Geologists have been trying for years to discover the forces that give rise to the kind of volcanoes that form the so-called "ring of fire" around the Pacific Ocean that form an arc for 40,000 kilometers in a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and or plate movements. The "Ring of Fire" has 452 volcanoes and is home to over 75% of the world's active and dormant volcanoes, which are produced when one of the tectonic plates that make up planet's crust plunges beneath another plate, a process called subduction.
Understanding the process that produces arc volcanoes is important because, among other things, most of the world's major deposits of such metals as silver, copper and molybdenum occur in these formations.
What was unclear was what factors controlled when, how and at what depth fluids and molten rock from these subducting plates are released, giving rise to the molten magma in the Earth's mantle that would then come spewing to the surface in the form of a volcanic eruption as happened in the St. Helen's eruption in 1980 (see video below).
The new findings will force a rewriting of textbooks and encyclopedias, says Timothy Grove, Professor of Geology Timothy Grove of the Department of Earth, Atmospheric and Planetary Sciences at MIT .The conventional understanding has been that the depth to these descending slabs under arc volcanoes is always 100 kilometers, but recent analysis shows that in fact the depth can vary considerably, from around 60 km to more than 170 km, depending on a number of factors.
Grove says the discovery of this variability in depths led his team to question why that is. One key variable turned out to be the characteristics of a particular mineral called chlorite that forms in the mantle above the oceanic crust. Chlorite contains a large amount of water, and this water is released when the chlorite breaks down at specific combinations of temperature and pressure. Chlorite breakdown occurs at particular depths in the Earth's mantle determined by the exact angle of the slab as it plunges downward. "The stability of this mineral is the key factor in our paper," Till says, because that's what limits the melting process to such a narrow range of conditions. The speed at which the two plates are converging, the team found, has relatively little effect on the melting depth.
"By knowing that process, we can independently come up with a model for the thermal structure below these volcanoes, and why arc magmas come from these certain depths," Till says. Until this research, she says, "we were still missing that link in how arc volcanoes form."
Posted by Jason McManus.
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