Wednesday, October 09, 2013

Geologic Hula

I mentioned in the comments to Geologic Dance that something completely different explains the change in magnetic alignment in the Hawaiian Islands. In fourth grade, the California public school curriculum includes circuits and magnetic fields. I don't know how much most kids understand, but I played around with iron fillings and magnets with my kid to assess her understanding and help her make connections to the natural world.

You'll need a jar of iron fillings, which we purchased at the gift shop of a science museum.  For easy cleanup, I laid a piece of paper down on the box lid before sprinkling some iron fillings on top.

We also used common household objects such as yoga bricks, a ceramic magnet from the hardware store and a box lid.   We've successfully used both paper and plastic box lids.
Place the magnet underneath the lid and slowly rotate it.

At first, I dumped out too many fillings.

Then I put most of them back into the jar and tried again.

This is what happens to the iron in molten rocks when they first reach the surface of the earth. As they cool and harden, their alignment provide a record of the orientation of the earth's magnetic field at the time they froze into position.

In Geologic Dance, the difference in magnetic alignment between earth layers of the same age, but at different locations, tells the story of how the different locations have moved since they froze.  I related it to the Channel Islands and the nearby San Gabriel Mountains, which are ~1,200 million years old.  They rotate on a timescale of tens and hundreds of millions of years.

In contrast, the Hawaiian Islands are much younger, ~0.1 to 10.0 million years old.  Even if they didn't sit in the middle of the Pacific plate, they haven't had time to rotate yet.

So what causes the shift in magnetic alignment in lava flows of different ages?

The earth's magnetic field varies in time* and even reverses directions in a process called magnetic reversal.  The times between reversals are highly variable, but they tend to occur a few times each million years.  The magnetic alignment of lava flows in Hawaii tell the story of how the earth's magnetic field has shifted in recent (geologic) history!

You can read more about it in Lava flows reveal clues to magnetic field reversals.

* It's not just the magma motion in the core (and mantle) of the earth.  Space weather from the sun causes daily fluctuations in the .total. magnetic field felt here on earth.  This is more likely to affect measurements on satellites than on the surface of the earth.  But, it is a measurable effect that you have to take into account for certain uses.

1 comment:

  1. Somehow I missed this post- probably due to the enormous amount of paperwork for work. I have iron filings and you did a great explanation here for the non geologist. Thank you!