Slackjaw Sally here with more instrument deployment news! Along with turning on the multibeam, when we’re in international waters we pull a magnetometer (Maggy) behind the ship. Maggy measures magnetic field strength and detects variations in the magnetic field of the seafloor. A stronger magnetic field could be due to a sunken ship made of steel, or a rock formation containing grains of magnetite. Scientists use magnetic data to identify important archeological sites and to estimate the age and thickness of volcanic lava flows at mid-ocean ridges. Engineers use magnetic data to locate pipelines, undersea cables and bridge foundations.
How can magnetic data help us estimate age? Believe it or not, the ocean floor is striped. What!? Striped? No, not like black and white or colored stripes you see with your eyes. These are stripes of normal and reversed polarity of the magnetic field recorded by magnetic minerals in the lavas of the ocean floor. Where do these stripes come from? As new lava cools, the magnetic minerals in it record the current orientation of the Earth’s magnetic field. When the Earth’s magnetic field changes, the minerals in the new lavas record the new orientation of the field, but the old lava is already frozen solid and retains the orientation of the old magnetic field. Scientists map the magnetic stripes using data from magnetometers pulled behind boats, or in submarines run close to the seafloor, which determine the direction of magnetic field in the rocks. The nearly symmetrical pattern of stripes moving in both directions away from the mid-ocean ridge is a time stamp of the magnetic flip-flops or reversals which our planet has seen. There have been about 170 magnetic flip-flops in the last 76 million years. These stripes help geologists figure out how fast the tectonic plates are moving over our planet.
The magnetometer is used to find the stripes. Maggy is a cylindrical, 1-meter-long instrument pulled behind the ship on a cable about three times the length of the ship (to avoid magnetic effects of the ship). Maggy has a chamber filled with hydrogen-rich liquid, like kerosene or methanol, and a power source. A magnetic field is applied to the liquid to spin the protons and when that magnetic field is turned off, the protons spiral back to alignment with the Earth’s geomagnetic field. Maggy measures the frequency of proton spiraling to calculate the total geomagnetic field. As an Overhauser magnetometer, Maggy can resolve the magnetic field into vectors of strength, inclination (angle of the magnetic field line’s intersection with Earth’s surface) and declination (angle from geographic north). This allows Maggy to measure the strength and the direction of the magnetic field as we move along above the ocean floor.
Take a look at the figure below to see how the magnetometer data lines up with the magnetic stripes on the seafloor. When the red line is above the dotted line, the magnetic field is in the normal direction and when the red line is below the dotted line, the magnetic field is in the reverse direction. A normal field is what we see today.- our compass needles point towards the magnetic north pole. If the field were to flip, those needles would point towards our current magnetic south pole.
I’ll be back with more about spreading seafloors tomorrow. Right now, I’m feeling a magnetic attraction to snacks.
Your munch magnet,
Sally
Image courtesy of Jeff Gee (Scripps)
