the top of the map. And secondly that the Earth’s magnetic poles reverse every now and then in geological time.
Polarity chron
What I learnt this week was that the period of a geomagnetic event is a chron (or sometimes a polarity chron). I just love this word. It is pretty obscure so I doubt you will be able to drop it into your conversation (but ten points if you do).
The earth is a giant magnet
In 1600, William Gilbert (1540-1603) an English physicist became the first to suggest that the Earth itself is a giant magnet. The Earth has north and south magnetic poles and is surrounded by a geomagnetic field (geomagnetic refers to the magnetic properties of the earth—geo for Earth).
Surrounding the planet is the magnetosphere. It is an area of ionized particles (i.e., particles with an electric charge) affected by Earth’s magnetic field. The magnetosphere is formed by the interaction between the magnetic field and the solar wind, a stream of particles from the Sun.
The source of Earth’s magnetism lies in the core of molten iron about 7,000 km in diameter (half the planet’s diameter). It generates powerful electric currents that create the geomagnetic field. The materials of the outer core undergo convection (movement due to variations in temperature) that generates electric current and hence a magnetic field.
Paleomagnetism
The direction of the geomagnetic field has reversed itself many times in the past, meaning the north and south poles switch over. There have also been events when the reversal didn’t complete, most recently 35,000 years ago. The study of these changes is called paleomagnetism (paleo means ancient, early, prehistoric, primitive, fossil from Greek palai for long ago, far back). Paleomagnetism uses the study of iron laden sediments that show the direction of magnetic fields when they were laid down. This is a branch of chronostratigraphy (from stratum for layers)—the study of the sequence of rocks formed during periods of geologic time.
Meaning of chron
Chron you will recognise as the Greek word for time (from khronos, which has no known origin). It is in many words, e.g.: chronology, things listed in time order; chronicle, an historical story told over time; chronic to describe diseases that last a long time; and chronometer, a device for accurately measuring time.
Chronos was the Greek’s personification of time. Cronos (note the different spelling) also referred to as Cronus or Kronos, was the father of all the Greek gods (the Titans). He was Zeus’s father and Zeus eventually dethroned and imprisoned him. Cronos is often confused with Cronos.
Time for a change?
We are presently living in the Brunhes palaeomagnetic chron, which is characterised by normal magnetisation (i.e. the geomagnetic North and South Poles are in the northern and southern hemispheres respectively, with Australia still in the true northern hemisphere). It began about 780 000 years ago. There are fears that it might be changing soon (but soon means tens of thousands of years).