Scientists Revising Thoughts on Continental Plate Shifts

In a article on Phys, they report scientists have found clues in Alaska that has them rethinking how to continental crust forms based upon research published in Nature Geoscience.

A new study appearing in this week’s Nature Geoscience raises questions about one popular theory and provides new support for another, in which arc lava from the surface and shallow “plutons” – magma that solidified without erupting – are pulled down into the Earth at subduction zones and then rise up to accumulate at the bottom of the arc crust like steam on a kitchen ceiling. Scientists have found compelling evidence to suggest that this could have produced the vast majority of lower continental crust through Earth history.

The process, called relamination, starts at the edge of a continental plate, where an oceanic plate is diving under the continental plate and magma is rising to form a volcanic arc. As the oceanic plate dives, it drags down sediment, lava and plutonic rock from the edge of the arc. As arc material descends, minerals within it become unstable with the rising pressure and heat, and they undergo chemical changes. New minerals form, and chunks of the rock and sediment can break off. When those chunks are denser than the mantle rock around them, they continue to sink. But when they are less dense, such as those that form silica-rich granulites, they become buoyant and float upward until they reach the bottom of the arc crust and accumulate there.

For more information, see:

Mount Saint Helen’s Crystals Predict the Past

Scientific American reports that a team in England and Germany are using crystallized minerals formed in the volcano just before eruption to determine a timeline of volcanic activity, and possibly predication from a study in the May 25 issue of Science.

…the researchers report that crystals of the silicate mineral orthopyroxene from 1980 and from subsequent eruptions trace various injections of magma, as well as other chemical changes, within the bowels of the volcano.

The crystals contain concentric rings of differing chemical composition. Some orthopyroxene crystals, for instance, have a magnesium-rich core surrounded by an iron-rich rim; others have an iron-rich core and a magnesium-rich rim. Each type of crystal zonation can record the conditions of the magma reservoir from which it emerged.

“We chemically fingerprint each of those zones to determine how they formed,” says lead study author Kate Saunders, a volcanologist of the University of Bristol in England. The outer rim of an orthopyroxene crystal, she says, represents the most recent stage of crystal formation and typically grew just months before the crystal’s emergence in volcanic ejecta. That allowed the researchers to make precise estimates of when, and how, the crystals acquired their chemical forms. “Mount Saint Helens is really good—because the samples, we know exactly when they erupted,” Saunders says.

They hope that the study of these crystals will corroborate and offer insight into the historical timeline of erruptions, something researchers today can only guesstimate.

For more information, see “What’s the Point of Volcano Monitoring?” from Scientific American and “Linking Petrology and Seismology at an Active Volcano” from Science.