One of the findings that helped convince geologists on the American side of the Atlantic that continental drift was a reality was the discovery of sea-floor spreading – the idea that new oceanic crust is generated at the mid-ocean ridges, and that new crust pushes older crust out of the way, away from the mid-ocean ridge, thereby making the ocean basins wider and wider at about the rate your fingernails grow.
The discovery of sea-floor spreading came through geophysics – specifically, measurements of the earth’s magnetic field as it is preserved in formerly molten rock. When molten rock, magma, solidifies, tiny particles of magnetic minerals, mostly the mineral magnetite, an iron oxide, become frozen in place with the orientation of the magnetic field that’s present at the time the rock solidified.
That’s all well and good, but so what? It’s useful because the earth’s magnetic field has changed over time, and is changing right now. The positions of the north and south magnetic poles change, the strength of the magnetic field changes, and the whole system even reverses its polarity, so that the north magnetic pole becomes the south magnetic pole and vice versa.
There’s a lot of research and a lot of debate about how a magnetic field reversal happens. Increasing evidence shows that it may take place over a period of a few thousand or even a few hundred years – instantaneous, geologically. It must have something to do with flow in the molten outer core of the earth, where the magnetic field is generated by electrical currents in the liquid rock there. For now, let’s just recognize that these reversals have indeed happened in the geologic past. We can use measurements of the rocks with opposite polarity to figure out a lot of geological things.
|Reversal pattern at mid-ocean ridge|
As the different intrusive magmas solidify, they record the magnetic field in place at that time. Because the field reverses, we end up with alternating high and low magnetic values, reflecting the alternating polarity of the field resulting from reversals. In practice, this gives a uniform striping to the map of the magnetic field along mid-ocean ridges – symmetrical, alternating, long linear magnetic highs and lows. We can figure out things like the direction of spreading, its speed, and more. An incredibly useful tool for understanding plate tectonics.
I mentioned back in the Jurassic that almost all the present-day oceanic crust is of Jurassic age or younger, because most of the older oceanic crust has been subducted, recycled and melted down inside the earth. So we can’t use this tool for times before the Jurassic, at least not using oceanic crust, but we can look at the magnetic field frozen into other magnetite-bearing igneous rocks that are of all ages.
I’ve brought this topic up now, during the Cretaceous, because the Cretaceous was a time when the earth’s magnetic field did not reverse as it has so often at other times. For 38 million years, 121 to 83 million years ago, the earth’s magnetic field didn’t flip, a time called the Cretaceous Normal Superchron, normal because it was the same as the field orientation of the present day, and ‘chron’ refers to a period of one polarity or the other. A superchron is a long period with one polarity. We know Cretaceous superchron exists because we can measure the magnetic field in oceanic crust of Cretaceous age, and for that time interval, there are no alternating highs and lows that would reflect the reversal of polarity. Why was the earth’s magnetic field quiet for 38 million years, when it usually flips on average about every half-million years or so?
Since we don’t really understand how or why the field flips at all, we also don’t really understand why it would stay so stable for so long. Modeling of the dynamo, the earth’s electro-magnetic generator in the fluid outer core, gives some suggestions – maybe there is some kind of trigger mechanism to start a reversal – which begs the question, why were there no triggers during the Cretaceous superchron. Bottom line, we don’t know why there was a long period of no magnetic field reversals during the Cretaceous.
There’s lots of research going on into the earth’s magnetic field, including the phenomenon of reversals. We may be entering one now, as the overall field has been decreasing – a characteristic of a reversal – and the position of the north magnetic pole as it moves around in Arctic Canada has increased its annual rate of migration. But we really don’t know if those are real precursors, or evidence for the start of a magnetic reversal. In any case, there is no correlation between reversals and extinction events, though it has to be admitted that no reversal has happened since humans have had our wonderful electronic world. It’s kind of hard to imagine there would be no impact other than a compass needle pointing the opposite direction. If mankind can survive its other threats, perhaps someone will see what those impacts may be in the next few hundred or few thousand years.
—Richard I. Gibson
Why do periods of stable magnetic field exist?
Reversal chart from Wikipedia (public domain)
Sea floor spreading diagram from USGS (public domain)