Plate Tectonics

No one could figure out why the readings were going up and down all over the seafloor - until the science of paleomagnetism showed that what was being recorded was the polarity of the rocks on the ocean floor. The Earth's magnetic field is like a bar magnet: the positive end is currently at the north pole and the negative end at the south pole.  Magnetic field lines “wrap” around Earth as the positive pole tries to "reach" the negative one. This produces a "force field" around our planet, and helps to shield us from the sun's ultraviolet radiation. The interaction between our poles and the solar wind is what creates the auroras.

Image source:  "Sources of Ionizing Radiation in Interplanetary Space " by NASA/JPL-Caltech/SwRI is in the Public Domain

The magnetic field of the Earth is imprinted upon iron-bearing rocks when they form. New rock is forming at the mid-ocean ridges. When the magma cools to form a rock, whatever the polarity of the Earth is at that time gets imprinted in the minerals that comprise the rock. This helped support the continental drift hypothesis in two ways:

1. It provides a record of the magnetic reversals (polarity) of our planet. As the magma rises up to the surface at the mid-ocean ridges and cools, it gets imprinted with the magnetic data. This rock is then pushed aside as new rock forms, and so on. So, parallel to the mid-ocean ridges are "stripes" of paleomagnetic data, one side the mirror image of the other. This is what was causing the odd pattern on the magnetometer readouts.
   
   
   
   

   Image source:  "Sea-floor spreading and magnetic anomalies." by , Houghton Mifflin Company is licensed under CC BY-NC 4.0

   
   
   
2. The original latitude of the rock (it's birthplace) is also imprinted on the rock when it forms. For example, if a piece of oceanic crust is formed at the equator, that information is recorded in the iron-bearing minerals in the rock. As new crust forms, it moves with the rest of the crust away from the ridge axis. After millions of years, the rock is now located quite far from where it was "born". Using paleomagnetism, a scientist can test the rock and determine where it was formed.
   

   Here's an example:  A piece of basalt was collected from Necker Island, which is at a latitude of 23° N.  The rock subjected to a battery of tests, and it was discovered that the rock is 10 million years old, and paleomagnetic data indicated that the rock formed at 18° N.  We now know that Necker Island formed over the Hawaiian Hotspot and moved along with the Pacific Plate 5 degrees of latitude, or 1,000 kilometers, over 10 million years. Scientists used this to help prove that the plates do indeed move.

The Magnetic north pole ≠ true north. The magnetic poles also don't stay in one place; they wander over time.  Below is a Google Earth image showing how the North Pole has wandered from 1594 through the present.

Image source:  "Google Earth Imagery " Google Earth by, is in the Public Domain

 

If that's what the North Pole is doing, then the South Pole should be doing the same, just in reverse, right?  Um.  Not quite.

Image source:  "Google Earth Imagery " Google Earth is in the Public Domain

Why?  We're not sure, but it likely has to do with how the magnetic field forms in the Earth's core.

 

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copyright Sonjia Leyva 2022	CSULA| PCC