Earth's North Magnetic Pole hasn't always been in the northern hemisphere. Earth's magnetic field reverses at irregular intervals! Sometimes several million years pass between reversals. At other times, only a few thousand years go by between reversals. Earth's magnetic field has reversed 17 times in the last 4 million years. A magnetic reversal doesn't happen overnight. A reversal occurs gradually over the course of a few thousand years. During a reversal, Earth's magnetic field gradually weakens, eventually dropping almost to zero. The magnetic field intensity then slowly builds back up, but with the locations of the poles switched.
Many rocks record the orientation of the Earth's magnetic field at the time they are formed. It's like a little record of the magnetic field got frozen into that rock. So, by looking at ancient rocks, we can tell the polarity of the magnetic field of the Earth even back to ancient times!
In the 1950's, magnetometer instruments started to be towed by ships traversing the ocean. The magnetometer would make a magnetic survey of the sea floor. Magnetic signatures were more intense than those found on land and they were arranged in bands parallel to mid-ocean ridges. In fact, areas around mid-ocean ridges showed that the magnetic signatures in rocks on one side of the ridge were mirrored by magnetic signatures on the other side of the ridge. So survey results might look something like this (with N denoting northward orientation and S denoting southward orientation):
It was deduced that mid-ocean ridges are places where new ocean crust is being created by upwelling material from the deep interior of the Earth. Another way to say this is that mid-ocean ridges are places of seafloor spreading. Here molten rock rises up from the depth of the Earth and makes new crust. The new crust forms in bands on either side of the mid-ocean ridge, with the newly created band pushing the already-existing crust a little farther away from the mid-ocean ridge. And of course, as each new band forms, the current orientation of the Earth's magnetic field is frozen into it.
This passage is a Windows to the Universe Original.
1) The magnetic north pole of the Earth
2) According to the passage, which of these is a valid magnetic signature for a mid-ocean ridge region?
3) Mid-ocean ridges are NOT:
Now, go to the sea floor model that your teacher has set up in the classroom. Use your magnetometer to perform a magnetic survey of the sea floor model. Draw the line representing the mid-ocean ridge. Draw an upward facing arrow when your magnetometer reads north and a downward facing arrow when your magnetometer reads south. Draw these arrows in order on either side of the mid-ocean ridge. Label your arrows "north" or "south". Sketch: