Mapping Seamounts

When light and sound fall short, gravity maps the ocean floor

It has been said that we know more about the detailed geography and topography of the Moon or Mars than we do about the ocean seafloor on our own planet.  For example, the seafloor is not as flat as older images would suggest.  Seamounts, primarily the remains of undersea volcanoes that do not reach the surface, dot the abyss like so many termite mounds on an African savanna.  But until recently, we did not have an accurate count or accurate map of these underwater mountains.

Seamounts pose a challenge to navigation as shown by a recent collision of a naval submarine with an uncharted rise.  They have also been called "oases of life" as the greater elevational diversity supports greater biological diversity and biomass.  And now that we can map them accurately, they are thought to play an important role in global ocean circulation, creating turbulence that can lead to the upwelling of seawater from the depths toward the surface.

Despite this importance, ~75% of the seafloor remains unmapped with current state-of-the-art technologies that can capture the kind of detail shown in this image.  So perhaps three-quarters of these important geological features were, until recently, unknown to us.

But, thanks to gravity, that is no longer true!  We will get to that in a bit, but first some background.   

Fine-scale mapping of the ocean floor lags well behind mapping of the land surface.  The largest geologic feature on the planet is the connected set of mid-ocean ridges that extends for 40,000 miles, threaded through every major ocean.  Its existence was not considered a serious possibility until the 1950s, and even a rough global map like this one for the Atlantic region was not compiled until the 1970s.

Imagine not knowing about the Himalayas until 70 years ago, and not having even a rough map of those peaks until 45 years ago.

Early discoveries of increased seafloor elevation in the middle of the Atlantic came from soundings by sailing vessels, and then from efforts to lay trans-Atlantic communication cables.  That's like discovering the Himalayas while laying a telegraph line from India to China!

But those individual lines give only a one-dimensional view of the seafloor they cross.  Millions of such transects would be needed to generate even a rough 3-D map of the ocean floor.  But by focusing data collection in areas where the elevation of the seafloor changed abruptly, and combining those data with accumulating maps of seafloor earthquakes, initial rough images of this huge feature began to emerge.

Marie Tharp was a scientist at Columbia University who was at the center of this multi-decade project to map the full system of mid-ocean ridges.  Her telling of the history of this work and the personalities involved is fascinating reading and can be found here, including the role the results played in the development of the theory of continental drift or plate tectonics (which also remained unaccepted until the 1970s).  It is a story of dedication and endless, detailed data gathering and mapping.  Some of the earliest finished maps were actually paintings by a professional artist (an example is included in Tharp's story).

While a map like the one above appears quite detailed, in reality only major features are shown  The image suggests that most of the ocean floor is relatively flat, and that turns out not to be true.  Why have we not known that? 

The short answer is that light does not penetrate very far into the ocean. 

Most of the dramatic images that we see of the state and dynamics of the land and ocean surfaces are derived by sensing some sort of "light" or electromagnetic radiation.

"Visible" light (the ROYGBV of rainbows) is just a very narrow window in the full range of this form of energy.  While our eyes can only detect light in this very narrow range of wavelengths, we have invented instruments that can generate energy across a wide range of wavelengths, and sensors that let us detect or "see" this huge range of wavelengths as well. 

We use short wavelength, high energy x-rays for medical analysis.  Long wavelength radiation in the microwave region both cooks our food and, as radar, guides airplanes and maps rainfall.  The way that light reflects from rocks or leaves or water or soft tissues tells us a lot about chemical content, temperature, net energy balance, physiological condition, and a host of other characteristics.  

But the ocean floor remains dark in all of these wavelengths.  Even the very longest wavelengths can only penetrate tens of meters into seawater, so there are no electromagnetic signals, no "light," to be had from the abyss. 

In contrast, sound does penetrate seawater, also traveling in waves, but of pressure, not light.  In fact, changes in water pressure and temperature with depth in the ocean create what has been called a sound channel that allows sound waves to travel for thousands of miles, allowing the songs created by whales to communicate across vast distances.

Unfortunately, noise created by boats, construction or seismic experiments can travel as well, and can disrupt this amazing form of communication among the world's largest mammals.

While sound waves travel four times faster in water than in the air, both rates are effectively infinitely slower than light.  This is why you can calculate how far away that last bolt of lightning was by counting the seconds between lightning and thunder.  You see the flash immediately.  Traveling at about 720 miles per hour (to use a round number), sound waves take about 5 seconds to travel one mile, so you count the seconds between flash and thunder and divide by 5 to see how many miles away the lightning was.

Sound waves also "bounce" off solid surfaces.  A research vessel can create sound waves (a "ping" as in old submarine battle movies) and "listen" for the return signal.  The time required for the signal to return is a measure of the distance to that object, just like estimating the distance to that bolt of lightning.

Sonar, short for Sound Navigation and Ranging, is the term used to describe the use of sound waves to determine the distance to the seafloor or other objects.  Together, the timing and strength of the returned signal can yield both distance and orientation (does the surface face the signal or is it at an angle?).

A single sound source and receiver would be little better than a sounding transect for mapping the seafloor, but the method has been extended to map larger areas in a single pass with the development of sidescan and multibeam sonar. 

Both methods emit a wide swath of "pings" and listen for the return.  The math for turning the series of returns into a map gets complicated, but the results can be stunning.  This site includes a video that is a great introduction to the technology and the precision of the maps produced.

With this technology in hand, why does 75% of the seafloor remain unmapped at this level of precision?  Not surprisingly, these methods are time consuming and relatively expensive, and so have been deployed first in areas where precision has the most value.  For the rest of the abyss, we have had only a rough idea of its topography.

Until recently.

What is more basic and constant than gravity?  While physicists continue trying to explain why it exists, we use the basic equations discovered centuries ago to predict the movement of planets and the paths of rockets and airplanes.  We know that the measurable and predictable effects of gravity relate to the density or mass of two objects and how far apart they are.

But surely the gravitational force of the Earth is constant, yes?  In terms of its interaction with the planets and the sun, it is, but the strength of the gravitational field over a specific location on the Earth is affected ever so slightly by changes in mass and density of the material at the surface.  For example, soil saturated with water will exert a slightly different pull than dry soil; or the gravitational field over a glacier, or the Greenland Ice Cap, will vary slightly based on the thickness of the ice.

The method for mapping these ultra-subtle differences in gravitational pull by satellites seems to me to be one of the truly amazing technological marvels in the remote sensing toolbox. 

The Gravity Recovery and Climate Experiment Follow On (GRACE-FO) mission, a joint project by NASA and the German Research Center for Geosciences, consists of twin satellites traveling identical orbits at a constantly known distance from each other.  The absolute difference in location between the two is known with such precision that subtle changes in the distance between them reflects equally subtle changes in the gravitational pull from the surface! 

A first, high-impact product of the GRACE-FO mission was mapped changes in the total water content of ice caps and soils worldwide.  This image captures the loss of ice mass over Greenland and parts of Antarctica in a way that greatly enhances the value of continuing direct measurements of ice loss made on location.

And how does this relate to seamounts and the mapping of the ocean floor? 

A study from 2011 used globally available data on variation in gravitational fields caused by differences in depth of the water column to map more than 24,000 sea mounts more than 330 feet tall.  A very recent study by the same group used updated maps to identify an additional 19,000+, bringing the total to about 45,000.  The researchers also used detailed data from seamounts that had been mapped with sonar to describe the average shape of seamounts and so increase the precision and detail of their maps.

Earlier studies had used the spatially limited mapping of seamounts by sonar to predict how many seamounts there might be, but a statistical average does not help much if you are navigating a submarine!  These relatively high-definition maps of seamounts (that I can't include here due to copyright restrictions, but you  can find them) add incredible precision and value to our understanding of the variation in what turns out to be a not-at-all-flat seafloor. 

It's a new and more complex world for students of ocean currents and marine biology - all thanks to gravity.

Sources

Background information on seamounts and the image in the essay can be found here:

https://oceanexplorer.noaa.gov/facts/seamounts.html

https://oceanexplorer.noaa.gov/explorations/18ccz/background/seamounts/seamounts.html

https://en.wikipedia.org/wiki/Seamount

The role of seamounts in marine biodiversity and productivity are described here:

https://www.pnas.org/doi/10.1073/pnas.0910290107

https://marine-conservation.org/on-the-tide/5-reasons-ocean-seamounts-matter/

The map of the Mid-Atlantic Ridge is from:

https://www.ngdc.noaa.gov/mgg/image/2minrelief.html

And more information about Mid-Ocean Ridges can be found here:

https://oceanexplorer.noaa.gov/edu/learning/2_midocean_ridges/activities/midocean_ridge.html

Marie Tharp's story can be found here:

https://news.climate.columbia.edu/2020/07/24/marie-tharp-connecting-dots/

The image of the electromagnetic spectrum is from:

https://commons.wikimedia.org/wiki/File:Electromagnetic-Spectrum.svg

Information about how sound travels in water, and about the songs of whales can be found here:

https://oceanservice.noaa.gov/facts/sound.html

https://ocean.si.edu/ocean-life/marine-mammals/why-do-whales-sing

More on estimating distance to lightning by timing thunder can be found here:

https://www.weather.gov/safety/lightning-science-thunder

Information on sonar, including sidescan and multibeam methods, can be found here:

https://oceanservice.noaa.gov/facts/sonar.html

https://en.wikipedia.org/wiki/Sonar

https://en.wikipedia.org/wiki/Side-scan_sonar

https://oceanexplorer.noaa.gov/explorations/sound01/background/acoustics/acoustics.html

Technical descriptions of measurements of gravity fields are here:

https://en.wikipedia.org/wiki/Gravity_gradiometry

https://en.wikipedia.org/wiki/Gravimetry

Information on the GRACE-FO satellites and mission, including the map of changes in surface water content, can be found here:

https://earthobservatory.nasa.gov/images/147437/taking-a-measure-of-sea-level-rise-gravimetry

https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO

The two studies reported here are: 

2011 - https://academic.oup.com/gji/article/186/2/615/588187?login=false

2023 - https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022EA002331