Scientists listen to tremors beneath a waking volcano named Ol Doinyo Lengai

In the highlands of northern Tanzania, the ground near a volcano has been softly rumbling for a month. The tremors are faint, but they carry important clues about what’s happening deep inside the Earth.

Seismic tremors provide a real-time window into volcanic activity below ground. To capture these signals, scientists have been eavesdropping beneath Tanzania’s Ol Doinyo Lengai, the world’s coldest volcano.

Tremors from Ol Doinyo Lengai

For 15 months, a team led by Miriam Christina Reiss from Johannes Gutenberg-University Mainz tracked Ol Doinyo Lengai’s activities.

A network of seismometers placed around the volcano picked up the ground vibrations with remarkable clarity.

Analyzing the recordings revealed far more than routine tremor activity. For the first time, they made it possible to trace the source in 3D, zeroing in on the tremor’s exact location and depth beneath the volcano.

These specific sites revealed an unprecedented view of the volcano’s magma plumbing and its complex dynamics.

What makes Ol Doinyo Lengai cold?

Don’t let the term “cold” fool you. Ol Doinyo Lengai’s lava reaches 510°C (950°F), which is hot enough to burn through your boots. Yet it’s cooler than 90% of the world’s lava, which usually reaches temperatures of up to 1,250°C (2,282°F).

It is this volcano’s unique chemistry that makes it colder than average. 

Most volcanoes erupt basalt or silica-rich rhyolite, but Ol Doinyo Lengai produces natrocarbonatite lava. It’s the only active volcano of its kind on Earth.

Made primarily of sodium, potassium, and calcium carbonates, the carbonatite lava contains very little silica. This unusual mix lets it melt at low temperatures and flow with a viscosity close to that of water. 

As it moves, it rapidly cools, appearing black or brown. Eyewitnesses often describe the lava as resembling runny mud.

The volcano’s unusual chemistry has intrigued scientists for years. This new research has now shifted the spotlight.

It used Ol Doinyo Lengai’s unique volcanism, and its location at the longest continental rift in the world, as a natural laboratory to explore magma activity beneath the surface.

What do volcano tremors tell us? 

Deep underground, extremely hot, molten rock begins its journey to the surface. As magma rises, it presses against the surrounding crust with enormous force. Rocks crack and shift under the strain, triggering earthquakes.

Volcanic earthquakes are generally weaker than the ones caused by tectonic plate movements. However, near active volcanoes, even small tremors can be dangerous, as they indicate magma movement beneath the surface. 

Such movement often hints at a coming eruption and the danger of lava flows and ash clouds. But here’s the catch – not every tremor ends that way.

Volcano tremors arise from different sources. They can be triggered by rising magma, escaping gas, or fluids moving through underground cracks. Decoding which tremors correspond to which processes can help scientists interpret the volcano’s activity.

“For volcano seismology, it is extremely interesting to study these signals and wave types that arise when magma moves below the surface,” said Reiss in a recent report by Johannes Gutenberg-University Mainz.

To spot patterns in the volcano’s restless rumble, Reiss and her team turned to 15 months of seismic recordings. They focused on seven weeks of intense tremor activity from late February to early April, 2020.

Listening to the rumbles 

The researchers identified 677 hours of volcanic tremor signals, accounting for 40% of the total observed period. For this data, the team tapped into a network of seismometers, geophones, and infrasound sensors surrounding Ol Doinyo Lengai. 

The instruments were part of the SEISVOL project – Seismic and Infrasound Networks to Study the Volcano Ol Doinyo Lengai. This collaborative effort among several German research institutions focuses on volcanic activity in Tanzania’s Natron Basin, within the East African Rift System.

SEISVOL’s sensitive instruments detected subtle tremors generated by magma beneath active volcanoes.

By sorting the vibrations according to frequency, the researchers identified two prominent tremor types: narrow-band tremors and quasi-harmonic tremors.

Each tremor type revealed the secrets of a different layer of the volcanic system. To uncover what each has to tell, the researchers examined the signal types individually.

Gas from Ol Doinyo Lengai

The team started by mapping the deeper tremors. The narrow-band tremors were steady vibrations at frequencies between 2 and 4.5 Hz. Their origin was pinpointed at 4–7 kilometers (2.5-4.3 miles) beneath the volcano’s northern flank – its steep outer slope. 

These tremors revealed tube-like channels rising from a fault in the Natron Basin toward the surface. They outlined the hidden route the carbonatite magma follows as it rises through the volcano, releasing gases along the way. 

A previous study estimated that CO₂ gas begins to separate from the magma about 4 kilometers (2.5 miles) underground. The narrow-band tremors at this depth might be the sound of this gas release.

Shallow and deep tremors

The second type of tremor, the quasi-harmonic ones, hummed at a soft fundamental frequency of 1.9 Hz. This low frequency offered clues about how the vibrations are formed. 

Researchers suggested that these tremors come from penny-shaped cracks at the volcano’s base. They also pointed to fluids oscillating within cracks or from magma moving through interconnected chambers beneath the surface as possible sources of the tremors.

The two types of tremors do not occur in isolation. In some episodes, they alternate and show coordinated behavior. This points to a physical connection between the deep and shallow magmatic systems of Ol Doinyo Lengai.

Lessons from Ol Doinyo Lengai

The researchers used a specialized technique called the network covariance matrix approach to locate tremors. Unlike traditional methods, this doesn’t require clocking in the exact time each tremor starts. Instead, it recognizes patterns in how vibrations correlate across multiple sensors.

To achieve this, the team split the collected data into 10-minute chunks and then into overlapping 48-second segments. By comparing the signals between different stations, they separated real tremors from background noise. 

Once coherent tremors were mapped, the team built a virtual 3D grid of the volcano and subsurface. The result was an unprecedentedly precise mapping of each tremor source beneath Ol Doinyo Lengai. 

By mapping the intricate plumbing system beneath the volcano, the study deepens our grasp of volcanic processes. Having this knowledge at hand could improve eruption forecasting and help better protect communities living near active volcanoes.

The full study is published in Communications Earth & Environment. 

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