Tom Shaw Blogs: Compass Conundrums
This post will dig deeper into compass use by weaving geology, physics, planetary science and outdoor navigation all into one as well as sharing my experiences as a graduate student in the MOAEL program at SOU. This is the second of 3 blog entries on this topic.
As you may remember in my last post, I was in a precarious situation. I was standing on Grizzly Peak, taking the final field test for my Navigation Class and my compass needle was not pointing North. Grrr, Why was Big Red (The red end of your compass needle) not behaving? The troublemaker, in this case, was Grizzly Peak itself. Grizzly Peak is the remnant of an old volcano, and where you find volcanoes, you almost certainly will find basalt. Basalt is the , and it contains iron. That iron is gonna mess with your compass. Iron is among a that can be magnetized when placed in a magnetic field, and The Earth鈥檚 magnetic field just so happens to be strong enough to do the trick. The magnetization happens when is flowing out of the Earth鈥檚 crust and the iron is in liquid form. The liquid iron can more easily . This means nearly all basaltic rock on Earth is magnetic! So now you are beginning to see Big Red鈥檚 problem. When you place your compass near volcanic rock there is a good chance it will direct Big Red to point in some direction other than 迟辞诲补测鈥檚 location for Magnetic North.
Notice I said 迟辞诲补测鈥檚 Magnetic North. This is where it starts to get truly interesting. The Earth鈥檚 magnetic field is ALWAYS changing location. Where Magnetic North is today is NOT where it will be a year from now. Check these two links to see how the location of the Magnetic North Pole has been changing over the years.
As basaltic lava solidifies into basaltic rock the rock retains a magnetic field that points in the direction of the Earth鈥檚 magnetic field at that moment. This means if you put your compass next to a chunk of basaltic rock that hasn鈥檛 moved since it solidified your compass will point in the direction of the Earth鈥檚 magnetic field from when the rock first formed. In the case of Grizzly Peak, it is pointing in the direction of Magnetic North from 23 million years ago. This is exactly how scientists know where the Earth鈥檚 Magnetic North Pole was at different times in the past. This, however, is not very useful in helping me pass my Wilderness Navigation practical, however. Make sure to keep your compass off those basaltic rocks!
Check out this that talks about how the magnetic fields stored in the lava beds on a Hawaiian Volcano have shown that the Earth鈥檚 Magnetic poles sometimes completely flip direction. That鈥檚 right, on this Hawaiian volcano, Big Red will actually point South. As a matter of fact, these polar flips happen ever 200,000 to 300,000 years and we are If the poles flip in our lifetime be ready for compass use to take on a whole new direction.
And just for fun, you know what else will knock Big Red off of the Earth鈥檚 current magnetic field? Meteorites! In my next and last post on Compass, use look forward to reading about some interplanetary considerations when using your compass. You may remember I wrote earlier in this post, 鈥淭his means nearly all basaltic rock on Earth is magnetic!鈥, notice the word Earth is italicized. You鈥檒l learn why in the next post!
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