Thursday, October 6, 2016

TAN-1611 - Post 4

Gravity and Magnetics

For the last few days the seas have been a little on the rough side which generally means that the boat has to slow down; the multi-beam data generally suffers in these conditions because the ship is rolling around a lot more.  We are hoping that this will pass soon and we can get back to full speed.  Regardless, everyone is happy and ship-shape.  I find it surprising that the cooks on board are still able to provide fresh fruit and vegetables everyday, along with copious amounts of baked goods and hot meals.  We are very lucky indeed!

Moving on from last weeks blog on the multi-beam system I thought I would talk about the geophysics side of data collection today.  This involves measurements of gravity and magnetism at the seafloor using two instruments: a gravimeter, and a magnetometer, and is run by Fabio Caratori Tontini (marine geophysicist), along with Christian Timm and Rachel Barrett.

 The role of the gravimeter is to measure variations in the Earth’s gravitational field at the seafloor which occur due to differences in density and thickness of rocks.  Different rock types have different densities; Earth’s crust has a density of roughly 2.9g/cm3 whereas rocks at 100km depth have a density off roughly 3.3g/cm3.  So by measuring the variation in the gravitational field we can understand something about the different rock types present at different depths.  Gravitational anomalies can also be related to variation in the thicknesses of Earth’s layers (like the upper and lower crustal lithosphere).

The magnetometer is a  one and a half metre long torpedo shaped object that is towed behind the ship at a distance of 300 metres to make sure the metal of the ship does not interfere with measurements.  From its position behind the ship it measures changes in the seafloor magnetic field which is related to a variation rock types and age.  Different rock types have their own characteristics, a volcanic rock characteristically contains a number of iron based minerals which are magnetic, sediments on the other hand do not, generally speaking.  This means that the volcanic rocks will have a stronger magnetic signal than the marine sediments and we can see this in the magnetometer data. 

Fig. 4.  An image showing magnetic anomalies around Macauley Island. Red/purple indicates higher magnetism and blue shows lower magnetism.  The map shows how complicate the pattern of magnetism could be on the seafloor.

The data also allow Fabio to gain an understanding of relative age - the age of something relative to a reference with a known age.  A magnetic signature can only develop when a rock forms, so before a volcano erupts the magma cannot become magnetised because it is like a thick liquid and at a very high temperature.  After an eruption, minerals in the magma cool below their Currie temperature - the point at which they can take on a magnetic direction (called a magnetic polarity).  Importantly, the Earth has gone through several changes in the polarity of its magnetic field, where the position of magnetic North flips to the opposite direction.  It is these magnetic reversals that we compare the rocks against to give us an idea of the time the formed.

The aim of making these measurements is to say something about the variation in rock types, the variation in the age of formation, and to use alongside a geological model to understand the shape, composition, and thickness of the Earth’s outer layers.  The measurements also help Christian Timm, the voyage petrologist, to make a decision on where we should dredged for rock samples - a tough job if the rocks are hidden under several thousand metres of ocean!

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