Monday, March 9, 2015

Macauley Caldera

The first dive and the calm before the storm– March 8th 2015 – Macauley Caldera

We are currently located near Macauley Island, which is a small island (about 3 km2) that is part of what was a large volcano. Much of this volcano would have protruded from the ocean surface several thousand years ago. However, a large eruption about 6,300 years ago is thought to have caused much of the volcano to collapse, forming what geologists call a caldera (a collapsed volcano), and leaving just this small island above sea level. The caldera is about 9-10 km long and 5-6 km wide, which encompasses an area about the same size as Wellington harbour!).

Macauly Island from the HMNZS Wellington

At Macauley, our mission has two broad objectives. First, will be to locate all major hydrothermal vent sites within the Macauley caldera. These locations are of ecological significance, and can be sources of copper, zinc and gold mineral deposits. The second objective will be to assess what are called ‘sediment waveforms’. These are basically concentric ‘rings’ of sand that radiate outwards from the caldera flanks and which stand up to 100 m from the seafloor, becoming progressively smaller with distance from the volcano (see image below).
The purpose of mapping these features is to determine how they formed. There are currently two different ways these types of structures are thought to have formed. Firstly, by submarine landslides (or slumps), and secondly (considered more likely) by the volcanic ash column created during the caldera forming eruption hurtling into the sky then buckling under its own weight, causing it to collapse back down into the volcano and cascading into the sea. Either of these two scenarios could have potentially generated a tsunami, which means that assessing the likelihood of either scenario is an important contribution to assessing tsunami hazard in the region. By using ground penetrating sonar aboard Sentry, we can get an idea of the internal fabric of the waveforms.  For example,do the waveforms have a chaotic internal fabric, suggesting submarine landslides, or a more layered structure, indicative of collapse of the ash column?

Macauley caldera and sand waves, image created by Susan Merle of NOAA

(Macauley caldera and sand waves, image created by Susan Merle of NOAA)
We deployed Sentry for her first dive of this mission on March 7th 2015. Unfortunately, she resurfaced about 30-60 minutes after deployment due to communication issues between Sentry and the ship. In this case, as Sentry approaches the seafloor she wants to be able to “see” the seafloor; her sonar on-board was struggling to do so, so then she will obey commands in the programming to abort and come to surface, unless overridden by separate commands sent from the ship. With the communication (‘comms’ as they like to call it) problems between vehicle and ship, Sentry did not get an overriding command to keep diving and so surfaced. The good news is that on the morning of Sunday 8th March Sentry was deployed and in the evening (about 12 hrs later) she was successfully recovered! Now it is time to look at the data she collected on this first dive. About 33 GB of multibeam data alone were collected during the dive!
In other news, the weather the last couple of days has been fantastic! We have had a barbeque, passed near a huge pod of dolphins, and watched amazing sunsets.

Sunset, calm seas, and dolphins from the HMNZS Wellington

….But that is all about to change….We have had word that a large storm (potentially a category 4-5 cyclone) is heading our way. It is estimated to reach us on the weekend. This means that operations are likely to be either paused and we shelter behind Raoul Island (using it to block the brunt of the storm) until it passes, or cut and run (meaning an early return to Auckland). The first priority will be the safety of those on-board the ship. There is also the additional consideration about making sure that Sentry and any other equipment (including the ship itself) does not get damaged. Discussions are ongoing about the best strategy moving forward.


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