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|
|Gravitometer located in the engine room of the HMNZS Wellington|
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|