The ancient volcanoes, now inactive, on which the island of Bermuda sits were formed in a completely unique way, scientists have discovered. The finding not only solves a long-standing mystery about the volcanic origins of the island, but also describes a new way in which volcanoes form.
When studying a rock core sample taken from Bermuda, drilled since 1972, geoscientists discovered the first direct evidence that the material that is within the transition zone of the Earth's mantle, a layer rich in water, crystals and melted rock, it can seep to the surface to form volcanoes.
Researchers have known for a long time that volcanoes form when tectonic plates converge, or as a result of mantle plumes that rise from the core-mantle boundary to make hot spots in the earth's crust.
But finding that material in the transition zone of the mantle, some 250 to 400 miles below the crust of our planet, can cause the formation of volcanoes is something new for geologists, according to the National Science Foundation.
"We found a new way to make volcanoes," said geologist Esteban Gazel, an associate professor at Cornell University and lead author of the article published in Nature. "This is the first time we have found a clear indication of the transition zone deep within the Earth's mantle that volcanoes can be formed in this way."
According to Gazel, the researchers expected the data to show that the volcano was a mantle plume formation, an emergence of the deeper mantle, like Hawaii. This was not what they found.
"I first suspected that Bermuda's volcanic past was special when I tested the nucleus and noticed the various textures and mineralogy preserved in the different lava flows," said the article's co-author, Sarah Mazza, of the University of Münster, Germany. "We quickly confirm extreme enrichments in trace element compositions. It was exciting to review our first results … the mysteries of Bermuda began to develop. "
From the core samples, the researchers detected geochemical signatures from the transition zone, which included larger amounts of water encased in the crystals than those found in the subduction zones. The water in the subduction zones is recycled back to the surface of the Earth. There is enough water in the transition zone to form at least three oceans, according to Gazel, but it is the water that helps the rock melt in the transition zone.
The geoscientists developed numerical models and discovered a disturbance in the transition zone that probably forced the material of this deep mantle layer to melt and leak to the surface. It is believed that this occurred about 30 million years ago and provided the basis on which Bermuda is based today.
Despite more than 50 years of isotopic measurements in oceanic lavas, the peculiar and extreme isotopes measured in the lava core of Bermuda were something scientists had never seen before.
With the knowledge of this new model for the manufacture of volcanoes, Bermuda can not be alone: there could be other volcanoes in the Atlantic Ocean that were formed by the same process or similar processes, said Mazza. "We just have not found them yet," she says.
Gazel said the research provides a new connection between the transition zone layer and the volcanoes on the surface of the Earth.
"With this work we can show that the transition zone of the Earth is an extreme chemical reservoir," he said. "We are now beginning to recognize its importance in terms of global geodynamics and even volcanism."
Read the full study here.
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