Deep connections: In the subsurface of Hawaii, volcanologists have uncovered previously unrecognized magma transport routes — and a connection between the volcanoes of Mauna Loa and Kilauea. It consists of pancake-like stratified shallow veins that lie at a depth of 35 to 50 kilometers. Persistent earthquake swarms suggest that hot magma flows through this distribution network into the crust and from there into the magma chambers of volcanoes, the researchers report in Science.
Hawaii is one of the most volcanically active areas on earth, as evidenced by recurring eruptions of the Kilauea and in November 2022, an eruption of Mauna Loa, previously dormant for 40 years. Cause of this fiery activity is a hotspot: Hawaii moves over you Mantelplume away, which transports hot magma from the deep mantle to the earth’s crust.
“Blind zone” below 15 kilometers depth
But while the magma chambers and channels directly beneath the volcanoes are relatively well studied, it remains unclear how magma from the slightly offset hotspot and upper mantle enters these reservoirs. “We know quite well what the magma is doing in the shallow part of the system above 15 kilometers depth,” explains lead author John Wilding of the California Institute of Technology. “But everything below that has been speculation so far.”
The reason for this: the measuring network of the US Geological Survey detects thousands of weak tremors in the subsoil of the Big Island every year, which can be traced back to magma movements. In the case of earthquakes from greater depths, however, the hypocentres might not be determined with sufficient accuracy because too much noise covered the signal. That’s why Wilding and his team have now used an adaptive algorithm to evaluate earthquake data from the last 3.5 years.
Network of shallow magma layers
Using this method, the volcanologists identified more than 192,000 weak tremors originating from a large area south of the two volcanoes Kilauea and Mauna Loa. By pinpointing the focus of the tremors to within a kilometer and analyzing the wave patterns, they were able to use this data to, to a certain extent, examine the subsoil in this area.
The surprising result: At a depth of 36 to 51 kilometers below the earth’s surface lies a previously unrecognized network of seismic mantle ridges and volcanic channels. The superimposed layers resemble a stack of pancakes—except the magmatic pancakes are five to six kilometers across and 300 meters thick. The individual magma layers are separated from each other by rock layers 500 meters thick.
According to the research team, this is the first time magmatic structures at this depth have been directly mapped. “Before, we knew little regarding how magma is transported and stored deep beneath Hawaii,” says Wilding. “Now, for the first time, we have a high-resolution map of an important part of this pipeline system.”
Magma influx for Kilauea and Mauna Loa
Thus, in the upper mantle beneath Hawaii, there exists an extensive complex of horizontal mantle ridges between which magma flows. This magmatic conduit network extends from an area near Pahala on the island’s southeast coast in two branches to the volcanoes of Kilauea and Mauna Loa. “The sill complex might thus serve as a common source of magma for Kilauea and Mauna Loa,” Wilding and his colleagues write.
Molten molten rock from the off-island mantle plume might be transported through these conduits below Big Island and its large volcanoes. “This level of interconnectivity is remarkable because geochemical and seismological data previously suggested that Hawaiian volcanoes each have their own conduit systems through which they receive magma from different parts of the mantle plume,” the volcanologists say.
Not an isolated case?
The newly identified line network beneath Big Island raises the question of whether such seismic mantle ridges exist beneath other volcanic islands. Similar earthquake swarms have also been observed on other island volcanoes, including La Reunion, the Cape Verde island of Fogo and the Canary Island of El Hierro. “Especially at the Fogo, the complex earthquake swarm occurred at a depth comparable to that under Pahala, and it was also suspected that it was a matter of seismic mantle sills,” reports the team.
According to Wilding and his colleagues, it is therefore quite likely that such extensive magma transport networks also exist under other island volcanoes. Future investigations with more precise seismic measurement methods and analyzes might clarify this. (Science, 2022; doi: 10.1126/science.ade5755)
Quelle: California Institute of Technology, Science