Day 17: What happens to a coconut in a subduction zone
By Jamie Menzies
Well, not a lot really…
Over the past day or so the RV Investigator has been stopping at different parts of the Pocklington Trough to collect rock samples. Oceanic troughs and trenches are formed when one tectonic plate slides underneath another. Perhaps the most famous example in the world is the Mariana Trench – here the Pacific plate subducts underneath the smaller Mariana plate as it moves westwards. This creates a topographic depression in the seafloor which is actually the deepest point in the oceans, deeper than Mt. Everest is high!
Diagram of a subduction zone showing an oceanic plate subducting beneath a continental plate. Source: Wikimedia commons.
Route of the RV Investigator as of 1300 24/08/2019. We’ve been tracking the edge of the Louisiade Plateau (flatter area to the South) where it meets the Pocklington Trough, which you can see as the darker line running south west to north east.
In this area we’ve found lots of interesting rock samples and many of them have been altered probably due to the active tectonics in the area. Perhaps the most interesting is this green serpentinite. Serpentinisation is a type of metamorphic transformation which happens to rocks in the upper mantle and involves heat and water. The water causes rocks to oxidise and changes the mineral olivine into serpentine, magnetite (a magnetic mineral) and others. The geologists aren’t totally convinced that this rock is serpentinite, but that will be confirmed back on land.
Serpentinite, probably a rock from the base of the oceanic crust as it transitions to the upper mantle. The foliation, or layers, are caused as the minerals get squashed under pressure Image: Jamie Menzies.
In the same dredge we also discovered a coconut! It’s probably a first for science to have a coconut and a piece of the mantle in the same dredge. It was picked up near to the coast of the Solomon Islands, meaning the lonely coconut likely bobbed along on the sea surface carried by currents until eventually it became waterlogged and sank to the bottom of the ocean. It was still in pretty good condition.
A rather fresh, albeit smelly, coconut. Image credit: Edward Clennett, University of Sydney
We’ve had an exciting few days on the Louisiade Plateau, and are excited for what’s to come as we transition south towards the Lord Howe seamount chain.