When we dream of landscapes, we might imagine rolling valleys or rugged mountains. But there is a whole landscape hidden from human view: the secret world of the seafloor.
Authors
- Jessica Irving
Associate Professor in Global Seismology, University of Bristol
- Elizabeth Day
Senior Teaching Fellow in Geophysics, Imperial College London
Half of Earth's oceans are more than 3.2km deep. Beneath them lie cavernous plains untouched by sunlight, vast gaping trenches made by Earth's tectonic plates shifting, and ranges of underwater mountains on which no human has ever set foot.
We have better maps of the surface of the Moon than of these secret landscapes of the seafloor. However, the international 2030 seafloor project has an ambitious aim: to create a definitive map of our oceans.
To date, despite huge efforts, less than a third of our oceans have been fully mapped . But one unexpected way to help understand what's beneath the surface may come from a project one of us (Jessica) works on called Mermaid - a mission that was originally designed to detect earthquakes.
Earth's deepest region, the Marianas Trench, plunges 2km deeper than Mount Everest is high. But along the ocean floors, there are also tens of thousands of mountains which rise upwards: seamounts. Traditionally mapped by ships, modern satellite missions are revealing more information about these - indeed, it's estimated that the number of known seamounts may double thanks to these space-based observations.
What's on the seafloor?
The seafloor is, typically, geologically much younger than the continents that make up Earth's dry land. New rock is formed at mid-ocean ridges that snake across the Earth's major oceans. These host hydrothermal vents where conditions are so different to the surface that astrobiologists compare them to other planets.
While the major mid-ocean ridges were being mapped 70 years ago , other underwater mountains dotted across the oceans are much less well known. These seamounts are often of volcanic origin and can grow so large that their summits escape the ocean, becoming islands. From its summit to its base at the floor of the Pacific Ocean, for example, Hawaii's dormant volcano Mauna Kea is taller than Everest.
Many seamounts are topped with coral reefs which have drowned as they sank too far below the ocean surface. But these drowned reefs remain important hotspots of biological diversity in our oceans, hosting both bottom-dwelling and swimming lifeforms.
A small number of seamounts are currently growing - some of which will eventually become Earth's newest islands. For example, if Vailuluʻu seamount keeps growing, it will become the newest island in the Samoan Archipelago.
New seamounts are still being discovered . It may seem odd to miss a mountain when you're making a map of a landscape, but they can be hard to find below the ocean.
How are scientists trying to map the seafloor?
Traditional methods of mapping the seafloor involve using ships to estimate the ocean's depth. New advances involve autonomous underwater vehicles, which can estimate seafloor depth, and satellite missions, which can "feel" the changes in gravity caused by seamounts.
Another indirect approach comes from EarthScope-Oceans, the consortium which operates Mermaid - a project sending small robots deep below the ocean surface to detect earthquakes.
Mermaid robots float at depths of about 1.5km, where the water pressure is 150 times that at the surface. These robots listen for pressure waves generated by signals from distant earthquakes in Earth's solid interior. Since 2018, one fleet of Mermaid sensors, deployed in the South Pacific Ocean, has recorded thousands of waves associated with earthquakes.
But in 2022, scientists realised that Mermaid robots had recorded something else: waves travelling through the ocean from a volcano. The violent underwater eruption of Hunga Tonga-Hunga Ha'apai, a South Pacific underwater volcano, was the biggest in nearly 150 years . As well as causing volcanic lightning and sending plumes of ash tens of kilometres into the sky, the eruptions sent pressure waves into the waters of the Pacific.
Mermaid sensors heard these waves thousands of kilometres away from the volcano. At some of these sensors - scattered across the ocean over vast distances - the sounds were virtually identical. But where the sounds were different, recent research has revealed that seamounts were often to blame.
Seamounts block energy travelling through the ocean. This opens the prospect of using pressure waves from underwater explosions and eruptions to listen for "acoustic shadows" caused by unknown seamounts. In other words, finding seamounts by listening to the pressure waves they interrupt.
The future of deep ocean landscapes
As we explore the seafloor, human impact on it will become more apparent. While some researchers are discovering exotic lifeforms such as deep-sea snailfish in the oceans' deep trenches, others are detecting signs of microplastic waste in trench-dwellers such as deep sea scavenging amphipods (which look a bit like shrimp).
The seafloor is rich in mineral deposits, many of which are elusive on land - including minerals critical for battery construction. For example, polymetallic nodules rich in rare earth elements litter the ocean floor.
Areas of elevated seafloor like seamounts are especially likely to host cobalt-rich deposits - one of many critical minerals needed for the green energy transition and to meet UN sustainability goals .
However, exploration and active mining in the delicate ecosystems that surround these hidden worlds is controversial , because of the harm it can cause.
If we want to know where resources lie - and where the ocean floor most needs our protection - it is vital we understand the landscapes of the seafloor.
Jessica Irving has received funding from the National Science Foundation to work with MERMAID. She is a member of the Earthscope Oceans Science Committee and was involved in the research study described in this article. Dr Irving acknowledges useful input from Dr Joel Simon of Bathymetrix, who led the MERMAID research into the Hunga Tonga Hunga Ha'apai eruption.
Elizabeth Day is part of the Membership Committee of the Royal Astronomical Society and also sits on the Royal Astronomical Society's Education and Outreach grants panel.
