Earth scientists reveal surprising path magma takes beneath volcanoes

Magma has been seen to travel an unusual path beneath volcanoes, offering information on the mechanisms underlying eruptions.

In a recent study from Imperial College London researchers, it was shown that rising magma does not necessarily follow the quickest, most direct route to reach volcanoes at the surface.

The research could shed some insight into the reasons why some volcanoes are more active than others and why volcanic activity changes over time.

What causes volcanoes at the Pacific Ring of Fire?

When two tectonic plates clash, the more dense plate may sink beneath the other (or subduct) and plunge between 10 and 100 kilometers deep into the Earth's mantle. 

These harsh conditions force away trapped water, as well as other volatile substances. Such fluids make up a crucial component of magma at volcanic arcs such as those found at the Pacific Ring of Fire. 

However, the route of these fluids through the Earth's interior – from the subducting plate to the volcanic arc – needs to be better understood.

Where does magma erupt?

The team used ocean-bottom seismometers to gather seismic (earthquake) data from a subduction zone in the Eastern Caribbean. This area gave rise to the volcanic islands of the Lesser Antilles. 

Their approach looked at waves generated by earthquakes. These either slow down or speed up as they pass through various materials. Additionally, waves also experience changes in their energy.

Knowing that hot and molten rock is highly 'attenuating' (or absorbing), the researchers used this understanding to identify where they were present at depth. They then used this data to create a precise 3D model of the subsurface. 

Unusually, the investigation discovered that the strongest seismic attenuation zone was displaced laterally from beneath the volcanoes. 

The map led the authors to conclude that as water is forced away from the subducting plate, it is dragged deeper, melting the mantle beneath the volcanic front.

After that, the melting congregates at the base of the underriding plate. It's then moved back towards the volcanic arc.

"Scientific views in this much-debated subject have traditionally fallen into two tribes. Some believe the subducting plate mostly controls where the volcanoes are, and some think the overlying plate plays the biggest role," said Lead author Dr. Stephen Hicks. 

"But in our study, we show that the interplay of these two driving forces over hundreds of millions of years is key to controlling where eruptions occur today."

He also highlighted that the team's research provides critical information about the mechanisms underlying volcanic eruptions. This could improve our understanding of the formation and replenishment of the magma reservoirs beneath volcanoes.

The paper was published in Science Advances on February 1 and is the result of an international collaboration between scientists from the United Kingdom, the United States, Germany, and Trinidad.