Back when Pangaea was the big continent on campus, a vast ocean called the Panthalassa covered the rest of the planet. With such an incredible area, it should come as no surprise that the expanse was not featureless. The rock record around the Pacific contains evidence of ancient volcanic arcs (like those along the Tonga Trench today). These would have formed where separate plates that made up the Panthalassa Ocean floor collided, driving (or "subducting") one plate down into the mantle. Later, those volcanic arcs ended up plastered onto the edges of continents as the Atlantic Ocean opened within Pangaea and oceanic plates were consumed by encroaching continents.
We've been uncertain about the exact location of those volcanic arcs in the Panthalassa. "Paleomagnetism" allows geologists to reconstruct the latitude of the rock at the time it formed, which is one of the ways we reconstruct the locations of continents in the past. Magnetic crystals in magmas align with the Earth’s magnetic field, but are locked into position as the rock solidifies. But there is no "prime meridian" in Earth’s magnetic field, so this information can’t tell you anything about the longitude of a rock.
In order to answer this question, a group of geophysicists analyzed seismic velocity data from the mantle beneath the Pacific. Seismic waves from earthquakes can be used in much the same way that doctors use CAT scans. Analyzing the data from many, many earthquakes generates a three-dimensional image of the Earth’s interior, as things like temperature, density, and phase (liquid or solid) alter the velocity and direction of seismic waves.
Since subducted plates are much colder than the rest of the mantle (and heat exceedingly slowly), they can be identified using seismic imaging. Using realistic estimates of sinking velocity, the researchers predicted where they might find the plates that sank at the ancient subduction zones of the Panthalassa. Lo and behold, potential signs of the plate showed up right where they expected it.
The signal doesn’t stand out as well as more recently subducted plates due to its location in the lower mantle, which helps explain why it hadn’t been identified by earlier analyses. As Douwe van der Meer, one of the authors, told Ars via email, "In my research I combine plate tectonics with deep mantle tomography, which not many people in the world do. It required a consistent, focused research effort to put the 'Panthalassa puzzle' together and, to some extent, to be daring enough to send it out for public scrutiny."
Finding the subducted remnants of this plate allowed the group to make a more confident reconstruction of the Triassic-Jurassic era planet. The volcanic arcs along the subduction boundary formed a north-south line roughly down the middle of the Panthalassa. This gave the researchers the rare opportunity to name some geographic features: two regions of the Panthalassa Ocean divided by the plate boundary. They called the eastern portion the Thalassa Ocean (a conservative choice) and designated the western part the Pontus Ocean. The plate beneath the Pontus was the one subducted at the plate boundary, which they called Telkhinia.
Pontus and Thalassa were sea deities in Greek myth, and their children, the Telkhines, were destroyed by the gods and sent to the underworld for misusing their powers. "I thought this was very suitable because their remains are detected several thousand kilometers deeper at present," wrote van der Meer.
In addition to working out the location of these volcanic arcs, the study will also help researchers reconstruct the history of the Pacific plate. "[F]or any future model of the Panthalassa Ocean, the presence of the Telkhinia subduction zone needs to be taken into account," wrote van der Meer. "Previous models simply extrapolated the spreading ridges until they [encountered] a continent. We have now proven that it was not that simple, and the tectonic history was complicated."