Guardians of the Caldera

Four hundred and eighty kilometers from the Oregon Coast lies an active submarine volcano known as Axial Seamount. It is one of the most studied underwater volcanoes in the world. The site is home to the first underwater observatory, the  New Millennium Observatory (NeMO) . Which was later followed by the world's first cable powered observatory, the Regional Cabled Array (RCA), funded by the National Science Foundation as part of the Ocean Observatories Initiative. The RCA includes fiber optic cables running the length of the ocean floor from Pacific City Oregon, across the Juan de Fuca Plate to Axial, a first-of-its-kind program championed by UW’s Dr. John Delaney. The RCA became operational in 2014. With high power and equally high bandwidth cables that power an intricate arrangement of 150 scientific instruments allowing scientists, educators and the public to access real-time data streaming to shore at the speed of light. This year, Axial became home to yet another first. 

During the third leg of the  VISIONS ’22 expedition , which provides annual maintenance to the RCA and offers marine science/engineering field experience to undergraduates, the Axial Acoustic Array was deployed from the UW research vessel the  Thomas G. Thompson  with the help of the Canadian Remotely Operated Vehicle (ROV) ROPOS. This project, proposed by UW marine geophysicist Dr. William Wilcock and Applied Physics Lab (APL) Engineer Dana Manalang, and funded by the National Science Foundation (NSF), consists of a network of Sonardyne Fetch sensors mounted on tripods built by the APL. The platforms are located on the eastern and western walls of Axial Caldera, as well as two sites within the caldera itself ~ 1500 m beneath the oceans’ surface. The Fetch instruments include a pressure gauge, temperature sensor, and velocimeter and provide important insight into the behavior and function of the Cascadian Subduction Zone.

Axial is a part of the Juan de Fuca mid-ocean ridge, which forms where the Pacific and Juan de fuca tectonic plates meet, and where they intersect with the Cobb-Eickelber seamount chain. It acts as a spreading center, with increased magma supply from the Cobb hotspot, it expands at a rate of 6 cm per year. 60% of the Earth's resurfacing comes from spreading centers over timespans of hundreds of millions of years.

Having erupted in 1998, 2011 and again in 2015, the Axial Seamount is the most active submarine volcano along the Juan de Fuca spreading center and an important region of study for what it can tell us about the behavior of subduction zones.

At the seamount's summit, is the Axial Caldera, an 8 kilometer long by 3 kilometer wide rectangle open at the southeastern edge and who's floor (at 100 m deep) lies 3/4 of a mile below the ocean's surface at the deepest part of the caldera. Here you will find a landscape formed by magmatic activity seemingly unlike any other and active  hydrothermal vents  with life forms that couldn't have even been imagined merely decades prior.

At the west and east walls of the caldera there are faults angled outward at 60 degrees, which are presumed to meet underneath the northern ridge. If this is true, they form a ring fault that may play an integral role in the behavior of the inflation of the magma chambers.

Below the caldera you will find a primary and secondary magma chambers. The main chamber is 1.1 to 2.3 kilometers deep and shallower on the southeastern side, similar to the caldera itself. The secondary magma chamber lies 5 kilometers to the east of the primary. It is the eruptions of these magma chambers that shape the landscape we see above.

To make sense of this alien world there are more than 20 instruments stationed within the caldera itself, spread between five sites: Central caldera, Eastern Caldera, the Ashes Vent Field and the International District hydrothermal vent field (which has two sites). All of the site instruments are connected to the RCA via Primary Node PN3B. 

The  Ashes vent field  is an area of hydrothermal activity east of the caldera boundary fault. With active hydrothermal vents referred to as "chimneys" like Mushroom and Inferno, that boast vibrant chemosynthetic communities surrounding their mineral smoke billowing vents. Instrumentation at this site includes: a medium powered J-box, an HD camera, a long-duration osmotic fluid sampler, a three-dimensional thermistory array, a bottom pressure-tilt instrument, CTD (Conductivity, Temperature & Depth) and a multi-beam sonar system.

The  International District  lies outside the southeast boundary fault of the caldera. A landscape shaped by submarine lava flow, fields telling the stories of previous eruptions and regular magmatic activity. In addition to lava channels to the west and extinct chimneys, the international district has the largest chimney deemed El Guapo that also plays host to an abundant chemosynthetic community.

It is believed that the chemosynthetic communities that form around hydrothermal vents like El Guapo may unlock secrets of the early formation and evolution of life on earth.

As stated, the International District has two monitoring sites, one to monitor high temperature venting processes and the other for low temperature processes. The instrumentation at these sites include: a medium powered J-box, a mass spectrometer, adaptive diffuse fluid and microbial DNA samplers, two instruments to measure high-temperature vent fluid and volatile chemistry, a digital still camera, a seismometer, bottom-pressure tilt instrument and current meter.

The  Eastern Caldera  does not offer the same overwhelming active geological formations on its surface that the thermal vent fields do. However, this site is important in the monitoring of the geophysical processes of the caldera. Here instrumentation monitors seismic behavior, as well as the vertical inflation and deflation of the magma chambers within Axial. Instrumentation at this site includes: a medium powered J-box, a hydrophone, a bottom pressure & tilt meter, short-period seismometers and a broadband seismometer.

The  Central Caldera  site sits at a depth of 1523 m and is another important site for the study of geophysical proccesses within the caldera. Both the Central and Eastern sites were chosen for the flation and deflation behavior of their magma chambers and for the level of seismic activity. During the eruption in 2015, the cabled array measured over 8,000 earthquakes!

You can learn more about the instruments installed at each site on the Interactive Oceans website for the Regional Cabled Array:  https://interactiveoceans.washington.edu/instruments/  

ROV Ropos was born in 1986, a HYSUB 5000 remotely operated vehicle, brought into the world by International Submarine Engineering (ISE). This first generation of ROPOS was operated by the federal Canadian Department of Fisheries and Oceans (DFO). It would make 377 dives before being lost at sea during a severe storm in 1996.

The second generation ROV ROPOS was also built by ISE. However, the ROV would now be operated by the Canadian Scientific Submersible Facility, members of which helped in designing the upgraded version of the first generation ROPOS.

In all, the second generation ROPOS would complete 572 dives from 1996 to 2006 serving a variety of different purposes including mid-ocean ridges, gas hydrate, habitat surveys, cabled seafloor observatories and search & recovery operations.

In 2005, the  Canadian Scientific Submersible Facility  really set about making ROPOS their own with 2.33 million dollars in upgrades that would bring ROPOS' capabilities into the modern era. Including a new frame, 3000-volt submotor and a fiber-optic telemetry system.

Subsequent upgrades continued to be made, like a launch and recovery crane in 2006 and the addition of HD Cameras and LED lighting systems from 2007 to 2012. Followed by hydraulic system upgrades in 2013.

Funding for ROPOS development and upgrades were the collective efforts of the Canadian Department of Fisheries and Oceans, Natural Resources Canada, Natural Sciences and Engineering Research Council of Canada, The Canadian Scientific Submersible Facility, the Canadian Foundation for Innovation and the British Columbia Knowledge Development fund.

To learn more about ROPOS and its role in the deployment of the Axial Acoustic Array an interview was conducted aboard the RV Thomas G. Thompson with CSSF Operations Manager Keith Tamburri.

 AZA Fetch  was designed by a company specializing in marine acoustic technology company called Sonardyne.

As stated by the product description, some of the AZA Fetch features include: autonomous recalibration to avoid drift over time, 10 year battery life, regularly calibrated pressure sensor, fine vertical movement measurement capabilities, time stamped data, they are also compatible with third party sensor technologies. The AZA Fetch is rated for up to 3000m. They can communicated acoustically, via bluetooth or via direct cabled connection. The glass orb is surrounding by a PVC casing with a steel guard and titanium ports. The company states that they are ideal for environmental surveys and seabed deformation monitoring.

The ability of the Fetch to remove drift via self calibration is expected to allow for greater sensitivity in the measurement of seafloor pressure. If the A-O-A calibration of the AZA Fetch proves effective, it could allow for better observation of subduction zone slip events over greater time scales.

The Sonardyne Fetch tripods stationed in and around the caldera can communicate acoustically by sending sound waves between transponder pairs, but only one of the four is attached to the RCA cable to relay information back to researchers in real-time. Based on proximity and acoustic ranging between the towers, scientists will be able to track inflation and deflation of the volcano and horizontal spreading of the ocean floor around the caldera.

While there is already an extensive record of vertical movement of Axial Seamount starting in 1987, there are limitations to the initial system’s ability to differentiate between magma chamber inflation and faulting activity.

In addition to monitoring vertical movement, the tracking of horizontal movement of the caldera may offer new insights into the behavior of the caldera. The horizontal component of the Axial Acoustic Array is intended to paint a more complete picture of Axial movements around inflation, deflation, and uplift related to faults on the east and west sides of the caldera. These ring faults have shown to have seismic activity leading up to Axial eruptions with movement of 3 meter in the east fault and 1 meter in the west fault. The ability to take finer measurements, that were not previously possible, is necessary to confirm that the movements of the two faults are linked. These faults have also been found to have aseismic movement. It is the hope that the use of Sonardyne Fetch array, in conjunction with the data already being collected by the RCA, can help to more clearly identify the role of magma chamber inflation and deflation in identifying the volcano’s eruption patterns.

One of the most exciting aspects of the array is the role that it can play in communicating with and directing Underwater Autonomous Vehicles (AUVs). This capability may allow a resident AUV within Axial Caldera to closely monitor the volcano’s behavior and overlying ocean waters prior to, during and after an underwater eruption: this would be the first experiment of its kind along a mid-ocean ridge spreading center where 70% of the volcanism on Earth occurs. In addition, AUVs play an essential role in collecting high-quality bathymetric imagery and have even been used to identify previously unknown hydrothermal vent sites.

CSSF (n.d.). ROPOS. Retrieved November 17, 2022, from https://www.ropos.com/index.php/ropos-rov/meet-ropos  

Manalang, D., & Delaney, J. R. (2016). Axial Seamount - Restless, wired and occupied: A conceptual overview of resident AUV operations and technologies. OCEANS 2016 MTS/IEEE Monterey. https://doi.org/10.1109/oceans.2016.7761305  

Manalang, D., & Wilcock, W. (n.d.) Axial Acoustic Array Proposal Retrieved September 1, 2022

OOI. (n.d.). Regional Cabled Array. OOI Regional Cabled Array. Retrieved November 17, 2022, from https://interactiveoceans.washington.edu/ 

PMEL, NOAA. (n.d.). Axial Seamount - hydrothermal vents. Retrieved November 17, 2022, from https://www.pmel.noaa.gov/eoi/axial_site.html 

Sonardyne. (2021, July 28). Fetch Aza. Sonardyne. Retrieved November 17, 2022, from https://www.sonardyne.com/products/fetch-aza/ 

US Department of Commerce & NOAA (2009, February 1). What is a hydrothermal vent? NOAA's National Ocean Service. Retrieved November 18, 2022, from https://oceanservice.noaa.gov/facts/vents.html