Hot Upper Mantle Beneath the Tristan da Cunha Hotspot From Probabilistic Rayleigh-Wave Inversion and Petrological Modeling
https://doi.org/10.1002/2017GC007347
https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017GC007347
Explained to a child
Volcanoes usually form where tectonic plates move, like at the edges of continents. But some volcanoes, like the ones near Tristan da Cunha (the remotest island in the South Atlantic Ocean), appear in the middle of a plate... and this is a bit of a mystery!
We wanted to understand why. We thought it might be because there is a super-hot area deep inside the Earth called a mantle plume rising up under the island. But since we cannot dig 100 km into the Earth, we had to get creative!
We used special instruments placed on the ocean floor to listen to vibrations from faraway earthquakes. These vibrations (called seismic waves) move at different speeds depending on how hot the rocks are inside the Earth. By measuring those speeds, we could figure out the temperature deep below Tristan da Cunha.
We discovered that the rocks are hotter than normal down there, though not as hot as under places like Hawaii (remember where Lilo & Stich leave?). This suggests a hot plume might be rising up, melting the rocks just a little, and causing volcanoes to form right in the middle of the ocean. So, even without digging, we can learn what is going on deep underground, just by listening to the vibrations of the Earth!
Explained to a friend
We studied the volcanic hotspot beneath Tristan da Cunha, a remote island in the South Atlantic. Like Hawaii, Tristan is part of a chain of volcanic islands thought to be formed as a tectonic plate moves over a stationary heat source deep in the Earth's mantle, known as a hotspot.
Many scientists believe that hotspots are created by mantle plumes, columns of hot rock rising from deep within the Earth, possibly from near the core-mantle boundary, over 2800 kilometers down. But whether these plumes actually exist, and how they behave beneath different hotspots, remains a major question in Earth science.
To investigate whether a mantle plume lies beneath Tristan da Cunha, we deployed a network of ocean-bottom seismometers on the seafloor around the island. These instruments recorded vibrations from distant earthquakes, allowing us to measure how fast seismic waves travel through the Earth beneath Tristan.
Seismic wave speeds are influenced by temperature: hotter rock slows down the waves, while colder rock speeds them up. By analyzing these wave speeds, we created a detailed image of the subsurface structure and inferred the temperatures beneath the island.
Our results revealed a distinct zone of unusually slow seismic waves between about 70 and 120 kilometers beneath the surface. This low-velocity zone suggests that the rock is hotter than normal. Although the wave speeds were slightly faster than those observed beneath Hawaii (where a massive plume is in place), they still indicate a significant thermal anomaly.
We also determined that the lithosphere (the rigid outer shell of the Earth) is relatively thin beneath Tristan, about 65 to 70 kilometers thick. This supports the idea that hot material is rising from below and weakening the plate.
Using models of how temperature affects seismic wave speeds, along with what we know about mantle rock composition, we estimated that the temperature beneath Tristan is about 50 to 120 degrees Celsius hotter than the typical oceanic mantle. That puts the mantle potential temperature at around 1410 to 1430 degrees Celsius.
This makes Tristan da Cunha hotter than the average oceanic region, though not as extreme as places like Hawaii or Iceland. Our models also suggest that there is a small amount of partial melting occurring beneath the island (less than 1%) which is enough to support volcanic activity without leading to the massive eruptions seen in more active hotspots.
All of these observations (slow seismic velocities, high temperatures, and a thin lithosphere) point to the presence of a mantle plume beneath Tristan da Cunha. While it may not be as intense as the plume beneath Hawaii, it still plays a key role in driving volcanic activity in the region.
Through this study, we have provided strong geophysical evidence that a moderately hot upwelling from the deep mantle exists beneath Tristan da Cunha, helping to explain its long-lived volcanism and place in the global hotspot system.
Explained to an expert
Understanding the enigmatic intraplate volcanism in the Tristan da Cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it. We measured phase-velocity curves of Rayleigh waves using cross-correlation of teleseismic seismograms from an array of ocean-bottom seismometers around Tristan, constrained a region-average, shear-velocity structure, and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot.
The ocean-bottom data set presented some challenges, which required data-processing and measurement approaches different from those tuned for land-based arrays of stations. Having derived a robust, phase-velocity curve for the Tristan area, we inverted it for a shear wave velocity profile using a probabilistic (Markov chain Monte Carlo) approach. The model shows a pronounced low-velocity anomaly from 70 to at least 120 km depth. Vs in the low velocity zone is 4.1–4.2 km/s, not as low as reported for Hawaii (∼4.0 km/s), which probably indicates a less pronounced thermal anomaly and, possibly, less partial melting.
Petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle (mantle potential temperature of 1,410–1,430°C, an excess of about 50–120°C compared to the global average) and a melt fraction smaller than 1%. Both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65–70 km, consistent with recent estimates from receiver functions. The presence of warmer-than-average asthenosphere beneath Tristan is consistent with a hot upwelling (plume) from the deep mantle. However, the excess temperature we determine is smaller than that reported for some other major hotspots, in particular Hawaii.
Hot upper mantle beneath the Tristan da Cunha hotspot from probabilistic Rayleigh-wave inversion and petrological modeling
Published in Geochemistry, Geophysics, Geosystems, 2018.
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Bonadio, R., Geissler, W. H., Lebedev, S., Fullea, J., Ravenna, M., Celli, N. L., et al. (2018). Hot upper mantle beneath the Tristan da Cunha hotspot from probabilistic Rayleigh-wave inversion and petrological modeling. Geochemistry, Geophysics, Geosystems, 19, 1412–1428. https://doi.org/10.1002/2017GC007347