Deep Volcanic Flows
In the depths of the hot and turbulent earth, it is anything but peaceful: as large quantities of liquid water carve their way through the rock as fluids, causing magma pockets to form. A research team led by the University of Münster, has shown that the fluids flow a lot faster through solid rock than previously thought. In the Chinese Tian Shan Mountains, fluids pushed their way to the earth’s mantle from great depths in just 200 years rather than in the course of tens or even hundreds of thousands of years. The researchers from Münster, Kiel, Bochum, Erlangen, Bethlehem (USA) and Lausanne (Switzerland) present their findings, based on an innovative combination of fieldwork, geochemical analysis and numerical calculations, in the current issue of the journal Nature Geoscience.
When tectonic plates move towards each other and push over each other at the edges, so-called subduction zones are formed. Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma.
Liquid rock forms and rises through volcanoes as magma. Magma is a mixture of molten or semi molten rock, volatiles and solids. This magma feeds the many volcanoes throughout the world that occur along the convergent plate boundaries and form the Ring of Fire, a volcanic belt that encircles the Pacific Ocean.
The fluids are commonly assumed to flow through the rock in a defined flow system. Geologists call these structures veins.
During field work in the Chinese part of the Tian Shan Mountains (Celestial Mountains), the research team found structures in the rocks they were studying which can be ascribed to massive fluid flows at a great depth. "Our investigation has shown that a great deal of fluid must have flowed through a rock vein at about 70 km depth and that this fluid has obviously already covered a distance of several hundred meters or more - the transport of such large quantities of fluid over such a great distance has not been demonstrated by anyone before us" explains Timm John from the Institute for Mineralogy, University of Münster.
"And the most exciting thing is that this amount of fluid flowed through the rock in what is for geological processes a very short time, only about two hundred years", adds Nikolaus Gussone of the same institute.
The release of fluids from minerals in the descending plates is a large-scale and continuous process that takes place at depths up to two-hundred kilometers and takes millions of years. During this time, the fluids first accumulate before they flow. As the researchers have now shown for the first time, the released fluids then flowed through the plate on their way to the mantle in pulses in a relatively short time along defined flow paths. "It’s like a reservoir that continuously fills and then empties in a surge through defined channels" Timm John points out. "The fluid release is focused in space and time, and is much faster than expected - almost like a jet through solid rock".
It is also possible that such focused fluid releases are associated with the occurrence of earthquake events in subduction zones. To be able to demonstrate such relations, however, intensive research is still needed.
The RUB’s petrologists were involved in modelling the chemical data. This enabled the research team to determine the time it took the fluids to make their way to the mantle. Determining the time scales of various geological processes is a particular expertise of Bochum’s petrologists. Among other things, they use minerals and rocks with zones that exhibit a different chemical composition.
For further information see Jet.
Celestia Mountain by Timm John via University Munster.