Hikurangi subduction zone expedition #375 – blog 4
In March 2018, JOIDES Resolution, a large scientific research vessel, headed out to sea to research the Hikurangi subduction zone on expedition #375. This article is the fourth blog from Aliki Weststrate, IODP (International Ocean Discovery Program) Outreach Educator. This is her account of a voyage full of excitement, challenges and science!
It’s week 4, and we’re halfway through expedition #375 to study the Hikurangi subduction zone. Americans call this half-way point ‘hump day’, so there were lots of funny photos of camels with humps pinned up around the ship. Usually we have a dance party to celebrate, but we had one last week, so I think we’re breaking tradition and may have another in week 6. It’s important to balance the long 12-hour shifts with leisure time!
Hump day celebrations
The middle of a United States voyage is traditionally celebrated as hump day. It was no exception for expedition #375, and funny camels appeared everywhere around JOIDES Resolution. Two months is a long time to be working with no weekends off, in an enclosed space, so every cause to celebrate is welcomed!
This week, we moved 90 km east of Gisborne to our second site, which has a pragmatic name – U1520. The site is next to the Tūranganui Knoll, which is a 840 m high seamount (an extinct underwater volcano).
The water depth is 3,522 m and is deeper than our first site (U1518) because we are further out from the coastal platform and in a deep ocean basin. We are drilling into sandstone and mudstone rather than into the fault zone as we did before at U1518.
Ship monitors showing ship location
The ship’s TV monitors on each level show the exact location of JOIDES Resolution with a red circle. This contour map shows JOIDES Resolution’s position at site U1520, just to the left of an ancient volcano called a seamount.
Our mission here
We are collecting sediment cores from up to 1,050 metres below seafloor (mbsf) to study the state and composition of the material here. This material is currently part of the Pacific tectonic plate, and it will eventually (over millions of years) be carried westward as the Pacific plate moves into the trench as if it were on a conveyor belt.
Some of the sediment will be dragged under (subducted into) the Australian plate. The rest will be bulldozed and become part of the accretionary wedge – this is a mass of sedimentary material scraped off the Pacific oceanic crust during subduction and piled up at the edge of the Australian continental crustal plate. You can see it in the coloured map here as the purple, bluish, and green ‘hillsides’ leading up to U1517.
3D map of expedition sites
This 3D image shows expedition #375 next to the Tūranganui Knoll.
The Pacific plate is getting denser as it cools and nears the continental Australian plate and begins to subduct (dive under) between drilling sites U1520 and U1518. The subducting material that is not dragged under gets bulldozed up onto the hanging wall above the fault zone. This appears on the map as the purple, bluish and green ‘hillsides’ leading up to U1517.
The geological before and after
Understanding what this proto or input sediment is made of and how it behaves before it is dragged into and above the fault zone means we can compare the two sites. We can conduct experiments on this input material by putting it through fluid and pressure tests to mimic what might happen to it as it is transported by the subduction conveyor belt deep into the slow slip source area of the fault zone.
Reasons for taking core samples
In response to questions asked by Otumoetai Intermediate School students, expedition Co-chief Scientist Dr Demian Saffer shows how sediment and rock in a core sample change as the drill goes down deeper and deeper. Geologists on board the research ship JOIDES Resolution take small geological samples back to their home laboratories for further study.
Note: This video footage was bounced off a satellite during a Skype session on board the ship. The beeping in the background is the cryomagnetometer – learn more about this specialised lab machine in the video What do core samples tell us?
If you find the sound quality difficult, please refer to the transcript.
We are also measuring and recording its current state – lithology, fossil types, palaeomagnetic properties, temperature, porosity, density, shear strength and chemistry.
All the experiments we are doing now in the JOIDES Resolution labs we will continue and extend back on land after the voyage. The tests should show us how this input material might behave once it is subjected to the forces of the megathrust zone 20 km west of here. It might also shed light on why there are slow-slip events in this region too.
What do core samples tell us?
Dr Demian Saffer and Aliki Weststrate from JOIDES Resolution expedition #375 tell us how scientists read the clues found in core samples taken from deep under the ocean.
Point of interest: Why do you think Aliki compares magnetic reversals to a barcode in the rock?
Note: This video footage was bounced off a satellite during a Skype session with Otumoetai Intermediate School students. If you find the sound quality difficult, please refer to the transcript.
Our research questions
Are the rocks and sediments here near the seamount unusual? What are they made of?
Are they dense and compacted out here in the basin at U1520, and do they release fluid as they are heated and buried on the way into the fault zone and slow slip area?
How does this incoming sediment react when it comes under increasing friction as it nears the subduction zone?
Will all of it be subducted, or will some be bulldozed onto the hanging wall above?
What are we seeing here in the cores?
The cores we are bringing up are getting progressively older the deeper we get, and we know this from the type of fossils the palaeontologists are seeing under their microscopes – which is no surprise. What is most noticeable to the untrained eye is how hard and brittle this core is compared to our first site above the fault zone.
Core from 800 mbsf
This core is from 800 mbsf (metres below seafloor) in the Hikurangi subduction zone basin. It has been split in half so geologists can describe it and take samples for experiments. It is mudstone with some nuggets of volcanic debris scattered through it (called basaltic clasts). The fossils in them have been dated to about 40 million years old.
At our first site above the fault zone, the core was quite wet and soft, so it was easy for the scientists to get samples out themselves using small cylinders and tubes. Here out at site U1520, the trained technicians are having to use the circular saw to cut samples for the geologists. The cores are almost as hard as cement, with very little fluid in them.
Cutting a core sample
Usually the core samples (long tubes of sediment and rock the drill pipe brings up from below the seafloor) are soft enough to take samples from. Here, in deep ocean and depths, it has become cemented, so a circular saw is needed to cut pieces off for lab testing.
Related content
This article gives some background to the scientific research vessel JOIDES Resolution.
Read Aliki’s other blog articles here.
Many scientists use rock core samples, such as Dr Phil Shane who is looking at our volcanic history. Find out more about his work reading rock core samples.
Learn more about seamounts and the biodiversity found around these underwater mountains in this video.
Activity idea
Your students may like to try making a core sample and examining it for features like particle size, colour variation and layering in this activity.
Useful links
Watch this YouTube clip about JOIDES Resolution and its research.
Read Aliki’s blog on seamounts and slow slips for the JOIDES Resolution here.
Learn more about the international science vessel JOIDES Resolution and more about expedition #375.
Watch the expedition #375 trailer – what is our mission at the Hikurangi subduction zone?
Follow the International Ocean Discovery Program – JOIDES Resolution Science Operator website for the information and resources on JOIDES Resolution expeditions around the world.
Acknowledgement
This article was written by staff at GNS Science working as part of the ANZIC IODP Consortium.
GNS Science
GNS Science, Te Pū Ao, is New Zealand’s leading provider of Earth, geoscience and isotope research and consultancy services. Its purpose is to understand natural Earth system processes and resources and to translate these into economic, environmental and social benefits.
