SBMA Science Series: How Deep is the Ocean
The SBMA will be releasing a series of science articles over the next few months to educate and empower the readers on the fundamental science concepts and characteristics involved with Deep Ocean Seabed Minerals Research and discoveries.
The Cook Islands includes fifteen islands and reefs scattered across the southwest Pacific Ocean. Our Exclusive Economic Zone (EEZ) extends two hundred nautical miles from each island and covers an area of almost two million square kilometres. But not only is our EEZ very large, it is also very deep; many kilometres deep.
The Forming of Our Islands
Over a period of time, these islands came about from volcanic eruptions that formed large underwater volcanoes called seamounts [1]. These tall seamounts rise from the seafloor all over the world [2], with our islands and reefs resting on the taller ones that emerged from the sea before erosion from rain and waves. This is very different from continents such as Australia and North America, which have large areas of relatively shallow seafloor connected to their land.
In the Cook Islands, the seamounts that support our islands formed millions to tens of millions of years ago. (I.e. The volcanic rocks of Rarotonga are an exception as they are ‘only’ about 1.4 million years old [3]) These seamounts are tall— ranging from five to six thousand metres. To illustrate this, take Mount Everest, reaching almost nine thousand metres tall. However, the base of Mount Everest is already at four thousand metres, so the mountain itself is actually similar in height to our seamounts!
Another interesting geological feature, besides our seamounts, is a massive underwater volcano called the Manihiki Plateau. This was formed by a massive outpouring of volcanic rocks about 125 million years ago and today stretches from Manhiki and Rakahanga all the way to Pukapuka and Nassau [4][5].
How Deep is Deep?
The deepest part of our EEZ is more than 6,000 metres deep. This is more than halfway of the Mariana Trench, which the deepest known part of the ocean (~11,000 metres deep).
The average water depth of the Cook Islands EEZ is about 5,000 metres [6], and almost all of our EEZ deeper than 2,500 metres. But how deep really is 5,000 metres? To give a reference, it is about the distance from Avarua Church to just past the airport, or 1.5 times higher than the height at which Air Raro flies between the islands of the southern group.
As you travel deep into the sea, the weight of the water on top of you grows as pressure. 5,000 meters is a lot of pressure on the organisms that live there. It is similar to having fifty elephants standing on your head [7]. Fish and other life are capable of surviving at these depth because their bodies have adapted ways to compensate for the pressure.
What’s going on at the Seafloor?
As you might have realised, all of our deep seafloor isn’t the same. There is a whole underwater landscape down there. The Cook Islands Seabed Mineral Authority recently published a map of the seabed [8]. They identified all sorts of landforms, such abyssal plains, volcanic knolls , ridges, plateaus, troughs, trenches, to name a few. This map was well received overseas and even won an award. Of special interest are the abyssal plains, because this is where explorers have found polymetallic nodules that one day might be worth harvesting. Recent studies estimate they are found across an area of as much as 900,000 km². That is more than three times the land area of New Zealand! Come on, let’s explore this vast area!
Although this great water depth is favourable for the formation of metal-rich nodules, it makes exploration for them difficult, expensive and risky. Simple tasks like seafloor sampling take a long time—taking at least three hours to recover a single sample from the seafloor. Survey vessels with all their crew, and fuel bills can cost more than $50,000 a day, so that single sample could cost more than $6,000 dollars. Even then, it is not unusual for a sampling device to come back empty or to be lost. In that case, if you really need that sample, you simply have to try again, leading to more costs.
Also, despite what the movies show, sending people to the deep ocean floor is expensive, dangerous and ineffective. Deep sea development thus relies on unmanned remotely controlled tools, and this industry is setting new standards in working effectively and remotely. Equipment that can cope with these extreme conditions requires highly specialised technology, similar to the technology used in sending rockets to the moon!7 As the picture shows, while the equipment might seem large when you stand next to it, against the scale of the ocean it seems really tiny.
Most of our EEZ is mapped by satellite data, and constrained by bathymetry sonar tracks collected by ships. One of the benefits of the exploration for polymetallic nodules will be the accurate mapping of our deep seafloor, to the resolution of a few metres. There is still much on the Cook Islands seabed that remains undiscovered, but learning more is sure to lead to a deeper understanding of our sea floor resources and its environmental habitats.
References
1. National Oceanic and Atmospheric Administration. What is a seamount? Ocean Exploration Facts 1 https://oceanexplorer.noaa.gov/facts/seamounts.html (2020).
2. Kim, S.-S. & Wessel, P. New global seamount census from altimetry-derived gravity data. Geophys. J. Int. 186, 615–631 (2011).
3. Jackson, M. G. et al. Contrasting Old and Young Volcanism from Aitutaki, Cook Islands: Implications for the Origins of the Cook–Austral Volcanic Chain. J. Petrol. 61, (2020).
4. Winterer, E. ., Lonsdale, P. ., Matthews, J. . & Rosendahl, B. . Structure and acoustic stratigraphy of the Manihiki Plateau. Deep Sea Res. Oceanogr. Abstr. 21, 793–813 (1974).
1. National Oceanic and Atmospheric Administration. What is a seamount? Ocean Exploration Facts 1 https://oceanexplorer.noaa.gov/facts/seamounts.html (2020).
2. Kim, S.-S. & Wessel, P. New global seamount census from altimetry-derived gravity data. Geophys. J. Int. 186, 615–631 (2011).
3. Jackson, M. G. et al. Contrasting Old and Young Volcanism from Aitutaki, Cook Islands: Implications for the Origins of the Cook–Austral Volcanic Chain. J. Petrol. 61, (2020).
4. Winterer, E. ., Lonsdale, P. ., Matthews, J. . & Rosendahl, B. . Structure and acoustic stratigraphy of the Manihiki Plateau. Deep Sea Res. Oceanogr. Abstr. 21, 793–813 (1974).
5. Taylor, B. The single largest oceanic plateau: Ontong Java–Manihiki–Hikurangi. Earth Planet. Sci. Lett. 241, 372–380 (2006).
6. GEBCO Compilation Group. GEBCO 2023 Grid. (2023) doi:10.5285/f98b053b-0cbc-6c23-e053-6c86abc0af7b.
7. Yancey, P. Life under pressure - 100 elephants on your head. https://schmidtocean.org/cruise-log-post/life-under-pressure-100-elephants-on-your-head/ (2014).
8. Browne, R., Parianos, J. & Murphy, A. Geomorphology of the Cook Islands, tropical South Pacific Ocean. J. Maps 1–7 (2023) doi:10.1080/17445647.2023.2169889.
Author: Ray Wood Co Author: Cook Islands Seabed Minerals Authority
Source: Originally published in Cook Islands News: 13th April 2024 and Revised: 19th April 2024