sea90E - an oceanographic expedition to the Indian Ocean



	Magnetometers (“Maggie”) and Attracting Sharks Magnetic profiling On the RV Roger Revelle, one of the ways we map the ocean floor is by using a magnetometer. This instrument measures magnetic variation with considerable accuracy. Since the magnetic field of the earth has already been mapped in detail by satellites such as MAGSAT and by other surface ships and aircraft, we can look at the small differences in our 90 East Ridge survey area to learn even more about the earth’s composition. The map below was prepared by combining multiple magnetic survey methods. (www.geomag.usgs.gov) This map shows the baseline magnetic field in the Indian Ocean. Did you know? The magnetic field of the Earth reverses at irregular intervals! The reversal takes 5,000 years or less, with the intensity of the field declining to almost zero and then building back again -- but then the field intensity builds up in the opposite direction! That means that the magnetic North Pole hasn't always been in the northern hemisphere. It is thought that the Earth's magnetic field has reversed 17 times in the last 4 million years. This map shows lines of where the current magnetic field is strong and weak. It also shows the location of the current north and south poles. The figure below shows this reversal in magnetic poles. Because of magnetic force reversal, we can learn something really interesting about rocks. (and confirm something we also know about SHARKS) Many rocks record the orientation of the Earth's magnetic field at the time they are formed. It's like a little record of the magnetic field got frozen into that rock. So, by looking at ancient rocks, we can tell the polarity of the magnetic field of the Earth even back to ancient times! Here our team unreels Maggie's yellow cord. (left to right: Shari, Brandi, Dax, Meghan and just visible to the right of the winch is Amy) Our magnetometer instruments are towed behind the ship on a long yellow cable. The magnetometer makes a magnetic survey of the sea floor. Magnetic signatures are arranged in bands parallel to mid-ocean ridges. In fact, mid-ocean ridges have a magnetic signatures in rocks on one side of the ridge were mirrored by magnetic signatures on the other side of the ridge. What does this mean? Well, we think that mid-ocean ridges are places where new ocean crust is being created by upwelling material from the deep interior of the Earth. Another way to say this is that mid-ocean ridges are places of seafloor spreading. Here molten rock rises up from the depth of the Earth and makes new crust. Remember Hotspots? The new crust forms in bands on either side of the mid-ocean ridge. Each newly created band pushes the already-existing crust a little farther away from the mid-ocean ridge. And of course, as each new band forms, the current orientation of the Earth's magnetic field is frozen into it. In the slide below, a part of our onboard seminar sessions: Krishna, a scientist from India, presented some of the magnetic data that suggests that parts of the ridge moved from one major plate to another during a part of the formation period. Krishna is a specialist in magnetic analysis. Based in part on the findings of his research, we think that the formation of the Ninetyeast Ridge is somewhat more complex than is explained by a more simple hotspot theory. AND, this is why we are here collecting magnetic data and rocks! The map above of the coast of California shows the magnetic signature of these magnetic reversals. We are mapping the 90 East Ridge in the same way. Here "Maggie" and her twin sister wait to become shark bait once again. The solid metal object in the case above "Maggie" is a pinger which is used to send out a "ping" sound when we dredge. (The pinger doesn't really attract sharks.) "Maggie” is a useful tool in our research because it provides important data that can be used to map magnetic anomalies and date ridge sequences. The age of the ridge can be determined by comparing geochemical and magnetic ages. If they both agree, we confirm the time that the area formed. Now, isn’t that cool? But, did I tell you about the sharks? No? Well, it turns out that sharks have special sensors up above their nose that pick up “Maggie’s” signal. "MAGGIE" ATTRACTS SHARKS! Take a look at these teeth marks. The coil that generates the electric magnetism is surrounded by almost 2 inches of tough material; good thing, huh? Here Brandi extracts part of a shark's tooth from "Maggie" with a metal tool. Note the deep, repeated slashing cuts to the left. We had wondered why we don’t ever get to go swimming while we are at sea. Well, here is one good reason why. updated July 4, 2007 from the Indian Ocean
			High.Seas.HIGHTECH/ analyze!ocean@voyages, by lots of us... How to Become a Captain, by the Captain The Search for the Indo-Pacific Humpback Dolphin, by Leslie Nemazi. Rocks Rock! (or ) The Fresh Cut Surface of Beautiful Ocean Rock, by Evelyn Mervine Exploring the Bridge of the RV Roger Revelle Dredging Operations onboard the R/V Revelle, by Amy Eisin Geochemistry of Volcanic Rocks, by Fred Frey Cool Expedition Discoveries Already!, by Will Sager Anatomy of a Seamount Survey, by Will Sager M agnetometers (“Maggie”) and Attracting Sharks, by Rory Wilson Acoustics: Substitute for Superman Vision?, by Will Sager What is Bathymetry?, by Will Sager Hotspots and Ninetyeast Ridge: What’s a hotspot?, by Will Sager