While studying the Earth’s layers deep beneath Bermuda’s sandy pink shores and turquoise tides, geologists found evidence of volcanic material rising from the mantle’s transition zone—a layer rich in water, crystals and melted rocks—suggesting the possibility of a third mechanism for volcano formation.
Previously, volcanoes were thought to form through one of two mechanisms; either when two tectonic plates subduct (one moves beneath the other), or as a result of mantle plumes that rise from the core-mantle boundary to make hotspots at Earth’s crust.
“We found a new way to make volcanoes,” Esteban Gazel, an associate professor of Earth and Atmospheric Sciences at Cornell University, said in a news release. “This is the first time we found a clear indication from the transition zone deep in the Earth’s mantle that volcanoes can form this way.”
The new research—published in the journal Nature—is based on the analysis of an 800-metre core sample drilled in Bermuda in 1972. For the last few decades, the massive cylinder of rock had been stored at Dalhousie University in Nova Scotia.
“I first suspected that Bermuda’s volcanic past was special as I sampled the core and noticed the diverse textures and mineralogy preserved in the different lava flows,” co-author Sarah Mazza of the University of Münster, Germany, said. “We quickly confirmed extreme enrichments in trace element compositions. It was exciting going over our first results … the mysteries of Bermuda started to unfold.”
To figure out where the island’s ancient volcanic material came from, Mazza analysed the cross-section for isotopes, trace elements, and evidence of water and other volatile materials trapped in the rocky layers.
“We were expecting our data to show the volcano was a mantle plume formation—an upwelling from the deeper mantle–just like it is in Hawaii,” Gazel said.
However from the sample, scientists determined that the now-dormant volcano that created Bermuda formed after a disturbance in the mantle’s transition zone which caused volatile-rich material to begin percolating toward the surface around 30 million years ago.
The main giveaway for this is the tremendous amount of water trapped inside the core sample’s crystals, along with geochemical signatures from the transition zone. There is enough water in the transition zone to form at least three oceans, according to Gazel, but it is the water that helps rock to melt in the transition zone.
This suggests that the transition zone, which is located at a depth of 410-660 km, is an important chemical reservoir for the Earth by bringing material from that depth to the surface.
Gazel explained that this research provides a new connection between the transition zone layer and volcanoes on the surface of Earth. “With this work we can demonstrate that the Earth’s transition zone is an extreme chemical reservoir,” said Gazel. “We are now just now beginning to recognize its importance in terms of global geodynamics and even volcanism.”
This post was written by Samantha Billups and edited by Ella Mercer.