Magma takes the long way around

Magma takes the long way around

A new study in Science Advances shows how molten rock from Earth’s mantle, which eventually erupts, does not always take the shortest, most direct path available to reach volcanoes at the surface. These new insights help us to inform what drives the type and rate of volcanic eruptions and the make-up of erupted magma.

The published paper results from an international collaboration between scientists from the United Kingdom, the United States, Germany, and Trinidad and has involved Oxford Earth Scientists Prof. Mike Kendall and Prof. Jon Blundy.

When two titanic tectonic plates collide, one plate can sink (subduct) beneath the other, plunging into Earth’s mantle. These subduction zones are responsible for some of Earth’s most hazardous earthquakes and volcanic eruptions. However, it remains poorly understood how magma forms originally and what controls the exact position of volcanoes on top of the overlying plate.

Lead author, Stephen Hicks, said, “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. But in our study, we show that the interplay of these two driving forces over 100s of millions of years is key to controlling where eruptions occur today”.

Subducting oceanic plates act as giant reservoirs, transporting water into the deep Earth. These fluids enter the plate through fractures and faults formed during its birth (at mid-ocean ridges) and where it later bends beneath Earth’s deep ocean trenches. Water gets locked into fractures and bound into minerals within the plate.

Subducting plates are subjected to high pressures and temperatures as they plunge to 10s-100s kilometres depth. These extreme conditions cause the locked-in water (and other volatile elements) to be driven off. These fluids, which melt the warm mantle above, are the key ingredient of magma that eventually erupts around volcanic arcs at the edges of Earth’s oceans (such as the Pacific Ring of Fire). Yet the pathways that fluids and melt take deep within the Earth, from the subducting plate to the volcanic arc, cannot be directly seen nor easily inferred from what is erupted.

When seismic energy from earthquakes travels through different materials, the waves either slow down or speed up. Along with these speed changes, the energy of waves is also absorbed (attenuated). Hot and molten rock is particularly attenuating: it zaps energy from seismic waves as they travel through it.

The study used earthquakes to map seismic attenuation in 3D, similar to how a CT scan maps the internal structure of our bodies. As part of a multi-million-pound Natural Environment Research Council (NERC) funded project, the researchers collected seismic data from a subduction zone in the Eastern Caribbean that resulted in the Lesser Antilles’ volcanic islands.

Cross sections through the 3D models.

Stephen Hicks said, “Our knowledge of fluid and melt pathways has traditionally been focussed on subduction zones around the Pacific. We decided to study the subduction of the Atlantic instead because the oceanic plate there was formed much more slowly, and it subducts more slowly than the Pacific. We felt these more extreme conditions would make fluid and melt pathways more imagable using seismic waves. Hicks continues, “Because many subduction zones lie underwater, rather than land, we needed to deploy ocean-bottom seismometers to build an accurate 3-D picture of the subsurface”.

Unusually, the study found that the zone of strongest seismic attenuation at depth was laterally offset from beneath the volcanoes. These images led the authors to conclude that once water is expelled from the subducting plate, it is carried further downwards, leading to mantle melting behind the volcanic front. Melt then ponds at the base of the overriding plate before it is likely transported laterally back toward the volcanic arc.

Schematic view of dehydration and melting processes in the mantle wedge beneath the Lesser Antilles Arc (LAA) based on our combined interpretation of seismic attenuation and velocities.

Schematic view of dehydration and melting processes in the mantle wedge beneath the Lesser Antilles Arc (LAA) based on our combined interpretation of seismic attenuation and velocities.

The research paper can be found here:

This news article was written by lead author Stephen Hicks (who conducted the research whilst at Imperial but who is now at UCL) and is shared with permission.