Far below Bermuda's pink sandy beaches and turquoise tides, geologists have discovered the first direct evidence that the material deep in the earth's mantle transition zone – a layer of water, crystals, and molten stone can percolates to the surface to form volcanoes.
Scientists have long known that volcanoes form when tectonic plates (traveling on the earth's mantle) converge or as a result of mantle plumes rising from the core mantle boundary to make hotspots at the Earth's crust. But getting evidence that material derived from the mantle's transition zone between 250 and 400 miles (440-660 km) below the earth's crust can cause volcanoes to be new to geologists.
"We found a new way to make volcanoes. This is the first time we found a clear indication from the transition zone deep in the earth's mantle that volcanoes can form in this way," says senior author Esteban Gazel, associate professor at the Department of Earth and Atmospheric Sciences At Cornell University, the research was published in Nature .
"We expected our data to show that the volcano was a mantle plume – an uproar from the deeper mantle – just like in Hawaii," Gazel said. 30 million years ago, a disturbance in the transition zone caused a stir of the magma material to rise to the surface, forming a hibernating volcano under the Atlantic and then forming Bermuda.
With a 2,600 foot (over 700 meter) core sample drilled in 1972, at home at Dalhousie University, Nova Scotia co-author Sarah Mazza at Münster University in Germany, assessed the cross section of isotopes, trace elements, evidence of cotton content and other volatile materials. The assessment gave a geological, volcanic history of Bermuda.
"I presumably assumed that Bermuda's volcanic past was special when I gathered the core and noticed the various structures and mineralogens preserved in the different lava flows," Mazza said. "We quickly confirmed extreme enrichment in track compositions. It was exciting to cross our first results … The Bermuda mysteries began to evolve."
From the nuclear samples, the group discovered geochemical signatures from the transition zone, which included larger amounts of water encapsulated in the crystals than was found in subduction zones. Water in subduction zones recovers back to the earth's surface. 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.
The geologists developed numerical models with Robert Moucha, a professor of soil science at Siracusa University, to detect transient disturbances likely to force materials from this deep coat layer to melt and percolate to the surface, Gazel said.
Despite more than 50 years of isotope measurements in oceanic lavor, the remarkable and extreme isotopes saturated in the Bermuda lava nucleus had not been observed before. But these extreme isotopic compositions made it possible for researchers to identify the unique source of lava.
"If we start to look more carefully, I think we will find these geochemical signatures in several places," says co-author Michael Bizimis, associate professor at the University of South Carolina.
Gazel explained that this research provides a new connection between the transition zone layer and the volcanoes on the earth's surface. "With this work, we can demonstrate that the Earth's transition zone is an extreme chemical reservoir," Gazel said. "We are now starting to recognize its importance in terms of global geodynamics and even volcanism."
Said Gazel: "Our next step is to explore more places to determine the difference between geological processes that can lead to volcanoes in the volcanoes and determine the role of the mantle's transition zone in the evolution of our planet."
Diamond inclusions suggest free-flowing water at the boundary between upper and lower sheath
Sarah E. Mazza et al., Sampling of the Volatile Rich Transition Zone during Bermuda, Nature (2019). DOI: 10,1038 / s41586-019-1183-6
From the deep mantle of the earth, scientists find a new way volcanoes (2019, May 15)
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