Monday, August 2, 2010

The Magnetic Barcode of the Seafloor

Hello! My name is Duayne Rieger and I am starting on my third year as a PhD student in Geology and Geophysics at Yale University. My research interest is seismic anisotropy in Earth’s upper mantle and its relation to global tectonics. Today however, I am going to talk about magnetics data that can be collected out at sea, rather than seismic data, being that there will be plenty of time to talk about the seismic data we are collecting.

As we cruise through the open pacific, over the Shatsky Rise, we are making measurements of Earth’s magnetic field strength. The reason for collecting these data is to observe anomalies in Earth’s magnetic field strength that are produced by rocks in the seafloor. By measuring these anomalies, we can gain insight into the age and history of the seafloor. Here is how!

At spreading ridges, where two oceanic plates diverge from one another, new oceanic crust is formed through the partial melting of Earth’s upper mantle. As the new oceanic crustal rocks freeze from the partial melts, the magnetic minerals in the rocks take on the orientation of (or align with) the Earth’s magnetic field. If Earth’s magnetic field remained constant in time, than the entire seafloor would possess the same magnetic orientation. However, throughout Earth’s history its magnetic field has not been constant but in fact has been reversing. A magnetic reversal is where the magnetic north and south poles of Earth switch positions. Because, as just mentioned, oceanic crust takes on the magnetic orientation of the Earth’s field at the time of its formation, these reversals in Earth’s magnetic field can be seen in lines of reversing magnetic orientation along the seafloor that are parallel to the spreading ridge. This process is illustrated in the figure above.

So when making magnetic field strength measurements, the measurements will be slightly larger when we are over oceanic crust that possesses the same magnetic orientation as the Earth currently does and will be slightly smaller when over seafloor with the opposite magnetic orientation. Then by knowing when these magnetic reversals happened in Earth’s history, we can learn more about the age of the seafloor.

The magnetic lineations around the Shatsky Rise are very interesting. They present evidence that the Shatsky Rise was formed at a triple junction. A triple junction is where three, rather than two, oceanic plates spread apart from each other from three different ridges. The magnetic lineations around a triple junction are not all parallel to each other, as you would expect if the seafloor were produced at one ridge, but in fact have angles in them to match the geometry of the multiple spreading ridges. This is illustrated in the figure to the left.

The bottom figure is a bathymetric map of the Shatsky Rise with the magnetic lineations included. The red lines in the figure are there to illustrate the angle in the magnetic lineations. Since the magnetic lineations around the Shatsky rise have an angle in them, scientists have reason to believe that it was formed at an ancient triple junction. Magnetic lineations around other oceanic volcanoes (also know as Large Igneous Provinces) may suggest that they too were created near ancient triple junctions. Is there a connection between oceanic super volcanoes and triple junctions? That is one of the many questions that we are out here to answer!

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