Understanding the Moho may help predict volcanic eruptions

 The molten rock that feeds volcanoes can be stored in the Earth’s crust for as long as a thousand years, a new study suggests.

And this knowledge may help scientists to better forecast when eruptions occur and manage the hazards, say the researchers from the University of Cambridge in the UK.

Writing in the journal Science, they describe using volcanic minerals known as “crystal clocks” to calculate how long magma can be stored in the deepest parts of volcanic systems. 

It is, they say, the first estimate of magma storage times near the Moho – the boundary of the Earth’s crust and the mantle. 

“This is like geological detective work,” says first author Euan Mutch. “By studying what we see in the rocks to reconstruct what the eruption was like, we can also know what kind of conditions the magma is stored in, but it’s difficult to understand what’s happening in the deeper parts of volcanic systems.” {%recommended 9412%}

Mutch and colleagues studied the Borgarhraun eruption of the Theistareykir volcano in northern Iceland, which occurred roughly 10,000 years ago, and was fed directly from the Moho.

To calculate how long the magma was stored at this boundary area, they used a volcanic mineral known as spinel, which is much like a tiny stopwatch or crystal clock.

They were able to model how the composition of the spinel crystals changed over time while the magma was being stored. Specifically, they looked at the rates of diffusion of aluminium and chromium within the crystals and how these elements are “zoned”.

“Diffusion of elements works to get the crystal into chemical equilibrium with its surroundings,” said co-author John Maclennan. “If we know how fast they diffuse we can figure out how long the minerals were stored in the magma.”

The researchers looked at how aluminium and chromium were zoned in the crystals, realising that this pattern was telling them something new about magma storage time. 

The diffusion rates were estimated using the results of previous lab experiments. The researchers then used a new method, combining finite element modelling and Bayesian nested sampling to estimate the storage timescales.

“We now have really good estimates in terms of where the magma comes from in terms of depth,” said Mutch. “No one’s ever gotten this kind of timescale information from the deeper crust.”

Calculating the magma storage time also helped the researchers determine how magma can be transferred to the surface. 

Instead of the classical model of a volcano with a large magma chamber beneath, the researchers say that instead, it’s more like a volcanic “plumbing system” extending through the crust with lots of small spouts where magma can be quickly transferred to the surface.