11. History in the Magnetic Patterns

Main Menu	11. Reading the History in the Magnetic Patterns
Expedition Menu 1. Introduction 2. Theory 3. Formation 4. Evidence 5. Earth's Interior 6. Heat Engine 7. Mid-ocean Ridge 8. On the Ridge 9. Seafloor Spreading 10. Magnetic Field 11. Magnetic History 12. Magnetic Patterns 13. The Plates 14. More on Plates
	Sometimes science happens by putting together diverse findings of other scientists with a spark of imagination! How to read the patterns of magnetic stripes, marine magnetic anomalies measured in the ocean basins? What does it all mean? Why are there mirror images of the magnetic patterns across the mid-ocean ridges? These questions were hotly debated by oceanographers, until.....
	The direction of the Earth's magnetic field is recorded in the lava once it cools below the Curie temperature (around 570^oC), which is the temperature at which the magnetism stabilizes in an iron-bearing rock, thus recording the direction of the Earth's magnetic field at that time. d. Notice how the magnetic direction alternates from one lava layer to the next AND that these alternations mark reversals in the direction of the Earth's magnetic field (see arrows in lines of force in diagram on right) -- in other words there were times (many times) in Earth history that the south magnetic pole changed places with the north magnetic pole -- and reversed back again -- amazing! This is now well-documented with a very good history of when each reversal took place!!!	Some land geophysicists came up with another idea -- a seemingly unrelated idea -- in part through their work in volcanoes in eastern California. It was widely known that many volcanic rocks have a magnetic signature because of their high iron (Fe) content. These scientists determined that the direction of magnetism in a thick stack of lava layers alternated in opposite directions from one layer to the next (see black and white layers to left). They proposed that the change in magnetic direction from one layer to the next was caused by a flip or reversal in the direction of the Earth's magnetic field at the time the lavas were erupted and cooled to become solid rock. Watch Animation - See the directions of arrows, showing the direction of the Earth's magnetic field, reverse!
Created By: Don Reed Dept. of Geology San Jose State University ©Copyright 2008 Last Updated on Sept. 22, 2008		Let's use this evidence to explain the patterns of marine magnetic anomalies