Into the underworld
Sanne Cottaar explores the mountains beneath our feet.
If we could cut a huge slice out of the Earth, what would we see? School textbooks suggest the inside would present neat layers: a thin surface crust, sitting on a mantle of gloopy magma, with a dense core within.
In reality, the mantle is largely solid rock. Given huge lengths of time and enough pressure, it gradually moves and deforms. The outer core, meanwhile, is a turbulent, liquid metal – its churning creates the planet’s magnetic field, which shields life on the surface from radiation.
These insights were hard won. Scientists have no textbook planet-slice to work with. The deepest humanity has ever cut into the planet is the Kola Superdeep Borehole, plumbed by the Soviet Union during the Cold War. The borehole reached depths of over 12 km, hitting 3 billion-year-old rocks and 180°C temperatures.
For a humbling sense of scale: deep earth scientists, who want to study the boundary between mantle and core, have to look 3,000 km down. The centre of the planet is deeper still, at over 6,000 km.
“Our history of looking into space is much longer than looking into our planet,” says Sanne Cottaar, Professor of Global Seismology in Earth Sciences. “It’s so hard to look down.”
Sanne climbing in Iceland.
Sanne climbing in Iceland.
In making use of earthquake waves, Sanne has seen deeper than most. By pairing an earthquake’s origin and its endpoints, she can give the planet something like a CT scan, where the seismic waves reveal hidden structures.
Growing up in the Netherlands, Sanne loved physics, but wanted to be more “grounded”: to work on natural events closer to home. Just maybe not too close.
“While I was doing my PhD in Berkeley, I experienced an earthquake for the first time. I thought there was a truck driving through my house! I definitely did not enjoy it.”
After 10 years at Cambridge, Sanne entrusts the shakier fieldwork to her colleagues, who monitor earthquakes and volcanoes close-up. Sanne piggybacks on data from as many local seismometer stations as she can, downloading their data to plug into her models. By extracting data from large earthquakes, she reveals the strange world beneath our feet.
When Earth rings like a bell
While interrogating earthquake waves for information, Sanne feels like a detective.
“It sometimes feels like the Earth is throwing clues at me. It’s my job to put it all together and solve the mystery.”
As our societies developed a need to monitor nuclear tests in the 1950s, a global network of seismometers was born. Since then, our data on quakes of all kinds has been increasing in quality, allowing better resolution of the world beneath the surface.
Sanne casts her gaze halfway into the Earth, to the planet’s strongest boundary: between the core and the mantle. She wants to know what sits on top of our core.
She hunts for earthquakes that will reveal the geometry of this region. Not just any earthquake will do. The ideal ones are deep, at just the right distance to seismometers on the surface.
In an earthquake, two parts of the earth move past each other causing a rumble. Large ones leave the planet ringing like a bell. After massive earthquakes, like Sumatra-Andaman on Boxing Day 2004, “the earth was ringing for months afterwards.”
Quakes also produce ripples in the Earth, which move outwards and downwards: shock pressure waves (in and out) and shearing waves (sideways). This is how humanity first figured out that Earth has a liquid core, when we saw that shearing waves couldn’t pass through it.
“I’m after waves that come down from the earthquake, and refract on the core-mantle boundary,” Sanne says. “Eventually, some of the energy travels up to a seismic station. By measuring a wave like that, we can pick up any anomalies in its path.”
Anomalies within the Earth alter the path of the seismic waves, making them bounce at strange angles. Just like light passing through a lens, the seismic waves hit an object and scatter to different places on the surface. Sanne infers how the earthquake’s wavefront moved through the Earth and maps out anomalous structures.
So, what structures are the seismic waves hitting?
Diagram of a seismic wave paths, travelling down from the crust, and deflecting off the core-mantle boundary (red).
Diagram of a seismic wave paths, travelling down from the crust, and deflecting off the core-mantle boundary (red).
Mountains beneath our feet
Buried in the data, Sanne has seen mountains: massive, dense rocks sitting on top of our core. These inner-Earth mountains can be many times the size of Everest.
“We call these deep mountains LLVPs: large, low-velocity provinces.”
There’s a mantle-mountain-range underneath Africa, and another one under the Pacific. They’re right beneath the cluster of hotspots for volcanic activity. Plumes of hot material come from the LLVPs and create islands on the surface.
“These features seem to play a role in shaping the surface. They may have influenced super-eruptions of the past, some of which caused mass extinction.”
Ed Garnero, seismologist at Arizona State University, has described LLVPs as “the largest things on the planet.” But despite their overwhelming size and tectonic importance, we know little about these masses. The earth sciences community has only started to map them in the last few decades.
An animation showing the deep Earth mountains (LLVPs), Cottaar and Lekic, 2016.
An animation showing the deep Earth mountains (LLVPs), Cottaar and Lekic, 2016.
Sanne is joined in this effort by her research group at Cambridge, the Deep Earth Explorers. The group’s work forms the core of an exhibit at the Sedgwick Museum, where members of the public can try their hand at triggering seismic waves.
“Our group also looks at smaller mountains on the core-mantle boundary called ULVZs: ultra-low velocity zones. These ones are even more anomalous – they must be made of a different material. We’ve found major ULVZs beneath Hawaii, Iceland and the Galapagos: all places with strong volcanism at the surface.”
These zones are mysterious in origin. Sanne and her colleagues think they could be ancient artefacts of Earth’s formation.
In comparing features like these to other planets, scientists are revealing why Earth has such favourable conditions for both plate tectonics and life.
Mars-quakes and moon-quakes
If you want to experience a snapshot of the deep earth, head to Iceland. Eruptions there may be fed by a mantle plume, creating lava fields and fire fountains on the surface. Their explosive phases are best viewed from a distance.
More generally, plate tectonics played a crucial role in forming Earth’s liveable surface. Volcanic eruptions released both the foundational gases of our early atmosphere and water vapour that condensed into our oceans. Life might have originated at hydrothermal vents caused by the spreading of plates. Earth scientists want to better understand these processes, and how they produced a fertile environment for life. Once again, this knowledge is playing hard-to-get.
“We don’t have another example of a planet with plate tectonics,” Sanne says. “In studying this, we have a sample size of one. And we don’t know how it started on Earth.”
Sanne with the Deep Earth Explorers group.
Sanne with the Deep Earth Explorers group.
In 2018, humanity placed its first high-sensitivity seismometer on Mars.
“It picked up Mars-quakes, giving us hints of what the interior structure of the red planet could be. Our view of exoplanets isn’t currently good enough to infer much about their composition, or the presence of plate tectonics.”
But earth scientists could have better luck in looking closer to home, at our planet’s satellite.
The moon’s origins are unclear, but it might be a remnant of a huge collision, between something the size of Mars and an ancient Earth. Some claim that the internal mountains on our planet, Sanne’s LLVPs, might be a remnant of this impact.
“We’ve observed deep moon-quakes during the Apollo missions, which are probably due to tidal forces. We suspect the moon has a core like Earth.”
Mars- and moon-quakes could soon reveal the internal organs of other terrestrial bodies. For now, Earth’s features and dynamics look to be unique.
We rely on scientists like Sanne to unravel the puzzle of life and its tempestuous host. By making use of Earth's most traumatic events, we can hope to know how deep our foundations go.
Published on 14 January 2025
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