About 75% of the world’s volcanoes live along the aptly named Ring of Fire. It’s logic. Squeezing a boundary between tectonic plates, the Ring of Fire is an open seam within the planet. But then there’s Hawaii, a chain of volcanic islands right in the middle of the Pacific Plate, far from any border. What feeds his fire?
Scientists have long theorized that columns of superheated rock – hot plumes crossing the mantle to the crust above – explain the Hawaiian Islands and other areas like them. Where these columns touch the surface, volcanic hot spots form and the ground erupts. For millions of years, inch by inch, Earth’s tectonic plates drag new soil to hot spots and form long volcanic chains.
The theory is old, but the detailed observation of the mantle plumes feeding these hot spots is fairly recent. “Theoretically, we know [plumes] must exist ”, Harriet Lau, geophysicist at the University of California at Berkeley Recount Quanta Magazine. “But they’re so hard to see seismically.”
Now, however, in a particularly vivid example, a team of scientists having filled out a card underworld nearly a decade in the making.
The result, beautifully visualized below for a functionality in Quanta, is one of the most detailed snapshots to date, and it’s surprisingly complicated. Instead of a simple vertical column rising up through the mantle, the structure resembles a tree, with roots near the nucleus, a trunk in the middle of the mantle, and finer branching structures growing near the surface.
The plume feeds one of the most active volcanoes in the world, the Piton de la Fournaise, on the French island of Reunion in the Indian Ocean. But it also drives an intensely volcanic region in East Africa, some 3,000 kilometers away. Going back in time to when dinosaurs still ruled the planet, it set an area known as the Deccan Traps ablaze. Today in modern India, the Deccan traps have released enough lava to bury California, Montana, and Texas.
See through the ground beneath our feet
The Hubble Space Telescope is certainly a wonder of the world. Imaging galaxies billions of light years away is impressive, but how exactly do you see through thousands of miles of rock? In a sense, geophysicists also build “telescopes”. But instead of detecting light, these systems collect and analyze the vibrations of the planet.
“People have a longer history and have an easier time looking at the stars,” said Cambridge University seismologist Sanne Cottaar. Recount Quanta Last year. “Looking down has actually been quite difficult.”
To create this particular model of the underworld, the team relied on data from one of the largest “telescopes” to date. In 2012, ships dropped 57 seismometers in the ocean around Réunion. The whole array, which also included 37 terrestrial sensors, spanned some 2,000 kilometers. Over the next 13 months, the sensors recorded subtle vibrations from seismic activity occurring halfway around the world.
When earthquakes shake the surface, they also ring inside the planet like a bell. By correlating a seismic event on one side of the world with the thrill it produces on the other, scientists deduce what happened in between. Seismic vibrations tend to move more slowly in warmer areas than in cooler areas, for example, so a mantle plume would slow their progress. With enough sensors and seismic events, researchers can build a model.
The model, in this case, was surprising.
Scientists agree that the mantle plumes underlying the hotspots are so floating and moving quickly that they should rise straight up. Diagonal branching paths in the data were unexpected.
The team proposes that they occur when temperature differences between hotter and colder materials cause certain areas of the plume to float more, pinching spots from the top of the trunk (or cusp) over time, one after the other. These spots rise vertically but appear to form diagonal branches because the older spots grew higher than the younger ones. Closer to the surface, where the upper mantle is less dense, they spread out and grow thin branches that feed surface volcanoes.
Because the team pulled data from both the land and sensors on the ocean floor – a traditional hole in seismic imagery – they form one of the most complete snapshots of a mantle plume. nowadays.
However, the construction of such models is difficult and depends somewhat on interpretation. Other artifacts, such as the mineral composition of rock, can also slow down seismic waves.
So while the research and model have been well received, the study of mantle plumes is still a work in progress for the scientific community.
Imaging the interior of the Earth is fascinating work. It can inform us about planetary changes on a continental scale and even augur future cataclysms.
The branch of the plume under East Africa may be divide the continent in two. In tens of millions of years, the southern branch of the plume could send a drop so titanic it will bring shame to the traps of the Deccan. Meanwhile, other places, like the supervolcano beneath Yellowstone, have yet to be connected to one of the two main plumes in the Earth’s mantle. A more comprehensive account of Yellowstone’s origin story will help us better understand (and perhaps predict) its explosive future.
This work is therefore a new page in the history of the epic forces that shape and cultivate the Earth as we know it, but there is undoubtedly a lot more to learn.