Melt transport times unexpectedly high
Scientists have successfully determined the permeability of the asthenosphere in the Earth's upper mantle and thus the rate at which melt rises to the Earth's surface: it flows up to 25 times faster than previously assumed. Thermo-mechanical and geochemical models on melt flows in volcanoes now have to be reconsidered.
A colossal centrifuge measuring about two meters in diameter is embedded in the floor in the cellar of the Department of Geosciences. It spins samples at 2800 rpm creating a radial acceleration of 3000 times the Earth's gravity. In full operation this centrifuge makes an infernal noise of 120 decibels. “That’s about as loud as if you were standing underneath an airplane”, says Max Schmidt, a professor from the Institute for Mineralogy and Petrology at ETH Zurich. The rim of the centrifuge reaches a speed of 850 km/h; if the machine is stopped by switching off the drive motor, it takes an hour to come to a standstill.
Schmidt joined ETH Zurich in 2001 with the idea of building a centrifuge in which, apart from increased acceleration, the temperature and pressure conditions characteristic of the Earth’s interior could be used to influence a sample. He was aided by a mechanic, an electronics technician and a company that specializes in producing centrifuges for sugar production or launderettes. After about one and a half years, the first “rough version” of this globally unique centrifuge was put into operation and improved continually. Now Schmidt’s research team has successfully used the centrifuge to determine the permeability of the asthenosphere – the area in the Earth’s upper mantle where the molten rock that feeds the volcanoes forms. The results were published in the science journal Nature.
The researchers simulated the asthenosphere’s melt transport conditions using basaltic glass from the mid-ocean ridge to represent the molten rock. In the experiment, the mineral olivine, which makes up two thirds of the Earth’s upper mantle, served as a matrix for the molten mass to flow through. They heated both to about 1300 degrees and exposed the mixture to a pressure of one gigapascal. The basaltic glass melted; based on the distance the molten mass covered through the olivine matrix when centrifuged at accelerations of 400 to 700g, the scientists were able to calculate the permeability directly by microscopically analyzing the samples prepared. This therefore enabled them to record specifically the constant which relates porosity (or melt volume) to permeability and that is important for thermo-mechanical models of melt transport.
Lava from the days of the Pharaohs
With a value of around ten, this constant is smaller by one and a half orders of magnitude than the value previously assumed for thermo-mechanical models. “Consequently, this also means that the magma speed in the mantle is one and a half orders of magnitude faster”, says Schmidt, “as the models calculated by James Connolly, an assistant professor at the institute, reveal.”
With such models, the flow behavior of magma melt in the basic tectonic areas of the Earth, such as on mid-ocean ridges upon which new oceanic crust forms, or on volcanically active tectonic plate margins – so-called subduction zones. For Schmidt, it is therefore clear that the existing models need to be revised in light of the constants now established. The melt that forms at a depth of about 120 kilometers does not need tens to hundreds of thousands of years to reach the Earth’s surface as was previously presumed; it only takes a few thousand years. “If a volcano erupts today, its magma did not form during the last ice age, but during the reign of the Pharaohs and around the birth of Christ”, states Schmidt.
This casts magmatism in a whole new light. Due to the rapid ascent, the
melt interacts much less with the rock it penetrates. This means that the
geochemical signals that bring the magma to the surface come from far greater
depths: “We are looking deeper than previously assumed”, says the mineralogist.
For the scientist, the rapid ascent also fits better with the fact that
volcanoes are only active for a few thousand years and the observation that
geochemical signals in the magma suggested a much faster rise until now.
Connolly JAD, Schmidt MW, Solferino G & Bagdassarov N. Permeability of asthenospheric mantle and melt extraction rates at mid-ocean ridges. Nature. 2009 November 11; 462: 209-213. doi:10.1038/nature08517