New research shows that colliding continental plates may draw down more water than previously thought. The results could explain the explosiveness of some volcanic eruptions, as well as the distribution of ore deposits such as copper, silver and gold.
The findings are based on the analysis of ancient magmatic rocks recovered from the Himalayan mountains which is a geologic formation that is the product of a subduction zone over millions of years.
Subduction zones occur as one tectonic plate slides beneath another. As a result, it can take ocean water with it, drawing it deep into the mantle, where the liquid can merge with rising magma. The more water the magma contains, the more explosive an eruption may be. Therefore, subduction zones are the sites of some of the strongest and most destructive volcanic eruptions in the world.
A new study finds that magma at subduction zones can contain up to 20 percent water content by weight, about double the maximum water content that has been widely believed.
Previously, estimating the amount of water drawn down in subduction zones was done by analyzing volcanic rock that had erupted on the surface.
Using a different approach, the team looked at ancient magmatic rocks called plutons that remained deep beneath the surface, never having erupted in the first place. They reasoned these rocks would be more pristine recorders of the water they originally absorbed.
The team combined water measurements with the composition of minerals in the rocks crystals and plugged these numbers into an equation to back-calculate the amount of water that must have been absorbed originally by magma, just before crystallizing into its rock form.
The calculation revealed that the arc magmas contained an original water content of more than 8 percent by weight.
The team’s new figures may help to explain why volcanic eruptions in some parts of the world are stronger and more explosive than others.
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