9. Magnetic Reversals In the 1950s, ocean-going research vessels recorded puzzling data based on the magnetism of the ocean floor. It was determined that the rock of the ocean floor had alternating bands of embedded iron oxides that pointed north and south. Thus, in 1963, the theory of the reversal of the earth's magnetic field was proposed and it has been a fundamental of earth science since. 1) Ship moves across view, towing magnetometer 2) magnetic stripes the magnetometer 'sees' then appear superimposed on ocean floor.

10. Scientists believe that the earth's magnetism is created by slow movements in the liquid outer core, caused by the rotation of the earth. The generation of the earth's magnetic field is a continuous, but variable, process that causes change in not only the intensity of the magnetic field, but also causes the Magnetic North Pole to move as well as the reversal of the earth's entire magnetic field. Lava, which hardens into rock, contains grains of iron oxides that point toward the magnetic pole as the rock solidifies. Thus, these grains are permanent records of the location of the earth's magnetic field. As new crust is created on the ocean floor (such as at the Mid-Atlantic ridge), the new crust solidifies, with its iron oxide acting like miniature compass needles. Scientists have matched the magnetic bands on either side of the Mid-Atlantic ridge out to the edges of the ocean. To determine the distance between the Americas and Europe and Africa at any point since Pangea, one need only to "roll back" the oceanic crust to the appropriate matching magnetic bands on either side of the ridge. Magnetic reversals helped to prove the theory of plate tectonics and continental drift. The earth's magnetic field has reversed approximately 170 times over the last 100 million years. The intensity of the magnetic field has been decreasing over time since it has been measured and some scientists expect that at the current rate of decline, there may be another magnetic reversal in approximately 2000 years. That should be enough time to replace all of our compasses.

12. Scientists have long known that the magnetic pole moves. James Ross located the pole for the first time in 1831 after an exhausting arctic journey during which his ship got stuck in the ice for four years. No one returned until the next century. In 1904, Roald Amundsen found the pole again and discovered that it had moved--at least 50 km since the days of Ross. The pole kept going during the 20th century, north at an average speed of 10 km per year, lately accelerating "to 40 km (25 miles) per year. At this rate it will exit North America and reach Siberia in a few decades. A National Geographic article in 2005 said that the pole has moved 685 miles (1,100 kilometers) over the over the past century

13. Globally the magnetic field has weakened 10% since the 19th century. As remarkable as these changes sound, "they're mild compared to what Earth's magnetic field has done in the past," says University of California professor Gary Glatzmaier. Sometimes the field completely flips. The north and the south poles swap places. Such reversals, recorded in the magnetism of ancient rocks (sea-floor spreading), are unpredictable. They come at irregular intervals averaging about 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows. According to Glatzmaier, the ongoing 10% decline doesn't mean that a reversal is imminent. "The field is increasing or decreasing all the time," he says. "We know this from studies of the paleomagnetic record." Earth's present- day magnetic field is, in fact, much stronger than normal.

14. To understand what's happening we have to take a trip... to the center of the Earth where the magnetic field is produced. At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2° of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core." Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has "hurricanes"--whirlpools powered by the Coriolis forces of Earth's rotation. These complex motions generate our planet's magnetism through a process called the dynamo effect.

15. So, what happens during a magnetic flip? Reversals take a few thousand years to complete, and during that time--contrary to popular belief--the magnetic field does not vanish, it just gets more complicated. Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unaccustomed places.

18. Convergent Boundary Two plates are moving towards each other, depending on the type of crust that is colliding mountain ranges or volcanoes may form Oceanic-Continental Boundary When oceanic crust collides with continental crust the oceanic crust is subducted (sinks) under the continental crust in a subduction zone and becomes magma again. Oceanic-Oceanic Boundary When oceanic crust collides with oceanic crust one plate also sinks under the other. As this happens a deep trench is formed on the sea bed. Magma rising up from the subduction zone leads to the formation of volcanoes. Given enough time (millions of years) the submarine volcanoes grow large enough to rise above sea level and become new volcanic islands.

19. Continental-Continental Boundary When continental crust collides with continental crust neither is significantly more dense than the other one so they are both pushed up and/or sideways. This folding and buckling creates fold mountains.

25. The Causes of Plate Tectonic Movement Ridge Push: The crust is higher at mid-ocean ridges this causes the oceanic lithosphere to slide down the hill, due to gravity, and push the rest of the crust Slab Pull: The oceanic plate that is sinking in the subduction zone is very dense, as it sinks it pulls the rest of the tectonic plate with it

26. Convection: Hot rock in the earth rises since it is less dense and cooler rock near the surface sinks (denser). This movement of rocks causes the oceanic lithosphere to move sideways, away from the mid-ocean ridge.