Summary: 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 ridgeAs new oceanic crust is generated at a mid-ocean ridge by upwelling magma, each new intrusion splits the previous rock into two, one slice on each side of the rift. The two slices are pushed aside by the new intrusion. Do this hundreds of times, and on both sides of a mid-ocean ridge you have pairs of almost identical stripes of rock representing the continual intrusion of new rock at the ridge axis. 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. Reversal chartThere have been at least 180 magnetic field reversals in the past 80 or so million years, with a seemingly random periodicity. The length of time that the magnetic field remains stable in either north or south polarity is also pretty variable, ranging from a million years or more to a few hundred years. The last major magnetic field reversal happened about 780,000 years ago, although there was a short-lived event about 41,000 years ago as well. 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
