Thursday, March 12, 2015

The magnetic and gravity surveys

The magnetic and gravity surveys: Friday 13th March 2015 – On the way to Devonport

You will recall from my earlier posts that in between Sentry deployments she requires a period of battery recharging. When Sentry is not in the water we have been running surveys assessing the magnetism of the rocks on the seafloor and variations in gravity. I thought it would be a good idea to focus a post on these surveys. Dr Fabio Caratori Tontini is in charge of these surveys, and he will assist me with this blog post.
Volcanic rocks contain the minerals magnetite and/or titianomagnetite and this enables their magnetic signatures to be measured. This is particularly useful in the case of this mission as it will tell us information about what rocks the ship is travelling over and can also provide insight into volcanic processes. We can measure the magnetism of the rocks on the seafloor by using a piece of equipment called a magnetometer (or ‘maggie’ for short) that is towed behind the ship, in our case at a distance of about 200m so that the sensor does not ‘see’ the effects of all the steel that is the ship!

Magnetometer being deployed by a Navy sailor, Fabio and Cornel
The gravitometer, as you might expect, measures variations in the Earth’s gravitational field on the seafloor. This is useful because different rocks have different densities influence the local gravity, which we can detect. Therefore, by measuring these variations in the gravity measurements we can determine information about the type of rocks that are on the seafloor and through modelling, get a good idea of their thickness. When combining these two measurements together, we can get good insight into the volcanic processes that have occurred in the past to form the volcanic edifices that are on the sea floor.

Gravitometer located in the engine room of the HMNZS Wellington
 One interesting preliminary finding here is that inside the large Macauley caldera we can see distinct magnetic and gravity ‘lows’ (see picture).  When volcanic rocks solidify, their magnetic properties (of magnetite) will lock into a single direction aligned to that of the Earth’s magnetic field at the time, hence they will all ‘point’ in the same direction. But if that material has been blown apart and redeposited with a random orientation, as would material ejected during an eruption, the corresponding previously coherent magnetic signature will effectively be cancelled out as many bits of rock will have fallen in many different directions.  Having both low magnetic and gravity anomalies therefore suggests that the material in this area is of relatively low density and the magnetism is very erratic (or demagnetised), almost certainly the consequence of a huge eruption with the collapse of the eruption column falling back down and infilling the caldera. 

maps showing the results from the gravity and magnetic surveys. Notice the orangey-browny coloured section within the gravity map and the blue area within the magnetic map indicating the centre of the caldera and material from the collapsed ash column is potentially

The gravity survey tells us that the rocks inside the caldera at Macauley volcano are of low density. This suggests that the material is not one consolidated mass. The magnetic survey shows the caldera infill material has no real magnetic anomaly. When considering these two pieces of evidence together (magnetism pointing in all directions and low density rocks) this suggests that the material at this location is likely to be unconsolidated material and not one large, dense mass of rock, such as lava. When we consider that this is the site of a volcano which we know to have erupted violently in the past we can then begin to make some interpretations about what this material could be. In all likelihood the material infilling the caldera is from the volcanic ash column after it collapsed, thereby filling the caldera caldera (see my crudely drawn image below).

Josh’s crudely drawn graphic of the potential sequence of events based on our gravity and magnetic surveys

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